Question 1

In a binary star system,an unseen component is found to have a mass of about 8 solar masses.If this were a normal star,then it would be visible,so it must be a collapsed object.Theoretical considerations tell us that it must be a

Accepted Answer

The fact that the object is not visible despite having a mass of 8 solar masses suggests that it is very dense and compact, meaning it is likely a collapsed object. A neutron star or a black hole are the only collapsed objects that could have a mass of 8 solar masses or greater. While it is possible for a neutron star to have this mass, it is more likely that it would collapse further into a black hole. Therefore, the best choice is a black hole.

Question 2

A nova is a sudden brightening of a star that occurs when

Accepted Answer

A nova occurs when a white dwarf star in a binary system accretes material from its companion star. As the material builds up on the surface of the white dwarf, it becomes compressed and heated until it undergoes a runaway fusion reaction, leading to a sudden increase in brightness. The explosion ejects some of the material into space, but the white dwarf remains intact and can repeat the process in the future.

Question 3

What is the difference between a nova and an X-ray burster? All of the following answers are correct EXCEPT one.Which one is the exception?

Accepted Answer

A nova and an X-ray burster both involve stars in close binary systems, not individual stars. In both cases, material is accreted from a companion star onto the surface of a white dwarf (in the case of a nova) or a neutron star (in the case of an X-ray burster), leading to the observed phenomena.

Question 4

The diameter of the black hole candidate Cygnus X-1 has been estimated by

Accepted Answer

The diameter of Cygnus X-1 is estimated by measuring how quickly it flickers in intensity, which relates to the size of the emitting region.

Question 5

At first,it was thought that nothing could ever escape from a black hole,yet astronomers are now locating black hole candidates by the X-rays they emit.Do they really see X-rays coming from a black hole?

Accepted Answer

The X-rays are emitted from highly compressed matter in the accretion disk outside the event horizon of the black hole. This matter is heated to extremely high temperatures due to the intense gravity of the black hole, causing it to emit X-rays. The X-rays are not coming from inside the black hole itself.

Question 6

A black hole can be thought of as

Accepted Answer

A black hole is a region of space where the gravitational pull is so strong that nothing, not even light, can escape from it. This is due to the extremely curved spacetime around it.

Question 7

One object that is believed to be a black hole in our Galaxy is

Accepted Answer

Cygnus X-1 is a powerful X-ray source believed to be caused by an accretion disk around a black hole. The other options do not meet the criteria of being a black hole in our galaxy.

Question 8

Which of the following is NOT one of the reasons we believe gamma-ray bursters are supernova explosions in distant galaxies rather than eruptions in the nuclei of active galaxies?

Accepted Answer

The answer of Which of the following is NOT one...

Question 9

What is believed to be the mass of the black hole candidate Cygnus X-1?

Accepted Answer

Based on multiple observations and calculations, the mass of Cygnus X-1 is believed to be approximately 7 solar masses.

Question 10

What prevents a neutron star from collapsing and becoming a black hole?

Accepted Answer

Neutron stars are held up by neutron degeneracy pressure, which is the repulsive force between neutrons due to the Pauli exclusion principle. This force balances the inward pull of gravity, preventing the neutron star from collapsing further. If the mass of the neutron star exceeds a certain limit, known as the Tolman-Oppenheimer-Volkoff limit, neutron degeneracy pressure is no longer sufficient to counteract the force of gravity, and the neutron star will collapse to form a black hole.

Question 11

What method is used by astronomers to infer the existence in space of a dark object with a mass of about 5 solar masses such as a black hole?

Accepted Answer

Astronomers can infer the existence of a dark object with a mass of about 5 solar masses such as a black hole by measuring the effect of its gravitational force on a companion object in a binary system. By observing the motion of the companion object, astronomers can calculate the mass of the unseen object and determine if it is consistent with the mass of a black hole.

Question 12

X-rays that come from the vicinity of a black hole actually originate from

Accepted Answer

X-rays that come from the vicinity of a black hole originate from just outside the event horizon, on the accretion disk. The intense gravity of the black hole pulls in surrounding matter, which heats up and emits X-rays before being engulfed by the event horizon.

Question 13

Type II supernovae show prominent lines of hydrogen in their spectra,whereas hydrogen lines are absent in spectra of Type Ia supernovae.Why is this? (Hint: Think about the type of star that gives rise to each of the two types of supernova. )

Accepted Answer

Type II supernovae result from the collapse of massive stars, which still contain significant amounts of hydrogen in their outer layers. Type Ia supernovae, on the other hand, originate from white dwarfs in binary systems that accumulate matter until they reach a critical mass and explode; these white dwarfs are composed mostly of carbon and oxygen, with no hydrogen layers, hence the absence of hydrogen lines in their spectra.

Question 14

Can a white dwarf explode?

Accepted Answer

White dwarfs can explode as a Type Ia supernova if they are in a binary star system and accumulate enough material from a companion star to reach the Chandrasekhar limit, leading to a runaway thermonuclear explosion.

Question 15

If a black hole is truly black and has an escape velocity greater than the speed of light such that no light can escape it,where do the X-rays come from in the black hole candidates so far identified?

Accepted Answer

X-rays are emitted from the matter surrounding the black hole, which is heated to extremely high temperatures as it is pulled towards the black hole's event horizon. This matter is often in the form of an accretion disk, composed of gas and dust that is spiraling towards the black hole. Friction and collisions within the disk cause it to heat up, emitting X-rays that can be detected by telescopes. The black hole itself is still black, as no light or radiation can escape its event horizon.

Question 16

What is the main observational difference between a Type I and a Type II supernova?

Accepted Answer

The main observational difference between Type I and Type II supernovae is the presence of hydrogen lines in the spectrum. Type II supernovae show prominent hydrogen lines, indicating that they still have a significant amount of hydrogen in their outer layers at the time of explosion. In contrast, Type I supernovae lack these hydrogen lines, suggesting they have lost their hydrogen envelopes prior to the explosion.

Question 17

X-ray bursters are objects in the sky that emit sudden bursts of X-rays in addition to a steady,low-level,X-ray emission.These bursts of X-rays are believed to be caused by

Accepted Answer

X-ray bursters are usually associated with neutron stars in binary systems, where the neutron star is accreting material from a companion star. The sudden burst of X-rays is thought to be caused by a thermonuclear explosion on the surface of the neutron star, triggered by the accumulation of material from the companion star. This process does not lead to the collapse of the neutron star into a black hole, but rather leaves it intact to repeat the process.

Question 18

What is the likely final fate of a star that has a mass of greater than 15 solar masses after completing its nuclear fusion-burning phases?

Accepted Answer

A star with a mass greater than 15 solar masses will eventually run out of fuel and undergo a supernova explosion. If the remnant core mass is greater than about 3 solar masses, the collapse will continue until it becomes a black hole.

Question 19

If the Sun were replaced by a 1-solar-mass black hole,then Earth would

Accepted Answer

The gravitational force that governs Earth's orbit depends on the mass of the Sun and the distance between Earth and the Sun. If the Sun were replaced by a 1-solar-mass black hole, the mass attracting Earth would remain the same, and so would the distance, meaning Earth would continue to orbit the black hole just as it orbits the Sun now.

Question 20

Pairs of jets carrying material and radiation outward in opposite directions have been detected (or inferred)for all of the following types of objects EXCEPT

Accepted Answer

While jets have been detected from all other types of objects listed, there is no evidence of jets coming from white dwarfs.

Quiz 12: Predicting the Violent End of the Largest Stars