February Exam Review

Question 1

where are stars born? and why?

Answer 1

stars are born in cold, relatively dense molecular clouds.

because they are cold enough for molecular hydrogen to form.

Question 2

what is a protostar?

Answer 2

a compact clump of gas that will eventually become a star.

Question 3

how are protostars created?

Answer 3

gravitational contraction of a molecular cloud fragment can create a protostar.

Question 4

what usually surrounds a contracting protostar?

Answer 4

often by a protostellar disk circling its equator.

Question 5

summarize the “pre-birth” stages of a star’s life.

Answer 5

1) protostar assembles from a cloud fragment and is bright in infrared light because gravitational contraction rapidly transforms potential energy into thermal energy.
2) luminosity decreases as gravitational contraction shrinks protostar’s size
3) core temperature and rate of fusion gradually rise until energy production through fusion balances the rate at which the protostar radiates energy into space. At the point, the forming star becomes a main-sequence star.

Question 6

what are the major phases of life of a low-mass star?

Answer 6

main sequence, in which the star generates energy by fusing hydrogen in the core.
red giant, with hydrogen shell-burning around an inert helium core.
helium-core burning, along with hydrogen shell-burning (star on horizontal branch on HR diagram)
Planetary nebula, leaving a white dwarf behind.

Question 7

how did past red-giant stars contribute to the existence of life on earth?

Answer 7

red giants created and released much of the carbon that exists in the universe, including the carbon that is the basis of organic molecules on Earth.

Question 8

what prevents carbon from fusing to heavier elements in low-mass stars?

Answer 8

electron degeneracy pressure counteracts the crush of gravity, preventing the core of a low-mass star from ever getting hot enough for carbon fusion.

Question 9

what determines the fate of a stellar core that has exhausted all its nuclear fuel?

Answer 9

a star’s final state depends on whether degeneracy pressure can halt the crush of gravity. A white dwarf is supported by electron degeneracy pressure. A neutron star is supported by neutron degeneracy pressure. If neutron degeneracy pressure cannot halt the collapse, the core becomes a black hole.

Question 10

what is a white dwarf?

Answer 10

a white dwarf is the inert core left over from a low-mass star, supported by electron degeneracy pressure.

Question 11

what is a white dwarf supported by?

Answer 11

electron degeneracy pressure.

Question 12

why can’t white dwarfs weigh more than 1.4 times the mass of the sun?

Answer 12

at masses greater than 1.4m, the white dwarf can no longer support its own weight with electron degeneracy pressure. The electron would have to “move” faster than the speed of light, which is physically impossible. So white dwarfs that become more massive than 1.4m must collapse.

Question 13

what is a nova?

Answer 13

a star showing a sudden large increase in brightness and then slowly returning to its original state over a few months.

Question 14

explain how a nova happens?

Answer 14

a white dwarf in a binary system can acquire hydrogen from its companion, which swirls toward the surface in an accretion disk. If enough hydrogen rains down on the white dwarf, the surface hydrogen layer will become so hot that it will ignite with nuclear fusion, essentially making a thermonuclear flash in which the star shines as brightly as 100,000 suns for a few weeks.

Question 15

a nova shines as brightly as how many suns?

Answer 15

100,000.

Question 16

what are white dwarf supernovae and what type is it?

Answer 16

type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf. It is type Ia.

Question 17

why are white dwarf supernovae good for measuring gigantic distances?

Answer 17

because they are nearly identical light curves, and they are so bright that they can be seen across the universe. We measure their distances from their apparent brightness in our sky.

Question 18

what is a neutron star?

Answer 18

the ball of neutrons created by the collapse of the iron core in a massive star supernova. It is like a giant atomic nucleus 10 km across but more massive than the sun.

Question 19

what is a pulsar?

Answer 19

rapidly rotating neutron star, that emits regular pulses of radio waves and other electromagnetic radiation at rates of up to one thousand pulses per second.

Question 20

what happens when a pulsar’s magnetic poles do not align with the poles of the rotation?

Answer 20

the beamed radiation from the hot spots sweep through space like a lighthouse and if it beams across the earth, we see it periodically appear and disappear - in pulses.

Question 21

how were neutron stars discovered?

Answer 21

they spin rapidly when they are born, and their strong magnetic fields can direct beams of radiation that sweep through space as the neutron star spins. We see such neutron stars as pulsars, and these pulsars provided the first direct evidence for the existence of neutron stars.

Question 22

what can happen to a neutron star in a close binary system?

Answer 22

neutron stars in close binary systems can accrete hydrogen from their companions, forming dense, hot accretion disks. The hot gas emits strongly in X rays, so we see these systems as X-ray binaries. In some of these systems, frequent bursts of helium fusion ignite on the neutron star’s surface, emitting X-ray bursts.

Question 23

what is a black hole?

Answer 23

a place where gravity has crushed matter into oblivion, creating a true hole in the universe from which nothing can escape, not even light.

Question 24

what would it be like to visit a black hole?

