Unit 13: The Structure of the Milky Way Galaxy
 
 

OVERVIEW

The structure of the Milky Way Galaxy is discussed: its constituents (stars, emission line nebulae, reflection and dark nebulae) and its large scale parts (nuclear bulge, disk, halo, corona). Evidence for dark matter is reviewed. The Galactic center is described. The observation of high-energy sources in the Galaxy is discussed and the possible causes of spiral structure in the disk are considered.

LEARNING OBJECTIVES

At the end of this unit you should be able to:

1. Describe and discuss the constituents of the MWG.

2. Describe and discuss the large scale parts of the MWG.

3. Summarize the evidence for dark matter in the Galaxy.

4. Describe the properties of the Galactic center.

5. Review the observational evidence for high-energy sources in the Galaxy (burst and non-burst sources) and possible causes.

6. Discuss the possible causes of spiral structure in the Galaxy.

 KEY WORDS Milky Way

Milky Way Galaxy

the Galaxy

spiral galaxy

spiral arms

stars

open clusters

globular clusters

emission line nebulae

planetary nebulae

supernovae remnants

reflection nebulae

dark nebulae

gas (ions, atoms, molecules)

dust (silicate grains surrounded by a layer of water ice or hydrogen)

nuclear bulge

disk

halo

Galactic corona (outer halo)

dark matter

non-luminous matter

small black holes

brown dwarfs

planet-sized bodies

exotic subatomic particles

neutrinos

Galactic center

heavily obscured by dust

massive black hole

Galactic nucleus

X-ray bursts

gamma-ray bursts

spiral structure

gas swept up by supernovae explosions

rotation of the Galaxy

density wave theory

 WRITTEN NOTES

The Contents of Our Galaxy

The galaxy that our Sun resides in is called the Milky Way Galaxy (sometimes simply called the Galaxy).

The Milky Way Galaxy is a spiral galaxy. The spiral structure exists in the "disk" part of our Galaxy which is about 100,000 light years in diameter. Thus, the "disk" is simply a large pancake-like structure which contains a greater concentration of stars in so-called spiral arms. The Sun lies in the disk of the Galaxy, about 27,000 light years from its center.

If you look out at the night sky during the right time of year, the large concentration of stars in the disk of the Milky Way Galaxy can be seen. To the unaided eye, this looks like a milky path of light across the sky, but with a telescope individual stars can be seen.

There are about 100 billion (1011) stars in the Milky Way Galaxy.

In addition to stars, the Galaxy is composed of gas and dust which forms some objects that are easy to visually detect. However, the Galaxy is also composed of gas and dust which is difficult to detect. Finally, the Galaxy contains a large amount of visually non-luminous dark matter which has been impossible to detect, except for its gravitational influence on other matter.

Astronomers often assign a variety of names to objects that are visible in the sky. These names can be specific to individual objects; they can be derived from proto-typing a class of objects; or they can be very generic names.

Excluding planets and smaller bodies, generic names for objects in the Galaxy are:

 1. Stars. These are objects which generate significant energy, either by nuclear fusion (when on the main sequence or in the giant or supergiant phase) or gravitational collapse (when forming and dying). They may reside alone, in binary or multiple star systems, in open clusters, or in globular clusters. A star is a visually luminous body.

2. Emission Line Nebulae. These are objects which contain gas that gives off electromagnetic radiation at specific energies (or wavelengths). Emission at a specific wavelength (called an emission line) occurs when an electron moves from an excited state in an atom or ion to a less excited state or the ground state. Planetary nebulae and supernovae remnants are types of emission line nebulae. An emission line nebula is a visually luminous body. Usually the light is red (sometimes green), depending on the wavelength of the dominant emission line.

3. Reflection Nebulae. These are objects which contain dust that reflects the light of nearby stars. A spectrum of a reflection nebula looks like the continuous blackbody spectrum of the starlight it is reflecting. A reflection nebula is a visually non-luminous body because it does not generate its own visible light.

4. Dark Nebulae. These are objects which contain dust that hinders our view of background stars. Thus, a dark nebula might look like a blank region of the sky where few or no stars are seen (except for foreground stars). A dark nebula is a visually non-luminous body.

 Note that a nebula may have gas and dust mixed together. The gas is in the form of ions, atoms, or molecules. The gas is mostly hydrogen. The dust is usually in the form of silicate grains surrounded by a layer of water ice or hydrogen.

However, some nebulae contain no dust and some nebulae contain a relatively large amount of dust. The relative presence of ions, atoms, molecules, and dust in a region of space is determined by the region's environment. Thus, a nebula's type depends on its environment.

If the environment is hot, there will be ions but no molecules or dust. If the environment is cool and the density of particles is higher than normal, there will be molecules and dust but no ions.

 Examples: Around a very hot star there are many ions but practically no molecules or dust. In these regions electrons drop down from highly excited energy states to lower energy states, giving rise to an emission line nebula. (Sometimes this is called an HII region since ions are involved and HII is the physics term for ionized hydrogen.) If a star is placed next to a large amount of dust and gas in a cool region, starlight reflects off the dust and a reflection nebula is seen. Finally, if a large amount of dust and gas are present, but no stars are nearby the region, our view of distant stars is obscured by the dust and a dark nebula is seen.

