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CHAPTER 16
The Milky Way Galaxy
CHAPTER OUTLINE
16-1 Our Galaxy
1. The Milky Way Galaxy is the galaxy of which the Sun is a part. From Earth, it appears as a
band of light around the sky.
2. About 200 billion stars make up the Milky Way Galaxy.
3. Most stars in the Galaxy are arranged in a wheel-shaped disk that circles around a bulging center.
4. The diameter of the Galaxy is about 50,000 parsecs (160,000 light-years).
5. The Sun and our solar system is about a third of the way out from the Galaxy’s center (8000 pc
or 26,000 light-years).
6. Interstellar dust and gas in our Galaxy prevented the Herschels (who measured the number of
stars along different directions in the 1780s) and Kapteyn (who measured the number and distance of stars along different directions in the early part of the 20th century) from getting accurate
star density counts from visual observations. These inaccurate data led them to the mistaken conclusion that the Earth is at the center of the Galaxy.
Globular Clusters
1. A globular cluster is a spherical group of up to hundreds of thousands of stars, found primarily in the halo of the Galaxy.
2. The average separation of stars near the center of a globular cluster is 0.5 light-year. Stars in
the region of the Sun average 4–5 light-years apart.
3. Shapley attempted to determine the Sun’s location in the Galaxy using globular clusters. In
order to determine the distance to these clusters, he used Leavitt’s discovery of the Cepheid variables’ period-luminosity relationship.
4. Shapley showed in 1917 that globular clusters are distributed evenly around the sky, about a
point 50,000 light-years from the Sun. His discovery also showed that the Galaxy is larger than
the Herschels had imagined.
5. In the 1920s, Oort and Lindblad studied the motions of great number of stars near the Sun and
found that there is a pattern in these velocities. They concluded that the center of the Galaxy is
thousands of light-years away in the direction of Sagittarius.
6. In 1930, the interstellar dust was discovered, resolving the conflict between the discoveries of
Shapley, Oort, and Lindblad and the star counts of Herschel and Kapteyn.
16-2 Components of the Galaxy
1. The Galaxy contains four components: the disk (which contains the Sun), the nuclear bulge,
the halo, and the galactic corona.
2. The disk is the large, flat part of the Galaxy that rotates in a plane around the nucleus. The disk
contains stars and most of the gas and dust in the Galaxy; it is about 1,000 parsecs thick.
3. Almost all O-type stars lie within about 100 parsecs of the galactic plane. The disk appears bluish because of the presence of the hot O and B main sequence stars.
4. The nuclear bulge is the central region of the Galaxy; it is about 2000 parsecs in diameter. It
contains both young and old stars and appears reddish because of the presence of many red giants
and supergiants.
5. The Galactic halo is the outermost part of the Galaxy; it is fairly spherical in shape and lies
beyond the spiral component. The outer halo is sometimes called the Galactic corona and may
contain large amounts of unseen matter.
6. Milky Way properties: Radius of disk = 80,000 light-years; Radius of nuclear bulge = 3000
light-years; Total radius of halo = 200,000 light-years; Sun’s distance from center = 26,000 lightyears; Sun’s orbital period = 250,000,000 years; Thickness of disk = 3000 light-years; Number of
stars = 200 to 400 billion.
Galactic Motions
1. If we assume that the average velocity of all globular clusters, relative to the Galactic center, is
zero, then we can use the Doppler effect to measure the velocities of the globular clusters relative
to the Sun and attribute the average motion that is observed to the motion of the Sun.
2. The Sun is traveling in a nearly circular path around the Galactic center at a speed of about 220
km/s.
3. With the radius of the Sun’s orbit equal to 8,000 parsecs, the Sun takes about 230 million years
to complete one revolution around the center of the Galaxy.
4. The galactic rotation curve is a graph of the orbital speed of objects in the galactic disk as a
function of their distance from the center.
5. A galactic rotation curve for our Galaxy indicates that large amounts of unseen mass orbit the
center far beyond the Sun’s orbit.
The Mass of the Galaxy
1. Oort and Lindblad discovered in 1927 that the Galaxy in the Sun’s neighborhood undergoes
differential rotation. This allows the use of Kepler’s third law to find the mass of the Galaxy inside the Sun’s orbit.
2. The mass of the inner Galaxy is estimated at 100 billion solar masses. Recent analysis of the
rotation patterns in the outer parts of the Galaxy indicates that the total mass of the Galaxy is
about 1012 solar masses (10 times more mass than calculated for the inner Galaxy).
16-3 The Spiral Arms
1. A spiral galaxy is a disk-shaped galaxy with arms in a spiral pattern.
2. In 1951, the spiral nature of our Galaxy was first hinted at by the distribution of O- and B-type
stars. Confirmation came from radio telescope observations of the 21-cm radiation.
3. The 21-cm radiation is radiation from atomic hydrogen, with a wavelength of 21.1 centimeters. It results from a transition that a hydrogen atom makes from a higher energy level to a lower
one.
4. Hydrogen gas clouds detected by 21-cm radiation are located at the same place as newly forming stars.
5. Applying the Doppler effect, astronomers use the 21-cm radiation to provide further evidence
for the spiral nature of the Galaxy.
6. Recent evidence suggests that the Milky Way Galaxy is actually a barred spiral galaxy.
16-4 Spiral Arm Theories
1. It may seem that the differential rotation of a spiral galaxy can explain the presence of spiral
arms. However, such a rotation would result in a fairly uniform disk.
