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Reference Desk
Frequently Asked Questions
What is a galaxy?
A galaxy is an enormous collection of gas, dust and billions of stars held      together by gravity. One galaxy can have hundreds of billions of stars and      be as large as 200,000 light years across.
What is the name of our galaxy?
The name of our galaxy is the Milky Way. Our Sun and all of the stars that      you see at night belong to the Milky Way. When you go outside on a dark night and look up, you will see a milky, misty-looking band stretching      across the sky. When you look at this band, you are looking into the densest      parts of the Milky Way, the "disk" and the "bulge."
Where is the Earth in the Milky Way galaxy?
Our solar system is at the edge of a spiral arm called the Orion Arm, and is about two-thirds      of the way from the center of our galaxy to the edge of the starlight. The      Earth is the third planet from the sun in our solar system of nine planets.
What is the closest galaxy like our own, and how far away is it?
The closest spiral galaxy is Andromeda, a galaxy much like our own Milky      Way. It is 2.2 million light years away from us. Andromeda is approaching      our galaxy at a rate of 670,000 miles per hour (1,078,260 kph). Five billion years from now      it will probably collide with our Milky Way galaxy.
What are the parts of a galaxy?
A galaxy contains stars, gas and dust. In a spiral galaxy like the Milky      Way, the stars, gas, and dust are organized into a "bulge," a "disk" containing "spiral arms," and a "halo." Elliptical galaxies      have a bulge-like central region and a halo, but do not have a disk.
Bulge
The bulge is a round structure made primarily of old stars, gas, and dust.      The outer parts of the bulge are difficult to distinguish from the halo. The      bulge of the Milky Way is roughly 10,000 light years across.
Disk
The disk is a flattened region that surrounds the bulge in a spiral galaxy.      The disk is shaped like a pancake. The Milky Way's disk is 100,000 light years      across and 1,000 light years thick. It contains mostly young stars, gas and      dust, which are concentrated in spiral arms. Some old stars are also present.
Spiral Arms
The spiral arms are curved extensions that begin at      the bulge of a spiral galaxy, giving it a "pinwheel" appearance.      Spiral arms contain a lot of gas and dust as well as young blue stars. Spiral      arms are found only in spiral galaxies.
Halo
The halo primarily contains individual old stars and clusters      of old stars ("globular clusters"). The halo also contains "dark      matter," which is material that we cannot see but whose gravitational      force can be measured. The Milky Way's halo may be over 130,000 light years      across.
Stars, gas, and dust
Stars come in a variety of types. Blue stars,      which are very hot, tend to have shorter lifetimes than red stars, which are      cooler. Regions of galaxies where stars are currently forming are therefore      bluer than regions where there has been no recent star formation. Spiral galaxies      seem to have a lot of gas and dust, while elliptical galaxies have very little      gas or dust.
How are galaxies classified?
Astronomer Edwin Hubble classified galaxies into four major types: spiral, barred spiral,      elliptical and irregular. Most of the nearby, bright galaxies are spirals, barred spirals or ellipticals.
Spiral galaxies have a bulge at the center and a flattened disk      containing spiral arms. Spiral galaxies have a variety of shapes and are classified      according to the size of the bulge and the tightness and appearance of the      arms. The spiral arms, which wrap around the bulge, contain numerous young      blue stars and lots of gas and dust. Stars in the bulge tend to be older and      redder. Yellow stars like our Sun are found throughout the disk of a spiral      galaxy. The disks of spiral galaxies rotate somewhat like a hurricane or a      whirlpool.
Barred spiral galaxies are spiral galaxies that have a bar-shaped      collection of stars running across the center of the galaxy.
Elliptical galaxies do not have a disk or arms. Instead, they are      characterized by a smooth, oval-shaped appearance. Ellipticals contain old      stars, and possess little gas or dust. They are classified by the shape of      the ball, which can range from round to oval (baseball-shaped to football-shaped).      In contrast to the disks of spirals, the stars in ellipticals do not all revolve      around the center in an organized way. The stars move on randomly oriented      orbits within the galaxy, like a swarm of bees.
