8) October 2012 | Anne’s Astronomy News
The Ghost Nebula, a reflection nebula in Cepheus
Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona ( http://www.caelumobservatory.com/index.html )
With this cosmic ghost I’m wishing you all a happy and spooky Halloween!
The Ghost Nebula (designated Sharpless 2-136 (Sh2-136) and vdB 141) is a rather isolated reflection nebula over 2 light-years across, located some 1200 light-years away at the edge of the Cepheus Flare molecular cloud complex in the constellation Cepheus.
It is nicknamed the “Ghost Nebula” due its spooky appearance and to several human-like figures with arms raised, rising up from the top of the cloud structure to the left of the bright reflection.
The complex process of star formation create dust clouds of many shapes and sizes. In the case of the Ghost nebula, spooky shapes seem to haunt this starry expanse drifting through the night. Of course, these shapes are also cosmic dust clouds. Several stars are embedded in the nebula, and their light gives it a ghoulish brown color.
Also cataloged as Bok globule CB230, the core of the dark cloud on the right side of this image is collapsing and is likely a binary star system in the early stages of formation, identified as BD+67 1300.
Bok globules are dark clouds of dense cosmic dust and gas within star-forming regions in which usually star formation takes place. They most commonly result in the formation of double or multiple star systems.
The name Sharpless comes from a catalog of 312 emission nebulae (H II regions). The first edition was published by Stewart Sharpless in 1953 with 142 objects (Sh1) and the second and final version was published in 1959 with 312 objects (Sh2). The “vdB” stands for “van den Bergh”; vdB 141 is reflection nebula number 141 in Sidney van den Bergh’s Catalog of Reflection Nebulae, created in 1966.
North is to the bottom… it looks much better upside down ;-)
October 30, 2012
The Fireworks Galaxy, a spiral galaxy in Cepheus and Cygnus
Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona ( http://www.caelumobservatory.com/index.html )
Today it’s my birthday, so let’s have a party… with fireworks!!
The Fireworks Galaxy (also known as NGC 6946, Arp 29, or Caldwell 12) is an intermediate spiral galaxy of about 20,000 light-years across, located just some 10 million light-years away, on the border between the constellations of Cepheus and Cygnus, and is moving away from us at approximately 45 kilometers per second.
The galaxy is undergoing a tremendous burst of star formation with no obvious cause. In many cases spirals light up when interacting with another galaxy, but this galaxy appears relatively isolated in space. An explanation for the high star formation rate is the recent accretion of many primordial low-mass neutral hydrogen clouds from the surrounding region.
The center of the Fireworks galaxy is home to a nuclear starburst itself, and picturesque dark dust lanes are seen lacing the disk, and it has widespread high-velocity clouds associated with the disk. The colors of the galaxy change from the yellowish light of old stars in the center to young, bright blue star clusters and reddish star-forming regions along its spiral arms. has widespread high-velocity clouds associated with the disc
NGC 6946 is called the “Fireworks Galaxy” because so many supernovae have been spotted there in the last hundred years. Nine supernovae (SN 1917A, SN 1939C, SN 1948B, SN 1968D, SN 1969P, SN 1980K, SN 2002hh, SN 2004et, and SN 2008S) have been observed in the galaxy. With this number of supernovae, NGC 6946 is leading the statistics, just one SN more than follow-up M83, the Southern Pinwheel galaxy.
The Fireworks galaxy is highly obscured by interstellar matter of our own Milky Way galaxy, as it is quite close to the galactic plane.
October 29, 2012
Messier 69, a globular cluster in Sagittarius
Image Credit: ESA/Hubble & NASA
Messier 69 (also known as NGC 6637) is a globular cluster of roughly 85 light-years across, with an estimated mass of 300,000 solar masses and an age of 13.06 billion years. It is located some 29,700 light-years away in the constellation Sagittarius (the Archer), quite close to the galactic center (only about 6,200 light-years distant), and is moving away from Earth at approximately 39.1 kilometers per second.
Globular clusters are dense collections of old stars. Messier 69, rather similar to its neighbor Messier 70 (with only 1,800 light-years separating the two objects), is one of the smaller and fainter globular clusters in Messier’s catalog.
Despite its age, Messier 69 is one of the most metal-rich globular clusters known. In astronomy, the term “metal” refers to any element heavier than the two most common elements in our Universe, hydrogen and helium. The nuclear fusion that powers stars created all of the metallic elements in nature, from the calcium in our bones to the carbon in diamonds. Successive generations of stars have, via supernova explosions at the end of their lives, built up the metallic abundances we see today.
Because the stars in globular clusters are ancient, their metallic abundances are much lower than more recently formed stars, such as the Sun. Studying the makeup of stars in globular clusters like Messier 69 has helped astronomers trace back the evolution of the cosmos.
In this picture, foreground stars look big and golden when set against the backdrop of the thousands of white, silvery stars that make up M69.
The image is made by the NASA/ESA Hubble Space Telescope, and is a combination of exposures taken in visible and near-infrared light by Hubble’s Advanced Camera for Surveys.
October 28, 2012
NGC 2787, a lenticular galaxy in Ursa Major
Image Credit: M. Carollo (Swiss Federal Institute of Technology, Zurich), NASA and The Hubble Heritage Team (STScI/AURA)
NGC 2787 is a barred lenticular galaxy of about 4500 light-years across, located approximately 24 million light-years away in the constellation Ursa Major. It is moving away from us at about 696 kilometers per second.
Lenticular galaxies are lens-shaped galaxies and, like spiral galaxies, they contain a disk, but like elliptical galaxies, they are usually short on dust and gas. They show little or no evidence of the grand spiral arms that occur in their more photogenic cousins. Lenticulars are relatively little studied, possibly because of their seemingly benign nature.
However, despite NGC 2787’s seemingly bland qualities astronomers did take a look at the center of this galaxy in order to understand what happens in the center and to help determine how lenticular galaxies formed, including the role of galaxy collisions and central black holes.
Pictures and evidence indicate that lenticulars can be both photogenic and scientifically interesting after all. For example, the above image of NGC 2787. Although the galaxy has no appreciable spiral structure or ongoing star formation, this image shows that the center of this lenticular galaxy does have an interesting structure. It contains a faint bar (not apparent in this image) and a bright nucleus, encircled with tightly wound, almost concentric, arms of dark dust.
Measurements of the speed of gas quite near the center of the galaxy show that it is accelerated to high speed, probably by a supermassive black hole that is quietly consuming small amounts of matter. Astronomers study this black hole to learn more about the relationship between supermassive black holes and their parent galaxies. NGC 2787 may be in the latest stages of an ultra-luminous infrared galaxy (ULIRG).
There are about a dozen globular clusters hovering around NGC 2787 visible in this image. What appear to be stars are, in fact, gravitationally bound families of hundreds of thousands of ancient stars orbiting the center of the galaxy.
This image was made with the Hubble Space Telescope using Hubble’s Wide Field Planetary Camera 2 in January 1999, by combining light from blue, green and infrared filters.
October 27, 2012
IRAS 13208-6020, a protoplanetary nebula in Centaur
Image Credit: ESA/Hubble & NASA
IRAS 13208-6020 is a bipolar protoplanetary nebula in the constellation Centaur, formed from material that is shed by a dying central star. The nebula has two very similar outflows of material in opposite directions and a clearly visible dusty ring surrounding the central star.
Protoplanetary nebulae are clouds of dust and gas formed from material shed by an aging central star similar in mass to our Sun. For such a star death is a long process. After billions of years, the hydrogen fuel that powers the star begins to run out. The star balloons to great size and becomes a red giant. Then the core shrinks and heats up. At the same time the star ejects its cooler outer layers, enveloping itself in clouds of gas, but the core is not yet hot enough to make the gas itself glow on its own. Instead, the gas is merely reflecting the light from the star.
But as the star continues to evolve, it becomes hot enough to emit strong ultraviolet light. At that point it will have the power needed to make the gas glow, and will become a real full-fledged planetary nebula. But before the nebula begins to shine, fierce winds of material ejected from the star will continue to shape the surrounding gas into intricate patterns that can only be truly appreciated later once the nebula begins to glow. All that is left after this process is the exposed, hot and dead core, known as a white dwarf.
