University of Michigan - Department of Astronomy

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Version: intro


The Structure of the Milky Way Galaxy

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

- Gary Snyder "Burning Island"

Overview


Introduction

The Galactic Disk

Galactic longitudeFigure 1: Top-down illustration of our Milky Way Galaxy.

 

When Galileo pointed his telescope at the band of light across the sky that we see as the Milky Way, he found that it resolved into stars. Thus, the Milky Way represents a preferred location for stars on our sky, indicating that the Sun is located within a giant disk of stars. However, it was not until the mid-20th century that astronomers developed a clear understanding of the structure and size of our Milky Way Galaxy. How does the milky band of light across the sky translate into a spiral disk galaxy? What are the different parts of this galaxy, and where are they on the sky?

The Galactic disk contains not only stars, but also almost all the gas and dust in our Galaxy. Since stars form out of this gas, the youngest stars are located closest to the plane of the disk. The gas and young stars are organized into spiral arms (Figure 1). The entire disk is rotating at 220 km/s. We now know that the disk is about 100,000 LY across and only 1000 LY thick.

The Galactic Halo

M13Figure 2: M13 globular cluster in Hercules

 

Globular clusters are dense, giant star clusters, with 105 - 106 stars. M13 in Hercules is the best known northern-hemisphere globular cluster, and it can be seen with the naked eye in very dark skies (Figure 2). 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 found in a vast, spherical Galactic halo of stars, surrounding the Galactic disk. The halo also contains millions of individual field stars. However, the halo is extremely sparse and contains only 2% of the Galaxy's stars, most of which are in globular clusters (Figure 3).

 

Modal of the Milky WayFigure 3: Modern model of the Galaxy

 

This halo with globular clusters allowed astronomers to identify the Sun's location in our Galaxy, by mapping their positions and estimating their distances. William Herschel noted in the early 19th century that the clusters were asymmetrically distributed across the sky. In the early 20th century, Harlow Shapley used pulsating variable stars to determine the distances to nearby globular clusters. Since all globulars are fairly similar, he used the apparent sizes and brightnesses of the farther ones to estimate their distances from Earth. Shapley correctly assumed that the clusters are in a spherical cloud whose center coincides with the Galactic center, allowing him to estimate the distance of the Sun from the Galactic center (Figure 3).

Unfortunately, Shapley did not know about interstellar dust, which obscures starlight. With dust in the line of sight, the clusters seem fainter and farther away than they really are. Thus, Shapley estimated the size of our Galaxy to be several times larger than its actual size.

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 about 26,000 LY from the Galactic 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 (Figure 3). 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 is an SBb galaxy, and has a mid-sized bulge, composed of billions of stars. These are also old stars, although not quite as ancient as those of the Galactic halo. Were it not for all the dust blocking our view, the Galactic bulge would be extremely bright and light our night sky to daytime levels of illumination. The bulge is about 15,000 LY in radius. It is also associated with the Galactic bar, which is an elongated stellar structure of similar size (see Figure 1).

The Milky Way bulge and bar are extremely difficult to observe because of all the dust from the disk that blocks our view. 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, -30º 02'. The dust-free "window" is about 1° in diameter on the sky.

Galactic Coordinates

We often need to understand the position of objects in relation to where they are located in the Galaxy. For example, is an object near or far from the plane of the Milky Way? How can we visualize that? Ordinary equatorial coordinates are not intuitively useful for doing this. Thus, it is convenient to define yet another coordinate system, Galactic coordinates, which are defined by the plane of the Galaxy. In the Galactic coordinate system, the Galactic equator is defined by the plane of the Milky Way, and it includes both the Galactic Center and the Sun. The Galactic poles represent the coordinate system's axis, perpendicular to the Galactic disk (Figure 4).

Figure 4: Galactic CoordinatesGalactic Coordinates

 

 

Galactic longitude and Galactic latitude are abbreviated as (l, b), respectively. Note that you are again standing at the origin of this coordinate system, just as for the equatorial and altitude-azimuth coordinate systems. In the Galactic coordinate system, the zero longitude is not arbitrary, but is the longitude of the Galactic Center. Conveniently, there is a bright point source at the exact dynamical center of the Milky Way Galaxy, the supermassive black hole, Sgr A*. The Galactic anticenter is the point on the sky diametrically opposite the Galactic Center. Longitude is defined counter-clockwise, looking down from the North Galactic Pole (see Figure 1). It is measured in degrees on a parallel to the Galactic equator, like longitude on the Earth, and azimuth. Similarly, Galactic latitude is measured in degrees of arc from the Galactic equator (the plane of the Milky Way) toward the Galactic poles (Figure 4), analogous to latitude on Earth, altitude, and declination. For example, the coordinates of the Galactic anticenter are (l, b) = (180°, 0°).

Note that Galactic coordinates are yet another way of specifying an object's position on the sky, not its position in the Galaxy! Figure 5 shows a map of the entire sky in Galactic coordinates, marked with the positions of energetic X-ray sources seen by the Swift mission. Using Galactic Coordinates allows us to see the relationship of the sources, if any, to the structure of our Galaxy.

Figure 5: All-sky map of Swift X-ray sources in Galactic coordinates

gamma ray sky map in galactic coordinates

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Updated: 08/27/14 by MSO

Copyright Regents of the University of Michigan.