ASTR 1230 (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
The picture at the right shows the concentration of stars to the
Milky Way, as seen from Cerro Tololo Interamerican Observatory in
Chile. Click for an
enlargement. Here is a
wide-field image showing the strong asymmetry in the northern
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
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.
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, and they had no
relationship to the Sun. There were thought to be no more than
a few thousand of them (those visible to the naked eye).
(1) Post-Copernican Structure
Copernicus (1543) showed that the apparent motions of the Sun and
stars in the sky were not intrinsic to them but rather were caused by
motions of the Earth. Copernicus argued that the Earth was a spinning
planet in orbit around the Sun and did not occupy a central position
in the universe.
With the consequent demise of the crystalline sphere model, it was
possible to conceive of the stars being distributed in depth in space,
possibly even to infinity. One of the earliest such concepts,
by Thomas Digges (ca. 1580), with "the orb of stars fixed infinitely
up," is shown below (click for full version):
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
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.
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 is a star would only come much later because it
proved very difficult to actually measure the distances to the stars.
That was first accomplished in 1838 by Friedrich Bessel, who used the
parallax to measure the distance of 61 Cygni. This method
revealed that even the nearest stars are over 200,000 times more
distant than the Sun, a fact that would have flabbergasted Copernicus,
Galileo, or Huygens.
Knowing the distances,
astronomers could estimate the luminosities of the stars, and those,
combined with the application of physics to the spectra of stars,
proved that the stars and the Sun had similar intrinsic properties.
See Lecture 5 for more
(2) Deep Telescopic Probes
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 (luminosity), the counts at each
magnitude can be converted into star densities at different distances.
We know now that stars have a large range of luminosities,
but the technique still works if stars have the same average
luminosities 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 they placed the Sun in a special
location and therefore had enough of an "anti-Copernican" flavor to
make some astronomers uncomfortable. It was important to find a
tracer other than ordinary stars.
used globular star
clusters 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 25,000 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?
C. STRUCTURE OF OUR GALAXY
Shapley's picture has been refined considerably. An edge-on sketch of
our star system based on our current understanding is shown at right 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 more inclined view of our Galaxy is here.
- The "spiral arms"
are conspicuous because they contain bright generations of young
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 25,000 light years from the center.
The whole galaxy would be roughly 75,000 light years across.
- [Reminder: 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 dense, central part of the Galaxy is inflated into a more
spherical structure, or bulge. The bulge in
the galaxy shown at the top of the page is
redder than the spiral arms because its stars are older, on
average. This is true of our Galactic bulge as well. Some matter in
our Galaxy is distributed in a low density
spherical halo that extends to large distances. The
globular clusters Shapley studied are associated with the halo.
- Younger stars, interstellar gas, and dust are
concentrated to the disk, or "plane," of the Galaxy.
Interstellar dust is the fine haze of
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 Galactic plane.
The stars we can easily see are therefore mostly associated with the
disk. Owing to the interstellar dust, we can see only to distances of
a few thousand light years in the plane of our galaxy with ordinary
(optical-band) telescopes. Because this region is roughly
symmetrical, star counts misled early astronomers into
thinking we were near the center of the Galaxy.
Panoramic mosaic of Milky Way.
explanation and orientation.
D. THE MILKY WAY
- The visual-band photographic panorama above shows a
360o view from the Sun's location of the Galactic plane,
which we see, of course,
edge-on. Our view of the central part of the Galaxy is partly
obscured by dust clouds, which produce the dark blots and rifts in the
picture. For more information on the panorama and identification of
important features, click here.
- When we look in the plane of the Galaxy, we see many
stars, often bright ones---e.g. in Scorpio, Cygnus, Auriga,
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 band of diffuse light in the night sky we
call the "Milky Way."
The center of the Galaxy is in the direction of Sagittarius,
(at the center of the picture above). To get a sense for the density
of stars in this direction,
see this remarkable
The Galactic "anti-center" is 180 degrees away from Sagittarius in the
direction of Auriga (at the left and right edges of the
picture). 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 disk in this
- When we look perpendicular to the Galactic plane, we see
few stars. The Galactic poles are the directions
exactly perpendicular to the plane (at the top and bottom of the
picture); 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
- 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 shown at the beginning of this section,
but made at infrared wavelengths by the 2MASS All-Sky Infrared Survey
(directed by UVa Prof. Mike Skrutskie). At these wavelengths, we
can see the bulge and inner disk of the galaxy without interference
for more information on the 2MASS project.