A protoplanetary nebula is a relatively short-lived phenomenon, so finding one is a rare opportunity for astronomers to learn more about them and to observe the beginning of the formation of planetary nebulae (hence the name protoplanetary, or preplanetary nebulae).
Despite their name, planetary nebulae and (thus) protoplanetary nebulae have nothing to do with planets. The name of planetary nebulae arose because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through early telescopes.
Although there are lots of (proto)planetary nebulae shaped like IRAS 13208-6020, with twin outflows of material resembling a butterfly, its shape is really strange. A star all alone in space would normally emit gas in a rough sphere around itself, but in this case something is shaping the wind. Most likely it’s some sort of companion: a star orbiting very close in, perhaps. Or, it’s possible that the central star had planets, and as it was dying it expanded, engulfing those planets. As they orbited inside the star, frying the whole time, they would help accelerate the star’s spin. That too could produce the bipolar shape of the gas around the star.
This picture was created from images taken with the NASA/ESA Hubble Space Telescope, using the High Resolution Channel of Hubble’s Advanced Camera for Surveys. Images taken in visible light and in infrared light have been combined to create this picture.
Usually, (proto)planetary nebulae are faint in the infrared, but this one is quite bright in that light; the gas itself doesn’t emit much infrared light, but the star sure does, and the gas is reflecting it.
October 26, 2012
The Cartwheel Galaxy, a ring galaxy in Sculptor
Image Credit: ESA/Hubble & NASA
The Cartwheel Galaxy (ESO 350-40) is a ring galaxy of some 150,000 light-years across (which is slightly larger than our Milky Way galaxy), located 496 million light-years away in the southern constellation Sculptor. It is speeding away from us at about 9050 kilometers per second and rotates at 217 kilometers per second. The galaxy has a mass of about 3.85 billion solar masses.
Previously, scientists believed the bright blue ring marked the outermost edge of the galaxy, but the latest observations detect a faint disk, not visible in this image, that extends to twice the diameter of the ring. This means the Cartwheel is a monstrous 2.5 times the size of the Milky Way.
The blue ring of starburst reveals billions of newly born stars, caused by a rare and spectacular head-on collision with a smaller companion approximately 200 million years ago (i.e., 200 million years prior to the image).
When the nearby intruder galaxy — possibly one of the two galaxies on the left side of the image — passed through the Cartwheel Galaxy, the force of the collision caused a powerful shock wave through the galaxy, much like the ripples produced when a stone is dropped into a lake.
Moving at the high speed of more than 300,000 km per hour, the shock wave swept up gas and dust, producing the expanding starburst ring around the galaxy’s center. The most recent star burst (star formation due to compression waves) has lit up the outermost ring of the Cartwheel.
Star formation via starbursts result in the formation of massive and extremely luminous stars. When these stars explode as supernovae, they leave behind neutron stars and black holes. Some of these neutron stars and black holes have nearby companion stars, and become powerful X-ray sources as they pull matter off their companions.
While most galaxies have only one or two bright X-ray sources, the Cartwheel contains an exceptionally large number of them. The brightest X-ray sources in the Cartwheel are likely black holes with companion stars, and appear as the white dots that lie along the rim of the image, because many massive stars formed in the rim.
The Cartwheel Galaxy was once a normal spiral galaxy similar to our home galaxy, the Milky Way, before it underwent the collision, with spiral arms winding outward from the galaxy’s center. The galaxy is beginning to take the form of a normal spiral galaxy again, as seen in the faint arms or spokes between the outer ring and the bulls-eye shaped nucleus.
Usually a galaxy is brighter toward the center, but the collision actually smoothed out the interior of the galaxy, concentrating older stars and dust into the inner regions, like the calm after the storm of star formation.
The Cartwheel Galaxy is one of the most dramatic examples of the small class of ring galaxies, and it is an unusual opportunity to study new stars, because many stars would ordinarily take much longer to form.
This image was taken with the NASA/ESA Hubble Space Telescope.
October 25, 2012
LH 72, a small group of stars embedded in a nebula in the LMC
Image Credit: ESA/Hubble, NASA and D. A. Gouliermis
LH 72 is a young and bright OB association embedded in a dense nebula of hydrogen gas, located about 160,000 light-years away in one of the largest known star-forming regions in the Large Magellanic Cloud (a small irregular satellite galaxy of our Milky Way galaxy) in the constellation Dorado.
An OB association is a loosely organized stellar grouping that usually contains 10–100 stars of type O and B — these are high-mass stars that have short but brilliant lives. There are several such groupings of stars in the Large Magellanic Cloud. Just like the others, LH 72 consists of several high-mass young stars with ages ranging from 5 to 30 million years. Their ages are increasing from the south to the north end of LH 72. The OB association lies in a supergiant shell of gas called LMC-4.
Much of the star formation in the Large Magellanic Cloud occurs in such supergiant shells. These regions of interstellar gas are thought to have formed due to strong stellar winds and supernova explosions that blew away dust and gas around the stars creating wind-blown shells. The swept-up gas eventually cools down and fragments into smaller clouds that dot the edges of these regions and eventually collapse to form new stars. Over a period of several million years, thousands of stars may form in these supergiant shells, which are the largest interstellar structures in galaxies.
LMC-4 is, with a diameter of about 6000 light-years, the biggest of these shells, and it is also the largest in the Local Group of galaxies that is home to both the Milky Way and the Large Magellanic Cloud.
LH 72 is the only OB association embedded in significant amounts of ionized neutral gas that lies within LMC-4. Thus, LH 72 may hold a special clue to LMC-4’s formation. Studying gas-embedded young associations of stars like LH 72 is a way of probing the supergiant shells to understand how they formed and evolved.
This image was taken with Hubble’s Wide Field Planetary Camera 2 using five different filters in ultraviolet, visible and infrared light.
October 24, 2012
NGC 1398, a large, barred spiral galaxy in Fornax
 Image Credit & Copyright: Adam Block/Sean & Renee Stecker/NOAO/AURA/NSF ( http://www.caelumobservatory.com/obs/bestofaop.shtml )
NGC 1398 is a large, barred spiral galaxy with a diameter of approximately 135,000 light-years, located about 65 million light-years away in the constellation Fornax, and it is part of the Fornax cluster of galaxies. NGC 1398 has a mass of some 300 billion solar masses. It is moving away from us at roughly 1400 kilometers per second.
It is a strikingly symmetric galaxy that features a bar with a very bright central bulge and a dense bright inner ring with a rope-like structure. The ring is actually two spiral arms that are closed in on each other. In contrast to its well-defined center, NGC 1398 has rather faint, patchy, or flocculent, nearly circular spiral arms which start tangent to the ring and extend out close to a full revolution, accented by bright regions.
NGC 1398 had one known supernova explosion, northeast of its nucleus: SN 1996N.
October 23, 2012
N 164, an emission nebula in the LMC
Image Credit: ESO
N 164 is a bright emission nebula and star-forming cloud, located within the Large Magellanic Cloud — an irregular satellite galaxy to our Milky Way galaxy — about 157,000 light-years away in the constellation of Dorado.
New hot stars inside the nebula are forming out of interstellar gas and dust, and they are causing the nebula to glow. The heating of the gas by these stars increases the pressure, and this causes the nebula to expand, pushing outwards against their surroundings. When you look carefully at N 164, the nebula reveals locations where the expansion is encountering resistance by denser clouds of gas, producing bright, thin rims.
October 22, 2012
The Eyes, a galaxy pair in Virgo
Image Credit & Copyright: Ken Crawford, Rancho Del Sol Observatory ( http://www.imagingdeepsky.com  and  www.aicccd.com )
The Eyes (also called Markarian’s Eyes) are two galaxies (NGC 4438 (left) and NGC 4435), about 52 million light-years away and some 100,000 light-years apart in the constellation Virgo. They are rather prominent in Markarian’s Chain, a string of galaxies in the central core of the Virgo Galaxy Cluster.
NGC 4438, the larger galaxy, is thought to have once been a spiral galaxy that was strongly deformed by collisions in the relatively recent past. It has a highly distorted disk and long tidal tails. A lane of obscuring dust is visible just below its weak nucleus and young stars are visible to the left of its center.
