The Milky Way: the galaxy we know the most about (by far)

Key points: Parts of a spiral galaxy: nucleus, bulge, disk, halo, dark matter; population I and II stars; formation of the Milky Way

As we have already discussed, what we see of the Milky Way in visible light is so strongly affected by interstellar dust that it is difficult to get an overall impression of the shape of the system.

optical mosaic showing whole Milky Way

When we observe the Milky Way in the infrared (so interstellar dust does not block our view), we get a much better view. The boxy appearance and asymmetry in the bulge - wider to the left - shows that it is barred with the near end of the bar to the left. (From 2MASS, DIRBE projects) milkyway2mass.jpg (281187 bytes)
It looks a lot like what we see when looking at another Sc galaxy edgewise (also called "edge-on"). This one does not appear to have a bar. NGC 891 in the near infrared

Mapping of spiral arms of the Milky Way:

Map of Milky Way from above A map of the Milky Way deduced from spiral arm tracers is to the left. Spiral arms can be located with 21-cm line radiation, HII regions, O and B stars.

In the Milky Way, visible light tracers like HII regions or O and B stars can only locate nearby spiral arms but dust prevents using these tracers on a wide scale in the Galaxy.

21-cm line radiation from atomic hydrogen is at such a long wavelength that it can penetrate dust and delineate arms on the other side of the galaxy. Because it is an emission line, it can be used to measure radial velocities. The velocities can be used to determine where along the line of sight the gas lies that is producing the line -- for example,  the parts of the galaxy closest to the center are rotating around the center at specific velocities set by the gravitational field -- analogous to how the inner planets in the Solar System orbit the Sun at specific speeds and with specific periods given by Kepler's Laws.


Astronomy Picture of the Day, R. Hurt, Glimpse Team


We get enough of a hint of spiral structure to prove that we live in a spiral galaxy, but detailed studies of spiral arms are best made in galaxies we can "see" better.

Here is what we see when we look at another barred Sc galaxy "face-on" (which of course we cannot do with the Milky Way!).


We conclude that the Milky is a barred Sc galaxy. Although it is difficult for us to see its grand design, because we live inside it we can study many aspects of it in far more detail than we can for other galaxies, deducing properties of spiral galaxies in general.


The structure of the Milky Way (and other spiral galaxies) consists of:

1.) bulge (roughly spherical system of old stars surrounding the nucleus)

2.) disk (flat system centered on bulge that contains the spiral arms, with their molecular clouds and young stars)

3.) halo (large spherical system of old stars containing globular clusters, some isolated stars, and surrounding the whole visible galaxy)

4.) nucleus (small region in the very center that may contain black hole) - called the "Galactic Center" for the Milky Way

5.) dark matter

Here are some more views of the Milky Waybuttonbook.jpg (10323 bytes)

Stellar Populations and Correlation of Stellar Properties with Location in the Galaxy

Population I, Population II are names given to broad categories of stars in the galaxy, according to where they are found, what kinds of orbits they circulate on, how rich they are in heavy elements, and their typical ages:

  Population I Intermediate Population II
Orbits Circular Elongated Very elliptical
Distribution patchy slightly patchy smooth
Shape spiral arms disk spherical
Central concentration none slight strong
Thickness (pc) 120 400 2000
Elements heavier than hydrogen and helium (%) 3-4 0.4-2 0.4 or less
Total Mass(M) 2x109 5x1010 2x1010
Age (yr) 108 109 1010
Typical Object Molecular Clouds, Open clusters, HII regions, OB stars Sun Globular clusters, RR Lyrae star
Orbits of population one stars in disk Population I includes the younger stars in the disk/plane of the galaxy. Because these stars formed recently, they have all be enriched in heavy elements produced in previous generations of stars.
Orbits of population two stars, extend out of plane Population II is the older stars that tend to lie around the center and in globular clusters, and hence have orbits that take them well out of the disk/plane. Many of these stars were among the first to form, and hence they tend to be almost pure hydrogen and helium, not enriched by previous generations of stars because there were no previous generations. From Gene Smith,


If we put all these lines of evidence together, we get a consistent picture of the Milky Way as a typical spiral galaxy.

m109_mw.gif (156484 bytes) A view of a very similar galaxy (M109) as our Galaxy would look from outside. It has a central  bulge and around it a distribution of globular clusters and a diffuse halo of stars (all population II). Around the bulge is the disk, with young O and B stars, emission nebulae, open clusters, and other markers for population I. Gas lies generally in the plane and extends well beyond where there is much starlight. The dark matter extends well outside any of the visible galaxy into and beyond the halo. (by G. Rieke, based on an image from Bill Keel,

Why are the properties of stars correlated with position in the galaxy?

The Milky Way formed from a large, rotating cloud that collapsed; the old halo stars outline the original shape of the cloud, since they formed first. The bulge next and thus it also contains very old stars. The youngest stars lie in the disk, both because stars formed late there and because the disk has enough gas that they continue to form to this day.

galaxforma.jpg (519428 bytes) Here is a summary of how the galaxy formed

(From RAVE: Radial Velocity Experiment, Matthias Steinmetz, additions by G. Rieke)

How did the Milky Way formbutton.jpg (6796 bytes)

Test your understanding before going onbuttongrad.jpg (11232 bytes)

galaxyqult.jpg (16627 bytes)




Galaxy quilt, by Paula van der Zwaan, from


sirtflaunch.jpg (4413 bytes)

badtoworsea.jpg (19538 bytes)






Far Side by Gary Larson, from Peter Barthel,

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hypertext copyright.jpg (1684 bytes) G. H. Rieke

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