A. INTRODUCTION TO GALACTIC ASTRONOMY

The picture at the right shows the Milky Way as seen from Cerro Tololo Interamerican Observatory. Click for an enlargement.

Thus, even very simple observations about the distribution of stars in the sky can lead to interesting and important conclusions.

B. HISTORY

With the demise of the crystalline sphere model after Copernicus, it was possible to conceive of large---even infinite---distributions of stars in space. One of the earliest such concepts, by Thomas Digges (ca. 1580), is shown below (click for full version):





The possibility that the stars were at very large distances, such that they were vastly brighter intrinsically than they appeared to be, encouraged astronomers to suggest that the Sun and the stars were the same kinds of objects, merely viewed at different distances:
"Across the sea of space, the stars are other suns." --- Christiaan Huygens (1692)
(Proof that the Sun was a star would only come later, with the direct determination of stellar distances and application of physics to the spectra of stars; see Lecture 5).

Galileo made a fundamental discovery about our star system with his first small telescopes in 1610 when he was able to resolve part of the Milky Way into thousands of faint, previously undetected stars.
Galileo commented, "For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters." Up to that time, it was not obvious that the Milky Way---the faint, glittering band of light which seems to ring the sky---was directly related to any other astronomical phenomenon.

Telescopes made it possible to probe the structure of our star system by counting stars in various directions.
If you assume that all stars have the same intrinsic brightness, the counts at each magnitude can be converted into star densities at different distances.
We know now that stars don't actually have all the same intrinsic brightness, but the technique works if stars have the same average brightnesses in all directions.

With his large telescopes Herschel undertook a concerted program of this type ca. 1790 and concluded the distribution of the stars to be as follows:

Herschel found the Sun to lie near the center of this flattened distribution of stars. In 1910, Kapteyn made a much more sophisticated survey of star positions and motions but came up with essentially the same result, with the Sun in the center of a somewhat more flattened disk of stars.
These pictures were plausible, but had enough of an "anti-Copernican" flavor to make some astronomers uncomfortable (i.e. they placed the Sun in a special location). It was important to find a tracer other than ordinary stars.
In 1920, Shapley used globular star clusters (click for an example) as a tracer.
Clusters were valuable first because they are up to 100,000 times brighter than a single star like the sun and second because they contain RR Lyrae-type variable stars whose properties can be used as distance indicators. Click here for an animation showing how variables appear in a globular cluster.

Surprisingly, Shapley found that the globular clusters were centered at a point which was 30,000 light years away from the Sun! This is the true center of our star system, which is therefore much larger than previously imagined. Why had astronomers been misled for so long? See below.

C. STRUCTURE OF OUR GALAXY

• We live in a spiral galaxy, a large, disk-like, slowly rotating star system.

• Face-on, it would look somewhat like the picture at the top of this page. An artist's conception of a face-on view of our Galaxy is here.

• The "spiral arms" are conspicuous because they contain bright generations of younger stars; but the overall mass contrast between the arms and the background disk isn't as large.

• Our Galaxy is huge. It contains about 100 billion solar masses of material, and every star you can see, even with a moderately large telescope, is in our Galaxy.

• The Sun definitely resides in its outskirts, at a distance of about 28,000 light years from the center. The whole galaxy would be roughly 100,000 light years across.

• [Remember that a light year is the distance light travels in one year. This is about 10¹³ km. The parsec is a distance unit based on the size of the Earth's orbit. It is about 3.1 x 10¹³ km or 3.25 light years.]

• The central part of the Galaxy is inflated into a spherical structure, or bulge, and some matter is distributed in a thinner spherical halo that extends to large distances. The old globular clusters Shapley studied are associated with the halo.

• Younger stars, gas, and dust are concentrated to the disk, or "plane," of the Galaxy.
  Interstellar dust is the fine haze of smokelike particles that is distributed between the stars.
  Dust is visible as the dark lanes in the star forming regions illustrated in Lecture 5 and also in the dark rifts in the Milky Way in the picture at the beginning of the next section.
  Dust scatters and absorbs optical light. If there is enough dust in a given direction, it can totally obscure our view of distant regions. Like ordinary dust in Earth's atmosphere, which can produce strikingly red sunsets, interstellar dust also "reddens" starlight.