NGC 4435 is a compact barred lenticular galaxy with a bright core of more than 50% of its diameter, and has a relatively young stellar population on its central regions (age of 190 million years). It seems to be almost devoid of gas and dust, but does have a very faint extension in the opposite direction of NGC 4438. It appears completely free of any tidal disturbances.
There is a debate about whether NGC 4435 and NGC 4438 are interacting.
According to one theory NGC 4435 is to blame. The effects seen in NGC 4438 were caused when the two galaxies came within about 16,000 light-years of each other about 100 million years ago. But while the more massive NGC 4438 was damaged, the smaller NGC 4435 was significantly more affected by the collision. Gravitational tides from this close encounter are probably responsible for ripping away their stars, gas, and dust. NGC 4438 managed to hold on to much of the material torn out in the collision, while material from NGC 4435 was more easily lost.
Another theory is that the giant elliptical galaxy Messier 86, further away from The Eyes, was responsible for the damage to NGC 4438. Recent observations have found filaments of ionized hydrogen gas connecting the two large galaxies, suggesting that they may have collided in the past.
In rich clusters of galaxies, such as the Virgo Cluster, galaxy collisions are fairly frequent, so it’s possible NGC 4438 suffered from encounters with both NGC 4435 and Messier 86.
There is uncertainty surrounding the energy mechanism that heats the nuclear source of NGC 4438; this may be a starburst region, or an active galactic nucleus powered by a supermassive black hole.
In this image you can also see many distant background galaxies.
October 21, 2012
G1.9+0.3, the youngest known supernova remnant in the Milky
Image Credit: X-ray (NASA/CXC/NCSU/S.Reynolds et al.); Radio (NSF/NRAO/VLA/Cambridge/D.Green et al.); Infrared (2MASS/UMass/IPAC-Caltech/NASA/NSF/CfA/E.Bressert)
Supernova remnant G1.9+0.3 was created by the explosion of a star about 25,000 light-years away in the constellation Sagittarius, near its border with Ophiuchus. Although the explosion occurred about 25,000 years ago, its light reached Earth just about 110 years ago (around the year 1900), what makes it the youngest known supernova remnant in the Milky Way Galaxy. (Prior to this discovery in 1984, the youngest-known Milky Way supernova remnant was Cassiopeia A, at about 330 years.) The expanding remnant is now some 2.6 light-years across.
The radio image from NRAO’s Very Large Array (VLA) obtained in 1985 (shown in blue) is overlaid by NASA’s Chandra X-ray Observatory image from early 2007 (in orange). The remnant had expanded by a surprisingly large amount, about 16% since 1985. The difference in size between the two images gives clear evidence for expansion of the original supernova explosion. This rapid expansion and young age for G1.9+0.3 was confirmed by a new VLA image obtained in early 2008.
The original supernova explosion – most likely a Type Ia supernova – was not seen in optical light about 145 years ago because it occurred close to the center of the Galaxy, and is embedded in a dense field of gas and dust. This made the supernova about a trillion times fainter, in optical light, than if it had been unobscured. Fortunately, the expanding gas cloud from the explosion shines brightly in radio waves and X-rays for thousands of years. X-ray and radio telescopes can see through all that obscuration and show astronomers what they had been missing otherwise.
Supernova remnants are caused when the debris thrown outwards by the explosion crashes into surrounding material, generating a shell of hot gas and high-energy particles that glows brightly in X-rays, radio waves and other wavelengths for thousands of years. (A Type Ia supernova is a result from the violent explosion of a white dwarf star.)
In the case of G1.9+0.3 the material is expanding outwards at almost 35 million miles (56 million kilometers) per hour, or about 5% the speed of light, an unprecedented expansion speed for a supernova remnant. Another superlative for G1.9+0.3 is that it has generated the most energetic electrons ever seen in a supernova remnant.
Astronomers regularly observe supernovae in other galaxies like ours. Based on those observations, researchers estimate about three explode every century in the Milky Way. If the supernova rate estimates are correct, there should be the remnants of about 10 supernova explosions that are younger than Cassiopeia A.
October 20, 2012
The Andromeda Galaxy, our large neighbour galaxy
Image Credit & Copyright: Dieter Beer ( http://www.starhopper.at/ ) & Patrick Hochleitner ( http://www.photonhunter.at )
The Andromeda Galaxy (Messier 31 or NGC 224) is a barred spiral galaxy of more than 220,000 light-years across, located approximately 2.54 million light-years away in the Andromeda constellation. It is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy (M33), and about 30 other smaller galaxies.
Although the largest, Andromeda may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and could be the most massive of the two. Mass estimates for Andromeda (including dark matter) give a value of approximately 1.2 trillion solar masses compared to 1.9 trillion solar masses for the Milky Way. But, Andromeda has a higher stellar density than that of the Milky Way. Andromeda contains one trillion stars: at least twice the number of stars in our Milky Way, which is estimated to be 200–400 billion. There may be several stellar black holes or neutron stars in Andromeda, which are heating incoming gas to millions of kelvins and emitting X-rays.
Andromeda appears to have also significantly more common stars than the Milky Way, and the estimated luminosity of Andromeda is about 25% higher than that of our own galaxy. However the rate of star formation in the Milky Way is much higher, with Andromeda producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. Andromeda contains two distinct populations of stars based on their metallicity, young, high velocity stars in the disk and older, red stars in the bulge.
The rate of supernovae in the Milky Way is also double that of Andromeda. This suggests that Andromeda once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation. Should this continue, the luminosity in the Milky Way may eventually overtake that of Andromeda.
Andromeda has a bright yellow nucleus, dark winding dust lanes and young star clusters, while reddish star-forming regions are dotting the clockwise wound-up spiral arms. These arms appear to be tightly wound, although they are more widely spaced than in our galaxy. There are two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years. These can be followed outward from a distance of roughly 1,600 light-years from the core.
Andromeda’s nucleus consists of two concentrations separated by 4.9 light-years. The brighter concentration, designated as P1, is offset from the center of the galaxy and has no black hole at its center. The dimmer concentration, P2, falls at the true center of the galaxy and does contain a supermassive black hole. The velocity of material spiralling around it is about 160 km/s.
There are approximately 460 globular clusters in Andromeda, including a dense and compact star cluster at its very center. The most massive of these clusters, Mayall II, has a greater luminosity than any other known globular cluster in the Local Group of galaxies. It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Mayall II (G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider Mayall II to be the remnant core of a dwarf galaxy that was consumed by Andromeda in the distant past.
Unlike the globular clusters of the Milky Way, that show a relatively low age dispersion, Andromeda’s globular clusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years.
In 2005, astronomers discovered a completely new type of star cluster in Andromeda. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that these stars are much larger (several hundred light-years across) and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.
In 2009, the first planet has been discovered in the Andromeda Galaxy. This exoplanet was detected using a technique called microlensing, which is caused by the deflection of light by a massive object.
Both Andromeda and the Milky Way are likely to have accreted and assimilated about 1–200 small galaxies during the past 12 billion years. The most important event in Andromeda’s past history was the merger with another galaxy that took place about 8 billion years ago. This violent collision formed most of its (metal-rich) galactic halo and extended disk and during that epoch Andromeda’s star formation would be very high, to the point of becoming a Luminous infrared galaxy during roughly 100 millon years.
Andromeda and the Triangulum Galaxy had a very close passage 2-4 billion years ago. This event produced high levels of star formation across the Andromeda Galaxy’s disk -even some globular clusters- and disturbed Andromeda’s outer disk.
While there has been activity during the last 2 billion years, this has been much lower than during the past. There have been interactions with satellite galaxies like M32, M110, or others that have already disappeared, absorbed by Andromeda that have formed structures like Andromeda’s Giant Stellar Stream and a merger roughly 100 million years ago that is behind a counter-rotating disk of gas found in the center of M31 as well as the presence there of a relatively young (100 million years) stellar population. During this epoch, star formation through Andromeda’s disk decreased to the point of nearly shutting down to increasing again relatively recently.
