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. With the exception of the Magellanic Clouds and the Andromeda galaxy, every nakedeye object in our sky is part of the Galaxy.
7. Interstellar dust and gas in our Galaxy prevented Herschel (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 Herschel 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 light-years; 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. It is now moving toward the constellation Cygnus.
3. With the radius of the Sun’s orbit equal to 8000 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).
Historical Note: The Shapley-Curtis Debate
1. In the late 1910s, there was considerable controversy about the size of the universe and
the nature of the spiral nebulae. In 1920 the National Academy of Sciences held a pubic
debate concerning the size of the universe and the nature of the spiral nebulae.
2. Harlow Shapley estimated a large size of our Galaxy, and concluded that the Magellanic Clouds, the Andromeda nebula, and other spiral nebulae were also part of it.
3. Heber Curtis estimated the Galaxy to be smaller and that the Andromeda nebula and
other spiral nebulae are outside and are other “island universes”.
4. We now know the nature of our Galaxy and other galaxies is closer to Curtis’ explanation. Shapley had made use of some incorrect data and misinterpreted observations of
Cepheid variables because it was not known at the time that there were different types.
Shapley was more correct in his ideas about the overall size of the universe however.
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 1940. 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 3.7 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.
Advancing the Model: The Milky Way: A Barred Warped Spiral Galaxy
1. Until recently, astronomers thought that the Milky Way is a standard spiral galaxy, but
evidence is accumulating that our Galaxy is a barred spiral galaxy with an elongated nucleus.
2. Evidence for the bar comes from observations of asymmetries in the 21-cm radiation
from the nuclear bulge, the motion of gas around the nucleus, and infrared observations
of the distribution of stars.
3. The distribution of atomic hydrogen and stars throughout the Galactic disk show that
the disk is warped and that the warp extends across the entire diameter of the Galaxy. The
warp could be explained as the result of gravitational interactions between the Galaxy’s
dark matter and the Magellanic clouds.
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 Popu-
lation 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.
Metal-rich, 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 the 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 orbits 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.