Question 1

The energy output of a bright quasar is equivalent to

Accepted Answer

Quasars are extremely luminous objects, far outshining individual galaxies. A single quasar can emit the energy equivalent to hundreds of Milky Way galaxies combined, making option D the correct choice.

Question 2

An astronomer is observing the radiation from a distant active galaxy and notices that the amplitude of the signal varies in strength regularly over a certain period. The maximum possible size for the source of this radiation can now be calculated from the

Accepted Answer

The maximum possible size of the source can be calculated from the period of the signal variation, not the amplitude. This is because the time it takes for the signal to vary (its period) can be related to the light travel time across the source, giving an upper limit on its size. The amplitude of the signal does not directly inform about the size of the source.

Question 3

The luminosity of the Milky Way Galaxy is about 10 billion times the luminosity of the Sun. During peak periods of brightness, the quasar 3C 279 has a luminosity about ten thousand times the luminosity of the Galaxy. Suppose the flux (energy per second per emitting area) from the Sun remains the same, but it grows in radius. How big would it have to be to produce the same luminosity as 3C 273 at its peak?

Accepted Answer

The answer of The luminosity of the Milky Way Galaxy...

Question 4

"It is starlike in appearance, showing very high redshift and an energy output of at least 100 galaxies from a small region about 1 light-day across." This statement describes which of these astronomical objects?

Accepted Answer

Quasars are extremely luminous active galactic nuclei, with very high redshifts, indicating that they are far away from Earth. They can outshine entire galaxies, emitting as much energy as hundreds of galaxies combined from a very compact region that is roughly the size of our solar system. This matches the description given.

Question 5

A quasar is now thought to be a

Accepted Answer

Quasars are extremely luminous objects located at great distances from Earth, believed to be powered by supermassive black holes at the centers of galaxies. They are among the most distant and energetic objects in the universe.

Question 6

Which of these is NOT a characteristic of active galaxies?

Accepted Answer

Active galaxies are characterized by their energetic processes, which include jets (A), strong emission lines in their spectrum (B), and bright, starlike nuclei (D). The characteristic not associated with active galaxies is C, Doppler shifts indicating a large rate of rotation. While Doppler shifts can be observed in active galaxies, they are not a defining characteristic; instead, they are more broadly related to the motion of astronomical objects and can be observed in various contexts, not exclusively in active galaxies.

Question 7

The fluctuation time for the brightnesses of quasars can be as short as a

Accepted Answer

The fluctuation time for the brightnesses of quasars can be as short as a day, indicating the compact size of the emitting region and the dynamic processes occurring within quasars.

Question 8

An active galaxy is measured to be 100 Mpc away. How many million years ago did the light observers see now leave this galaxy?

Accepted Answer

The light we see from an object 100 Mpc away left that object 326 million years ago because 1 Mpc is approximately equivalent to 3.26 million light-years.

Question 9

Which observations of the radiation from quasars indicate that quasars are physically very small objects compared with galaxies?

Accepted Answer

Rapid fluctuations in output, often in less than 1 day, suggest that quasars are physically small because the changes in brightness must occur across the entire object, implying a small size due to the speed of light constraint.

Question 10

What observations convince astronomers that the energy source of a quasar is physically very small?

Accepted Answer

Rapid fluctuations in brightness indicate that the energy source must be small, as changes over 1 day suggest the source is less than 1 light-day across.

Question 11

Compared with the total output of energy from the Milky Way Galaxy, what is the equivalent output of a typical quasar?

Accepted Answer

Quasars are extremely luminous and can be 100 times brighter than the Milky Way Galaxy, despite their energy coming from a relatively small region of space. This high luminosity is due to the accretion of matter by supermassive black holes at their centers.

Question 12

The surprising observational fact about quasars is that they appear to

Accepted Answer

Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light. They can outshine entire galaxies, yet they are compact, occupying a space similar to the size of our solar system. This immense energy production in such a small volume is what makes quasars so surprising and fascinating to astronomers.

Question 13

Suppose the X-ray signal from a quasar fluctuates with a period of 3 hours. What is the maximum size of the source?

Accepted Answer

The maximum size of the source is the distance light can travel in 3 hours, which is 10.8 au.

Question 14

What observational fact convinces astronomers that the source of energy in a typical quasar is physically very small?

Accepted Answer

Rapid variation in the energy output of a quasar indicates that the source must be physically small, as larger objects cannot change their brightness quickly due to the time it takes for changes to propagate across the object.

Question 15

Variations in the luminosity of quasars over the span of a few hours are an indication

Accepted Answer

Variations in the luminosity of quasars over short periods, such as a few hours, suggest that the emitting regions must be relatively small. This is because, according to the light travel time argument, only a small region can change brightness quickly enough to account for the observed rapid variations.

Question 16

What is the relationship between the size of an object and the rate at which its radiant output can vary?

Accepted Answer

Small objects can vary their radiant output more rapidly because they have less mass to heat up or cool down, allowing for quicker changes in temperature and thus radiant output.

Question 17

Which of these would NOT necessarily be categorized as "active galaxies"?

Accepted Answer

Active galaxies are characterized by their energetic processes and emissions, often linked to phenomena like radio galaxies, double-radio sources, and BL Lacertae objects, which are all associated with active galactic nuclei (AGN). However, the mere presence of a supermassive black hole in a galaxy (D) does not automatically categorize it as an "active galaxy"; it's the interaction and accretion processes around the black hole that can activate an AGN, making the galaxy "active."

Question 18

How can astronomers determine the size of an emission region in a very distant and unresolvable source?

Accepted Answer

The brightness variability method is used because the time it takes for light to cross the source limits how quickly the brightness can change, allowing astronomers to estimate the size of the emission region.

Question 19

The fact that quasars can be detected from distances from which even the biggest and most luminous galaxies cannot be seen means that

Accepted Answer

Quasars are among the most luminous and energetic objects in the universe, allowing them to be visible from great distances, far beyond the reach of even the largest galaxies' visibility.

Question 20

Why is it that astronomers will NOT see fluctuations in light output in times shorter than about 1 day when they observe an extragalactic source whose diameter is about 1 light-day?

Accepted Answer

Arrival times of light from different parts of a source that is about 1 light-day in diameter will indeed be different due to the vast size of the source. This variation in arrival times causes any fluctuations in light output that occur on timescales shorter than the time it takes light to travel across the source (in this case, about 1 day) to be smoothed out, as the light we observe at any given moment comes from different parts of the source that emitted their light at slightly different times.

Quiz 18: Quasars and Other Active Galaxies