The Andromeda Galaxy is approaching the Milky Way at about 100 to 140 kilometres per second (400 light-years every million years). The Andromeda Galaxy and the Milky Way are thus expected to collide in about 3.75 to 4.5 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.
Like the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most observed satellites are M32 and M110. M32 may once have been a larger galaxy that had its stellar disk removed by Andromeda, when they had a close encounter more than 200 million years ago, and underwent a sharp increase of star formation in the core region, which lasted until the relatively recent past.
M110 also appears to be interacting with M31, and astronomers have found in the halo of Andromeda a stream of metal-rich stars that appear to have been stripped from both of these satellite galaxies. M110 contains a dusty lane, which may indicate recent or ongoing star formation.
Because Andromeda is one of the brightest Messier objects, it is visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. However, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.
October 19, 2012
Messier 17 (the Omega or Swan Nebula), the central part
Image Credit: ESO/R. Chini
Messier 17 (also catalogued NGC 6618, and known as the Omega, Swan, Horseshoe, and Lobster Nebula) is a diffuse emission nebula and H II (star-forming) region, located roughly 5,500 light-years away in the heart of the Milky Way in the constellation of Sagittarius (the Archer). While the bright nebula seems to be roughly 15 light-years across, the total gaseous cloud extends to at least 40 light years.
It is one of the brightest and most massive star-forming regions of our galaxy. The total region of gas, dust and hot young stars has a mass of 30,000 solar masses, while the mass of the bright part of Messier 17 is an estimated 800 solar masses. This image of its central region shows vast clouds of gas and dust illuminated by the intense radiation from young stars.
In the centre of Messier 17 is a cluster of 35 massive young stars whose intense radiation makes the surrounding hydrogen gas glow. However, the actual number of stars in the nebula is much higher -up to 800, plus more than 1000 stars in formation on its outer regions. With an age of just 1 million years it is one of the youngest star clusters known.
To the lower right of the cluster is a huge cloud of molecular gas. At visible wavelengths, dust grains in the cloud obscure our view, but by observing in infrared light, the glow of the hydrogen gas behind the cloud can be seen shining faintly through.
Hidden in this region, which has a dark reddish appearance, the astronomers found the opaque silhouette of a disk of gas and dust. Although it is small in this image, the disc has a diameter of about 20,000 AU, dwarfing our Solar System (1 AU is the distance between the Earth and the Sun). It is thought that this disc is rotating and feeding material onto a central protostar — an early stage in the formation of a new star.
It is a popular target for amateur astronomers, who can obtain good quality images using small telescopes. With a dark observing site and a keen pair of eyes Messier 17 can even be seen with the naked eye.
This composite image is created by using data from ESO’s Very Large Telescope (VLT), at the Paranal Observatory in Chile, made at near-infrared wavelengths with the ISAAC instrument.
October 18, 2012
The Triangulum Galaxy, a nearby spiral galaxy
Image Credit & Copyright: Roth Ritter, Dark Atmospheres Astrophotography ( http://www.darkatmospheres.com/astro )
The Triangulum Galaxy (catalogued as Messier 33 or NGC 598) is a spiral galaxy, located approximately 3 million light-years away in the constellation Triangulum. With a diameter of about 50,000 light years (half the diameter of the Milky Way), the Triangulum galaxy is the third largest member of the Local Group (a group of galaxies which also contains the Milky Way, Andromeda and about 30 other smaller galaxies). However, the faintest outlayers seem to reach more far out, so that the true diameter may be some 82 million light-years. It is approaching our Galaxy at 24 kilometers per second.
Triangulum may be home to 40 billion stars, compared to 400 billion for the Milky Way, and 1 trillion (1000 billion) stars for Andromeda. The mass of the Triangulum Galaxy has been estimated between 10 and 40 billion solar masses.
The disk-shaped galaxy, surrounded by a faint halo, has prominent loosely wound arms of gas and dust that spiral out from the nucleus, and dark dust lanes. There is no bulge at the nucleus. The inner part of the galaxy has two luminous spiral arms, along with multiple spurs that connect the inner to the outer spiral features. The main arms are designated IN (north) and IS (south).
Although Triangulum is classified as unbarred, there may be a weak bar-like structure about the galactic nucleus. The nucleus of this galaxy is an H II (star forming) region, and it contains an ultraluminous X-ray source, which is the most luminous source of X-rays in the Local Group. However, the nucleus does not appear to contain a supermassive black hole.
Triangulum appears to be divided into two distinct components with a different star formation history between the inner disk (within a radius of 30 million light-years) and the outer disk and halo, and may be explained by a scenario of “inside-out” galaxy formation. This occurs when gas is accumulated at large radii later in a galaxy’s life space, while the gas at the core becomes exhausted. The result is a decrease in the average age of stars with increasing radius from the galaxy core.
The northern main spiral arm contains four large HII regions, while the southern arm has greater concentrations of young, hot stars. The estimated rate of supernova explosions in the Triangulum Galaxy is one supernova explosion every 147 years, on average. As of 2008, a total of 100 supernova remnants have been identified in the Triangulum Galaxy, and a majority of the remnants lie in the southern half of the spiral galaxy. Similar asymmetries exist for highly luminous concentrations of massive stars. The center of the distribution of these features is offset about two arc minutes to the southwest.
About 54 globular clusters have been identified in this galaxy, but the actual number may be 122 or more. The confirmed clusters may be several billion years younger than globular clusters in the Milky Way, and cluster formation appears to have increased during the past 100 million years. This increase is correlated with an inflow of gas into the center of the galaxy.
At least 112 variables have been discovered in Triangulum, including 4 novae and about 25 Cepheids. In 2007, a black hole about 15.7 times the mass of the Sun was detected in the galaxy. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days. It is the largest stellar mass black hole known.
Triangulum’s brightest and largest H II region is NGC 604, a diffuse emission nebula containing ionized hydrogen. As seen from Earth, NGC 604 is located northeast of the galaxy’s central core. It is one of the largest H II regions known, with a diameter of nearly 1500 light-years and a spectrum similar to that of the Orion Nebula.
NGC 604, may have undergone a discrete outburst of star formation about three million years ago. Astronomers counted 200 young hot massive stars (of 15 to 60 solar masses) which have recently formed here. This nebula is the second most luminous HII region within the Local Group of galaxies.
Triangulum is also home to the smaller H II regions NGC 588, 592 and 595. Other prominent HII regions in the galaxy include IC 132, IC 133 and IK 53.
One of the small Local Group member galaxies, the Pisces Dwarf (LGS 3), located a little more than 2 million light-years away, lies at a distance of 913 million light-years from both Triangulum and Andromeda. This means it could be a satellite galaxy of either galaxies. The Pisces Dwarf has a core radius of 483 light-years and 26 million solar masses.
Triangulum itself is a remote but gravitationally bound companion of the Andromeda galaxy. The distance of Triangulum from Andromeda is about 750,000 light-years. Triangulum is moving towards the Andromeda Galaxy and a clumpy stream of hydrogen gas and stars is linking Triangulum with Andromeda, what suggests that a past interaction between these two galaxies took place between 2-8 billion years ago, and a more violent encounter 2.5 billion years in the future.
The fate of the Triangulum Galaxy is unclear, but seems to be linked to its larger neighbor Andromeda. Suggested future scenarios for Triangulum range from being torn apart and absorbed by Andromeda fueling it with hydrogen to form new stars or losing all of its gas—thus, the ability to form new stars—to participating in the collision between the Milky Way and Andromeda, most likely ending orbiting the merger remnant of the latter two galaxies and fusing with it much later. Two other possibilities are a collision with the Milky Way before Andromeda arrives or an ejection out of the Local Group.
The Triangulum Galaxy is one of the most distant objects that can be viewed with the naked eye – under exceptionally good viewing conditions with no light pollution. Through a small telescope, the galaxy looks like a diffuse patch about the same size as the full Moon.
October 17, 2012
The Heart Nebula, an emission nebula in Cassiopeia
Image Credit & Copyright: J-P Metsavainio ( http://astroanarchy.zenfolio.com/ )
The Heart Nebula (IC 1805, Sh2-190) is an emission nebula and star forming region of almost 200 light-years across, located within the Perseus Arm of our Milky Way galaxy, about 7500 light-years away in the northern constellation Cassiopeia. It is nicknamed the Heart Nebula because it looks like a human heart.
