ASTR 130 (O'Connell) Lecture Notes
6. GALACTIC ASTRONOMY
Spiral galaxy NGC 1232 (ESO VLT)
A. INTRODUCTION TO GALACTIC ASTRONOMY
Even a casual familiarity with the sky reveals that the stars are
unevenly distributed. For instance, the region containing the
"watery" Zodiacal constellations like Capricorn, Aquarius, and Pisces
in the autumn sky, contains few bright stars compared to the area
between Lyra and Scorpio in the summer sky or the region of the
"Winter Hexagon." This raises the question of the spatial
structure of the star system in which the Sun resides.
The fact that the sky does not look the same in all directions
tells you immediately that the matter in the universe cannot be
distributed in a spherically-symmetric fashion about the Earth's
location. Our star system cannot be a sphere with the Earth at its
center. Thus, even very simple observations about the distribution of
stars in the sky can lead to interesting and important conclusions.
The study of the structure of our star system revealed the spatial
scale of the universe near the Earth, analogous to the way that
the study of the physics of the stars (in
Lecture 5) revealed the temporal scale of the universe.
Just as in the case of the temporal scale, the spatial scale of our
universe is vastly larger than anyone had expected.
B. HISTORY
The question of the structure of our star system would have made no
sense to pre-Copernican astronomers because in the old, geocentric
cosmologies, the stars were thought to be small luminous bodies fixed to a
sphere centered on the Earth and rotating about Earth once a day.
The stars had no distribution in depth.
With the demise of the crystalline sphere model, 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 large distances to the stars in such pictures also encouraged
astronomers to suggest that the Sun and the stars were the same
kinds of objects, merely viewed at different distances. (Proof
that the Sun was a star would only come later, with the
direct determination of stellar distances and application of
physics; 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. 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 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 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 found 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 (i.e. they placed the Sun in a special location) to make some
astronomers uncomfortable. 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. These were valuable first
because they are up to 100,000 times brighter than a single star like
the sun and second because they contain variable
stars whose properties can be used as distance indicators.
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
Shapley's picture has been refined considerably. An edge-on sketch of
our Galaxy based on our current understanding is shown below:
- 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. 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.
[A light year is the distance light travels in one
year. This is 1.0 x 1014 km or 3.25 parsecs.]
- The central part of the Galaxy is inflated into a spherical
structure, or bulge, and some matter is distributed in a
thinner spherical halo which 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
which 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." 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.
- 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 with an infrared
satellite (COBE). Here we can
see the bulge and inner disk of the galaxy without interference from
dust.
E. OTHER GALAXIES
Shapley had used a type of variable star to determine distances to
globular clusters. Soon afterwards, Hubble (1923) applied a
similar technique, using intrinsically luminous "Cepheid variables,"
to the brightest of the many faint, diffuse "spiral nebulae" which had
been first recorded about 125 years earlier. He was able to
demonstrate conclusively that
Messier 31 (the "great
nebula in Andromeda")
was an independent star system outside our own. Although
the more evocative term "island universes" was used for a while,
external star systems quickly became known as galaxies and
our own star system as the Milky Way Galaxy. ("Galaxy" is
derived from the Greek root for "milk.")
Note: Cepheid variables are the subject of ASTR 130 Lab No. 6.
Two galaxies in the northern hemisphere are visible with the naked eye
or binoculars: M31 in Andromeda and M33 in Triangulum. M33 is quite
faint, but M31 is readily visible on a dark night. This 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:
Since Hubble's discovery, astronomers have devoted tremendous effort
to probing the distant universe. We have found that there are
over 1 billion galaxies within reach of our best telescopes.
There are many types of galaxies, covering a wide range of
morphologies (shapes) and an enormous range of mass. Just as in the
case of our Sun in the context of other stars, our Galaxy is only
average in properties. We are only in the early phases of
understanding the life cycles of galaxies.
The Hubble Deep Field
Given their intrinsic brightnesses, galaxies can be detected at
very great distances. With the Hubble Space Telescope and large
ground-based telescopes, we have detected many galaxies over 10
billion light years away(!) Because of the finite speed of light, we
are viewing them as they were 10 billion years in the past. This
"time machine" therefore allows us to observe galaxy evolution in
progress. The "Hubble Deep Field" (part of which is shown above) is
the deepest image yet obtained of distant galaxies. 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.
We will not discuss galaxies further except to say that hundreds of
nearby ones are accessible to an 8-in telescope under dark sky
conditions. The views possible are, of course, much less detailed
than the deep exposure picture at the top of this page, though with
good conditions you would be able to distinguish shape, spiral
structure, dust lanes, and so forth. 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. A good source
of background information on observing bright galaxies is The Messier Catalog home
page.
Homework:
- 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 Astronomy
Library and in the Day Lab (267 Astronomy).
- You should be working on Labs 3 and 4.
Web links:
Last modified
24 May 2001 by rwo
M31-M33 map copyright © Hawaiian Astronomical Society;
other images in public domain. Text copyright © 2001 Robert W.
O'Connell. All rights reserved. These notes are intended for the
private, noncommercial use of students enrolled in Astronomy 130 at
the University of Virginia.
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