Stars & Galaxies- Chapter 27

A Congressman from Texas visits our National Observatory in Tucson, Arizona, and wants to be shown an object in the universe with the "biggest darn redshift you ever saw." What type of object should the astronomers show him?

A quasar.

An astronomer claims that the large redshifts of all quasars are caused by some new mechanism and not the expansion of the universe. The redshift tells us nothing, he says, about where any quasar is located. Which of the following would be a way to disprove his view of quasars?

Find a number of cases where a quasar seen in a cluster of galaxies has the same redshift as all the galaxies in the cluster.

What method would astronomers use to find the distance to a remote quasar?

Finding the redshift and using Hubble's Law.

When quasars "shine" (in visible light and other kinds of radiation) with a lot of energy, where (what location) does this huge amount of energy come from?

From an accretion disk around a supermassive black hole.

If quasars are at the distance most astronomers believe they are, then (for most luminous ones) their luminosities must be

Like the combined luminosity of a hundred (10^14) Suns.

The astronomer who first solved the problem of the strange red-shifts seen in quasar spectra and thus helped us understand that the quasars must be distant objects was

Maarten Schmidt.

Today, astronomers find compelling evidence that the energy source of the quasars and active galaxies is

Matter falling toward a supermassive black hole at the center of a galaxy.

How can supermassive black holes at the centers of galaxies help new stars to form in that galaxy?

New stars can form when the flow of particles from a black hole accretion disk or jet compresses the material some distance away from the black hole and starts the "clumping" that leads to the formation of stars.

How are galaxies and quasars related?

Quasars are active supermassive black holes at the centers of galaxies.

Why do astronomers think that there are fewer quasars today than there were billions of years ago?

Quasars are seen when the supermassive black hole at the center of a galaxy has lot of "food to eat' (stars and gas) and the amount of available food tends to decrease with time.

How do astronomers now explain the fact the energy emitting regions for quasars are so small?

Quasars are the result matter falling into a black hole; the vent horizons of even supermassive black holes are extremely small on the cosmic scale.

What makes astronomers believe that the energy source in quasars is only a few light months across (the distance light travels in a few months)?

Quasars show variations in their energy output that have a period of a few months.

Today we know that what all quasars have in common is that they appear to be small sources of energy with

Redshifts that indicate they are far away.

Which of the following observations is a convincing argument for the idea that quasars are located inside galaxies?

Relatively nearby quasars show "fuzz" around them with the same spectra and redshift as the quasar.

The Hubble Space Telescope has enabled astronomers to explore an active galaxy such as M87 in remarkable detail. Which of the following observations of M87 is NOT an important part of the web of evidence that shows it must have a supermassive black hole at the center?

The discovery of a gravitational lens in M87.

What observations in astronomy, made AFTER the discovery of quasars, was big help to astronomers in figuring out what quasars really were?

The discovery that the Milky Way Galaxy has a black hole at the center with enough mass for 4 million Suns.

A friend of yours who is a science fiction fan hears you talk about the fact that astronomers now believe that the mechanism for the large energy output of quasars involves a supermassive black hole. He challenges you, saying something like "Oh come on, every science fiction fan knows that nothing, not even light, can escape from a black hole! How can a black hole be an energy source?" How would you respond to his objection?

The energy we see from quasars comes from regions where matter is falling in; these regions are still outside the event horizon.

If a nearby galaxy still acts like a quasar today, what is the most likely explanation?

The galaxy must be undergoing a collision with another galaxy that is providing fresh fuel for its central black hole.

How do astronomers explain the energetic jets that come out of quasars and active galactic nuclei in opposite directions?

The jets are "spit out" from the chaotic accretion disks of supermassive black holes in directions that are perpendicular to the disk.

When astronomers make counts of how many quasars there are at different distances from us, what do they find?

The largest number of quasars can be seen at about the distance corresponding to a time when the universe was only 20% its current age (a time when the universe was still young).

How do quasars demonstrate that the universe evolves with time?

The number of quasars reached a maximum some time ago, and now the numbers have been declining.

If quasars often resemble little blue stars, what was it about them that so surprised astronomers when they were discovered?

Their spectral lines were at first hard to recognize and then turned out to have large redshifts.

Astronomers have established that quasars and active galaxies have a lot of mass at the their centers in a very small volume of space. Why can't this mass be i the form of a cluster of stars that are quite close to each other?

To fit as much matter into the cluster as we observe, the stars in the cluster must be so close to each other they would merge into a superstar and soon collapse into a black hole.

A distant quasar shows a large redshift- one so large, in fact, that the feature s we now see in visible-light region of the spectrum would be invisible to us, were it not for the redshift. What band of the electro-magnetic spectrum were these features most likely in, before the spectrum was redshifted?

Ultraviolet.

After several decades of observation, astronomers have concluded that quasars are

Very powerful and compact sources of energy at the centers of distant galaxies.