Question 1

Prior to the 1990s,most astronomers assumed that gamma-ray bursts came from neutron stars with accretion disks.How do we now know that this hypothesis was wrong?

Accepted Answer

Observations from the Compton Gamma-Ray Observatory demonstrated that gamma-ray bursts are isotropic, meaning they appear to come from all directions in the sky, which contradicts the hypothesis that they originate from neutron stars within our galaxy. This isotropy suggests a cosmological origin, far beyond our galaxy.

Question 2

How do we know that pulsars are neutron stars?

Accepted Answer

Neutron stars are incredibly dense and can spin at very high speeds due to their small size and conservation of angular momentum. No other known massive object can spin as fast as pulsars do, confirming their identity as neutron stars.

Question 3

Black holes,by definition,cannot be observed directly.What observational evidence do scientists have of their existence?

Accepted Answer

The strongest observational evidence of the existence of black holes comes from their gravitational interaction with other objects. Scientists can observe the effects of black holes on nearby stars and gas, and can use this information to infer the presence of a black hole even if it cannot be directly observed. The other choices are not directly observational evidence of black holes, but rather theoretical predictions or indirect detections.

Question 4

An advanced civilization lives on a planet orbiting a close binary star system that consists of a 15MSᵤn red giant and a 10MSᵤn black hole.Assume that the two stars are quite close together,so that an accretion disk surrounds the black hole.The planet on which the civilization lives orbits the binary star at a distance of 10 AU.
-Sometime within the next million years or so,their planet is likely to be doomed because

Accepted Answer

The red giant is expected to undergo a supernova explosion within the next million years, which will result in the release of an enormous amount of energy capable of destroying the nearby planet. The other options are not likely to happen in the given timescale. The jets from the accretion disk are not directed towards the planet, and even if they were, they are unlikely to reach it from such a distance. The red giant's total consumption in the accretion disk would not cause the planet to be destroyed. Tidal forces could be a problem, but they are not mentioned in the question, and the planet is not close enough to be affected by the black hole. Finally, the planet's orbit is not expected to change significantly during the timescale mentioned in the question.

Question 5

A 10-solar-mass main-sequence star will produce which of the following remnants?

Accepted Answer

A 10-solar-mass main-sequence star will end its life as a supernova, leaving behind a neutron star as the remnant.

Question 6

What is the Schwarzschild radius of a 100 million-solar-mass black hole? The mass of the Sun is about 2 × 10³⁰ kg,and the formula for the Schwarzschild radius of a black hole of mass M is:Rs = $ \frac{2 \mathrm{GM}}{\mathrm{c}^{2}} $ (G = 6.67 × 10-11 $ \frac{\mathrm{m}^{3}}{\mathrm{~kg} \cdot \mathrm{s}^{2}} $ ;c= 3 × 10⁸ m/s)

Accepted Answer

The mass of the black hole is 100 million times the mass of the Sun, which is 2 x 10^30 kg. So, M = 2 x 10^30 kg x 10^8 = 2 x 10^38 kg.
Using the formula for Schwarzschild radius, Rs = (2GM/c^2), we get:
Rs = (2 x 6.67 x 10^-11 x 2 x 10^38) / (3 x 10^8)^2
Rs = 2.98 x 10^10 meters
Rs = 29.8 million kilometers
Therefore, the Schwarzschild radius of a 100 million-solar-mass black hole is approximately 300 million km, which is option D.

Question 7

What is the ultimate fate of an isolated pulsar?

Accepted Answer

Pulsars are rotating neutron stars that emit beams of electromagnetic radiation out of their magnetic poles. Over time, they lose energy and spin down, causing their magnetic fields to weaken. Eventually, they emit too weakly to be detectable, effectively becoming invisible to observers.

Question 8

What causes the radio pulses of a pulsar?

Accepted Answer

Pulsars are rapidly rotating neutron stars that emit beams of radio radiation from their magnetic poles. As the star spins, these beams sweep through space, and if one of them crosses Earth, we observe a pulse.

Question 9

When we see X-rays from an accretion disk in a binary system,we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole.Suppose we then observe each of the following phenomena in this system.Which one would force us to immediately rule out the possibility of a black hole?

Accepted Answer

Sudden, intense X-ray bursts are characteristic of neutron star systems, not black holes. These bursts are thought to result from material suddenly being dumped onto the neutron star's surface, causing a thermonuclear explosion. Black holes, having no solid surface, cannot produce such bursts.

