Astronomy Ch.13 & Ch.14

How does it compare with the amount of energy released by the Sun during its entire main-sequence lifetime?

(EsupernovaExplosion/ESunTotal)~10^2

Provided following are various stages during the life of a high-mass star. Rank the stages based on when they occur, from first to last.

- contracting cloud of gas and dust - protostar - main-sequence O star - red supergiant - supernova - neutron star

Provided following are various elements that can be produced during fusion in the core of a high mass main sequence star. Rank these elements based on when they are produced, from first to last.

- helium - carbon - oxygen - iron

he Chandra X-Ray Observatory has detected X rays from a star system that contains a main-sequence star of spectral type B6. The X-ray emission is strong and fairly steady, and no sudden bursts have been observed. Which of the following statements are reasonable conclusions about this system? Check all that apply. -The main-sequence star must orbit a white dwarf. -The main-sequence star orbits either a white dwarf or a neutron star. -The main-sequence star must orbit a black hole. -The main-sequence star is emitting X rays. -The main-sequence star orbits either a neutron star or a black hole. -The main-sequence star must orbit a neutron star. -Gas from the main-sequence star makes an accretion disk around another object. -Some time in the next few decades, this system will undergo a nova explosion.

-The main-sequence star orbits either a neutron star or a black hole. -Gas from the main-sequence star makes an accretion disk around another object.

Which of these objects has the smallest radius? -a 1.2 MSun white dwarf -a 0.6MSun white dwarf -Jupiter

-a 1.2 MSun white dwarf

The Sun's main sequence lifetime is

1.02 x 10^10 years

How old must a star cluster be if it no longer contains any O stars (stars with a surface temp. above 30,000 K)?

10 billion years old

You observe a star cluster and find the hottest star in the cluster is 20,000 K (corresponding to a B-type star). The age o f the cluster is

10 million years.

Determine the main-sequence lifetime of a Type A star (having the surface temp. of 10,000 K). What is the main-sequence lifetime?

5 x 10^8 years

When the core collapses, a hydrogen-burning shell forms. Fusion begins here because this shell heats up as the core collapses. Why does the shell heat up?

As the core and surrounding gas collapse, potential energy is converted into heat.

Once the escape velocity of an object exceeds the speed of light, that object is a [...].

Black Hole

If Jupiter turned into a black hole,

Earth would continue orbiting the Sun, unaffected by this event.

What would happen if the Sun suddenly became a black hole without changing its mass?

Earth's orbit would not change.

Why can the core temperature of a high-mass star nearing the end of its life become much greater than that of a low-mass star?

Gravitational contraction can release more thermal energy in the core of a massive star.

Why is the min. temp. at which helium can fuse higher than that for hydrogen?

Helium nuclei have higher charges, so they repel each other more strongly than hydrogen nuclei.

Listed following are characteristics that describe either high-mass or low-mass stars. Match these characteristics to the appropriate category.

High Mass Stars: -late in life fuse carbon into heavier elements -end life as a supernova -have higher fusion rate during main sequence life Low Mass Stars: -the Sun is an example -have longer lifetimes -end life as a planetary nebula -final corpse is a white dwarf

What are the phases of life for a Main Sequence Star?

Hydrogen-burning shell, Red giant, Helium-burning, Double-shell burning, Planetary nebula, Cooling white dwarf

The sun is now a double-shell-burning red giant. What happens to the star?

It gets brighter, bigger, and cooler.

If the position of the cannon does not change, how does the escape velocity depend on the radius of the planet?

It remains the same regardless of the planet radius.

Viewed from a distance, how would a flashing red light appear as it fell into a black hole?

Its flashes would shift to the infrared part of the spectrum.

From Part B, you know that from afar we'll never see the in-falling rocket cross the event horizon, yet it will still eventually disappear from view. Why?

Its light will become so redshifted that it will be undetectable.

What would happen to a neutron star with an accretion disk orbiting in a direction opposite to the neutron star's spin?

Its spin would slow down.

What is the key observation needed to determine whether the compact object in Part C is a neutron star or a black hole?

