The Andromeda ``Nebula''
The Andromeda Galaxy (M31) [Robert Gendler] |
|
The Andromeda Galaxy (M31) [Robert Gendler] |
To determine if Andromeda was another ``island universe'' -- a galaxy like the Milky Way -- we needed to find out its distance. For this we had to find brighter objects which could be used as standard candles. A special kind of star, known as a Cepheid variable, turned out to be just what was required.
|
Cepheid variables are giant stars. Instead of shining
steadily, they vary in brightness, following a regular
pattern. The time a variable star takes to go through one
cycle is known as its period.
Cepheid have a number of advantages as standard candles:
|
|
| Leavett was studying Cepheid variable stars in the Large Magellanic Cloud, a small satellite galaxy orbiting the Milky Way. She noticed that the average brightness of these stars was related to their periods. Because all these stars are equally far away, this implied there was a relationship between period and average luminosity. |
Large Magellanic Cloud [Wikipedia] |
| No Cepheid variable was close enough for a parallax measurement, but some were found in star clusters whose distances could be measured by main-sequence fitting. This enabled astronomers to determine the period-luminosity relationship, and turn Cepheid variables into standard candles. |
|
|
In 1924, Hubble found Cepheid variable stars in Andromeda. He
measured their periods and used the period-luminosity
relationship to determine their luminosities. Luminosities
and average brightnesses in turn yielded distances.
Andromeda turned out to be much further away than anything associated with the Milky Way, 2.4 million light years, or dAnd = 736 kpc = 0.736 Mpc . At this distance, Andromeda was about twice the size as the Milky Way! With this, astronomers realized that the Milky Way was just one of the countless galaxies scattered about the universe. |
Astronomy Picture of the Day [NASA] |
Astronomy Picture of the Day [NASA] |
Astronomy Picture of the Day [NASA] |
Astronomy Picture of the Day [NASA] |
Astronomy Picture of the Day [NASA] |
Astronomy Picture of the Day [NASA] |
Astronomy Picture of the Day [NASA] |
Astronomy Picture of the Day [NASA] |
Astronomy Picture of the Day [NASA] |
Hubble and others sorted galaxies into several different
classes -- ellipticals, spirals, and irregulars -- on the
basis of their appearance.
|
|
| Galaxies like company. The Milky Way and Andromeda, together with several dozen smaller galaxies, make up the local group. The nearest big cluster of galaxies is the Virgo Cluster (right), which contains about a thousand galaxies! |
|
Big, ``regular'' clusters like these are rich in elliptical galaxies. Spiral galaxies are more often found in groups and in ``irregular'' clusters.
The Hercules Cluster of GalaxiesHercules contains a larger fraction spiral galaxies than Virgo, and some of these galaxies are colliding. |
Hercules Cluster of Galaxies [UA] |
|
Even before the nature of galaxies was understood, it was
known that their spectra, while showing basically the
same patterns of lines as nearby stars and star-forming gas
clouds, tended to be shifted toward the red.
This shift follows a simple rule: the change in wavelength Δλ is always a fixed fraction of λ0, the wavelength measured locally. This fraction is known as the redshift: z = Δλ ⁄ λ0 . |
|
Redshifts are due to relative velocity: other galaxies are moving away from us at speeds v = c z, where c is the speed of light.
Astronomy Picture of the Day [NASA] |
|
The lines in this spectrum are all shifted by the same factor; for example:
| Line | λ | λ0 | Δλ | z = Δλ ⁄ λ0 | v = c z |
| (Å) | (Å) | (Å) | (km ⁄ sec) | ||
| Hβ | 5009 | 4861 | 148 | 0.0304 | 9120 |
| Hγ | 4472 | 4340 | 132 | 0.0304 | 9120 |
|
Plotting redshifts of galaxies against their distances, Hubble
found that each was proportional to the other:
v = H0 d . Modern measurements give H0 = 72 km ⁄ sec ⁄ Mpc . |
|
Hubble's discovery had two key implications. First, it provides an easy way to estimate the distance to a galaxy: take a spectrum, measure the redshift, and calculate d. Second; it shows that the universe is expanding.
The Cartoon History of the Universe |
| It's natural to think of a finite sphere of galaxies expanding into nothingness. But there's no evidence the universe as an edge -- looking further and further, we just see more and more galaxies in all directions... |
|
|
It's natural to think of a finite sphere of galaxies expanding
into nothingness. But there's no evidence the universe as an
edge -- looking further and further, we just see more and more
galaxies in all directions...
From a mathematical point of view, it's much harder to describe a finite universe like this one. The preferred model of the universe is infinite in all directions. |
|
|
The visible parts of galaxies turn out to be only a few
percent of the mass in the universe. Zwicky noticed this
while studying orbital speeds of galaxies in galaxy clusters.
If the visible stars were the only matter present, the
galaxies should orbit with speeds of about
100 km ⁄ sec.
In fact, the speeds were typically 10 times larger, or about 1000 km ⁄ sec! If galaxy clusters are held together by gravity, their total masses have to be about 100 times the visible mass. In other words, galaxies are only about 1% the mass of a cluster. |
Astronomy Picture of the Day [NASA] |
Hot Gas in Galaxy ClustersHot gas (temperature T = 107 K) in galaxy clusters provides another line of evidence for dark matter. This gas would easily escape the feeble gravity of cluster galaxies; the gravity required to hold it in place requires, again, about 100 times the visible mass in the galaxies. The amount of hot gas is about 10 times the mass in galaxies. 90% of the cluster's mass is still in an unknown form! |
X-ray/Optical Image of MACSJ1423 [Harvard] |
Another line of evidence is provided by gravitational lensing, the bending of light by strong gravitational fields. When we look at galaxy clusters, we see images of background galaxies distorted into arcs. The amount of mass required to do this is about the same amount required to explain the speeds of cluster galaxies and retain the hot gas.
The rotation of galaxies provides more evidence of dark
matter. If galaxies only contained visible matter, stars
orbiting at large radii R would follow Kepler's third
law, with orbital speeds
|
Galaxy rotation curve [Wikipedia] |
Rubin observed that the orbital speeds are roughly constant, instead of inversely proportional to the square root of the radius. This implies that each galaxy has a halo of dark matter, probably roughly spherical, enveloping the visible stars and gas.
``The Mice'' (NGC 4676) [STScI] |
Computer simulations show that galaxies ``stick together'' when they collide, and that the end product of a merger between spiral galaxies is an elliptical galaxy.
Following this idea, some people have suggested that Hubble, like da Vinci, put his secret thoughts into mirror writing:
| Last: 12. Structure of the Milky Way | Next: 14. Black Holes and Quasars |
|
Joshua E. Barnes
(barnes@ifa.hawaii.edu)
Last modified: November 16, 2006 http://www.ifa.hawaii.edu/~barnes/ast110_06/trotn.html |
|