Chapter 15: Galaxies

A 360-degree photographic panorama of the Milky Way showing our edge-on view of the galaxy.

Astronomers often make a simple distinction between the primary elements of a main sequence star — hydrogen and helium — and all the elements with higher atomic numbers in the periodic table. Elements beyond helium can only be found in a star after a star from a previous generation has left the main sequence and ejected those heavier elements into space. Therefore, the proportion of elements beyond helium is a measure of the total amount of previous star formation in the region of space that contains the star. Astronomers refer to elements other than hydrogen and helium as "heavy elements," even though they are not all very heavy, and they also refer to them as "metals," even though they are obviously not all metallic elements!

The grand design spiral galaxy M51, also known as the Whirlpool galaxy, as seen by the Hubble Space Telescope

Stars in the halo, called halo population stars, live in a spherical swarm that extends far above and below the disk of the galaxy. The halo contains many individual halo stars and a few hundred majestic globular clusters. All these stars are moving on elliptical orbits that loop in and out of the galactic plane from any direction. Halo stars were first identified due to their rapid motions. The Sun is traveling around the disk at a speed of 225 km/s. Stars near us in the disk show relatively small Doppler shifts of 10 to 20 km/s with respect to the Sun.

Distribution of star populations in Milky Way

Astronomers have slightly confusing terminology for the stellar populations. Population I is the term for the disk population, not because these stars formed first, but because they were the first type of stars astronomers became familiar with, located near the Sun. Population II is the term for the halo population, the type of star discovered second. We can also use the more descriptive terminology of disk population and halo population. Stars with only materials found in the big bang are called population III stars, and no examples have been found, although some stars that have compositions indicative of having only 1 or 2 supernovas contributing to their composition have been found and are sometimes called population III stars. These stars are found in the galaxy's halo.

The halo has ten times more stellar mass than the disk. The many stars in the halo are not readily apparent for two reasons. First, they are often at large distances from the Sun, so their light is dimmed by the inverse square law of radiation. Second, the halo population is old enough that the most massive and luminous stars left the main sequence long ago. Halo stars are mostly low-mass main-sequence stars and white dwarfs, stars that are intrinsically dim. Remember also that the stellar census does not include dark matter. The total mass of the galaxy is about 5 × 10¹¹ solar masses, most of it lying far beyond the visible halo and disk. Adding the stellar mass of the three populations, they only account for about 15% of the mass of the galaxy. The bulk of the galaxy is composed of enigmatic dark matter.