| Discovery of Galaxies Key points: How it was established that other
galaxies are island universes of stars; standard candles and distances; the distance
ladder
While the Milky Way was considered to be a thin, distorted disk of stars with the
sun near the center in the early 20th century, other galaxies were confused with gaseous
nebulae, and were assumed to be part of Milky Way.
The best pictures showed other galaxies to have spiral
arms, but they also appeared smooth and were thought to be a special kind of gaseous
nebulae, the "spiral nebulae".
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By the 1920s, a debate was ranging among astronomers about
whether the spiral nebulae were gaseous objects (HII regions) or separate "island
universes" like the Milky Way. Vesto Slipher had obtained spectra that showed them to have continuous spectra, not emission lines,
consistent with their being made of stars.
Harlow Shapley thought that the Milky Way was so big that it
had to dominate the Universe, and thus that the spiral nebulae were gaseous and part of
the Milky Way. Part
of his evidence was they avoided the plane of the Milky Way and they would have been found
over the whole sky if they were independent systems. Both of these arguments were wrong,
because Shapley did not appreciate the effects of interstellar dust
and extinction. Extinction toward the globular clusters had made their stars too dim and
consequently Shapley had placed them too far away, and extinction in the plane of the
Milky Way hid the spiral nebulae behind it.
On the other hand, many astronomers did not yet accept
Shapley's arguments that the solar system was to the side of the Milky Way -- again
because extinction hid the true shape of the system from most
types of observation.
Shapley and Heber Curtis "debated" the two views on April 26, 1920.
Shapley was under consideration for director of Harvard College Observatory, and did not
want to do anything that would be unnecessarily provocative and undermine his position. He
gave a lecture that was very watered down, with many popular ideas that were
uncontroversial. Curtis gave a more advanced talk, and achieved extra points for technical
sophistication for bringing his outline on projection slides that provided the audience a
written version while he talked! Both debaters were right (Shapley -- shape of the Milky
Way; Curtis -- size of the Milky Way, spirals are island universes) and wrong (Shapley --
spirals are gaseous; Curtis -- sun is near center of Milky Way (Kapteyn model)). Shapley's
main item of evidence -- measurements by van Maanen that indicated that the spiral arms
moved perceptibly with time -- was soon shown to be wrong. More on the debate at http://antwrp.gsfc.nasa.gov/diamond_jubilee/debate_1920.html
Why was this debate so important
| Here is one of Hubble's photographic plates of the center of M33,
with Cepheid variables marked. If you look closely (and if the original resolution has
been preserved from plate to journal to scanner to web page to your computer), you will
see that the gray "nebulosity" is broken up into little specks that are
individual stars. (The plate is a negative, as was the convention for publishing results
-- the stars look black and the sky white) (From ApJ, 63, 236, 1926) |
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Hubble marked the Cepheid variable stars on the plate above. Here are
his measurements of some of the Cepheid variable light curves (some of these stars are in
other regions than the one in the picture above). Cepheids
have a Period-Luminosity relation that makes them good standard candles and allowed Hubble
to prove that M33 is so far away that it cannot be part of the Milky Way. |
This proved the "island universe" model, that the "spiral
nebulae" are galaxies like the Milky Way.
Why do we need to determine the distances to the galaxies
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The group of galaxies called "The
Local Group" to which the Milky Way belongs has played a crucial role
in our determination of distances to galaxies. |
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Here is a sketch of the arrangement of the local group -- in three
dimensional projection. Each of the two large galaxies (M31 = Andromeda and the Milky Way)
has a retinue of small galaxies.
And here is a close up of the space around the Andromeda Galaxy, showing its set
of satellite galaxies in more detail. Galaxies on solid stalks are above the grid plane
and those on dashed stalks are below.
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Although Hubble proved the "Island Universe" hypothesis by
measuring distances in the Local Group with Cepheid variable stars, Cepheids cannot be detected in very distant galaxies. Therefore,
astronomers have reached out by calibrating distance indicators useful over ever greater
distances. All of these tests are built on "standard candles", objects where we
think we know the luminosity and so can assume that the apparent brightness is determined
by the distance through the inverse r2 law. The
local group galaxies were to a large degree where the distance
indicators were explored, tested, and established. With improvement in instruments, we
were then able to use them on galaxies much farther away.
Distance Scale "Ladder"
The series of steps we use to measure distances is described as a
ladder, with each rung dependent on the ones below supporting it. These rungs we list
below in the order of the distance range they work over:
Radar: Radar is the most accurate way to measure distances in the solar system, giving
an accurate foundation for parallax.
Parallax: Modern space observatories let us measure parallax to 0.001 arcsec, directly
determining distances to hundreds of parsecs.
Main sequence fitting: Since we know that similar stars should appear at the same place
on the H-R diagram, we adjust the distances of cluster H-R diagrams until their main
sequences overlap. From the placement of stars on the H-R diagrams, we can estimate their
absolute magnitudes and thus determine both the true and relative distances to the
clusters.
Cepheids: directly
calibrated in Milky Way, for example from clusters with distances from main sequence
fitting. Can be observed to ~20 Mpc
Brightest stars:
relatively constant and useful over about 4x the distance that Cepheids are
Novae: reach about
the same peak brightness as the brightest stars and serve as a check
Globular clusters:
the brightest ones have about the same brightness and are visible over slightly greater
distances than the brightest stars. Also useful for ellipticals.
21-cm line width:
the width of the HI 21-cm line reflects the rotation rate of the galaxy which in turn is a
measure of the mass. Most galaxies have nearly the same ratio of mass to luminosity, so if
we know the mass we can compute the luminosity (technically, this correspondence is called
the Tully-Fisher relation).
Supernovae:
can be seen over huge distances (to ~ 50% of the age of the Universe). White dwarf based
supernovae are expected to all have similar peak luminosities.
Test your understanding before going on
 An illustration from Chinese
mythology of the Milky Way, from Postel & Guerrero http://www.artistexpo.com/Collprints.html |
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"Dark Matter" by G. Rieke |
Click to return to syllabus |
| Click to return to the Milky Way |
hypertext  G. H. Rieke |
Click to go to Dark Matter |
