Introduction to Galaxies

Stars do not exist as individual objects, but most often are found in double systems called binary systems, where two stars are orbiting each other. But, stars are not only grouped together in pairs, triples, or even quartets, but also in much larger collections called galaxies. A galaxy is a vast collection of stars, gas, dust, space, and dark matter that is invisible to the eye, but whose presence is detected by its gravitational influence. During the middle of the 19th Century, astronomers were constructing bigger telescopes with greater light-gathering power. Many of the "nebulae" observed and catalogued by Charles Messier in the 1700's were now resolving themselves into collections of numerous stars, but there was still widespread disagreement as to the true nature of these "spiral nebulae." The proof that these objects were galaxies of stars came on December 30,1924. Edwin Hubble was working atop Mt. Wilson with a new 100-inch telescope and he was able to photograph a few bright galaxies. He noticed individual stars as well as several Cepheid Variable Stars with an apparent magnitude of 18. This kind of star is in a class called "supergiants," and for such large stars to have such a faint apparent magnitude means that their location must be very far away ... external to our own Milky Way. What was once considered parts of our Milky Way galaxy were now discovered to be distant objects akin to our galaxy, and the more astronomers probed deep space, the more galaxies they found. Modern photographs have revealed many galaxies in space everywhere.

The purpose of this unit is to take a look at the structure of our home galaxy, the Milky Way, and then compare our place to other galaxies. This unit is therefore divided into the following pages:

Our celestial home, The Milky Way

Galaxy shapes and interactions

Formation of Galaxies ... a look at the beginning, and a nice article on Galaxy Formation from Sky & Telescope Magazine, October, 2004 issue.

Galaxy images ... a look at a collection of fantastic photographs of various galaxies, including the most recent HST Ultra Deep Field!

How many galaxies ... the Hubble Deep Field Lab

You can proceed forward to The Milky Way, or go to the Syllabus .

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