Q&A: Galaxies

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... Q&A: Galaxies (cont'd) ...
8. How are galaxies classified today?

Hubble's "Tuning Fork" Diagram

See reference: http://cse.ssl.berkeley.edu/Segwayed/lessons/classifying_galaxies/tunfork.htm

Today we classify galaxies mainly into two major groups following Hubble's examples, as shown above. Elliptical galaxies range from round shapes (E0) to oval shapes (E7).

Spiral galaxies have a pinwheel shape and are classified according to their bulge, as well as how tightly their arms are wrapped around the bulge. They range from Sa, which has a large bulge and tight, smooth arms, to Sc, which has a small bulge and loose, lumpy arms.

Barred spiral galaxies, classified as SB, are pinwheel-shaped and have a distinct "bar" of stars, dust, and gas across their bulge. They range from an SBa, which has a bar across its large bulge and tight, smooth arms, to an SBc, which has a bar across its small bulge and loose, lumpy arms.

Irregular galaxies have no definite shape but still contain new stars, gas, and dust. The chart below summarizes the properties of the main classes of galaxies.

Properties of the main classes of galaxies
Spiral/Barred Spiral
Shape and structural properties Have disks of stars, gas, and dust containing spiral arms that attach to a central bulge. Sa and SBa have the largest bulges. SB galaxies have central bars. Have neither disk nor arms. Stars are distributed evenly from near circular to oval (football). Have no definite structure.
Stellar content Have both young and old stars. Halos consist of old stars only. Contain mostly old stars. Contain both young and old stars.
Gas and dust Disks contain gas and dust. Halos contain little gas or dust. Have little or no gas or dust. Have lots of gas and dust.
Star formation Stars form largely in spiral arms. Have little or no star formation. Have lots of star formation.
Stellar motion Gas and stars rotate around the center of the galaxy. Stars move on randomly oriented orbits like a swarm of bees. Stars and gas have irregular orbits.

9. Who is Edwin P. Hubble and what has he to do with galaxies?

Edwin P. Hubble revolutionized cosmology by proving that galaxies are indeed "island universes" beyond our Milky Way galaxy. 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 with the Hubble Tuning Fork (see question 8).

Edwin Powell Hubble was born in Kentucky, where he grew up observing the habits of birds and animals. In 1910 he received his undergraduate degree from the University of Chicago and studied law under a Rhodes Scholarship at Oxford University. Later he changed his mind and completed a Ph.D. in astronomy at Chicago's Yerkes Observatory in 1917. He had several other interests, and for a while he thought of becoming a professional boxer. He also enjoyed basketball, and even answered a call in World War I to serve in the infantry.

Hubble 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.

10. Galaxy names are identified by a group of letters and numbers. What do they stand for?

Scientists classify galaxies in different catalogs. The most common catalog is NGC, which stands for New General Catalog. Other catalogs include M (Messier), ESO (European Southern Observatory), IR (Infrared Astronomical Satellite), Mrk (Markarian), and UGC (Uppsala General Catalog). Sometimes a galaxy appears in more than one catalog and can have more than one name.

The numbers following the letters, such as Mrk917 (Sc) or NGC1433 (SB), indicate the galaxy's entry in the catalog and are often related to the galaxy's relative position in the sky.

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Q&A: Galaxies