The Milky Way

The Milky Way is visible to the naked eye as a swath of hazy white stretching across the night sky. The Milky way stretches 360^o around the celestial sphere.

One of the observations Galileo made with his telescope was that this hazy stretch of light when observed through a telescope resolved into far more stars than could be seen with the naked eye.

William Herschel (who discovered Uranus in 1781) also observed, with a much better telescope than Galileo made the same observation and concluded that the “haze” seen with the naked eye is actually billions of stars, crowed together.

The conclusion astronomers came to in the late 1700’s was that the sun is part of a disk of stars that has much higher density of stars than the surrounding regions. We call this disk of stars a galaxy.

Nature of our Galaxy

Astronomers today are very confident they understand the basic construction of the galaxy and the Sun’s place in it.

The galaxy has a central bulge where the gas, stars and dust are the densest. The bulge is in the constellation Sagittarius and is about 27,000 light years away.

The main part of the galaxy is the disk of stars and dust that has an approximate radius of 50,000 light years.

Globular clusters (clusters of hundreds of thousands of gravitationally bound stars) exist in a halo surrounding the central bulge.

The Sun takes 250 million years to orbit the sun once. It has made about 20 complete orbits in its 5 billion year life.

Sun’s position in the Galaxy

William Herschel with his sister Caroline set about mapping this disk of stars. They did not have the tools to determine the distance to the stars, so they simply counted the stars in each direction. The direction with the highest density of stars should be the center of the galaxy.

They found about the same density of stars in every part of the Milky Way and concluded that the sun is approximately in the center of their diagram.

Another astronomer a century after the Herschel work, Jacobus C. Kapteyn in the early 1900’s, was able to take into account the distances to the stars that the Herschels had merely counted. He determined that the density of stars decreased in all directions as distance from the sun increases. This supported the idea that Sun must be at the center of the disk.

This idea seemed to be a throwback as the work of Copernicus, Galileo, Kepler and Newton had all suggested that Earth was not a special place at the center of the universe. It seemed strange then that our Sun would have a special place at the center of our galaxy.

Indeed, the Herschels and Kapteyn made in principle the same mistake as Ptolmey and Aristotle. Our position in the universe only appears special because that is the position from which we make all our observations.

The Herschels and Kapteyn assumed that they were seeing everything looking at the galactic disk from Earth.

Their mistake was not taking into account the interstellar matter, which is particularly dense in the galactic disk, in block out the light from more distant stars.

At the same time that Kapteyn was making his observations, a man by the name of Harlow Shapely was trying to determine the suns position in the galaxy by measuring the distances to globular clusters.

Globular clusters are old clusters that consist of hundreds of thousands of stars and lie outside the galactic disk.

These clusters contain Cepheid variable stars. Using the relationship of the period of the light cycle of a Cepheid to its luminosity, Shapely was able to measure the distances to 93 globular clusters.

Shapley found far more globular clusters in one direction than in any other. He assumed that the clusters would all be orbiting the center of the galaxy and thus predicted the center of the galaxy to be toward the constellation of Sagittarius, thousands of light years away.

Since the globular clusters are outside the galactic disk, they were not obscured by the dust that confused Kapteyn and Herschel. So his calculation was more accurate than the earlier observations.

Shapley had overestimated the size of the Galaxy (due to the dimming of the clusters by intersteller matter) nevertheless this observation pulls the sun away from the center and increases dramatically the size of the galaxy from what the Herschels predicted. (Much like the Copernican model did.)

Rotational Curve

We would expect since it is still the force of gravity that pulls the stars in their orbits around the sun that they would obey Kepler’s Law, that the further they were from the center the galaxy.

However, when we measure the revolution rates of stars at different distances from the sun we find that they do not obey Kepler’s Law.

This implies that there is mass that we do not see that spread out outside the Sun’s orbit around the galactic center, that effects the stars orbits.

We do not know where the mass is or what its nature is. Some think the “missing” mass consists of black holes orbiting the galactic center in an extended halo beyond the globular clusters. Others suspect it is cold dust that does not radiate or reflect any light.

Spiral Nature of the Galaxy

In the 1924, Edwin Hubble discovered Cepheid variable stars in other nebula, which confirmed that they were well outside the Milky Way that indeed these were other galaxies or island universes.

Many of these galaxies have a spiral shape with distinct arms that curl away from the center.

In 1951 astronomers found that bright, young O and B type stars were clustered at certain distances in our own galaxy.

That same year 21-cm radiation was discovered. It was predicted that cold hydrogen gas would emit light in the radio wave part of the EM spectrum at about a 21-cm wavelength. With more sophisticated radio telescopes astronomers were able to detect this radiation for the first time.

This allowed astronomers to locate interstellar dust that had been invisible before. It was found from the Doppler shift of this radiation that the gas was clustered at certain distances much like the O and B type stars.

These two observations as well as the fact that many other spiral galaxies can be seen, led astronomers to believe that the Milky Way has a spiral nature as well.

Spiral Arm Theories

It is not well understood how a spiral galaxy keeps its arms well defined for so long. It is easy to show that just differential rotation will quickly lose its spiral nature and appear random.

One theory is that there is a “density wave” that is propagating around the galaxy. This wave would slow down any material that tries to move through it and create a region of higher density, which would also ignite star formation. This would account for the young stars in the arms and the concentration of hydrogen gas (21-cm radiation)

The competing theory is kind of the opposite. It is not the high density that causes star formation it is the star formation that causes the high density. Stars are constantly being formed, the massive stars that go through their lifetimes quickly and explode as supernova will cause new stars to form. So it is only the arms that are being constantly supplied with new stars, so the differential rotation causes the spiral arms.

Formation of the Galaxy

Astronomers are able to measure the age of different parts of our galaxy. For example, in the sun’s region of the galaxy there are very few white dwarf stars. This implies that our region of the galaxy is relatively young as WD stars are the final stages of small and middle sized stars. This means the outer portion of the galactic disk is less than 10 billion years old.

The globular clusters’ ages are measured directly as most of the stars were formed at the same time. These clusters are between 11 and 14 billion years old.

The galactic nucleus, which is very difficult to see due to gas and dust, contains low mass stars that have already reached the red giant stage. Further, these stars are 2^nd generation stars, meaning they formed from the left over material of previous stars. This is the oldest portion of the galaxy.

Although there is no conclusive model for the formation of the Milky Way. The best one so far is much like the formation of stars.

-A very large cloud of hydrogen gas collapses under its mutual gravitational attraction.

-In the central, densest regions star begin to form.

-In the out regions clumps of gas swirl together and form clusters in the galactic halo.

-The gas swirling toward the center flattens into a disk and forms spiral arms of star formation. (globular clusters are massive enough and the density of gas low enough that they are not “captured” by the disk.)

There are several other theories of galaxy formation, some involving collisions between smaller galaxies. Currently, however, there is not a better model the one described above.