Chapter 16: Galaxies: Normal and Active

Elliptical galaxies

A galaxy that is round or elliptical in outline; it contains little gas and dust, no disk or spiral arms, and few hot, bright stars.

Spiral galaxies

A galaxy with an obvious disk component containing gas; dust; hot, bright stars; and spiral arms.

barred spiral galaxies

A spiral galaxy with an elongated nucleus resembling a bar from which the arms originate.

lenticular galaxies

A galaxy with an obvious disk and nuclear bulge but little or no visible gas and dust and no hot, bright stars.

Irregular galaxies

A galaxy with a chaotic appearance, large clouds of gas and dust, and both population I and population II stars, but without spiral arms.

megaparsec (Mpc)

A unit of distance equal to 1 million pc.

distance indicators

Object of known brightness that astronomers use to find distance—for example, Cepheid variable stars and supernovae. Sometimes called a standard candle.

distance scale

The combined calibration of distance indicators used by astronomers to find the distances to remote galaxies.

look-back time

The amount by which you look into the past when you look at a distant galaxy; a time equal to the distance to the galaxy in light-years.

Hubble law

The linear relation between the distance to a galaxy and its radial velocity.

Hubble constant

A measure of the rate of expansion of the Universe; the average value of velocity of recession divided by distance; about 70 km/s/megaparsec.

rotation curve method

The procedure for finding the mass of a galaxy from its rotation curve.

Gravitational lensing

The effect of the focusing of light from a distant galaxy or quasar by an intervening galaxy to produce multiple images of the distant body.

Rich clusters

A cluster containing more than 1000 galaxies, mostly elliptical, scattered over a volume about 3 in diameter.

Poor clusters

An irregularly shaped cluster that contains fewer than 1000 galaxies, many spiral, and no giant ellipticals.

Local Group

The cluster of about 40 nearby galaxies of which our Milky Way Galaxy is a member.

tidal tails

A long strand of gas, dust, and stars drawn out of a galaxy interacting gravitationally with another galaxy.

galactic cannibalism

The theory that large galaxies absorb smaller galaxies.

ring galaxy

A galaxy that resembles a ring around a bright nucleus; thought to be the result of a head-on collision of two galaxies.

starburst galaxies

A bright blue galaxy in which many new stars are forming, thought to be caused by collisions between galaxies.

radio galaxies

A galaxy that is a strong source of radio signals.

active galaxies

A galaxy that is a source of excess radiation, usually radio waves, X-rays, gamma-rays, or some combination.

active galactic nuclei (AGN)

The central energy source of an active galaxy.

Seyfert galaxies

An otherwise normal spiral -galaxy with an unusually bright, small core that fluctuates in brightness; thought to indicate the core is erupting.

double-lobed radio galaxies

A galaxy that emits radio energy from two regions (lobes) located on opposite sides of the galaxy.

double-exhaust model

The theory that double radio lobes are produced by pairs of jets emitted in opposite directions from the centers of active galaxies.

hot spots

In radio astronomy, a bright spot in a radio lobe.

quasars

Small, powerful source of energy thought to be the active core of a very distant galaxy.

unified model

The attempt to explain the different kinds of active galaxies and quasars by a single model.

Why can't the evolution of galaxies go from elliptical to spiral?

Elliptical galaxies do not have the gas and dust needed for the star formation found in spiral galaxies.

Why can't the evolution of galaxies go from spiral to elliptical?

Spiral galaxies have a distinctive disk that would remain even after star formation ended.

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.

What is the Hubble law a relationship between?

• the recessional velocity of a galaxy

• the distance of a galaxy

What must actually be measured to use the Hubble law to determine the distance to a galaxy?

a galaxy's redshift

What must be known to estimate the distance to a galaxy with the Hubble law?

• the recessional velocity

• the Hubble constant

How can collisions affect the shapes of galaxies?

They are twisted and distorted by tidal forces.

What evidence suggests that Seyfert galaxies have suffered recent interactions with other galaxies?

About 25 percent of Seyfert galaxies have peculiar shapes.

What evidence is there that quasars occur in distant galaxies?

Quasars are often surrounded by hazy features whose spectra resemble those of normal galaxies.

Why are there few quasars at low redshifts and at very high redshifts but many at redshifts between about 2 and 3?

The peak in quasar numbers at redshifts of 2 to 3 suggests that this represents a time in the universe when the rate of collisions between galaxies was at its maximum. At higher redshifts we are looking back to a time when most galaxies had yet to form and collide. Lower redshifts represent more recent times, when most smaller galaxies have been gobbled up and expansion of the universe has carried the galaxies away from each other.

Hubble discovered the universe is expanding by plotting which of the following?

velocity against distance