The very brightest part of this nebula (the knot at the bottom right) is separately classified as NGC 896 (also designated IC 1795). The Heart Nebula is the western neighbor of the Soul Nebula (NGC 1848) and together they form the Heart and Soul Nebulae. They both are part of a complex of star forming regions that lie at the edge of a large molecular cloud.
The Heart Nebula is formed by plasma of ionized hydrogen and free electrons. The image, coloured in the Hubble palette, is showing a mix of glowing interstellar gas and dark dust clouds. The nebula appears relatively devoid of stars because of the obscuring dust. However, fierce winds radiated from massive stars, in an open star cluster near the center, shaped the nebula.
This small open star cluster of about 30 light-years across and approximately 1.5 million years young, is named Melotte 15. It contains a few bright stars nearly 50 times the mass of our Sun, and many more dim stars that are only a fraction of our Sun’s mass. The cluster used to contain a microquasar that was expelled over a million years ago.
October 16, 2012
The Double Cluster, two open star clusters in Perseus
Image Credit & Copyright: Roth Ritter, Dark Atmospheres Astrophotography ( http://www.darkatmospheres.com/astro )
The Double Cluster (also known as Caldwell 14) consists of the open star clusters NGC 884 and NGC 869 which are close together within the Perseus arm of the Milky Way galaxy, in the constellation Perseus, quite close to the constellation Cassiopeia. NGC 884 (left) and NGC 869 (right) are at distances of 7600 and 6800 light-years away, respectively. The Double Cluster represents the jeweled handle of Perseus’s sword.
The clusters are blueshifted, with NGC 869 approaching Earth at a speed of 22 kilometers per second and NGC 884 approaching at a similar speed of 21 kilometers per second.
Separated by just a few hundred light-years, each cluster contains hundreds of young, hot stars, much younger and hotter than the Sun. Among them are more than 300 blue-white super-giant stars, that are many thousands of times more luminous than our Sun, and a few orange stars. The clusters are both part of the so-called Perseus OB1 association, an assembly of extremely massive stars.
The clusters’ ages, based on their individual stars, are pretty young. NGC 869 is 5.6 million years old and NGC 884 is 3.2 million years old, making them among the youngest star clusters known in the galaxy. In comparison, the Pleiades are over 100 million years old.
Open clusters are believed to originate in the same general area of space from the same local gas clouds. Due to their age similarities, it is likely that both NGC 884 and NGC 869 were a product of the same star-forming region.
The Double Cluster is a great target for binoculars and small telescopes, but also visible to the unaided eye from dark locations as a hazy patch. It nearly marks the radiant point of the Perseid meteor shower, which peaks annually around August 12 or 13.
October 15, 2012
The Red Spider Nebula, a planetary nebula in Sagittarius
Image Credit: ESA & Garrelt Mellema (Leiden University, the Netherlands)
The Red Spider Nebula (NGC 6537) is a “butterfly” or bipolar (two-lobed) planetary nebula located near the heart of the Milky Way, in the northwest of the Sagittarius constellation. The exact distance from Earth is not known; estimations vary from 1900 up to 4000 light-years. Therefore it’s diameter is also unknown. The Red Spider is moving toward us at 17.3 kilometers per second.
Planetary nebula have nothing to do with planets except that to early astronomers these round objects looked like the planets Uranus and Neptune. Planetary nebula are the last stage of life for stars like our Sun. After billions of years, stars reach a point where there is little hydrogen gas to burn. To help convert their stellar furnaces to burn other elements such as helium, the star balloons in size to become a red giant. Eventually, however, the star collapses back on itself. This increases the temperature at its core and most of the stars material is catapulted into space, forming a bubble around the star, often in the form of a “butterfly” or two-lobed structure as in the case of the Red Spider. Then the star cools down to become a white dwarf.
The surface temperature of the central white dwarf in the Red Spider is an incredible 540,000 degrees F, more than 50 times hotter than the Sun, what makes it one of the hottest stars known.
It produces a powerful and hot (≈10,000 K) wind blowing with a speed of 2000-4500 kilometers per second, which has generated waves 100 billion kilometres high. The waves themselves move outwards at a slower rate of 300 km/s. These winds are what give this nebula its unique ‘spider’ shape and also contribute to the expansion of the nebula.
The gas walls of the two lobed structures are not at all smooth, but rather are rippled in a complex way. These waves are driven by stellar winds radiating from the hot central white dwarf, much as a wind passing over a lake can generate waves on the water. Supersonic shocks formed when the local gas is compressed and heated in front of the rapidly expanding lobes.
The Red Spider has an ‘S’-shaped symmetry of the lobes – the lobes opposite each other appear similar. This is believed to be due to the presence of a companion to the central white dwarf (none of which are visible in the image).
This Hubble Space Telescope image was obtained on 12 September 1997 with the Wide Field and Planetary Camera 2 in five different filters. Here, light from sulphur ions is displayed in red, nitrogen ions in orange, and ionised hydrogen (H-alpha) in green, while atomic oxygen is in light blue and ionised oxygen in dark blue.
October 14, 2012
Circinus Galaxy, an active spiral galaxy in Circinus
Image Credit: Andrew S. Wilson (U. Maryland) et al., WFPC2, HST, NASA
Circinus Galaxy (ESO 97-G13), is an active spiral galaxy (Type Seyfert II), located about 13 million light-years away in the Circinus constellation, just below the Galactic plane. The Circinus Galaxy is one of the closest known active galaxies to the Milky Way. It is receding form us at 434 kilometers per second.
Seyfert galaxies are galaxies with extremely bright nuclei that produce spectral line emission from highly ionized gas. The centres of Seyfert galaxies have active galactic nuclei (AGN), and usually contain supermassive black holes with masses between 10 and 100 million solar masses. Seyferts are classified as Type I or II, depending upon whether the spectra show both narrow and broad emission lines (Type I), or only narrow lines (Type II).
Circinus Galaxy consists of extended spiral arms and, at the center, an active galactic nucleus, where matter glows brightly before spiraling into a supermassive black hole. The inner spiral arms of the galaxy contain hydrogen-rich star forming regions.
The galaxy is undergoing tumultuous changes, as hot gas, colored pink, is being ejected out of the spiral galaxy from the central region. Much of Circinus’ tumultuous gas, however, is concentrated in two rings. The outer ring, located about 700 light-years from the center, appears mostly red and is home to tremendous bursts of star formation. An inner ring, inside the green disk, is visible only 130 light years from the center.
Circinus Galaxy was a home for the Type II supernova SN 1996cr, that has been identified over a decade after it exploded. The supernova was first singled out in 2001 as a bright, variable object in a Chandra image, but it was not confirmed as a supernova until years later.
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum. Type II supernovae are mainly observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies.
The Circinus Galaxy went unnoticed until the 1970s because it is so obscured by material in the plane of our own Galaxy. However, the galaxy can be seen with a small telescope.
This image is taken by the Hubble Space Telescope.
October 13, 2012
The Stingray Nebula, a young planetary nebula in Ara
Image Credit: Matt Bobrowsky, Orbital Sciences Corporation and NASA/ESA
The Stingray Nebula (designated Henize 1357, Hen 3-1357) is the youngest known planetary nebula, located about 18,000 light-years away in the southern constellation Ara. It has a relatively small size (some 0.16 light-year across, one-tenth the size of most other known planetary nebula). However, it is about 130 times larger than our Solar System and is continuously expanding. It is named the Stingray because its shape is similar to the catfish.
This nebula was created when a dying star (a supergiant) exhausted its shell of gases, and now shines like a neon light. This shell of gas is illuminated by the star remaining in the center of the nebula. As the nebula continues to move away from the star, the core of the star – which has become a white dwarf — is heated and illuminates the gas.
Forty years ago the Stingray nebula was still a protoplanetary nebula in which the gas had not yet become hot and ionized. It is now considered to be a planetary nebula, as its central star has warmed up long enough in the last 40 years to light the gases. Such young nebulae are found very rarely, because it takes 100 years for a dying star to turn into a visible planetary nebula, which means very little considering the life of a star. Typically, stars live for millions of years. This nebula will help astronomers to understand this short and still poorly known transition phase which leads to the formation of a new planetary nebula.