Measure Doppler shifts in the spectrum of the main-sequence star so that you can determine the mass of the compact object.

The following items describe observational characteristics that may indicate that an object is either a neutron star or a black hole. Match each characteristic to the correct object; if the characteristic could apply to both types of object, choose the bin labeled "Both neutron stars and black holes."

NS: may emit rapid pulses of radio waves. may be in a binary system that undergoes X-ray bursts. BH: is detectable only if it is accreting gas from other objects. can have a mass of 10 solar masses. Both: may be located in an X-ray binary. may be surrounded by a supernova remnant.

Why doesn't a high-mass star fuse its iron into heavier elements, like uranium?

No rest-mass energy is released due to fusion of iron.

Each item below describes an observation of a hypothetical supernova. Classify each observation as either "Not surprising" if it fits in with our current understanding of supernovae, or "Surprising" if the observation would cause us to rethink our understanding of supernovae.

Not surprising: A white dwarf supernova in a galaxy of only old stars. Two massive star supernovae occur in the same young star cluster. A massive star supernova leaves behind no detectable compact object. A massive star in a binary system explodes. Surprising: An isolated star like our Sun explodes as a white dwarf supernova. A young (5 million years) star explodes as a white dwarf supernova.

Type of Main Sequence with shortest lifetime

O stars

For the Earth, where is the force of gravity the highest?

On the surface

If you could measure the orbital speeds of particles in an accretion disk around a black hole, you would notice that:

Particles near the center are moving fastest.

The distance from the center of the black hole to the event horizon id called the...

Schwarzschild radius

Why are red giant stars rare compared to main-sequence stars?

The amount of time a red giant lasts is small compared to the lifespan of a main-sequence star.

In the dying stages of our Sun, it will fuse helium into carbon. After that what will occur?

The core contracts, but the carbon never fuses.

For distances greater than the Earth's radius, how does the force of gravity due to the Earth compare to that of the black hole?

The forces are the same.

Why did the helium flash cause the core to expand?

The increased temperature in the core caused the pressure to increase.

Why does the core collapse once fusion stops?

The pressure in the core can no longer support the weight of the overlying layers.

Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each word only once.

The radius of a white dwarf is determined by a balance between the inward force of gravity and the outward push of "electron degeneracy pressure." A(n) "nova" occurs when hydrogen fusion ignites on the surface of a white dwarf in a binary system. A(n) "massive star supernova" occurs when fusion creates iron in the core of a star. A white dwarf in a close binary system will explode as a supernova if it gains enough mass to exceed the "white dwarf limit (1.4 solar masses)". A(n) "accretion disk" consists of hot, swirling gas captured by a white dwarf (or neutron star or black hole) from a binary companion star. A(n) "white dwarf supernova" can occur only in a binary system, and all such events are thought to have the same luminosity.

Why does the sun's luminosity increase when shell burning begins?

The rate of hydrogen fusion is higher than during the main-sequence stage.

Fusion is now occurring in the core as well as in a shell. Yet the sun's luminosity has decreased, indicating that the rate of fusion has fallen. Why?

The temperature in the hydrogen-burning shell has decreased.

From the viewpoint of an observer in the orbiting rocket, what happens to time on the other rocket as it falls toward the event horizon of the black hole?

Time runs increasingly slower as the rocket approaches the black hole.

The ball's trajectory does not depend on the size of the planet, but only on its mass and the distance between its center and the cannon. True or false?

True

Match the items below with the correct type of supernova.

WD: Can only occur in a binary system. Spectra always lack strong hydrogen lines. Can occur in a very old star cluster. Star explodes completely, leaving no compact object behind. Has a brighter peak luminosity. MSS: Black hole or neutron star left behind. Can only occur in a galaxy with ongoing star formation.

The following items describe observational characteristics that could indicate that an object is either a white dwarf or a neutron star. Match each characteristic to the correct object.

WD: may be surrounded by a planetary nebula. emits most strongly in visible and ultraviolet. may be in a binary system that undergoes nova explosions. NS: may be in a binary system that undergoes X-ray bursts. can have a mass of 1.5 solar masses. may be surrounded by a supernova remnant. may repeatedly dim and brighten more than once per second.

