The Realm of the Nebulae

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The universe is populated with galaxies -- some like our own Milky Way, others very different. Recognizing other galaxies and estimating their distances led astronomers to the realization that the universe is expanding.


The Andromeda ``Nebula''

The Andromeda Galaxy
The Andromeda Galaxy (M31) [Robert Gendler]

Distances to Other Galaxies

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:

  • Easily recognized
  • Extremely luminous
  • Luminosities can be measured
  Light curve of a Cepheid

Leavitt's Law: the Period-Luminosity Relationship

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.   The Large Magellanic Cloud
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.   Period-Luminosity Relationship

The Distance to Andromeda

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.

  Variables in Andromeda
Astronomy Picture of the Day [NASA]

Galaxies and Their Environments

Now that astronomers knew how to recognize galaxies, they began to compare them in detail, and discovered many different types.

They also noticed that galaxies were not sprinkled completely at random; instead, they were often found in groups or clusters.

The Needle Galaxy: A Disk Galaxy Viewed Edge-On

The Needle Galaxy
Astronomy Picture of the Day [NASA]

The Sunflower Galaxy: A Spiral Galaxy Viewed at an Angle

The Sunflower Galaxy
Astronomy Picture of the Day [NASA]

NGC 1365: A Barred Spiral Galaxy

NGC 1365
Astronomy Picture of the Day [NASA]

The Whirlpool Galaxy: A ``Grand Design'' Spiral

Astronomy Picture of the Day [NASA]

The Sombrero Galaxy: A Disk With a Large Bulge

The Sombrero Galaxy
Astronomy Picture of the Day [NASA]

NGC 1316: An Elliptical Galaxy

NGC 1316
Astronomy Picture of the Day [NASA]

M32: A Nearby Elliptical Galaxy

Astronomy Picture of the Day [NASA]

Sextans A: A Small Irregular Galaxy in the Local Group

Sextans A
Astronomy Picture of the Day [NASA]

Galaxy Classification

Hubble and others sorted galaxies into several different classes -- ellipticals, spirals, and irregulars -- on the basis of their appearance.

Hubble's galaxy classification
  Hubble's galaxy classification

Groups and Clusters

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!   The Virgo Cluster

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 Galaxies

Hercules contains a larger fraction spiral galaxies than Virgo, and some of these galaxies are colliding.

  Hercules galaxy cluster
Hercules Cluster of Galaxies [UA]

The Redshift

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 .

  Galaxy spectra at z=0 and z=0.1

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.

Redshift: An Example

The Tadpole galaxy
Astronomy Picture of the Day [NASA]
  Spectrum of young star cluster

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)
5009 4861 148 0.0304 9120
4472 4340 132 0.0304 9120

The Expansion of the Universe

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 diagram

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.

Was it Something We Did?

Galaxies are fleeing because they HATE us!
The Cartoon History of the Universe

The Universe Has No Center

Expansion from MW   Expansion from Galaxy 2   Expansion from Galaxy 3
Observed from MW   Observed from Galaxy 2   Observed from Galaxy 3

The expansion looks the same no matter where we are: all observers see other galaxies moving away from their galaxy, with speeds proportional to distances. The expansion of the universe does not define a center.

The Universe Has No Edge

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...   Finite universe expands into nothing

The Universe Has No Edge

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.

  Finite universe expands into nothing

The Universe Has An Age

The simplest assumption we can make is that the galaxies move away from each other at constant rates. If that's true, we can ask how long ago all galaxies were ``on top of each other''.

Consider a galaxy now at a distance of d = 100 Mpc; its speed away from us is

v = H0 d = (72 km ⁄ sec ⁄ Mpc) × (100 Mpc) = 7200 km ⁄ sec .

Assuming the galaxy's speed has been constant, it covered this distance in time

t  =  d 

 =  100 Mpc

7200 km ⁄ sec
 =  3.09×1021 km

7200 km ⁄ sec 
 =  4.29×1017 sec   =  13.6 Gyr .

(The distance d actually cancels out of this calculation, and you would get the same answer for any other choice of d.)

13.6 Gyr is a reasonable estimate for the age of the universe!

Dark Matter

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.

  The Coma galaxy cluster
Astronomy Picture of the Day [NASA]

Hot Gas in Galaxy Clusters

Hot 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 comparison of MACSJ1423
X-ray/Optical Image of MACSJ1423 [Harvard]

Gravitational Lensing by Clusters

Galaxy cluster with lensed galaxies

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.

Galaxy Rotation Curves

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
v  ∝  1 

  Galactic rotation curves
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.

What is This Dark Stuff?

We have a good idea of what the dark matter is not.

Our best guess is that the dark matter is some kind of sub-atomic particle. Like neutrinos, these particles must be almost completely indifferent to ordinary matter. (Neutrinos were actually a leading candidate for the dark matter, but it turns out they can't provide the necessary mass.) The jury is still out on the true nature of the dark matter.

Galaxy Formation

Dark matter determines how galaxies form. At early times, mass was very smoothly distributed throughout the universe. Gravity pulled the dark matter into clumps which grew by merging with each other. Visible galaxies grew at the centers of these clumps of dark matter.   Clustering of dark matter

This scenario predicts that galaxies can merge when their parent clumps or halos of dark matter fall together. Supporting this idea, we find examples of merging galaxies throughout the universe.

The Mice at Play

Image of the Mice
``The Mice'' (NGC 4676) [STScI]

A Model of the Mice

Model of the Mice

Galaxy Transformation

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:

Hubble scheme flipped
Last: 12. Structure of the Milky Way Next: 14. Black Holes and Quasars

Joshua E. Barnes (
Last modified: November 16, 2006
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