Galaxies-I

By the 1700’s the old notion that the Earth was the center of the Universe was overthrown by the success of Newton’s theory of universal gravitation, a theory which explained the motion of the planets around the Sun. It became respectable to see the stars as other suns, like our Sun, but scattered through space, with the possibility of other planets around those suns and even intelligent life on those planets. And so the question arose: where is our Sun in this universe of stars? Answering this question required 200 years, roughly from 1750 to To answer it, astronomers started from careful observations of the distribution of stars and star clusters in the night sky.

The Milky Way is a hazy band of light visible to the naked eye in the evening sky. In a telescope it resolves into clouds of stars. In 1750 Thomas Wright suggested that the existence of the Milky Way proves that the stars are not randomly distributed, but distributed in a flattened disk with the Sun off to one side. In 1785 William Herschel counted stars in 683 regions of the sky and used these counts to estimate the size of the disk and the direction to the center. horizon Milky Way Scutum star cloud Sagittarius star clouds

Herschel’s picture of the system of stars (1785). Herschel’s estimated position of the Sun. Illustration by Wright showing stars confined to a plane (1750). Direction of Sagittarius star clouds The system of stars which contains our Sun is now called the Galaxy

A composite of 77 photographs showing details of the star clouds in the Milky Way. By the early 1900’s the dark areas were understood to be obscuring clouds of dust and gas. This source of interstellar absorption invalidated the star counts Herschel (and later astronomers) used to estimate the size of the Galaxy.

This is a more recent color mosaic of the Milky Way created by an amateur astronomer.

Open clusters are a type of star cluster that typically contain 100 to 1000 stars. They are found where stars are forming out of the interstellar medium. The Pleiades in Taurus (photo from Bok, 1976) The distribution of star clusters helps us understand the structure of our galaxy.

The Milky Way in Cygnus. Open clusters, yellow circles on this chart, are concentrated along the Milky Way. Gaseous nebulae, in green, are also found concentrated along the Milky Way. “Veil nebula” supernovae remnant NorthAmerican nebula

An example of a gaseous nebula is the “North American nebula,” NGC 7000, near Deneb in Cygnus. Note the evidence of dark clouds by comparing star counts in the region on the right to the region on the left.

Star clusters condense out of the dust and gas between the stars. HII regions are gaseous emission nebula glowing from the ultraviolet light from hot young stars, the red color is distinctive

Globular clusters, in contrast to open clusters, are concentrated in one part of the Milky Way. This picture of Sagittarius star clouds contains one-third of all known globulars in an area less than 3% of the sky. A typical globular cluster has 500,000 stars.

The distribution of open clusters in the sky as seen from Earth, in a coordinate system aligned with the Milky Way. The center (marked by a cross) is in Sagittarius. The horizontal line is the plane of the Milky Way. The distribution of the globular clusters in the sky. They are strongly concentrated in the direction of Sagittarius. North Galactic Pole South Galactic Pole North Galactic Pole South Galactic Pole

Harlow Shapley proposed, circa , that the center of the distribution of the globular clusters was the true center of the Galaxy.This was slowly accepted. The position of the Sun is marked S in this diagram. The position of the center of the Galaxy in Shapley’s model is marked C. The gray area around S is about the size of Herschel’s model of the Galaxy from star counts. (Trumpler, 1930) Magellanic Clouds

Though Shapley’s distances were overestimated, his idea was correct. By the 1950’s the overall size and the shape of the Galaxy was thought to be well understood. The Galaxy consists of a disk population, Population I, of relatively young stars and a halo population, Population II, of older stars, remnants of the formation of the Galaxy. The central bulge also contains older stars. Two small satellite galaxies, the Large and Small Magellanic Clouds, slowly orbit our Galaxy. Disk Halo Central Bulge

The Disk: The open clusters are Population I and found in the disk of the galaxy. The distribution of HII regions, open clusters, and dark molecular clouds where stars are forming is not uniform in the disk. These objects trace out spiral arms. The best measurements of the spiral arms are made by observing the 21-cm radio emission of neutral hydrogen gas. Neutral hydrogen is concentrated in the spiral arms. 21-cm radiation is emitted when the spin of the electron in the hydrogen atom changes from spin-up to spin-down. The hydrogen emits a photon with an energy equal to the energy difference of the two states: Higher energy: spin-upLower energy: spin-down

This is a radio map of the 21-cm radiation from the disk of our galaxy. The dark areas of strong 21-cm emission outline the spiral arms. Maps like this only became possible after WWII.

IR view of the Milky Way (2MASS) showing central bulge.

Inside the bulge is the center of the galaxy. We will study this in more detail later.

An artist’s conception of the Milky Way Galaxy seen face-on. This is based on radio maps and other sources of data. The yellow dot is the position of the Sun: 10 kpc from the center of the Galaxy. 30 kpc

From the motions of stars in the galaxy, and the 21-cm data, we know that the galaxy rotates. The Sun takes ~250 million years to complete one orbit around the galactic center. The inner stars complete their orbits in shorter times. This differential rotation would destroy the spiral arms though, winding them up in a few hundred million years. If the galaxy is billions of years old, why do the spiral arms persist?

Density wave theory is one solution. The density wave, like a water wave, moves through the disk causing stars to form by compressing the gas and dust in the disk, but hardly affecting the stars at all.

Another theory is self-propagating star formation, where “waves” of star formation can form bands of young stars in the galactic disk. 1) Cluster of stars form out of part of an interstellar cloud. 2) The hottest stars ionize the gas surrounding gas and an HII region forms. The HII region compresses the cloud nearby, causing new star formation. 3) The most massive stars become supernovae and the expanding shock waves also cause star formation. 4) Differential rotation then shears these regions into arcs we see as spiral arms.

How a wave of star formation propagates through the interstellar medium. Successive waves continually recreate the bright stars and HII regions that trace out the spiral arms.

Both theories have strong points (and weak points), perhaps both processes play a role in our galaxy. If we could see our galaxy face-on, it would look very much like this. Note the color difference between disk, nuclear bulge, and core. HII regions show as red patches in the arms.

NGC 7331 is thought to be similar to our galaxy.

NGC 7331 in IR showing dust and star formation regions.

If we could see our Galaxy edge-on from the outside, it would look like this. The Milky Way Galaxy contains roughly 100 billion stars and our Sun is one star among these, located roughly 30 thousand light years from the center of the Galaxy in the disk. What about other galaxies?