$$\require{cancel}$$

# 15.8 Stellar Populations

One of the most interesting discoveries about our galaxy is that it contains a range of star types of different composition, age, distribution, and orbital geometry. Smaller collections of stars that have formed together and are held together by gravity — stellar groups and associations, and globular clusters. However, there are also major components of the Milky Way galaxy that share properties. Large collections of stars that share properties of age or chemical abundance or motions are called stellar populations.

Around us in the disk of the Milky Way are stars that are relatively young. The spiral arms of the disk are traced by star-forming regions that are full of hot, young stars. These stars travel in circular orbits around the galactic center, like the Sun, and are called disk population stars. (In fact, disk stars have a slight vertical motion that makes them undulate as they orbit the galaxy.) Only a small percentage of the mass of disk population stars consists of elements heavier than helium, including carbon, oxygen, silicon, and iron. Compared to the Sun, these disk stars have heavy element abundance ranging from 1/10 to 2 times the Sun's abundance.

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 the 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!

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 of the 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.

Imagine that we are whipping around a circular racetrack. All the cars around us are going at roughly the same speed, but there is a smaller relative motion due to the cars that pass us and the cars that are being passed by us. Early in the 1900s, astronomers found stars at high galactic latitude with large Doppler shifts of 100 to 200 km/s. Such high speeds represent the motion of halo stars plunging toward or away from the disk. There are even a few near the Sun that are speeding through the solar neighborhood. Stars of the halo population are nearly pure hydrogen and helium with only a fraction of a percent of heavier elements, or 1/1000 to 1/30 of the Sun's abundance.

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 galaxies halo.

Astronomers now know that the division of all stars into two primary populations is an oversimplification. The orbits and volume of space occupied by disk and halo stars are different, but in other respects each population shows a range of properties. Halo stars generally show a tiny fraction of elements heavier than helium, whereas disk stars show a range of 1 to 5% in their abundance. Since halo stars formed with 10 to 50 times less of the important planet-forming materials such as silicon, oxygen, nickel, and iron, we might speculate that planets (and perhaps living organisms too) are rare around halo stars. Halo stars are old and have an age range of about 9 to 13 billion years, as indicated by the ages of globular clusters. Disk stars have an enormous range in age. The Sun is 4.5 billion years old, and a few disk stars are even older. Most disk stars are much younger and the age range goes all the way down to zero — we see stars that are just forming now.

The bulge of our galaxy represents an intermediate population. Within a few kiloparsecs of the galactic center, there are not only stars that are old and red but also stars that are relatively young and blue. The bulge population is most similar to the halo population but it includes younger stars with a larger proportion of heavy elements.

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.

A 360-degree photographic panorama of the Milky Way showing our edge-on view of the galaxy. Click here for original source URL.

The grand design spiral galaxy M51, also known as the Whirlpool galaxy, as seen by the Hubble Space Telescope. Click here for original source URL.

"," 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!

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 of the 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 small Doppler shifts of 10 to 20 km/s with respect to the Sun.

Imagine that we are whipping around a circular racetrack. All the cars around us are going at roughly the same speed, but there is a smaller relative motion due to the cars that pass us and the cars that are being passed by us. Early in the 1900s, astronomers found stars at high galactic latitude with large Doppler shifts of 100 to 200 km/s. Such high speeds represent the motion of halo stars plunging toward or away from the disk. There are even a few near the Sun that are speeding through the solar neighborhood. Stars of the halo population are nearly pure hydrogen and helium with only a fraction of a percent of heavier elements.

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 galaxies halo.

Distribution of star populations in Milky Way. Click here for original source URL.

Astronomers now know that the division of all stars into two primary populations is an oversimplification. The orbits and volume of space occupied by disk and halo stars are different, but in other respects each population shows a range of properties. Halo stars generally show a tiny fraction of elements heavier than helium, whereas disk stars show a range of 1 to 5% in their abundance. Since halo stars formed with 10 to 50 times less of the important planet-forming materials such as silicon, oxygen, nickel, and iron, we might speculate that planets (and perhaps living organisms too) are rare around halo stars. Halo stars are old and have an age range of about 9 to 13 billion years, as indicated by the ages of globular clusters. Disk stars have an enormous range in age. The Sun is 4.5 billion years old, and a few disk stars are even older. Most disk stars are much younger and the age range goes all the way down to zero — we see stars that are just forming now.

The bulge of our galaxy represents an intermediate population. Within a few kiloparsecs of the galactic center, there are not only stars that are old and red but also stars that are relatively young and blue. The bulge population is most similar to the halo population but it includes younger stars with a larger proportion of heavy elements.

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.