THE MILKY WAY GALAXY

The Milky Way is our own galaxy , a huge swirling mass of 100 billion stars, gas and dust. To us, it has the appearance of a milky band of light across the sky. Galileo's telescope revealed that it is composed of many stars.

SIZE OF THE GALAXY

William Herschel in the 18th century observes there are almost no stars dimmer than a certain brightness. Concludes those are the most distant stars.

Jacobus Kapteyn, 1922, presents more detailed work along these lines, taking into account differences between stars of different spectral types, and their relative numbers. Conclusions:

• the Kapteyn universe is roughly pancake shaped
• the Sun is approximately at the center of the Universe
• the diameter of the Galaxy is about 3,000 pc (10,000 ly)

Wrong! Does not take into account absorption of starlight by interstellar dust: a systematic error

VARIABLE STARS

Some stars are known to vary in brightness. Examples:

• Algol "the Ghoul,", Beta Persei; varies from magnitude 2.3 to magnitude 3.5 every 20 hours -- an eclipsing binary.
• Mira "the Wonderful,", Omicron Ceti. Discovered in 1596 by Fabricius; varies from magnitude 2 or 3 to magnitude 10, with an irregular period of about 300 days. -- a helium-shell burning star.
• Several types of short period variables:
  • Cepheids
  • RR Lyrae stars,
  • Delta Scuti stars.

THE PERIOD-LUMINOSITY RELATIONSHIP FOR CEPHEID VARIABLES

Key fact, discovered by Henrietta Leavitt: the luminosity of a Cepheid variable is directly related to its period. Thus, measuring the period of a Cepheid star reveals its luminosity, and by comparing its luminosity to its apparent brightness, it is possible to determine the distance to the star.

Extends cosmic distance ladder out to as far as we can see Cepheids (5 Mpc or more, that is, out to neighboring galaxies)

SIZE OF THE GALAXY (concluded)

Harlow Shapley used Cepheid variables to measure the distance to the globular clusters.

• finds a spherical distribution about 30 kpc (30,000 pc) across
• this is the true size of the galaxy
• the Sun is not at the center of the distribution, but about 8 kpc out from the center

STRUCTURE OF THE GALAXY

Parts of the galaxy:

• the halo
  • roughly spherical, possibly flattened along the galactic equator
  • contains little gas or dust
  • globular clusters
  • few blue stars
  • halo stars are poor in heavy elements (W. Baade, 1940's)
• galactic disk: concentration of stars in the galactic plane
  • open clusters, star forming regions (emission nebulae, etc.)
  • most of the blue stars of the galaxy are confined to the disk
  • disk has spiral structure
  [Conclusion: the halo stars are old (1st generation), and little star formation occurs there. Most of the formation of new stars takes place in the galactic disk.]
• galactic bulge: a thickening of the galactic disk near the center of the Galaxy
• galactic center: a large mass is concentrated at the center of the Galaxy

OBSERVATIONS OF THE GALACTIC DISK AND EVIDENCE FOR THE SPIRAL STRUCTURE OF THE MILKY WAY

Optical observations in the plane of the galactic disk are limited by the dust. However, after correcting for absorption by dust, it is possible to plot the location of O- and B- (hot young stars) which tend to be concentrated in the spiral arms

Radio frequency observations reveal the distribution of hydrogen (atomic) and molecular clouds. The distance to the clouds is calculated from comparing velocities derived from Doppler shifts to the rotational motion of the Galaxy. These observations show the presence of the galactic bulge, and give further evidence for the existence of spiral structure in the disk.

Still, the details of the spiral arms of the Milky Way galaxy remain blurry.

GALACTIC ROTATION -- refers to the collective motion of the stars orbiting the center of the Galaxy.

• Stars near the center revolve more quickly than those at the rim, just as (by Kepler's 3rd law) in the Solar System, planets near the Sun revolve more quickly
• observed by measuring the velocities of stars near the Sun; those nearer the center of the Galaxy are passing us by, those further out are falling behind.
• Similar differential rotation observed in other nearby galaxies, such as Andromeda.
• The halo stars also orbit the galactic center, but not in as organized a way.)

Our Sun's orbital velocity : 250 km/s
Our Sun's orbital period ("galactic year"): 200-250 million years

THE FORMATION OF THE GALAXY

Observations of the differences between the halo and the disk suggest that the formation of the Galaxy may have proceeded in a way similar to the formation of the Solar System.

• The Galaxy forms from a cloud.
• conservation of angular momentum increases its rate of rotation as it contracts.
• increased rate of rotation causes an equatorial bulge that flattens most of the mass of the Galaxy into a disk
• halo stars are what's left of the stars that formed early on, before the Galaxy's contraction and rotation had flattened the rest of it.

ORIGIN OF THE GALAXY'S SPIRAL STRUCTURE

Some possible theories:

1. Spiral structures might in principle form from the differential rotation of the Galaxy; but they wouldn't last long, because the differential rotation would soon wind them very tightly around the Galaxy, which is not observed.
2. Spiral density wave theory: a spiral wave of compression and rarefaction passes through the stars of the Galaxy
   • Agrees with observation that blue stars are found at the trailing edge of the spirals (formed by compression of gas) and that there is more gas in the spirals than the space between them
   • However, calculations indicate that the spiral waves also wouldn't last as long as the Galaxy has
3. Formation of stars (or the death of stars in supernovae) produces a shock that then causes a new wave of star formation that produces...; however, calculations show that this model would produce pieces of spirals, rather than complete spirals

MASS OF THE GALAXY

Can be determined by a method similar to using Kepler's 3rd law to find the mass of the Sun. In the Galaxy, the orbital velocities of the stars are determined by the total mass of the Galaxy contained within that star's orbit. Thus, knowing how the rotation speed of the Galaxy varies with distance from the center (galactic rotation curves) reveals the distribution of mass in the Galaxy. Two key results:

• There is a large mass contained in a very small volume at the center of the Galaxy
• Much of the mass of the Galaxy is not observed: it consists neither of stars, nor of gas or dust, and it extends far beyond the visible part of the Galaxy

THE MISSING MASS PROBLEM

missing mass, called dark matter, may consist of brown dwarfs, black dwarfs, black holes, or exotic subatomic particles.

• A search for brown dwarfs is ongoing; they would be observed by the effect of their gravity on the light of stars
• Various theories of subatomic particles are being tested in laboratories.

THE GALACTIC CENTER

lies in the direction of the constellation Sagittarius -- stars near the center are much closer together than in our neighborhood

There is probably a huge black hole at the center of the Galaxy.

• from orbital velocity of gas, there is an object of a million solar masses in a volume less than 5 pc
• lots of radio energy coming from the galactic core. (Cores of other galaxies are observed also to emit strongly in the X-ray.) Probable source: accretion disk

---

Return to table of contents.

Previous section

Next section

your instructor

---

Copyright © 1996 M. S. Pettersen
Permission is granted to make copies for individual use, not for redistribution.
This document was last updated September 2, 1998.