2. There are two competing theories to explain the spiral nature of galaxies: the density wave theory and the self-propagating star formation theory.
The Density Wave Theory
1. The density wave theory was first proposed by Lindblad in 1960. It is a model for spiral galaxies that proposes that the arms are the result of density waves sweeping around the galaxy.
2. A density wave is a wave in which areas of high and low pressure move through the medium.
3. The density wave theory holds that stars revolve around the galaxy independent of the spiral
arms; the arms are simply areas where the gas density is greater than at other places. The density
waves cause the formation of new stars and glowing emission nebulae.
4. Sound waves are an example of density waves. However, the density waves of a spiral galaxy
move slower than the gas particles. Thus, since the orbital speeds of the gas and dust particles, as
well as stars, is greater than the speed of the density wave, these objects pass in and out of the
spiral arms.
5. As interstellar clouds enter the density wave, they are compressed and new stars are formed.
When we look at a spiral galaxy, the arms are obvious to us because they are the areas containing
the bright stars.
6. One problem with the density wave theory is the question of how the density wave is sustained
through the life of the galaxy. Also, observations of the Whirlpool Galaxy show that the spiral
arms penetrate farther into the nucleus than was previously thought possible.
7. It is possible that an asymmetric gravitational field in the nucleus might generate density
waves. Another possibility is that gravitational interactions between galaxies can generate and
sustain density waves.
8. The spiral arms produces by density waves are well defined. For galaxies that have poorly defined arms another theory has been proposed for their arms.
The Self-Propagating Star Formation Theory
1. The self-propagating star formation theory is a model for spiral galaxies that explains the arms
as resulting from a series of supernovae, each triggering the formation of new stars.
2. At the rate at which massive stars would be formed, differential rotation would cause spiral
arms to be formed and sustained.
3. This theory has the advantage of being able to explain how a spiral arm would begin since the
chain reaction starts with a single supernova.
16-5 The Galactic Nucleus
1. Observations in the early part of the 20th century revealed that the center of the Galaxy lies in
the direction of Sagittarius.
2. Because the presence of dust and gas in the Galactic plane dims visible light from the nucleus,
it wasn’t until the development of IR/radio and X-ray/gamma-ray astronomy that we could “look”
at the Galactic nucleus.
3. The observed number density of stars increases as we get closer to the Galactic center, down to
about 2 pc from the center. For distances closer than 2 pc, observations of the velocities of stars
suggest a nucleus harboring a black hole of mass about 3.5 million solar masses.
4. Radio observations show the presence of a disk of neutral gas 100 – 1000 pc from the center. A
molecular ring with mass at least 20,000 solar masses is 2 – 8 pc from the center. The velocities
of the ionized gas in this region increase as we move from 2 pc to 0.1 pc toward the center. These
measurements suggest the presence of a black hole at the center, in agreement with results obtained using stellar velocities.
5. The case of a large black hole at the Galactic center is also supported by observations of gamma rays emanating from a very small region in the nucleus; also, the X-ray variability from Sgr
A* (a very small unresolved radio source inside the Sgr A region) suggests that the hot, radiating
gas could not occupy a region bigger than about 1 AU.
16-6 The Evolution of the Galaxy
Age and Composition of the Galaxy
1. On average, Population II stars contain only about 1% of the heavy elements of Population I
stars.
2. The abundance of heavy elements decreases by a factor of 0.8 for each thousand parsecs from
the center of the Galactic disk.
3. Most Population I stars are found in the Galactic disk. Most globular clusters are made of Population II stars.
4. 70% of known globular clusters in the Milky Way have an average heavy-element content 1/20
that of the Sun. The remaining 30% contain about 1/3 the heavy-element content of the Sun.
5. Star clusters provide a convenient method of measuring the age of stars.
6. Since there are no white dwarfs near the Sun, this portion of the Galactic disk must be no older
than about 10 billion years.
7. The presence of O- and B-type stars primarily near the Galactic plane suggests that star formation does not occur to any great extent except along the plane.
8. Stars in the halo are older than those in the disk. The youngest stars in the globular clusters are
about 11 billion years old and the oldest are about 13 billion years old.
9. From relatively scant data, it is thought that the nuclear bulge must also be very old. Metalrich, long-lived K- and M-type stars predominate there.
10. The existence of a galactic corona of hot gas has been confirmed from FUSE data.
The Galaxy’s History
1. The Galaxy began as a tremendous cloud of gas and dust bigger than the present Galactic halo.
Mutual gravitation between the cloud’s parts pulled it together.
2. The center portion was the first to become dense enough for stars to form. Dense pockets in
orbit around the center became globular clusters.
3. The initial cloud had some rotation, and as it contracted it spun faster. The rotating matter
formed into a disk.
4. Density waves formed in the Galaxy’s disk, creating the spiral arms where star formation continues today.
5. In an alternative model, several separate clouds of gas merge to form than Galaxy rather than
one. High-velocity atomic hydrogen clouds have been observed since 1963; they have the mass of
a small galaxy and are 15,000 pc across. Their obits do not follow the approximately circular orbits of most Galactic objects.
6. Recent observations seem to support the alternative model of gradual formation of our Galaxy
through accretion of large hydrogen clouds.
7. It is also possible that our Galaxy grew from smaller galaxies by collisions. This “hierarchical”
picture seems to fit better with current theories on the present structure of the universe. Data from
Hipparcos support the idea of a dwarf galaxy colliding with our Galaxy some 10 billion years
ago. Also, the discovery in 2003 of a ring of stars around our Galaxy supports this model.