Irregular galaxies are galaxies that are neither spiral nor elliptical.      They tend to be smaller objects that are without definite shape, and tend      to have very hot newer stars mixed in with lots of gas and dust.
Galaxy names are identified by a group of letters and numbers. What do they stand for?
The letters indicate the catalog listing of the galaxies. Galaxies are listed      in several different catalogs. The most common catalog is NGC, which      stands for New General Catalog. Other catalogs include M (Messier), ESO (European Southern Observatory), IRAS (Infrared Astronomical      Satellite), Mrk (Markarian), and UGC (Uppsala General Catalog).
The numbers following the letters, such as Mrk 917 or NGC1433, indicate      a galaxy’s entry in the catalog and are often related to the galaxy’s      relative position in the sky.
Sometimes a galaxy appears in more than one catalog and can have more than      one name.
Why do astronomers study galaxies in ultraviolet light?
Galaxies emit all kinds of electromagnetic radiation, from x-rays to radio waves. From this "light," astronomers get a clear picture of what these galaxies look like. But very      distant galaxies pose a special problem. Light from these galaxies travels      great distances (billions of light-years) to reach Earth. During its journey,      the light is "stretched" due to the expansion of space. As a result,      much of the light from the most distant galaxies is no longer visible, but      has been shifted to the infrared where present instruments are less sensitive.      The only light now in the visible region of the spectrum comes from regions      where hot, young stars reside. These stars emit mostly ultraviolet light.      But because this light has been stretched, it appears as visible light by      the time it reaches Earth. Studying these distant galaxies is like trying      to put together a puzzle with some of the pieces missing. So, astronomers      are studying nearby galaxies in ultraviolet light to compare their shapes      with those of their distant relatives.
What are colliding galaxies?
When two galaxies are close enough, their gravitational attraction draws      them to each other. That attraction increases as the galaxies move closer.      The Milky Way and Andromeda are examples of two galaxies that will probably eventually      collide (no sooner than 5 billion years in the future).
In a direct encounter between two massive spiral galaxies such as these,      huge cold clouds of molecules (gas) will be compressed, and millions of new      stars will burst into life like a string of lights. As the galaxies      first swing by each other, their once orderly disks will become jumbled with      dust, gas, and brilliant blue star clusters. Then the galaxies may do a slow,      graceful U-turn and plunge into each other. This second encounter will trigger      another burst of star formation, which will drive the remaining gas and dust      from the combined system. As the stars settle into randomly oriented orbits,      the resulting system may take on an elliptical appearance.
How do astronomers measure the distances to galaxies?
Astronomers measure the distance to a galaxy in the same way we estimate      the distance to an oncoming car by the brightness of its headlights. We know      from experience how much light a car's headlights emits, so we can determine      how far away the car is.
To measure the distance to a galaxy, we try to find stars in that galaxy      whose absolute light output we can measure. We can then determine how far      away the galaxy is by observing the brightness of the stars. Such stars can      help us measure the distance to galaxies 300 million light years away.
If a galaxy is too far away for us to distinguish individual stars, astronomers      can use supernovae in the same manner, because the light output of supernovae      at their peak brightness is a known fact. Supernovae can be used to measure      the distance to galaxies as far as 10 billion light years away.
Who is Edwin P. Hubble and what has he to do with galaxies?
Edwin P. Hubble revolutionized cosmology by proving that the clouds of light      astronomers saw in the night sky were actually other galaxies beyond our Milky      Way.
His greatest discovery was in 1929, when he identified the relationship between      a galaxy's distance and the speed with which it is moving. The farther a galaxy      is from Earth, the faster it is moving away from us. This is known as Hubble's      Law. He also constructed a method of classifying the different shapes of galaxies.
Edwin Powell Hubble was born in Marshfield, Missouri. In 1910, he received      his undergraduate degree from the University of Chicago and studied law under      a Rhodes Scholarship at Oxford University. His true love, however, was astronomy, and he returned to the University of Chicago to earn a Ph.D. in that subject and work at the Yerkes Observatory. He served      in the infantry during World War I.
He once said that he "chucked the law for astronomy," knowing that      even if he was second-rate or third-rate, it was astronomy that mattered.
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