University of Michigan - Department of Astronomy

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Version: intro long


Stellar Populations and the Milky Way Halo

gazing far up the lanes of Sagittarius
richest stream of our sky—
a cup to the center of the galaxy!.

- Gary Snyder "Burning Island"

Introduction

In the early 20th century, the shape and extent of our Milky Way Galaxy were not yet known. It was generally believed that the Galaxy was roughly disk-shaped, about 20,000 LY in diameter and 6000 LY thick, with the Sun near the center. Astronomers arrived at this size and shape by studying the band of light we usually refer to as the Milky Way.

If you did the "The Milky Way Disk" activity, you already know that most of the stars, and especially the hot, young stars, reside in the aptly named Galactic disk. Numerous techniques were developed in the last century for accurately measuring the size, shape, mass, speed, and so forth. We now know the disk is about 100,000 LY across and only 1000 LY thick, and it contains most of the gas, dust, and younger stars. But there are stars belonging to other stellar populations in our Galaxy besides the disk.

The Galactic Halo

M13Figure 1: M13 globular cluster in Hercules

In the early 20th century, a young astronomer named Harlow Shapley began mapping the globular clusters. Globular clusters have hundreds of thousands to millions of stars, and they are spherical in shape. M13 in Hercules is the best known northern-hemisphere globular cluster, and it can be seen with the naked eye in very dark skies. Stars in globular clusters are ancient, low-mass stars, and were probably among the earliest stars to form in our Galaxy. The globular clusters are not found in the disk, but instead form a spherical stellar halo. The halo also contains millions of individual field stars, but only 2% of the Galaxy's stars are in the halo, and most of them are in globular clusters.

 

Modal of the Milky WayFigure 2: Modern model of the Galaxy

The discovery of the halo of globular clusters led to identifying the Sun's location in our Galaxy. There were two important parts to determining the Sun's position. The first was the distribution of the globular clusters on the sky. Herschel noted in the early 19th century that the clusters were not uniformly distributed on the sky. The second was the determination of the distance to these clusters. Shapley used RR Lyrae variable stars to determine the distances to nearby globular clusters, but telescopes of the time were unable to resolve individual stars in the more distant clusters. However, since globulars are all similar, he used their apparent sizes and brightnesses to to estimate their distances from Earth.

Unfortunately, Shapley did not know about interstellar dust, which dims and reddens starlight (see the Galactic Star Clusters activity for more about this, and how globular clusters are used to study stellar evolution). With dust in the line of sight, the clusters seem farther away than they really are. Thus, Shapley estimated the size of the Galaxy to be around 300,000 LY x 30,000 LY, with the Sun roughly 50,000 LY from the center.

There are an estimated 200 globular clusters in the Milky Way, and we can see about 150 of them clearly enough to have good measurements of their distances. Today, we know that the Sun is in the disk, roughly 26,000 LY from the center, or slightly more than half way out. The stellar halo extends about 200,000 LY in radius.

The Galactic Bulge

Another stellar population that is spheroidally shaped is the Galactic bulge. You know that the relative size and luminosity of a galaxy's bulge relative to its disk is an important criterion for classifying its galaxy type. The Milky Way bulge is extremely difficult to observe because of all the dust from the disk that falls in our line of sight. However, there is one line of sight which happens to be almost dust-free, and offers a view into the bulge. This line of sight is called Baade's Window, located at 18h 03m, -30d 02'. The dust-free region is about 1º in diameter.

Sag A*Figure 3: Sagittarius A*

Another way to view the central regions of our Galaxy is to use longer wavelength light, such as infrared and radio emission. These wavelengths penetrate dust more easily than visible light. The earliest radio astronomers quickly discovered that there is a strong radio source at the center, named Sagittarius A* (pronounced “A-star”). It also appears in X-ray and gamma ray observations. There are some links in the Resources to some beautiful images of this region.

Recently, infrared observations of stars orbiting the center of the Galaxy have narrowed the estimates of the size of Sgr A* to something smaller than the orbit of Mercury, and 4 million times the mass of the Sun. Astronomers have determined that it must be a supermassive black hole at the center of our Galaxy.

 

The Disk and the Solar Motion

The Sun is in the stellar disk, which is rotating at 220 km/s. It is often convenient to assume that the Sun's motion is perfectly circular, and moving at exactly 220 km/s. This idealized motion for the Sun is called the Local Standard of Rest (LSR). Astronomers measure velocities relative to the LSR instead of relative to the Sun's true velocity, because the LSR provides simpler geometry and motion. The point toward which the Sun is moving is called the apex, and the point away from which it appears to be moving is the antapex. We can consider the apex and antapex of the true solar motion, or of the LSR. In Figure 4, we can see some of the Galaxy's spiral arms, which are trailing as the Galaxy rotates.

Galactic longitudeFigure 4: Galactic longitude

If you did the Coordinate System or Celestial Sphere activity, you know about different coordinate systems astronomers use. There is also a coordinate system based on the Milky Way disk and centered on the Sun, which is the Galactic coordinate system. You may have seen this already if you did "The Milky Way Disk" activity.

In Galactic coordinates (l, b), the Galactic equator is set by the Galactic disk. The Galactic Center is defined to have both Galactic longitude and latitude of zero, (l, b) = (0, 0). Galactic longitude (l) is measured in degrees away from the Galactic Center as shown in Figure 4. Galactic latitude (b) is measured in degrees above or below the Galactic equator. The Galactic Anticenter is the point representing the direction exactly opposite the Galactic Center on the Galactic equator, (l, b) = (180, 0).

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Updated: 10/31/12 by SAM & MSO

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