E. OTHER GALAXIES AND THE FAR UNIVERSE
Our Galaxy is an astonishingly massive structure, and for several decades
at the beginning of the 20th century most astronomers believed
it constituted the entire universe. But it was soon realized that the
Galaxy is only one of innumerable building blocks of comparable or
larger scale in the universe.
Shapley had used RR Lyrae variable stars to determine distances to
globular clusters within our Galaxy. Shortly 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 1230 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, at a distance now estimated to be 2.5 million
light years (see picture at right; click for a larger view).
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. In
the southern hemisphere
the Large and Small Magellanic
Clouds are conspicuous; they are small satellite galaxies of
the Milky Way.
M31 is the most distant object you can see with the naked eye. The
locations of M31 and M33 are shown on the map below:
All four of the naked-eye galaxies are members of a loose association
of galaxies (including ours) called the Local Group. Of
these, only M31 is comparable in size to our own galaxy. Apart from
the Magellanic Clouds and M33, the other Local Group systems are
"dwarf galaxies," and are mostly not observable with small telescopes
despite their proximity.
Since Hubble's discovery, astronomers have devoted tremendous effort
to probing the extragalactic universe. The biggest concentrations of
massive galaxies nearby us lie in the direction of the constellations
Virgo and Coma. These regions are easiest to observe in the late
winter sky, and they transit near midnight in March.
Hundreds of nearby galaxies are accessible to an 8-in telescope under
dark sky conditions. The views possible with visual observing are, of
course, much less detailed than the deep exposure shown at the top
of this page, though with good conditions you would be able to
distinguish shape, spiral structure, and large dust lanes. Your 8-in
telescopes are capable of revealing the three primary galaxy
morphologies (spiral, elliptical, irregular). A good source of
background information on observing bright galaxies is The Messier
Catalog home page.
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 3130 using
a CCD camera.
The Lookback Effect
Given their large intrinsic brightnesses, galaxies can be detected at
very great distances. Because of the finite speed of light, we see
the galaxies not as they are today but as they were millions of
years ago. At M31's distance of 2.5 million light years, the
photons you see from it now left the galaxy 2.5 million years ago
(before modern humans evolved on the Earth). The brighter galaxies in
Virgo are 50 million light years away, and we see them at a "lookback
time" of 50 million years.
The Far 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, an enormous range of mass, and wide variations in star
formation histories and chemical content. Just as in the case of our
Sun in the context of other stars, our Galaxy is only
average in properties.
We are still early in our understanding of the life cycles of
galaxies. Only in the last 40 years, for instance, have we realized
that galaxies can undergo violent gravitational interactions with one
another, sometimes leading to "tidal disruption" or, alternatively,
"mergers" and wholesale transformation of morphologies.
think that our Galaxy will eventually merge with M31 (several billions
of years in the future).
Here is a supercomputer simulation of what such a
merger would look like. Note that the product of the interaction
looks nothing like the two spiral galaxies that went in.
The Hubble Ultra Deep Field
With the Hubble Space Telescope and large ground-based telescopes, we
have detected many galaxies over 10 billion light years away(!)
We therefore see them as they were 10 billion years in the
past. This "lookback time machine" 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 because the distances between galaxies were
smaller then). More information on how the Deep Field was imaged and
the scientific questions that can be pursued about the early evolution
of the universe using this data is
- Download, print, and read the webnotes for this lecture.
- Take the Review Quiz for week 7 on Collab
- Supplementary reading: The best resource for this material is a
good ASTR 121/124 textbook.
- Finish Lab 3 at the earliest opportunity.
May 2019 by rwo
M31-M33 map copyright © Hawaiian Astronomical Society.
Image of Milky Way over CTIO copyright © Roger Smith
(NOAO/AURA/NSF). Dust grain drawing by
Galaxy merger animation by John
Dubinski. Text copyright © 2000-2019 Robert W. O'Connell.
All rights reserved. These notes are intended for the private,
noncommercial use of students enrolled in Astronomy 1230 at the
University of Virginia.