One scenario for how a region in the Galaxy may change with time is as follows:

· Because of gravitational attraction or gas swept up by a supernova explosion, a region in space becomes very dense, but is cool. This region will contain large amounts of molecules and dust. If no stars are nearby, we see a dark nebula, but if stars are nearby we see a reflection nebula.

· After millions of years gravity may cause the gas and dust to collapse enough to begin fragmentation into proto-stars. Eventually the gravitational collapse causes the densities and temperature to become hot enough to start nuclear fusion, and very hot (O type) main sequence stars form.

· The hot stars ionize the existing gas and dust, and, eventually, there are only ions (near the star) and atoms left. Now we see an emission line nebula known as a HII region.

 The Large-Scale Parts of the Galaxy The large-scale structure of the Galaxy can be broken up into four different parts: 1. The Nuclear Bulge. This contains the galactic nucleus (which probably has a massive black hole at its center), densely packed old stars (Population II), and interstellar gas and dust. This is the center of the Galaxy. Overall this region has a flattened spherical shape which is about 16,000 light years in diameter. It does not exhibit spiral structure.

2. The Disk. Outside of the bulge is the pancake-like (or pinwheel-like) structure called the Galactic disk. It is very thin (its thickness is 2% of its diameter) and it exhibits concentrations of stars in a spiral pattern. The disk contains interstellar gas and dust and young stars (Population I), with the youngest stars often residing in the spiral arms.

3. The Halo. This is a large spherical region about 130,000 light years in diameter that surrounds the Galaxy and contains old stars (Population II), globular clusters (also Population II), and some interstellar gas. The amount of gas in the large halo is small compared to the amount of gas in the thin disk.

4. The Galactic Corona (sometimes called the outer halo). This region extends well beyond the normal galactic halo, possibly having a diameter of 200,000 to 300,000 light years. It is composed of non-luminous matter. We know this matter is present only because of its gravitational attraction on luminous matter. The matter may be in the form of small black holes, brown dwarfs (objects not massive enough to form stars), large planet-sized bodies, exotic subatomic particles, neutrinos, etc. 80% to 90% of the mass of the Galaxy is due to non-luminous matter in the galactic corona. [Probably 95% of the mass of the Universe is non-luminous!]

 The Galactic Center It is easy for dust to absorb visible light. Therefore, because of the large amounts of dust present in the disk of our Galaxy and because the Sun is located in the outer part of the Galactic disk, it is difficult for astronomers to get a clear view of the Galactic center in visible light.

Dust does not easily absorb infrared and radio electromagnetic radiation. Astronomers study the Galactic center by making observations at infrared and radio wavelengths.

[Observations of star forming regions, which are also usually heavily obscured by the dust in dense star forming gas clouds, are also often made at infrared and radio wavelengths.] Radio observations can be made easily from the ground, but only limited observations in the infrared can be made from the ground. The Kuiper Airborne Observatory, the Infrared Astronomical Satellite, and other NASA missions have made observations in the infrared. In the future, NASA hopes to have its Great Infrared Observatory (the Space Infrared Telescope Facility) in orbit.

Observations of the Galactic Center indicate very energetic activity occurs there. A massive black hole (106 solar masses) may exist in the Galactic nucleus.

 High-Energy Sources in the Galaxy High-energy sources in the Galaxy emit x-ray and gamma-ray radiation. The Earth's atmosphere absorbs X-ray and the most energetic gamma-ray radiation.

To study the high-energy sources observations from space must be made. NASA and other countries have put small satellites in orbit to make x-ray and gamma-ray observations. One of NASA's high-energy Great Observatories, the Gamma-Ray Observatory, was put into orbit in 1991. In the future, NASA will put the other high-energy Great Observatory into orbit, the Advanced X-Ray Astrophysics Facility.

 Some of the high-energy sources are bursters, giving off unpredictable, short bursts of radiation. X-ray bursts may occur when matter from an accretion disk falls onto a massive collapsed star. Gamma-ray bursts may occur when matter from a neutron star accretion disk falls onto the neutron star causing a thermonuclear explosion.

Many of the non-burst Galactic X-ray sources are associated with accretion disks around massive collapsed stars (white dwarfs, neutron stars, black holes) in binary systems. X-rays are generated in the hot accretions disks themselves.

Background gamma-ray radiation in the disk of the Galaxy probably results from the interaction of cosmic rays (fast moving charged particles like protons) created in supernovae explosions with the interstellar gas and dust.

 The Spiral Structure of the Galaxy The spiral structure of the Galaxy can be traced by observing the locations of galactic clusters, young O and B stars, and HII regions which are concentrated in the spiral arms.

There are two reasons why spiral structure may be present in a galaxy.

1. Gas swept up by supernovae explosions will be elongated into string-like structures where new stars will form. The rotation of the Galaxy, which can be observed by studying stellar proper motions and Doppler shifts, then orders these concentrations of stars into a spiral-like pattern.

2. The density wave theory of spiral structure predicts that the rotation and gravity of the mass in the galactic disk itself will cause spiral patterned regions in the Galaxy to form. The stars form and spend more time in these regions. While stars revolve around the galactic center they move more slowly in the spiral arms and more quickly when they are out of the spiral arms.

 READING ASSIGNMENT

Chapter 18

HOMEWORK

No homework due, but look at Ch. 18, Review Questions 24 and 28 for your own benefit.