Download Chapter22_New

Chapter 22 - The Milky Way
CHAPTER 22
THE MILKY WAY
CHAPTER OUTLINE AND LECTURE NOTES
1. Interstellar Matter
To show the students why emission nebulae have the distinctive reddish color they do, I
turn down the light and turn on a hydrogen discharge tube. The excitation mechanism may
be different for HII regions, but the emission line output is the same.
2. The Shape and Size of the Galaxy
I like to use the subject of the size and shape of the galaxy as an example of the way that
science often proceeds by fits and starts with some wrong turns. I treat the work of
Herschel and Kapteyn, though basically wrong, with respect.
3. The Rotation of the Milky Way
Rotation curves for other galaxies (the one for the Milky Way is shown in Figure 22.30) are
the basis for the discovery that there is considerable dark matter in galaxies, so it is
probably a good idea to emphasize that the orbital speed of a star or gas cloud depends on
all the matter inside its orbit, not just the stars and other luminous material.
4. The Spiral Structure of the Galaxy
If you have had the good fortune of never having to deal with urban freeway traffic, let me
assure you that not only can traffic knots develop around slow moving vehicles, they can
also develop and become self-perpetuating so that they really resemble the spiral density
waves in galaxies. I suspect this happens most easily on heavily traveled highways with
relatively long distances between interchanges. I used to see such knots almost every day
when I worked at Ft. Meade, Maryland, and commuted to work on the BaltimoreWashington Parkway. I would often come up to such a knot, slow down with the other
traffic in the knot, and then come out of it without ever seeing any reason, such as a stalled
or slow moving car, for the knot to exist. I’m sure this is a well-known traffic theory result,
but I haven’t found a reference that I can quote. The winding dilemma for differentially
rotating material arms is shown in an animation on the CD that accompanies this textbook.
5. The Center of the Galaxy
The measurement of the orbital periods of the stars that orbit the galaxy within as little as
60 AU of the center has just about eliminated any other explanation for the central
concentration of matter in the Milky Way except for a several million solar mass black
hole.
22-1
Chapter 22 - The Milky Way
6. The History of the Galaxy
To show how more or less random cloud motions in the early history of the galaxy could
have given rise to the organized rotation we see today, I put a beaker with about 2 inches of
water in it on an overhead projector. I sprinkle a small amount of lycopodium powder and
then use a stirrer to agitate the water in as random a manner as I can. The students can then
watch the currents and eddies collide and merge until a general sense of rotation is
established. I usually do this several times to show that the amount of net angular
momentum and the direction of rotation can vary from one case to another. By the way, for
what its worth, lycopodium powder is made from a kind of club moss and is used in physics
demonstrations, fireworks, and surgery.
KEY TERMS
dark matter — Matter that cannot be detected or has not yet been detected by the radiation it
emits. The presence of dark matter can be deduced from its gravitational interaction with
other bodies.
dark nebula — A dense interstellar cloud containing enough gas and dust to block the light of
background stars. The dimming of background stars gives the appearance of a region with
no stars.
density wave theory — A theory to account for the spiral arms of spiral galaxies. According to
the density wave theory, spiral arms are the crests of waves moving through a galaxy like
water waves move through water.
extinction — The dimming of starlight due to absorption and scattering by interstellar dust
particles.
galactic bulge — A somewhat flattened distribution of stars, about 6 kiloparsecs in diameter,
surrounding the nucleus of the Milky Way.
galactic disk — A disk of matter, about 30 kiloparsecs in diameter and 2 kiloparsecs thick,
containing most of the stars and interstellar matter in the Milky Way.
galactic equator — The great circle around the sky that corresponds approximately to the center
of the glowing band of the Milky Way.
galactic halo — The roughly spherical outermost component of the Milky Way, reaching to at
least 30 to 40 kiloparsecs from the center.
galactic latitude — The angular distance of a body above or below the galactic equator.
galactic longitude — The angular distance, measured eastward around the galactic equator,
from the galactic center to the point on the equator nearest the direction to a body.
