Part 14: Galaxies
John P. Pratt
Our Milky Way Galaxy
- There is a band of light across the sky easily seen when away from city lights called the Milky Way.
- In ancient Greece it was correctly attributed by Democritus to being many unresolved stars.
- With the telescope, Galileo confirmed that proposal.
- The galaxy is disk shaped, so when we look through the thick part, we see a band across the sky.
- Herschel began mapping the galaxy as a disk.
- The Milky Way contains at least 100 billion stars, perhaps a trillion.
The Sun's location
- Before interstellar reddening was discovered, it looked like we were in the center of the galaxy.
- When Cepheid variables were discovered in globular clusters, we could determined their distances.
- Shapley showed in 1918 that globular clusters are centered around Sagittarius.
- He correctly deduced that that must be the direction of the center of the galaxy.
- The galactic bulge at the center of the galaxy was also discovered.
- The Milky Way is about 100,000 ly in diameter, and the sun is located 2/3 out from the center.
- Until 1924 it was thought that spiral galaxies might be gaseous nebulae forming into stars.
- Then the new 100" telescope on Mt. Wilson showed that the Andromeda Galaxy is made of stars.
- Cepheids were discovered among them, indicating that they are distant from our galaxy.
- Studying other galaxies helps us understand our own better.
Galactic Latitude and Longitude
- Astronomers have defined galactic latitude and longitude along the axis of the galactic disk.
- Making plots of stars in those coordinates helps show the structure of the galaxy.
- Open clusters and associations are almost all located in the galactic plane.
- Globular clusters are almost all located in a spherical halo around the entire galaxy.
- The galaxy's flattened disk shape implies that it is rotating.
- To measure our rotation, we refer to a reference system of the other galaxies.
- We are rotating toward Cygnus (the Northern Cross) and away from Canis Major (the Big Dog).
- The sun is in orbit around the galactic center, with a period of 240 million years.
- Differential Rotation: Following Kepler's laws, the stars near the center revolve faster.
- The age of the galaxy is thought to be about 12-16 billion years.
- The glaxay's mass can be determined by
Mapping the Spiral Arms
- O and B stars fall along spiral arms
- We are in a spiral arm, called the Orion arm, because most of the stars in Orion are in it.
- Other arms can be mapped using 21-cm radio emissions from H I clouds along the spiral arms.
- The arm closer than us to the center is the Sagittarius arm, farther is the Perseus arm.
Origin of Spiral Arms
- The galaxy is not rotating as a solid disk, like a pinwheel, but the center rotates faster.
- If the arms date back to the formation of the galaxy, they should be wound up tightly.
- The bright stars tend to be found along the inner edge of the spiral arms.
- The density-wave theory is that a wave compresses the gas to form hot stars.
- The stochastic star-formation theory is that supernovae expanding gases compresses
neighboring clouds to condense into star clusters, which are then sheared by differential
rotation to form spiral arms. This makes great sense because arms don't last long.
Two Star Compostions
- Stars in the solar neighborhood are Population I stars, containing 2-4% heavy elements.
- Stars in globular clusters or passing through the disk are Population II, with no heavies.
- Pop II globular clusters contain RR Lyrae stars, like Cepheids, but contant luminosity.
- Pop II are thought to be very old because no heavy elements and place in H-R diagrams.
- Nearby Pop II are call high velocity stars because they are not in circular orbits like us.
Formation of the Milky Way
- The old view is that globular clusters formed originally and are all very old, while the disk then
collapsed and later formed the young stars.
- Measurement of ages of globular clusters now shows that many of them are younger.
- Also, many stars in the central bulge are younger also.
- Hence, we now think that the galaxy was formed by the merging from smaller units.
The Galactic Center
- The galactic center is brighter in radio wavelengths than the sun, called Sagittarius A.
- The center is somewhat bar-shaped, called a stellar bar.
- There is a central star cluster with 10 million times the density of stars as our local neighborhood.
- Gamma rays are emitted that appear to be from electron-positron annihilation.
- Many astronomers think there might be a massive black hole at the center of the galaxy.
The Local Galaxies
Distance to the Nearest Galaxies
- Many thought that spiral galaxies were local nebulae condensing into solar systems.