This image shows how the outer shells of gas are collimating the continuing outflow of gas from the central star — an important observation, as the process of how these outflows become collimated has not been well understood.
Stingray’s central white dwarf star faded by a factor of three between 1987 and 1995. Observations also revealed a companion to the white dwarf, seen diagonally above it at 10 o’clock in this image. Astronomers suspect that the companion account for the complex shapes and rings of this planetary nebula.
A spur of gas (green) is forming a faint bridge to the companion star due to gravitational attraction. The image also shows a ring of gas (green) surrounding the bright central white dwarf, with bubbles of gas to the lower left and upper right of the ring.
The wind of material propelled by radiation from the hot central star has created enough pressure to blow open holes in the ends of the bubbles, allowing gas to escape. The red curved lines represent bright gas that is heated by a “shock” caused when the central star’s wind hits the walls of the bubbles. The colors shown are actual colors emitted by nitrogen (red), oxygen (green), and hydrogen (blue).
October 12, 2012
Image Credit:  R Jay Gabany  (Blackbird Observatory)
The Umbrella Galaxy (designated NGC 4651 and Arp 189) is a spiral galaxy located on the outskirt of the Virgo Cluster of galaxies of which it is a member, at a distance not well determined that ranges from 35 to 72 million light-years in the constellation of Coma Berenices. It is receding from us at 788 kilometers per second.
About 50 thousand light-years across, this galaxy has a faint umbrella-shaped structure (right) that seems to extend some 50 thousand light-years farther, beyond the bright galactic disk. It is known as the Umbrella Galaxy due to this umbrella-shaped structure.
The Umbrella Galaxy has an active galaxy nucleus (AGN) and a fine spiral structure. Unlike most spiral galaxies of the Virgo Cluster, NGC 4651 is rich in neutral hydrogen, also extending beyond the optical disk, and its star formation is the typical for a galaxy of its type.
The giant cosmic umbrella is composed of stellar streams, being the remnants of a much smaller galaxy that has been torn apart by NGC 4651’s tidal forces, something that explains why the Umbrella Galaxy has been included on Halton Arp’s Atlas of Peculiar Galaxies as Arp 189 –as a galaxy with filaments. (Arp’s Atlas of Peculiar Galaxies is a catalog of 338 peculiar galaxies produced by Halton Arp in 1966.)
Studies using radiotelescopes of the distribution of its neutral hydrogen show distortions on NGC 4651’s outer regions and a gas clump associated with a dwarf galaxy that may have born in the event that produced the mentioned stellar streams.
The galaxy can be seen with amateur telescopes.
October 11, 2012
The Ghost Head Nebula, an emission nebula in the LMC
Image Credit: NASA/ESA & Mohammad Heydari-Malayeri (Observatoire de Paris, France)
The Ghost Head Nebula (NGC 2080) is an emission nebula and star-forming region of some 55 light-years across located about 168,000 light-years away in the southern constellation Dorado. The Ghost Head is one of a chain of star-forming regions lying south of the 30 Doradus (Tarantula) nebula in the Large Magellanic Cloud (LMC, a satellite galaxy to our Milky Way galaxy).
Two bright regions, called the “Eyes of the Ghost”, named A1 (left) and A2 (right), are very hot, glowing ‘blobs’ of hydrogen and oxygen. The bubble in A1 is produced by the hot, intense radiation and powerful stellar wind from a single young, massive star. A2 has a more complex appearance due to the presence of more dust, and because it contains several young stars in a newly formed cluster, still obscured by their originating dust cloud The massive stars in A1 and A2 must have formed within the last 10,000 years since their natal gas shrouds are not yet disrupted by the powerful radiation of the newly born stars. These stars together have begun to create a bubble in the nebula with their outpourings of material, called stellar wind.
The presence of stars also greatly influences the color of the nebula. The light from the nebula captured in this image is emitted by two elements, hydrogen and oxygen. Astronomers use the separated colors produced by these elements to investigate star-forming processes in the Ghost Head Nebula. The colors explain much about the nature of such nebulae.
The red and the blue light are from regions of hydrogen gas heated by nearby stars. The green light on the left comes from glowing oxygen. The energy to illuminate the green light is supplied by a powerful stellar wind (a stream of high-speed particles) coming from a massive star just outside the image. The white region in the center is a combination of all three emissions and indicates a core of hot, massive stars in this star-formation region. The intense emission from these stars has carved a bowl-shaped cavity in the surrounding gas.
This image was taken by the NASA/ESA Hubble Space Telescope on March 28, 2000.
October 10, 2012
Hickson Compact Group 44, a group of galaxies in Leo
Image Credit: Stephen Leshin ( http://sleshin.startlogic.com/ )
Hickson Compact Group 44 (also known as the NGC 3190 Group) is a gravitational bound group of bright galaxies containing three spiral galaxies and an elliptical galaxy, located about 60-100 million light-years away in the constellation of Leo. Two dwarf galaxies are also part of the group but are not included in the Hickson catalog.
NGC 3190 in the center is spiral galaxy of some about 75,000 light-years across that we see nearly edge on, NGC 3193 on the left is an elliptical galaxy, NGC 3187 above NGC 3190 is another (S-shaped) spiral, but with a prominent central bar, and NGC 3185 on the lower right is also a barred spiral, but its bar is subtle and much smaller relative to its overall size.
Galaxies, like stars, frequently form groups. A group of galaxies is a system containing more than two galaxies but less than the tens or hundreds typically found in a cluster of galaxies. A most notable example is the Local Group of Galaxies, which houses over 30 galaxies including our Milky Way, Andromeda, and the Magellanic Clouds.
A Hickson Compact Group (HCG for short) is a collection of four or five gravitational bound galaxies in close physical proximity to one another, published in a list of 100 objects (462 galaxies) by the Canadian astronomer Paul Hickson in 1982. These groups usually contain large quantities of diffuse gas and are dominated by dark matter. Strong galaxy interactions result and merging is expected to lead to the formation of one, most likely elliptical, larger galaxy.
They are among the densest concentrations of galaxies known, comparable to the centers of rich galaxy clusters. Compact groups are worthy of intense study as they provide a rich opportunity to study galaxy interactions and mergers.
Compact groups are relatively short lived entities that form via mergers of galaxies within loose subsystems and groupings. Simulations predict that merging of the group members should proceed rapidly within one billion years. Hickson groups are therefore snapshots at various stages in this merging process. Astronomers believe they may represent an intermediate stage between loose groups and individual galaxies. A better understanding of the nature of HCGs could help explain galaxy formation on a larger scale in the early Universe. Compact groups are surprisingly numerous, and may play a significant role in galaxy evolution.
Signs that galaxies in Hickson 44 are either slowly merging or gravitationally pulling each other apart are present. In particular, NGC 3190 and 3187 show signs of gravitational interaction. NGC 3190, the dominant edge-on spiral galaxy, shows of warping of its dust lane on the side nearer to NGC 3187, and NGC 3187 (also known as ARP 316a) has developed tidal tails well above and below its disk plane. If you look closely, you may also notice a faint halo around NGC 3190, and also a very subtle smudge of light between it and NGC 3193, a bridge of stars being shared between the two giants as they embrace in their cosmic dance.
The bright source on the upper left in this image is a foreground star. In the background you will find hundreds of tiny yellowish and reddish objects. These are not stars, but extremely distant galaxies, hundreds of millions of light years away.
October 9, 2012
Saturn’s moon Dione 
Image Credit: NASA/JPL/Space Science Institute
Dione is one of Saturn’s smaller moons and is, at 1123 km (or 700 mi) across, the 15th largest moon in the Solar System. It is more massive than all known moons smaller than itself combined, and is the third densest of Saturn’s moons (after Enceladus and Titan).
Dione orbits Saturn once every 2.74 days at a distance of about 377,400 km (234,000 mi) — roughly the same as that between Earth and its moon, and is locked in a synchronous orbit similar to that of Rhea. This causes the same face of the moon to point towards Saturn at all times.