22-2
Chapter 22 - The Milky Way
galactic nucleus — The central region of the Milky Way.
globular cluster — A tightly packed, spherically shaped group of thousands to millions of old
stars.
HII region — A region of ionized hydrogen surrounding a hot star. Ultraviolet radiation from
the star keeps the gas in the HII region ionized.
interstellar matter — Gas and dust in the space between the stars.
interstellar reddening — The obscuration, by interstellar dust particles, of blue starlight more
strongly than red starlight.
kiloparsec (kpc) — A unit of distance, equal to 1000 parsecs, often used to describe distances
within the Milky Way or the Local Group of galaxies.
microlensing event — The temporary brightening of a distant object that occurs because its
light is focused on the Earth by the gravitational lensing of a nearer body.
reflection nebulae — A cloud of interstellar gas and dust that is luminous because the dust it
contains reflects the light of a nearby star.
rotation curve — A plot of the speed of revolution of the stars and gas in a galaxy versus
distance from the center of the galaxy.
scattering — The redirection of light in random directions when it strikes atoms, molecules, or
solid particles.
Sgr A* — A small, bright source of radio emission, possibly the accretion disk of a black hole,
that probably marks the exact center of the Milky Way.
spiral arm — A long, narrow feature of a spiral galaxy in which interstellar gas, young stars,
and other young objects are found.
stellar population — A group of stars that are similar in spacial distribution, chemical
composition, and age.
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
1. The star must emit a large amount of ultraviolet radiation. Cool stars don’t emit much of
their radiation in the ultraviolet.
2. O stars emit more ultraviolet radiation than B stars.
22-3
Chapter 22 - The Milky Way
3. An HII region forms about a hot, young star in a molecular cloud and begins to expand.
When the HII region reaches the edge of the molecular cloud, it begins to expand rapidly in
the lower density region outside the cloud. When the star leaves the main sequence and
becomes cooler, the HII fades out.
4. There is little atomic hydrogen in an HII region.
5. The hot phase is heated by shock waves from supernovas.
6. Most bright nebulae (HII) would look approximately round.
7. The reflection nebula is bluer than the star that illuminates it.
8. 0.06
9. The dust is nearly transparent in the infrared.
10. The diffuse light is bluer than the typical bright stars in the galaxy.
11. The core forms at relatively high temperatures in outflows from cool stars. The icy outer
layer can only form at the much lower temperatures of interstellar space.
12. Interstellar dust prevented Herschel and Kapteyn from seeing far enough to learn the true
shape and size of the galaxy.
13. He explained that the galaxy is flat so we can only see distant stars (that form the glow of
the Milky Way) when we look along the disk rather than out of it.
14. The observer’s planet is located near the center of the galaxy.
15. More matter is required to account for the speed of rotation of the galaxy than can be found
in visible forms such as stars and nebulae.
16. Young objects
17. Interstellar dust prevents us from seeing far enough at visible wavelengths to discern the
entire spiral pattern.
18. If the arms were “material arms” they would have become wound extremely tightly during
the history of the galaxy.
19. In the vicinity of the spiral arms, the gravity of the material in the arms has an effect on the
speeds of orbiting matter.
20. About every 400 to 500 million years
21. Stars are formed in spiral arms. O and B stars have such short lives that they don’t survive
long enough to leave the spiral arm in which they form.
22. Rapidly orbiting gas near the center shows that there are several million solar masses of
material within the inner 0.1 pc of the galaxy.
23. The low density in the innermost 2 pc of the galaxy suggests that explosions periodically
blast matter out of the central region.
24. The gas in the galaxy has become flatter with passing time. Old objects show the shape of
the galaxy long ago whereas young objects show the present shape of the galaxy.
25. The galaxy has become flatter and richer in heavy elements as time passes.
Problems
1. 110 million years
2. 4 × 1011 solar masses
3. The mass of A is 9 times the mass of B.
4. 10 million solar masses
5. 0.0017 seconds of arc
22-4
Chapter 22 - The Milky Way
Figure-based Question
1. About 4
22-5