Chapter 16

The Milky Way GalaxyThe Milky Way in Infrared: The COBE Project, DIRBE,

NASA

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).

Figure 16.02

Figure 16.03: The region around the Galactic Center Courtesy of Bill Keel, University of Alabama

6. Interstellar dust and gas in our Galaxy prevented the Herschels and Kapteyn 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.

1. A globular cluster is a spherical group of up to hundreds of thousands of stars, found primarily in the halo of the Galaxy.

Globular Clusters

Figure 16.08b: A VLT image of 47 Tucanae, 16,000 light-years away in the constellation Tucana.

Courtesy of R

ubina Kotak and H

enri Boffin, E

SO

 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 tried to determine the Sun’s location in the Galaxy using globular clusters.

To determine the distance to these clusters, he used Leavitt’s discovery of the Cepheid variables’ period-luminosity relationship.

Figure 16.09: Determining the relationship between the periods and absolute luminosities of RR Lyrae variable stars to find distance

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 stars near the Sun and found 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.

Figure 16-11

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 – It 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.

Galactic Motions

1. If we assume 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.

Figure 16.13a: If all the mass of the Galaxy were concentrated at its center, the galactic rotation curve would follow Kepler’s laws.

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.

Figure 16.14: Locations of the greatest concentrations of O and B stars

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.

Figure 16.15b: Our Galaxy in 21-cm radiation

Courtesy of NASA Goddard Space Flight Center

 5. Applying the Doppler effect, astronomers use the 21-cm radiation to provide further evidence for the spiral nature of the Galaxy.

Figure 16.16: (a) The Sun and three hydrogen clouds. (b) The 21-cm radiation observed along the line of sight

6. Recent evidence suggests that the Milky Way Galaxy is actually a barred spiral galaxy.

Figure 16.17: Our Galaxy probably looks like the NGC 2997 spiral galaxy.

© A

nglo-Australian O

bservatory, photography by David M

alin

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.

Figure 16.18: Stars starting out in a straight line from the Galaxy's center would spiral but would lose the pattern in a few rotations

 

2. There are two competing theories to explain the spiral nature of galaxies:

– the density wave theory– the self-propagating star formation theory

The Density Wave Theory

1. The density wave theory holds that the arms of spiral galaxies 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.

– Sound waves are an example of density waves.

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. 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.

Figure 16.20a: The orbits of objects in the Galaxy are very nearly circular.

Figure 16.20b: A cloud of dust and gas is about to enter a spiral arm.

Figure 16.20c: The higher density of gas in the arm causes the cloud to compress, resulting in star formation.

Figure 16.20d: As the cloud passes through the spiral arm, bright, massive stars make the arm appear brighter.

Figure 16.20e: The bright stars burn out before they exit the spiral arm, as another cloud of dust and gas begins to enter the arm.

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.

Figure 16.22: Clusters near the galactic center

Courtesy of D. Figer (STScI) and NASA

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.

Figure 16.23b: Inner region of the Galactic nucleus—the center of the strongest emission

(b) Courtesy of NRAO

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 the hot, radiating gas couldn’t occupy a region bigger than about 1 AU.

Fig. 16-26a

Courtesy of N

AS

A/C

XC

/MIT

/F.K

. Baganoff et al.

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

– 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.

Figure 16.28a: According to a leading theory of galactic evolution, the Galaxy began as a sphere of gas and dust about 13 bya.

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.

Figure 16.28b: As the sphere collapsed toward the center, stars and clusters

formed.

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.

Figure 16.28c: The present state of the Galaxy

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 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.

– The discovery in 2003 of a ring of stars around our Galaxy supports this model.