ASTR WEEK 10

Conditions inside of and near a black hole can be described using Einstein's general theory of relativity. The theory's basic premise is that matter curves spacetime, and this curvature tells matter how to accelerate. Based on general relativity, how do black holes affect spacetime, matter, and radiation in their region?

- Matter and radiation that fall inside a black hole's event horizon can no longer be detected by an outside observer. - Around a black hole and within the event horizon, spacetime is curved to the extent that space folds over on itself. - As matter and radiation approach a black hole, they react to the curvature of spacetime by significantly changing their direction of motion

According to the figure, a meter stick in a spaceship traveling at half the speed of light would appear to have a length of

0.87 meter

Which of the following is NOT an observational feature associated with the binary-star system Cygnus X-1, which is suspected of containing a black hole?

A small, dark region in the space near the visible companion indicates that light cannot escape from that region, which is the characteristic feature of a black hole.

Although we cannot directly observe black holes, we can use theoretical models and Einstein's general theory of relativity to hypothesize about the structure of black holes. Label the main parts of a black hole on the figure below.

Arrow: Schwarzchild radius Center: Singularity Outline of Circle: Event horizon

Detecting a black hole is a challenging endeavor for astronomers. Why is it so difficult for astronomers to observationally detect black holes?

Black holes have an escape speed that is greater than the speed of light.

Stellar-sized black holes form when a star explodes in a supernova, leaving behind enough material that collapses to a distance within the black hole's Schwarzschild radius. As the core of the star collapses, its gravity becomes so strong that nothing—not even light—can escape from it. Sort the following stars according to whether they could or could not collapse into black holes.

Could collapse into a black hole: - Eta Carinae (100 solar masses) - XI Persei (40 solar masses) Could not collapse into a black hole - The Sun (1 solar mass) - Betelgeuse (18 solar masses) - Sirius A (2 solar masses) - Spica A (11 solar masses)

If the Sun were magically to turn into a black hole of the same mass,

Earth's orbit would remain unchanged.

What process is responsible for producing the rapid spin rate of millisecond pulsars?

Gas spiraling in from a nearby companion transfers angular momentum to the pulsar, increasing the rate of its rotation.

What will happen to an isolated neutron star that accumulates more than about 3 solar masses of material?

Gravity will overcome the neutron degeneracy in its interior and form a black hole.

Why is light increasingly redshifted near a black hole?

It must expend energy to escape the gravitational pull of the black hole.

X-ray bursters are caused by a process similar to the process in which other object?

Nova

Why aren't all young neutron stars seen as pulsars?

Only some neutron stars are oriented so that their beams sweep in the direction of Earth.

At what stage of its life will our Sun become a black hole?

The Sun will never become a black hole.

The properties of neutron stars help explain their relationship to pulsars. Which of the following is (are) important when explaining how a pulsar generates the radiation we detect?

The magnetic field lines in a neutron star are squeezed close together, creating an extremely intense magnetic field. Neutron stars can rotate extremely rapidly, as quickly as 30 times a second or more.

If the Sun were replaced by a 1-solar-mass black hole, what would be the gravitational pull of the black-hole "Sun" on Earth?

The same

Why does the existence of planets around a millisecond pulsar come as a surprise?

The supernova explosion that formed the pulsar would have blown away any planets originally there.

What do the observed locations of gamma-ray bursts tell us about them?

They must be extra-galactic because they occur everywhere in the sky.

Which of the following is NOT a reason that observable pulsars are found at the centers of some, but not all, supernova remnants?

Type II core-collapse supernovae do not leave behind rotating neutron stars.

A neutron star is about the same size as

US city

Listed following are distinguishing characteristics of different end states of stars. Match these to the appropriate consequence of stellar death.

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

Which of the following is NOT a characteristic feature of a neutron star?

a radius about the size of the Earth's

If there is a black hole in a binary system with a blue supergiant star, the X-ray radiation we may observe would be due to the

accretion disk of material falling into the black hole

Gamma-ray bursts are observed to occur

approximately uniformly over the entire sky.

What effect predicted by Einstein's theory of general relativity is confirmed during a solar eclipse?

deflection of light from distant stars by the gravitational field of the Sun

What keeps light (and all other forms of radiation) from escaping a black hole?

gravity

The best place to search for black holes is in a region of space that

has strong X-ray emission.

The X-ray emission from a neutron star in a binary system comes mainly from

heated material in an accretion disk around the neutron star.

Why do black holes emit neither radiation nor other information? The answer to this question is related to the concept of escape speed—the speed necessary for one object to escape the gravitational pull of another object. The escape speed from an object depends entirely on its mass and radius. Consider a star of a given mass. If the star remains the same mass, but decreases in radius, its escape speed will _____.

increase

A neutron star's immense gravitational attraction is due primarily to its small radius and

large mass

Black holes result from stars having initial masses

more than 25 times the mass of the Sun.

What have astronomers detected in the center of the Crab Nebula?

neutron star

Black holes...

prevent anything inside the event horizon from escaping

The best evidence for supermassive black holes in the centers of galaxies is

rapid gas motion and intense energy emission.

The most rapidly "blinking" pulsars are those that

spin fastest

Neutron stars and pulsars are associated with

the collapse and supernova explosion of massive stars

What defines the event horizon of a black hole?

the radius at which the escape speed equals the speed of light