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 an obvious question: what is 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 uniform fashion about the Earth's location. Our
star system cannot, for instance, 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
A question about "the structure of our star system" would have made no
sense to pre-Copernican astronomers because in the ancient geocentric
cosmologies, the stars were thought to be small luminous bodies fixed to a
crystalline sphere centered on the Earth and rotating about Earth once a day.
In this model, the stars had no distribution in depth.
With the demise of the crystalline sphere model (after 1550), 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. [The picture at the
right shows the Milky Way as seen from Cerro Tololo Interamerican
Observatory. Click for an enlargement.]
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 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 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. 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 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
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. 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."
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
Shapley had used RR Lyrae variable stars to determine distances to
globular clusters. Soon afterwards,
Hubble (1923) applied a
similar technique, using intrinsically luminous Cepheid
variables, to estimate the distance to the brightest of the many
faint, diffuse "spiral nebulae" which had been first recorded about
125 years earlier. [Note: Cepheid variables are the subject of ASTR 130 Lab No. 6.]
By this method, Hubble was able to demonstrate conclusively that Messier 31
(the "great nebula in Andromeda") is 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.")
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.
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:
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 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
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 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.
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 using
a CCD camera. 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
Day Lab (268 Astronomy).
- You should finish Lab 3 and move on to Lab 4 at the earliest opportunity.
Web links:
Last modified
October 2004 by rwo
M31-M33 map copyright © Hawaiian Astronomical Society.
Image of Milky Way over CTIO copyright © Roger Smith
(NOAO/AURA/NSF). Galaxy merger animation by John
Dubinski. Text copyright © 2000-2004 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.
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. [The picture at the
right shows the Milky Way as seen from Cerro Tololo Interamerican
Observatory. Click for an enlargement.]
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 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 using
a CCD camera. A good source
of background information on observing bright galaxies is The Messier Catalog home
page.
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.
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