Dione’s density is 1.48 times that of liquid water, suggesting that about a third of Dione is made up of a dense core (probably silicate rock) with the remainder of its material being ice. At Dione’s average temperature of 87 K, or −186°C), ice is very hard and behaves like rock.
Its icy surface consists of heavily cratered terrain, moderately cratered plains, lightly cratered plains, and areas of tectonic fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres (62 mi) in diameter, while most of the craters in the plains areas are less than 30 kilometres (19 mi) in diameter. The largest crater is called Amata and is 241 km (150 mi) in diameter. These craters lack the high relief features seen on Mercury and the Moon.
Some of the plains are more heavily cratered than others. Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains areas present on the leading hemisphere. Heavy cratering would normally be expected on the leading edge of a tidally locked satellite.
This suggests that during the period of heavy bombardment, Dione was tidally locked to Saturn in the opposite orientation. Because Dione is relatively small, it would have only taken an impact leaving a 35 km (22 mi) crater to spin the moon around. With many craters on Dione exceeding 35 km (22 mi), it is possible that the moon has been spun around more than once throughout its long history. The pattern of cratering since then and the bright albedo of the leading side suggests that Dione has remained in its current orientation for several billion years. However, the fact that Dione seems to have spun exactly 180 degrees is a mystery.
Dione is a world riven by enormous fractures on its trailing hemisphere. It has long, deep, steep-sided depressions, ridges and long narrow depressions. One of the most striking features of this moon is a network of bright ice cliffs – with lengths of tens to hundreds of kilometers, often cutting through plains and craters — created by tectonic fractures.
These bright canyon ice walls (some of them several hundred meters high), were likely created by tectonic fractures. The walls are bright because darker material falls off them, exposing bright water ice. These fracture cliffs suggest Dione experienced tectonic activity in its past. They could be a mature phase of the so-called tiger stripes on Enceladus. At lower right is the feature called Cassandra, exhibiting linear rays extending in multiple directions.
Very fine ice powder (equivalent to cigarette smoke) from Saturn’s E-ring constantly bombards Dione. The dust in the E-ring ultimately comes from Enceladus, which has prominent geyser activity.
The unmanned Cassini probe, which flew by Dione, detected a thin layer of molecular oxygen ions around the moon, what makes Dione even more interesting than previously thought. Cassini’s instruments were unable to directly detect water from the exosphere, but it seems that highly charged particles from the planets’ powerful radiation belts could split the water in the ice into hydrogen and oxygen.
Dione was first imaged by the Voyager space probes. It has also been probed three times from closer distances by the Cassini orbiter, in 2005, 2010 and 2011.
October 8, 2012
IC 1396, a large emission nebula in Cepheus
Image Credit & Copyright: J-P Metsavainio ( http://astroanarchy.zenfolio.com/ )
IC 1396 is a large and comparatively faint emission nebula and star forming region over 100 light-years across, located about 2,400 light-years away in the constellation Cepheus. It is energized by the bright, bluish central multiple star called HD 206267.
The nebula mixes glowing cosmic gas and dark dust cloud, and there is a scattering of hot stars, quite widely separated from each other, inside the nebula. Among the intriguing dark shapes within IC 1396, is the Elephant’s Trunk nebula (IC 1396A), seen within the bluish cavity in blue to the lower left in this image. This is a dense globule of gas and dust of more than 20 light-years long, being eroded by the radiation pressure from HD 206267. It is commonly called the Elephant’s Trunk nebula because of its appearance at visible light wavelengths.
The Elephant’s Trunk nebula is a site of star formation. Over 250 young stars in and around the Elephant’s Trunk are identified. Several very young (less than 100,000 years old) stars and two older (but still young, a couple of million years) stars are present in a small, circular cavity in the head of the globule. Winds from these young stars may have emptied the cavity.
The combined action of the light from the massive star ionizing and compressing the rim of the cloud, and the wind from the young stars shifting gas from the center outward lead to very high compression in the Elephant’s Trunk nebula. This pressure has triggered the current generation of protostars.
Numerous other such globules of varying size are scattered throughout the nebula. Some of these are positioned between us and the bright clouds of gas, and so appear as inky black silhouettes. These dense globules can protect themselves from the star’s harsh ultraviolet rays.
The brightest star (38,000 times brighter than the Sun) in the frame, at the right, is mu Cephei. It is a red supergiant star with a diameter larger than the orbit of Saturn, some 2536 times the diameter of the Sun. It is one of the largest stars we know of. Also called Herschel’s Garnet Star, mu Cephei is a variable star that varies in magnitude from 3.4 to 5.1 over a period of approximately 730 days.
October 7, 2012
Markarian 273, an ultraluminous infrared galaxy in Ursa Major
Image Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)
Markarian 273 (also designated UGC 8696) is an ultraluminous infrared galaxy with a bizarre structure, located about 500 million light-years away in the constellation Ursa Major. It is one of the most luminous galaxies when observed in the infrared. The galaxy is receding from us at 10,971 kilometers per second.
The galaxy has an intense region of starburst, where 60 solar masses of new stars are born each year. It is an ‘active’ galaxy, producing radiation in its nucleus at such a rate that the output cannot be accounted for by stars alone. It may well harbour a black hole in its centre.
Markarian 273 has an intricate central region and a striking tail that extends diagonally towards the bottom-right of the image. The tail is about 130,000 light-years long and is strongly indicative of a merger between two galaxies. In both parts of Markarian 273 dark dust lanes can be clearly seen.
Near-infrared observations reveal that the galaxy has a double nucleus, whose two components are separated by about 2200 light-years. So, we are probably witnessing the merger of two galaxies, whose nuclei we can still see.
This image is part of a large collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.
October 6, 2012
M1-67, a wind-nebula surrounding the Wolf-Rayet star WR 124
Image Credit: ESO
 M1-67 is the youngest wind-nebula around a Wolf-Rayet star (WR 124) in our Galaxy, located some 10,000-15,000 light-years away in the constellation Sagittarius. The stellar wind is responsible for the formation of the intensely hot ring nebula. In fact, M1-67 originates from several outbursts that occurred inside the bow shock around WR 124. The nebula is expanding at a rate of over 150,000 kilometers per hour, and is currently 2.98 light-years across, not large enough to have come into contact with the interstellar medium. M1-67 is estimated to be no older than 10,000 years.
Wolf-Rayet (WR) stars are extremely rare and short-lived super-hot stars (in this case 50,000 degrees Kelvin) which start their lives with dozens of times the mass of our Sun, but rapidly loose most of it through a powerful stellar wind, with speeds up to 2000 km/s (1240 mi/s). They are also highly luminous, from tens of thousands to several million times the luminosity of the Sun, although not exceptionally bright visually since most of their output is in far ultraviolet and even soft X-rays. WR stars are candidates for being progenitors of long-duration Gamma Ray Bursts. Often occurring in binary systems, they are doomed, within a few million years, to explode as supernovae.
WR 124 has an estimated mass of 20 solar masses. In the past, the star could have been much larger, but a large portion of its mass has blown away through the intense stellar winds of the Wolf-Rayet phase. It may lose about three-quarters of its initial mass during its Wolf-Rayet phase. WR 124 is one of the hottest Wolf-Rayet stars known.
The fierce stellar wind coming from WR 124 does not flow smoothly into space but has instabilities which make it clumpy. Hot clumps of gas are being ejected into space at speeds of over 160,000 kilometers per hour (100,000 miles per hour).
Observations of M1-67 reveal a wealth of small knots and substructures inside the nebula. Astronomers study how these structures have evolved and what they can teach us about stellar winds, their chemistry, and how they mix with the surrounding interstellar medium, before the star will eventually blow everything away in a fiery supernova explosion.
Remarkable in M1-67 are vast arcs of glowing gas around the star, which are resolved into filamentary, chaotic substructures, yet with no overall global shell structure. The clumps in the stellar wind are 160 billion kilometers (100 billion miles) wide glowing gas blobs. Each blob is about 30 times the mass of the Earth. As the blobs cool they will eventually dissipate into space.