Answer 24

you could orbit a black hole just like any other object of the same mass. However, you’d see strange effects for an object falling toward the black hole:

• time would seem slow for the object
• its light would be increasingly redshifted as it approached the black hole.
• the object would never quite reach the event horizon, but it would soon disappear from view as its light became so redshifted that no instrument could detect it.

Question 25

do black holes really exist?

Answer 25

no known force can stop the collapse of a stellar corpse with a mass above the neutron star limit of 2 to 3 solar masses, and theoretical studies of supernovae suggest that such objects should sometimes form. Observational evidence supports this idea.

Question 26

what is the maximum mass of a neutron star?

Answer 26

the max of a neutron star is about 3, it is determined by the maximum amount of mass that neutron degeneracy pressure can support.

Question 27

do black holes suck?

Answer 27

no not at large distances, if our Sun were replaced by a black hole, Earth would not be sucked into the black hole and the planets would continue to orbit normally.

Question 28

what property of a black hole determines its "size"?

Answer 28

depends on its mass, because the mass determines the size of the black hole's event horizon (schwarzschild radius), the boundary of the region from which not even light can escape.

Question 29

what would happen if you watched someone falling into a black hole?

Answer 29

you would see time slow down for them as they approached the black hole, and their light would be increasingly redshifted. They would not ever quite reach the event horizon, though they would soon disappear from view as their light became so redshifted that no instrument could detect it.

Question 30

state several ways in which high-mass stars differ from low-mass stars?

Answer 30

High-mass stars live much shorter lives than low-mass stars. High-mass stars have convective cores but no other convective layers, while low-mass stars have convection layers that can extend from their surface to large depths. Radiation supplies significant pressure support within high-mass stars, but this form of pressure is insignificant within low-mass stars. High-mass stars die in supernovae, while low-mass stars die in planetary nebulae. Only high mass stars can fuse elements heavier than carbon. A high-mass star may leave behind a neutron star or a black hole, while a low-mass star leaves behind a white dwarf. High-mass stars are far less common than low-mass stars.

Question 31

How do high-mass stars produce elements heavier than carbon?

Answer 31

How do high-mass stars produce elements heavier than carbon?

Question 32

What causes a supernova?

Answer 32

As a high-mass star ages, carbon and heavier elements can fuse to form ever heavier elements. Shells of increasingly heavy element fusion are created, like onion skins inside the star. However, since fusion of iron uses up energy instead of releasing energy, an iron core cannot support the weight of the outer layers. The collapse of this core — which occurs in a fraction of a second — results in a supernova that nearly obliterates the star (perhaps leaving a black hole or a neutron star).

Question 33

What is the primary topic of the general theory of relativity?

Answer 33

The general theory of relativity is primarily a theory of gravity, stating that the force of gravity arises from distortions of spacetime.

Question 34

What is spacetime?

Answer 34

Spacetime is the four-dimensional combination of space and time that forms the “fabric” of our universe.

Question 35

What is the equivalence principle?

Answer 35

The effects of gravity are exactly equivalent to the effects of acceleration.

Question 36

What do we mean by dimensions?

Answer 36

Each dimension represents an independent direction of possible motion. In three-dimensional space, the three dimensions of length, width, height are perpendicular to one another. A four-dimensional space has a fourth dimension perpendicular to all three of the others. We cannot visualize this, but it can still exist.

Question 37

How does mass affect spacetime?

Answer 37

Mass causes spacetime to curve, and the curvature of spacetime determines the paths of freely moving masses.

Question 38

How would an ordinary star, a white dwarf, and a black hole of the same mass differ in spacetime?

Answer 38

Because all three objects have the same mass, far from their surfaces they would affect spacetime in precisely the same way. Up close, however, the white dwarf would distort spacetime much more than the ordinary star, and the black hole would distort spacetime so much that it essentially would form a bottomless pit—a true hole in the universe.

Question 39

How have experiments and observations verified the predictions of the general theory of relativity?

Answer 39

Observations of the change of Mercury’s orbit match that predicted by Einstein’s theory. Observations of stars during eclipses and photos of gravitational lensing provide spectacular confirmation of the idea that light can travel curved paths through space. Gravitational redshifts observed in the light of objects with strong gravity confirm the slowing of time predicted by general relativity.

Question 40

How do we measure the age of a star cluster?

Answer 40

Main-sequence turnoff Massive blue stars die first, followed by white, yellow, orange, and red stars Main-sequence turnoff point of a cluster tells us its age.

Question 41

What are the two types of star clusters?

Answer 41

Open clusters contain up to several thousand stars and are found in the disk of the galaxy. Globular clusters contain hundreds of thousands of stars, all closely packed together. They are found mainly in the halo of the galaxy.

Question 42

How do we measure the age of a star cluster?

Answer 42

Because all of a cluster’s stars we born at the same time, we can measure a cluster’s age by finding the main sequence turnoff point on an H–R diagram of its stars. The cluster’s age is equal to the hydrogen-burning lifetime of the hottest, most luminous stars that remain on the main sequence.