The Birth of the Milky

The structure, composition, and motion of the Milky Way hold the keys to its origin. Though the theory of galaxy formation is far from complete, we have a fairly good picture of how our Galaxy might have formed.

This image of the center of the Milky Way galaxy was generated by the Very Large Array. The loops and filaments apparent are related to the presence of magnetic fields. The Sgr A* source (thought to be the center of our galaxy) is located in the lower right of the image.

(Image from NRAO)

Ten to fifteen billion years ago, an enormous cloud of gas began to collapse under its own gravity. The cloud, like all the universe, would have been mostly hydrogen. It would have had a mass equal to that of all the stars and gas in the Milky Way, or several hundred billion solar masses.

The stars that formed first as the great cloud collapsed assumed randomly oriented elliptical orbits—with no preferred plane. Today, the oldest stars in our Galaxy—which are those in the Galactic halo and Galactic bulge—ring with the echoes of their earliest days. The Galactic halo is a vestige, a souvenir of the galaxy's birth. And the globular clusters in the halo may have even formed prior to the cloud's collapse.

Star Words

Mass-to-light ratio is the ratio of the total mass of a galaxy (detected by its gravitational effect) to the mass in luminous matter. If there were no "dark matter," the mass-to-light ratio would be 1.

Star Words

Mass-to-light ratio is the ratio of the total mass of a galaxy (detected by its gravitational effect) to the mass in luminous matter. If there were no "dark matter," the mass-to-light ratio would be 1.

In a gravitational collapse process very similar to that which formed the solar system (see Chapter 11, "The Solar System Home Movie,") the great cloud began rotating faster around a growing mass at the center of the Galaxy. The rotation and collapse caused the clouds of gas to flatten into the Galactic disk, leaving only stars in the halo.

Since the Galactic disk is the repository of raw materials, it is the region of new-star formation in the Galaxy. The Galactic halo is out of fuel and consists only of old (cool) redder stars. The orderly rotation of stars and gas in the Galactic disk stands in contrast to the randomly oriented orbits of stars in the halo and the Galactic bulge.

"What you see is what you get," the popular saying goes. Such is not always the case in Galactic affairs.

Using Kepler's Third Law, we can calculate the mass of the Galaxy. This mass (expressed in solar masses) can be derived by dividing the cube of orbit size (expressed in astronomical units, or A.U.) by the square of orbital period (expressed in years). And Newton told us that at a given radius, all of the mass causing the rotation can be considered to be concentrated at a point at the center of rotation. For a system in "Keplerian rotation" (like a planetary system), we would expect the velocities of rotation to decrease as we looked farther and farther out, much as Jupiter orbits more slowly than, say, Mercury.

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment