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.

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.
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.
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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