Question 1

What is the Chandrasekhar limit?

Accepted Answer

The Chandrasekhar limit is the mass limit to the total mass of a white dwarf, beyond which the electron degeneracy pressure will be overcome and the core will collapse, leading to a supernova explosion. It is named after the Indian astrophysicist Subrahmanyan Chandrasekhar. The current value of the Chandrasekhar limit is about 1.4 solar masses.

Question 2

The estimated rate at which supernova explosions occur in a spiral galaxy such as our own is about once every _____ years.

Accepted Answer

The answer of The estimated rate at which supernova explosions...

Question 3

A Type II supernova is the:

Accepted Answer

A Type II supernova is the explosion of a massive star after it has exhausted all its nuclear fuel and its core has reached a state where it cannot support itself against its own gravitational collapse. The collapse produces an extremely dense core of mostly neutrons, known as a neutron star or possibly a black hole. This collapse sends a shock wave through the rest of the star, causing it to explode in a powerful supernova. The fusion of silicon in the core produces a core made up of iron nuclei, which is why it cannot support itself and collapses.

Question 4

In this chapter,we consider high-mass stars.What is considered to be the lower mass limit for a high-mass star?

Accepted Answer

High-mass stars are defined as those that have a mass greater than 8 times the mass of our Sun, making 8 M the lower mass limit for high-mass stars.

Question 5

A nova is an explosion involving a white dwarf.Can a white dwarf become a nova more than once? Why or why not?

Accepted Answer

A white dwarf can indeed become a nova multiple times if it is part of a binary system where it accretes matter from its companion star. When enough material accumulates on the surface, it can trigger a thermonuclear explosion, seen as a nova. This process can repeat if the white dwarf continues to accrete matter.

Question 6

Measurements from distant galaxies indicate that supernovae should occur at a rate of five per century in a spiral galaxy such as the Milky Way,but only three have been recorded in this Galaxy in the past 1000 years.Explain.

Accepted Answer

The dense gas and dust in the Milky Way plane would have prevented us from observing most supernovae in the Galaxy, which would make it appear as if the rate of supernovae is lower than expected.

Question 7

Astronomers believe the binary companions of the white dwarfs which produce Type Ia supernovae are probably:

Accepted Answer

The answer of Astronomers believe the binary companions of the...

Question 8

Type II supernovae show prominent lines of hydrogen in their spectra,whereas hydrogen lines are absent in spectra of Type Ia supernovae.Explain.(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 have significant amounts of hydrogen in their outer layers, visible in their spectra. Type Ia supernovae, however, originate from white dwarfs in binary systems reaching a critical mass and exploding, and these white dwarfs are composed mostly of carbon and oxygen, with no hydrogen lines in their spectra.

Question 9

From observations of supernova explosions in distant galaxies,it is predicted that there should be about five supernovae per century in our Galaxy,whereas we have seen only about one every 300 years from Earth.Explain.

Accepted Answer

Interstellar dust in the galactic plane has obscured our view of most supernovae in our Galaxy, leading to a lower number of observed supernovae compared to the predicted rate.

Question 10

There is a mass limit for a star in the white-dwarf phase,the Chandrasakhar limit,beyond which the electron degeneracy pressure can no longer support the star against its own gravity.This limit is:

Accepted Answer

The Chandrasakhar limit is 1.4 solar masses. Beyond this limit, electron degeneracy pressure is unable to prevent gravitational collapse, leading to a supernova explosion and the formation of a neutron star or black hole. Options A and B are incorrect as they are either too low or too high for the Chandrasakhar limit. Option D is also incorrect as it is the upper mass limit for a neutron star, not the Chandrasakhar limit.

Question 11

The spectrum of a Type Ia supernova can be identified because it lacks evidence of:

Accepted Answer

Type Ia supernovae are produced by exploding white dwarf stars, which lack hydrogen. Therefore, their spectra do not show evidence of hydrogen.

Question 12

Suppose you see a news announcement that astronomers have discovered a nova in a distant galaxy.What do you immediately know about the star that gave rise to this nova?

Accepted Answer

A nova occurs in a binary system where one star is a white dwarf and the other star is either a main-sequence star or a giant star. The white dwarf accretes material from the companion star until a runaway nuclear reaction occurs on its surface, causing a sudden and intense increase in brightness. Therefore, option A is the only likely option. The other options involve a neutron star or a black hole, which are not associated with novae.

Question 13

What result might you expect in a binary system containing a white dwarf with a mass of about 1.4 M?

Accepted Answer

A white dwarf with a mass of about 1.4 M (known as the Chandrasekhar limit) is unstable and can undergo a Type Ia supernova. In this process, the white dwarf accretes matter from its companion star until it reaches a critical mass and ignites a runaway fusion reaction that destroys the star entirely.

Question 14

Why do astronomers believe the binary companions of the white dwarfs that produce Type Ia supernovae are MOST probably main-sequence stars?

Accepted Answer

The answer of Why do astronomers believe the binary companions...

Question 15

Which observation has provided the MOST direct evidence that Type II supernovae are caused by the collapse of the core of a massive star?

Accepted Answer

Neutrinos were detected from supernova SN 1987A, providing direct evidence for the collapse of the core of a massive star during a Type II supernova. Neutrinos are produced during the collapse and subsequent explosion of the star's core, and their detection was a strong confirmation of this mechanism. The other options listed provide indirect evidence or are unrelated to the collapse of massive star cores.

Question 16

At the center of the remnant of a Type Ia supernova you would expect to find:

Accepted Answer

Type Ia supernovae occur when a white dwarf in a binary system accretes enough material from its companion to reach a critical mass and explode. The explosion completely disrupts the white dwarf, leaving no remnant like a black hole or neutron star.

Question 17

Can a white dwarf explode?

Accepted Answer

A white dwarf can explode as a Type Ia supernova if it is in a binary system and accretes enough mass from its companion to reach the Chandrasekhar limit, leading to a runaway nuclear reaction.

Question 18

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

Accepted Answer

The main observational difference between a Type Ia and a Type II supernova is that hydrogen lines are prominent in the spectrum of a Type II supernova but absent in that of a Type Ia. This is because Type II supernovae originate from the explosion of massive stars, which still have hydrogen in their outer layers, while Type Ia supernovae arise from the explosion of white dwarfs, which have already shed their outer hydrogen layers.

Question 19

A Type Ia supernova is the:

Accepted Answer

A Type Ia supernova occurs when a white dwarf in a binary star system accretes mass from its companion star, causing the white dwarf to reach a critical mass and ignite a runaway fusion reaction that destroys the star. This type of supernova does not involve the collapse of a massive star or a helium flash in a red giant.

Question 20

What result might you expect in a binary system containing a white dwarf with a mass considerably below 1.4 M?

Accepted Answer

A white dwarf with a mass below 1.4 M is not massive enough to undergo a Type Ia supernova, which requires the white dwarf to reach the Chandrasekhar limit. However, if the white dwarf is in a binary system and accretes matter from its companion star, it can reach a temperature and density high enough to trigger a runaway fusion reaction and result in a nova.

Quiz 12: The Deaths and Remnants of Stars