Our Milky Way Galaxy is a collection of about 200 billion stars, concentrated mostly onto a disk. Just as the Earth is not the center of the solar system, our Sun is not the center of the Galaxy, but located about 8,000 to 9,000 pc (8-9 kpc) from it.

If it were viewed from "above" the disk a set of spiral arms would also be apparent. This pattern outlines the location of massive young stars and the interstellar material from which they recently formed. The stars themselves orbit the center of the Galaxy, and their speeds at various distances from the center indicate how the mass of the Galaxy is distributed. In both our Galaxy and others, the amount of mass measured exceeds that which we can see emitting light, hence we have deduced the presence of "dark matter" whose nature is not yet known.

The stars and the spiral pattern do not move as one. Rather, the stars and interstellar gas and dust move through the spiral pattern in the same way that cars move through a traffic jam. The gas and dust get compressed as they pass through, increasing the density of material to the point where the higher density blobs can collapse and new stars can form. This process is also aided by compression due to stellar winds and supernova explosions.

It is in these regions that we find loose aggregations of hot massive stars that are rich in heavy elements such as iron (Fe). These are the Open or Galactic Clusters. By contrast, massive Globular Clusters are located in a more spherical distribution (the halo) centered on the center of the Galaxy. These are deficient in heavies, or "metal-poor".

These give us clues about the possible history of formation of the Galaxy, since the heavies are cooked up in the cores of the massive stars. Very roughly, the material that makes our Galaxy probably collapsed out of metal-poor material from which the globular clusters formed. These objects continue in orbits that retain this spherical infall memory. The leftover gas was flattened out into a disk, from which later generations of stars formed, and are still forming today. Stars that formed later benefited from having material already processed in older stars into heavier elements. However, there are numerous recent observations that suggest this hierarchical picture of galaxy formation is incomplete.

Many galaxies are also very "active" in that they emit lots of energy (light) from their cores using processes that have nothing to do with starlight. These are the quasars, Seyfert galaxies, radio galaxies, BL Lac objects, etc. These luminous cores seem to be powered by mass falling onto a supermassive black hole via an accretion disk. We will see accretion disks many times in this course.

H I is the most abundant atom in the universe. It also can emit a photon when the magnetic poles of its constituent proton and electron flip from being parallel to antiparallel. The photon has a wavelength of 21 cm, and is in many ways the most important spectral line in the universe. Astronomers use it to map out the positions and velocities of gas in galaxies.

Most molecules can form directly from individual atoms or simpler molecules. H₂, the most common molecule in the universe, cannot do this. It requires dust to form (or in some cases, a series of special reactions involving ions).

We find gas and dust in a variety of places throughout the Galaxy. One is in diffuse clouds of low-density atomic gas (with dust mixed in), and higher-density clouds dominated by molecular gas (and dust), the Giant Molecular Clouds. These GMCs are where we find star being born.

Giant Molecular Clouds (GMCs)

There are thousands of these clouds, which are so cold that the material is mostly molecular.

Typical characteristics:

Composition

We can observe the molecules by the emissions they produce. These can be produced when:

All transitions are quantized, as in the atom.

More than 65 molecules have been detected in GMCs, with molecules containing as many as 13 atoms each (HC₁₁N). Organic molecules are present. The amino acid glycine (C₂H₅O₂N) may have been detected!

Astronomers have identified numerous molecules in GMCs. There are well over 90 known so far, and the majority of them are atoms containing C, what are usually referred to as organic molecules.

The dust in interstellar space can also be studied spectroscopically. Dust can absorb, transmit, and scatter the incident radiation. If it does so in a manner that preferentially affects the light waves oriented in one direction, the light will become polarized. Absorbed radiation will heat up the grain, and it will give off thermal emission.

The dust, although it is a solid, produces spectral absorption bands that indicate the presence of carbonaceous material, silicates, and in the GMC ices.

Young Stars and Related Objects

Embedded Protostars, HH Objects, Bipolar Flows

When a clump of stuff in a GMC collapses gravitationally, it can form a star. Due to the conservation of angular momentum, much of the infalling material will be in the form of an accretion disk. As the material works its way down to the protostar. It heats up, and part of it will expand outward (often aided by a rotating magnetic field tied to the star and inner disk). We observe bipolar molecular flows with shock-heated Herbig-Haro Objects in them.

Evaporating Gaseous Globules

The star will gain mass until something shuts off the inflow. In some cases, it just uses up all the stuff that was collapsing (except what left in the bipolar flow or other stellar wind). In other cases, massive hot stars nearby, through their winds and ionizing photons, choke off the material prematurely. In some cases, we see these Evaporating Gaseous Globules (EGGs) at the edges of the clouds.

Proplyds

Eventually, these systems become totally detached from the densest parts of the clouds, and are visible as "freestanding" Protoplanetary Disk Stars (Proplyds), such as those seen in the Orion Nebula.

T Tauri Stars and Herbig Ae/Be Stars

Eventually, the obscuring cloud material begins to dissipate, and we see the young protostars. Stars 2 solar masses and less become T Tauri Stars (TTs), while the more massive ones are known as Herbig Ae/Be Stars (HAEBEs).

Isolated post-T Tauri Stars and post-Herbig Ae/Be Stars

Eventually, the disks in the TTS and HAEBEs begin to dissipate, leaving behind a collection of debris. Somewhere during this period of time, planets may form. These are sometimes referred to as post-TTs and post-HAEBEs (PHAEBEs).

The Vega Phenomenon, Vega-like Stars, Beta Pictoris, etc.

Eventually, the cores of these stars become so hot and dense that hydrogen fusion starts, and the star settles down to become a Main Sequence Star. Many of these are observed to still have debris around them. In some cases, we can still see the disk structure (the Vega-like stars, the best-known being Beta Pictoris).

The disk of Beta Pictoris, obtained with the Hubble Space telescope. The bottom figure is a detail of the central region, showing the presence of a warp most likely due to the presence of one or more massive planets.

Spectra of these disks are very similar to that of comets in our solar system, as we might expect if our solar system formed in the same manner as the other stars.

The silicate emission band in two HAEBE/PHAEBE stars, HD 163296 and HD 31648 (circles) compared to those in Comet Hale-Bopp and Comet Levy 1990 (curves).

One should continually keep in mind that these divisions are somewhat arbitrary. Some HAEBEs have HH objects, HR 4796 can be called a Vega-like stars or a PHAEBE, etc.

The photographic images included here are from the Space Telescope Science Institute, operated by the Association for Universities for Research in Astronomy, Inc., from NASA contract NAS5-26555 and are reproduced here under the conditions specified for educational use by AURA/STScI.

The spectra of HD 163296, HD 31648, Hale-Bopp, and Levy were obtained with the BASS spectrometer. This research was supported by the URC of the University of Cincinnati, the SRG of the American Astronomical Society, and the Aerospace Corporation IRD program.

The line drawings are strictly my own.