October 5, 2012
Messier 66, an intermediate spiral galaxy in Leo
Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona ( http://www.caelumobservatory.com/index.html )
Messier 66 (also known as NGC 3627 and Arp 16) is an intermediate spiral galaxy of some 95,000 light-years across. The galaxy is about 36 million light-years away in the constellation of Leo, and is moving away from us at 727 kilometers per second. It is the largest member of the famous Leo Triplet, a group of three interacting galaxies that include Messier 65 and NGC 3628.
Messier 66 has a well developed but not well defined central bulge. Its core and bar-like structure illustrates a concentration of older stars, and while the bar seems devoid of star formation, the bar ends are actively forming stars.
The prominent and unusual asymmetric spiral arms, laced with striking dust lanes and bright star clusters, are somewhat distorted. They seem to climb above the galaxy’s main disc and an apparently displaced nucleus.
This asymmetry is unusual; most often, dense waves of gas, dust and newly born stars wind about the galaxy’s centre in a symmetric way. Astronomers believe that Messier 66′s once orderly shape has most likely been distorted by the gravitational pull of its two neighbours.
Four supernovae have been observed in Messier 66: 1973R, 1989B, 1997bs and 2009hd.
Messier 66 is number 16 in Arp’s Atlas of Peculiar Galaxies, a catalogue of 338 peculiar galaxies produced by Halton Arp in 1966. Moreover, Arp assigned the number 317 to the Leo Triplett.
October 4, 2012
Messier 78, a reflection nebula in Orion
Image Credit: ESO/APEX (MPIfR/ESO/OSO)/T. Stanke et al./Igor Chekalin/Digitized Sky Survey 2
Messier 78 (also known as NGC 2068) is a reflection nebula and star-forming region of some 10 light-years across, located about 1,600 light-years away in the constellation of Orion, just to the north of Orion’s Belt.
Messier 78 is the brightest diffuse reflection nebula within the Orion B molecular cloud (LDN 1630) that also include NGC 2071, NGC 2067, NGC 2064 and NGC 2024 (the Flame Nebula). As a reflection nebula, M78 is a cloud of interstellar dust which shines in the reflected and scattered light of bright blue supergiant stars, among them the brightest, HD 38563A, and second-brightest HDE 38563B.
Spitzer Space Telescope exposes the depths of this dusty nebula with its infrared vision, showing stellar infants that are lost behind dark clouds when viewed in visible light.
Infrared investigations have given a clearer image of the open cluster of young stars which have formed in this nebula. From investigations astronomers concluded that much of the young, embedded star formation is occurring in clusters, including the formation of lower mass stars. They counted a number of 192 stars, spread over an area of 7′ angular diameter.
The two round greenish nebulae, looking like a pair of big eyeglasses, are actually cavities carved out of the surrounding dark dust clouds. The light from young, newborn stars are starting to carve out these cavities within the dust, and eventually, this will become one larger nebula. The extended dust is mostly dark but the edges show up in mid-wavelength infrared light as glowing red frames surrounding the bright interiors.
A string of baby stars that have yet to burn their way through their natal shells can be seen as red pinpoints on the outside of the nebula. Eventually these will blossom into their own glowing balls, turning this two-eyed eyeglass into a many-eyed monster of a nebula.
About 45 variable stars, young stars (less than 10 million years old) still in the process of formation as well as some 17 Herbig-Haro objects are known in Messier 78. These Herbig-Haro objects are presumably jets of matter ejected from young stars embedded in the nebulous matter of M78 where they have just formed.
Messier 78 is easily seen in small telescopes to the naked eye, but looks strikingly different, with dominant, dark swaths of dust. Spitzer’s infrared eyes penetrate this dust, revealing the glowing interior of the nebulae.
This is a three-color composite image taken with NASA’s Spitzer Space Telescope that shows infrared observations from two Spitzer instruments. Blue represents 3.6- and 4.5-micron light and green shows light of 5.8 and 8 microns, both captured by Spitzer’s infrared array camera. Red is 24-micron light detected by Spitzer’s multiband imaging photometer.
October 3, 2012
47 Tucanae, a bright globular cluster in Tucana
Image Credit & Copyright: J-P Metsavainio ( http://astroanarchy.zenfolio.com/ )
47 Tucanae (NGC 104) or just 47 Tuc is a globular cluster that contains millions of stars in a region of about 120 light-years across, making it one of the most massive globular clusters in the Milky Way galaxy. It is located some 16,700 light-years away in the southern constellation of Tucana (near the Small Magellanic Cloud). The cluster, which approaching us at roughly 19 kilometers per second, is estimated to be approximately 13.06 billion years old!
It is the second largest and second brightest globular cluster in the skies, outshone only by another southern globular cluster, Omega Centauri (NGC 5139). 47 Tucanae is also one of the densest globular clusters in the southern hemisphere. As its name “47 Tucanae” indicates, this object was first cataloged as a star and numbered the 47th in Tucana, before it was recognized as a cluster of stars.
Although the density of stars is very small at the outskirts of a globular cluster, near the centre it can be more than 10,000 times higher than in the local vicinity of our Sun. Just as bumps and jostles are much more likely in a crowded commuter train, so are encounters between stars in a densely populated cluster more likely than here in our quiet stellar backwater. These encounters can be as dramatic as collisions or even mergers. The crowded conditions make it extremely difficult to identify individual stars accurately.
Globular clusters efficiently sort stars by mass: heavier stars slow down and sink to the cluster’s core, while lighter stars pick up speed and move across the cluster to its periphery. However, there is no evidence (yet) for the existence of any black holes in 47 Tuc; Hubble Space Telescope data provides the strongest constraint on the mass of any possible black hole at its center.
47 Tuc contains hundreds of binary star systems: variable stars containing white dwarfs accreting from companion stars, and binaries containing neutron stars that are not currently accreting. Its dense core contains several exotic stars as well: at least 23 blue stragglers (unusually hot and bright stars thought to be the product of collisions between two normal stars, which move more slowly than the more typical, lighter stars) have been discovered.
Further, 47 Tuc has 23 known millisecond pulsars, the second largest population of pulsars in any globular cluster, and about a third of the total number of known cluster pulsars. These pulsars are thought to be spun up by the accretion of material from binary companion stars. The companion of one pulsar in 47 Tucanae, 47 Tuc W, seems to still be transferring mass towards the neutron star, indicating that this system is completing a transition from being an accreting binary to a millisecond pulsar. Red giant stars on the outskirts of the cluster are easy to pick out in this sharp image.
The core of 47 Tuc was the subject of a major survey for planets, to look for partial eclipses of stars by their planets. No planets were found, though 10-15 were expected based on the rate of planet discoveries around stars near the Sun. This indicates that planets are relatively rare in globular clusters. A later ground-based survey in the uncrowded outer regions of the cluster also failed to detect planets when several were expected. This strongly indicates that the low metallicity (common in old globulars) of the environment, rather than the crowding, is responsible.
October 2, 2012
The California Nebula, an emission nebula in Pereus
Image Credit & Copyright: J-P Metsavainio ( http://astroanarchy.zenfolio.com/ )
The California Nebula (NGC 1499) is a large emission nebula and star-forming cloud of around 100 light-years long, located some 1,000-1,500 light-years away in the Orion spiral arm of the Milky Way galaxy (where our Solar System is also located) in the constellation of Perseus, what makes it one of the nearest H II regions to Earth. It is so named because it appears to resemble the outline of the US State of California. It has a very low surface brightness and it’s very difficult to observe visually.
The California Nebula contains a hot blue-white star named Menkhib (also designated HD24912 and Xi Persei), by far the brightest star in this image. Menkhib is one of the hottest stars visible in the night sky; its surface temperature is about 37,000 Kelvin (about 66,000 degrees Fahrenheit, or more than six times hotter than the Sun). Because of its high temperature, it appears blue-white to the human eye. It has about 40 times the mass of the Sun and gives off 330,000 times the amount of light. Menkhib is a runaway star, and the fast stellar wind it blows is piling up in front of it to create a shock wave. This shock wave is heating up dust.
Menkhib, which is receding from us at about 20 kilometers per second, is part of an association of very hot stars that were born from the California nebula only a few million years ago. These massive and luminous stars are lighting up the nebula, as well as heating and ionizing it. In visible light, the ionized gas glows red, while in infrared light we see the heated dust.
October 1, 2012