Which of the following is true about observational evidence for black holes?

We have inferred the presence of super-massive black holes in many galaxies

What makes us think that black holes really exist?

We have observed orbital motions around objects so compact that according to current understanding, they must be black holes.

Match these to the appropriate consequence of stellar death.

White Dwarf: -in a binary system, it can explode as a supernova -has a mass no greater than 1.4MSun - typically about the size (diameter) of Earth - supported by electron degeneracy pressure Neutron Star: -usually has a very strong magnetic field -sometimes appears as a pulsar Black Hole: -size defined by its Schwarzschild radius -viewed from afar, time stops at its event horizon,

Which of these objects has the largest radius?

a 1.2 MSun white dwarf

Which of these black holes exerts the weakest tidal forces on an object near its event horizon?

a 10^6 MSun black hole

A supernova occurs when:

a massive star burns through successively heavier elements until it has an iron core, which will not release energy from fusion.

The event horizon of a 3-solar-mass black hole is roughly the same size as

a small town

Listed following are several astronomical objects. Rank these objects based on their mass, from largest to smallest. (Be sure to notice that the main-sequence star here has a different spectral type from the one in Part A.)

a typical black hole (formed in a supernova), a typical neutron star, a one-solar-mass white dwarf, main-sequence star of spectral type M, Jupiter, the Moon

If we see a nova, we know that we are observing

a white dwarf in a binary system.

If a star is twice as massive as the Sun, it's main sequence lifetime will be:

about 15% that of the sun

Which of these binary systems is most likely to contain a black hole?

an X-ray binary containing an O star and another object of equal mass

Which of these isolated neutron stars must have had a binary companion?

an isolated pulsar that pulses 600 times per second

If you were inside the rocket that falls toward the event horizon, you would notice your own clock to be running __________.

at a constant, normal rate as you approach the event horizon

If the Earth miraculously turned into a black hole without losing or gaining mass, a cannonball orbiting it would

continue in the same orbit.

After helium begins to fuse in the core, the Sun's radius [...] and its surface temperature [...].

decreases, increases

Where do gamma-ray bursts tend to come from?

extremely distant galaxies

Main Sequence Star stay alive as long as they

fuse hydrogen and helium in their core

The event horizon for a 10-solar-mass black hole is

larger than that of a 1-solar-mass black hole

Less massive stars will [...]; more massive stars will [...].

live longer, die quicker

Listed following are several astronomical objects. Rank these objects based on their diameter, from largest to smallest. (Note that the neutron star and black hole in this example have the same mass to make your comparison easier, but we generally expect black holes to have greater masses than neutron stars.)

main-sequence star of spectral type A, Jupiter, a one-solar-mass white dwarf, the Moon, a two-solar-mass neutron star, the event horizon of a two-solar-mass black hole

If all stars in a young star cluster formed at about the same time and you were to observe the star cluster over several billion years, you would see:

many supernovae early on, and then no more.

As the falling rocket plunges toward the event horizon, an observer in the orbiting rocket would see that the falling rocket __________.

slows down as it approaches the event horizon, and never actually crosses the event horizon

Some X-ray binaries are candidates for black holes because

the object in the center of the accretion disk is too massive to be a neutron star.

Listed following are several astronomical objects. Rank these objects based on their density, from highest to lowest.

the singularity of a black hole, a typical neutron star, a one-solar-mass white dwarf, a main-sequence star

Why are massive stars short-lived?

they guzzle fuel so quickly that they run out of gas soon despite their large gas tanks

In a massive star supernova explosion, a stellar core collapses down to form a neutron star roughly 10 kilometers in radius. The gravitational potential energy released in such a collapse is approximately equal to [(GM)^2]/r where M is the mass of the neutron star, r is its radius, and G=6.67×10^−11m^3/kg×s^2 is the gravitational constant. Using this formula, estimate the amount of gravitational potential energy released in a massive star supernova explosion.

~ 10^47 jouls