- In 1923 Cepheid variables were the key to discovered that galaxies are external to the Milky Way.
- Cepheids in the nearest spiral galaxy (Andromeda) now show that it is 2 million ly away.
- Novae and supernovae can also be used, but they are more rare.
- These methods overlap other methods to get distances to more distant galaxies.
- The nearest 15-20 galaxies are called our Local Group.
The Magellanic Clouds
- The two nearest galaxies in our Local Group are the Large and Small Magellanic Clouds.
- They are both near the south celestial pole, so-named because Magellan used them for navigation.
- They are close to the Milky Way (50 kpc), not far from the farthest globular clusters.
- If Milky was was a dinner plate, they would be oranges two feet away, with Andromeda another
plate twenty feet away.
- They are not spiral galaxies, but are irregular galaxies.
- Near the center bar of the LMC is the Tarantula Nebula, a huge cluster of blue supergiants.
- Both clouds have globular clusters of blue stars.
- A stream of hydrogen gas connects the two galaxies.
- They are important to astronomers because the many kinds of stars are at the same distance.
- In our Local Group, there are several dwarf elliptical galaxies, like giant globular clusters.
- Like globulars, they have Pop II stars and little gas or dust.
- There are about ten in our Local Group.
The Andromeda Galaxy
- The Andromeda Galaxy is a spiral galaxy, very much like ours, and is in our Local Group.
- It is about the same size, with a disk and a halo of globular clusters.
- Its central bulge and two companion galaxies are composed of red giant stars.
- Its nucleus also has a dozen X-ray sources.
Classifying Galaxies by Shape
- Galactic morphology is classifying galaxies by their shapes, which follow a clear pattern.
- Spirals may have tightly wounds arms and large central bulge (called Sa) or loose arms and
a small central bulge (called Sc, with Sb intermediate).
- Barred spirals have a bar-shaped nucleus, sometimes very prominent (SBa - SBc).
- Ellipticals range in size from dwarfs to giant ellipticals, are E0 to E7 (E0 spherical).
- Irregulars include many other shapes.
- When velocities of stars are measured to determine mass, it shows 90% of mass is too dark to see.
- This same result is found even though different techniques are used for measurements.
- This is an important area of research in modern astronomy.
The Expanding Universe
The Hubble Relation
- Hubble discovered the Hubble Law: the farther a galaxy is away from us, the larger its
- The redshift is interpreted as velocity away from us, so it appears that the universe is expanding.
- One can show that such a law would be observed from any galaxy in the universe.
- The Hubble parameter is the ratio of speed to distance, determines the age and size of the
- Some redshifts may not be due to such expansion: some galaxies at the same distance have
Clusters of Galaxies
- Beyond our Local Group, we find outer clusters of galaxies.
- Some clusters are small like our Local Group; the Virgo cluster has several hundred galaxies.
- Our Milky Way is probably included in an enormous supercluster of galaxies.
- The Virgo cluster is probably the center of our Local Supercluster.
- Dozens of other superclusters have been discovered.
- Active galaxies have much radio and X-rays coming from their nuclei, like ours.
- Seyfert galaxies have blue nuclei and broad emission lines, meaning high speed gases.
- Radio galaxies comprise about 1° of all galaxies.
- Some galaxies (like M82) appear to be exploding, with huge streams of matter streaming out
of the nucleus.
- Quasars (quasi-stellar radio sources) are strong radio sources coming from something that
appears as small as a stellar source.
- They were discovered to have extremely redshifted lines.
- They are thought to be galaxies incredibly bright and far away.
- If that is true then the best explanation so far for their source of energy is a supermassive black hole.
- Many appear to be near nearby galaxies; some think they were shot out of them; but if so,
one needs to explain why none are coming toward us and hence blue-shifted.
The Big Bang
- The Hubble law has led astronomers to believe in the Big Bang, namely that the entire universe
was born in a giant explosion.
- The main confirming evidence is the discovery of the so-called 3° background radiation.
- They take this very seriously and talk about when the universe was the size of a grapefruit.
- They write books about what occurred in the first incredibly small fractions of a second.
- You can read these on your own time.