Question 10

One foolhardy day,a daring major (let's call him Tom)in the space force decides to become the first of his race to cross the event horizon of the black hole.To add to the drama,he decides to go in wearing only a thin space suit,which offers no shielding against radiation,no cushioning against any forces,and so on.Which of the following is most likely to kill him first (or at least cause significant damage)? (Hint: The key word here is first.Be sure to consider the distances from the black hole at which each of the noted effects is likely to become damaging. )

Accepted Answer

The X-rays from the accretion disk are likely to cause significant damage first, as they are emitted from the hot gas and matter spiraling into the black hole, which Tom would encounter before reaching the event horizon.

Question 11

If you were to come back to our Solar System in 6 billion years,what might you expect to find?

Accepted Answer

In about 5 billion years, the Sun is expected to exhaust its nuclear fuel and evolve into a red giant, eventually shedding its outer layers and leaving behind a white dwarf. This process does not lead to the formation of a pulsar or a black hole, as those are the remnants of much more massive stars. The solar system will not remain the same, and it will not transform into a red giant at that stage since the red giant phase is a precursor to the white dwarf stage.

Question 12

How does the gravity of an object affect light?

Accepted Answer

According to general relativity, the gravity of a massive object can cause light to redshift, meaning that the wavelength of the light increases as it moves away from the source of gravity. This effect is known as gravitational redshift, and it has been observed in light coming from compact massive objects such as neutron stars.

Question 13

Which of the following statements about black holes is not true?

Accepted Answer

If the Sun were replaced by a black hole of the same mass, Earth's orbit would remain unchanged because the gravitational pull would be the same.

Question 14

In some cases,a supernova in a binary system may lead to the eventual formation of an accretion disk around the remains of the star that exploded.All of the following statements about such accretion disks are true except

Accepted Answer

The statement "several examples of flattened accretion disks being 'fed' by a large companion star can be seen clearly in photos from the Hubble Space Telescope" is not true. While accretion disks around neutron stars and black holes have been observed with the Hubble Space Telescope, they are not typically fed by a large companion star. Instead, the material in the accretion disk comes from the original companion star or from the expanding debris of the supernova explosion.

Question 15

What is the basic definition of a black hole?

Accepted Answer

A black hole is defined as any object from which the escape velocity exceeds the speed of light. This means that anything, including light, that comes too close to the black hole will be pulled in and cannot escape.

Note: Choices A, B, D, and E are all incorrect as they do not encompass the full definition of a black hole. While it is true that a black hole emits no light, it is not the defining characteristic. Dead stars may become black holes, but not all black holes are dead stars. Dark matter has nothing to do with the definition of a black hole, and not all galactic nuclei that can only be viewed in infrared are dead.

Question 16

Why do astronomers consider gamma-ray bursts to be one of the greatest mysteries in astronomy?

Accepted Answer

Gamma-ray bursts are considered one of the greatest mysteries in astronomy because the current evidence suggests that they are the most powerful bursts of energy that ever occur anywhere in the universe, but we don't know how they are produced.

Question 17

Through a bizarre (and scientifically unexplainable)fluctuation in the spacetime continuum,a copy of a book titled Iguoonos: How We Evolved appears on your desk.As you begin to read,you learn that the book describes the evolution of the people living in the star system described above.In the first chapter,you learn that these people evolved from organisms that lived 5 billion years ago.Which of the following statements should you expect to find as you continue to read this book?

Accepted Answer

The answer of Through a bizarre (and scientifically unexplainable)fluctuation in...

Question 18

How do we know what happens at the event horizon of a black hole?

Accepted Answer

We don't have direct observations or measurements of what happens at the event horizon of a black hole. Based on the predictions of general relativity, we expect that matter and radiation crossing the event horizon is trapped and cannot escape, which leads to the formation of an accretion disk and the emission of X-rays. However, our understanding of what happens beyond the event horizon is still incomplete and subject to ongoing research and debate. Physicists have not yet been able to create miniature black holes in the lab, and astronomers have not sent spacecraft through the event horizon of a black hole. While astronomers can analyze the light from matter within the event horizon of black holes, they cannot directly observe what happens at the event horizon itself.

Question 19

How does a black hole form from a massive star?

Accepted Answer

A black hole forms from a massive star through a process known as gravitational collapse. During a supernova explosion, if a star is massive enough for its gravity to overcome neutron degeneracy of the core, the core will be compressed until it becomes a black hole. This occurs when the core of the star is unable to support itself against gravity, causing it to collapse inward rapidly, causing a supernova explosion. The remaining core, which is extremely dense, may continue to collapse until it becomes a black hole.

Question 20

What do we mean by the singularity of a black hole?

Accepted Answer

The singularity of a black hole is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions. It is the point of zero volume and infinite density where the gravitational force is infinitely strong.

Quiz 18: The Bizarre Stellar Graveyard