Chapter 15: The Milky Way Galaxy

kiloparsec (kpc)

A unit of distance equal to 1000 pc, or 3260 ly.

disk component

All material confined to the plane of the galaxy.

halo

The spherical region of a spiral galaxy containing a thin scattering of stars, star clusters, and small amounts of gas.

central bulge

The spheroidal cloud of stars at the center of most spiral galaxies, including our Milky Way Galaxy.

spherical component

The part of the galaxy including all matter in a spherical distribution around the center (the halo and nuclear bulge).

rotation curve

A graph of orbital velocity versus radius in the disk of a galaxy.

Keplerian motion

Orbital motion in accord with Kepler’s laws of planetary motion.

galactic corona

The low-density extension of the halo of a galaxy; now suspected to extend many times the visible diameter of the galaxy.

dark matter

Nonluminous material that is detected only by its gravitational influence.

spiral arms

Long, spiral pattern of bright stars, star clusters, gas, and dust that extends from the center to the edge of the disk of spiral galaxies.

spiral tracers

Object used to map the spiral arms, for example O and B associations, open clusters, clouds of ionized hydrogen, and some types of variable stars.

spiral density wave theory

The conjecture that spiral arms in disk galaxies are caused by a pressure wave that rotates slowly around the galaxy, triggering star formation by compressing interstellar gas clouds.

self-sustaining star formation

The process by which the birth of stars compresses the surrounding gas clouds and triggers the formation of more stars; proposed to explain spiral arms.

metals

In astronomical usage, any atom heavier than helium.

Population I stars

Star rich in atoms heavier than helium; nearly always a relatively young star found in the disk of a galaxy.

Population II stars

Star poor in atoms heavier than helium; nearly always a relatively old star found in the halo, globular clusters, or the nuclear bulge of a galaxy.

galactic fountain

A region of the galaxy’s disk in which gas heated by supernova explosions throws gas out of the disk where it can fall back and spread metals through the disk.

top-down hypothesis

The hypothesis that the Milky Way Galaxy formed by gravitational collapse of a single large spinning cloud of gas. This hypothesis is now considered inadequate to explain many observed characteristics of the galaxy.

protogalaxy

A cloud of gas collapsing gravitationally to become a galaxy. Evidently, protogalaxies only existed in the early history of the Universe.

bottom-up hypothesis

The conjecture that the Milky Way Galaxy and other large galaxies formed mostly by collisions and combination of smaller galaxies and star clusters.

What evidence can you give that we live in a galaxy?

The milky band we see in the night sky indicates a concentration of stellar material in a plane that circles the sky. We have images of other galaxies that one could imagine stepping into and looking out to see a similar view across the sky.

Why is it difficult to give exact dimensions for the Galaxy's disk?

because we are on the inside looking out and interstellar dust

Why is it difficult to give exact dimensions for the Galaxy's visible halo?

because we are on the inside looking out

Why didn't astronomers before Shapley realize how large the galaxy is?

interstellar dust obscuring starlight

Contrast the motion of the disk stars to that of the halo stars. How do their orbits differ? Why?

Disk stars tend to have more circular orbits, while halo stars tend to have more elliptical orbits. Disk stars are much younger than halo stars, and formed from material that had already fallen into the galactic disk.

Why can't spiral arms be physically connected structures? What would happen to them if they were physically connected structures?

Differential rotation would destroy them.

How does self-sustaining star formation produce clouds of stars that look like segments of spiral arms?

Differential rotation drags the inner edge of the cloud ahead of the outer edge. This stretches the star formation region out into an extended spiral structure.

Why must astronomers use infrared and radio telescopes to observe the motions of stars around Sgr A*?

Interstellar dust obscures visible light that the stars around Sgr A* emit.

Could the black hole in the nucleus of the Milky Way Galaxy be the remnant of a single dead star? Why or why not?

No. This black hole has a mass of several million, far too massive to result from a single star.

Which of the following are evidence that spiral arms are sites of star formation?

blue stars

Which of the following stars don't live long enough to leave the spiral arms where they form?

the most massive blue stars

Which of the following best describes the sun?

• The sun left the spiral arm, where it formed, long ago.

• The sun has passed through a number of spiral arms since it formed.

• The sun has orbited the galaxy a number of times since it formed.

Gas clouds passing through a spiral arm are which of the following?

They are triggered into star formation.

A grand design spiral galaxy has which of the following attributes?

two main spiral arms

The spiral density wave is which of the following?

a natural instability in the rotating disk of a galaxy

Which of the following main-sequence stars live the shortest lives?

blue stars

Which of the following stars don't live long enough to leave the clouds of gas and dust from which they form?

the most massive stars

The expanding circles in this animation symbolize which of the following?

• intense light from newborn massive stars

• processes that can stimulate the formation of more stars

• shock waves from supernovae

Spiral arms show which of the following?

where most of the starlight is generated

Gas clouds, in which self-sustaining star formation is occurring, can be drawn out to form which of the following?

short spiral segments

Self-sustaining star formation may explain why some galaxies have which of the following?

spurs and branches

Explain how the rotation curve method of finding a galaxy's mass is similar to the method used to find the masses of binary stars.

They both involve the use of Kepler's 3rd law.