• The Sun is roughly centered vertically in the plane. The stars we can easily see are therefore mostly associated with the disk. Typically, with small telescopes, we can see only to distances of a few thousand light years in the plane. Because this region is roughly symmetrical, star counts misled early astronomers into believing we were near the center of the Galaxy.

Panoramic mosaic of Milky Way. Click for explanation and orientation.

D. THE MILKY WAY

• The visual-band panorama above shows our view of the Galactic plane, which is, of course, edge-on. Our view of the central part of the Galaxy is obscured by dust clouds, which produce the dark blots and rifts in the picture. For more information on the panorama, click here.

• When we look in the plane of the Galaxy, we see many stars, often bright ones---e.g. in Scorpio, Cygnus, Perseus, Orion, and Gemini. We also see the combined glow of millions of fainter, distant disk stars too faint to resolve individually. This is what produces the "Milky Way."
  On the star charts in Norton's Star Atlas, the Milky Way is shown in green shading.
  The center of the Galaxy is in the direction of Sagittarius, while the "anti-center" is in the direction of Auriga. The Milky Way is less conspicuous toward Auriga because the density of matter in the disk falls off with distance from the center, and we are looking toward the outer part of the Galaxy in this direction.

• When we look perpendicular to the Galactic plane, we see few stars. The Galactic poles are the directions exactly perpendicular to the plane; they lie in the constellations Coma Berenices (north) and Sculptor (south). These directions are free of dust, and here we can therefore see out of our Galaxy into extragalactic space.
  
  
• Dust obscures our view of the central part of the Milky Way at visible wavelengths. However, infrared telescopes can penetrate the dust haze, since dust has less effect on infrared light. Above is an image of the galaxy, similar to that at the beginning of this section, but made at infrared wavelengths by the 2MASS All-Sky Infrared Survey (directed by Prof. Mike Skrutskie, of UVa). At these wavelengths, we can see the bulge and inner disk of the galaxy without interference from dust. Click here for more information on the 2MASS project.

E. OTHER GALAXIES AND THE FAR UNIVERSE

M31 is the most distant object you can see with the naked eye; it is 2.1 million light years away, and the photons you see now from it left the galaxy 2.1 million years ago. The locations of M31 and M33 are shown on the map below:

We are still in the early phases of understanding the life cycles of galaxies. Only in the last 20 years, for instance, have we realized that galaxies can undergo violent gravitational interactions with one another, sometimes leading to "tidal destruction" or, alternatively, "mergers." We now think that our Galaxy will eventually merge with M31 (several billions of years in the future).
Here is a supercomputer simulation (10 MB) of what such a merger would look like. The shapes of the galaxies are transformed by the interaction.

The "Hubble Ultra Deep Field" (part of which is shown above) is the deepest image yet obtained of distant galaxies. Many of the galaxies in such deep images look disturbed or peculiar since they are still in the process of formation (and have recently interacted with others, as in the model shown above, because the distances between galaxies were smaller then). More information on how the Deep Field was imaged and the scientific questions which can be pursued using this data about the early evolution of the universe is available here.

Imaging with photographic or electronic cameras is needed to bring out full details. At right is a galaxy image taken by UVa undergraduates in ASTR 313 using a CCD camera. A good source of background information on observing bright galaxies is The Messier Catalog home page.

• Download, print, and read webnotes for this lecture.

• The best resource for this material is a good ASTR 121/124 textbook. Texts are available for consultation in the Day Lab (267 Astronomy).

• You should finish Lab 3 and move on to Lab 4 at the earliest opportunity.

Recommended:
Atlas of the Universe, a multi-scale map of our universe starting from the Solar System and extending outward to a scale of 15 billion light years. By Richard Powell.

Measuring the ages of other galaxies (O'Connell)
The Shapley-Curtis Debate (1920). Information on a famous debate concerning the structure of our Galaxy and the local universe. H. D. Curtis was one of the first PhD students in astronomy at UVa.
Background information on Edwin Hubble
Nick Strobel's Astronomy Pages for background information on stellar and galactic astronomy.
The Multiwavelength Milky Way
Background information on Cepheid variable stars and their use as distance indicators.
Hubble Space Telescope Image Gallery---best site for space-based pictures of astronomical objects.
Anglo-Australian Observatory Images Page--best site for ground-based pictures of astronomical objects.
The Messier Catalog--- information and links for the brightest nebulae, stars clusters, and galaxies
Tutorial on infrared astronomy
Information on the Hubble Deep Field