Astro 150 Spring 2024 Exam 3 Review (April 12, 2024)

When a star FIRST becomes a red giant, what's happening in its core?

• It's fusing hydrogen

• It's fusing helium

• It's compressing

• It's expanding

Answer explanation

A red giant star starts to become a red giant because the core is no longer fusing, but it's compressing, causing it to heat up and expand. Once it has heated up enough, hydrogen fusion will start in a shell around the core. The core will not fuse helium until later.

How can we see white dwarfs if they're so small?

• They're very hot

• Their gravity influences their companion stars

• Background light bends around them

• We only see them when they explode, and they explode violently

Which mass range of main sequence stars become neutron stars?

• < 8 Solar masses

• Between 8 and 25 solar masses

• Greater than 25 solar masses

• Greater than 50 solar masses

Answer explanation

Stars less than 8 solar masses become white dwarfs, stars between 8 and 25 solar masses become neutron stars. Stars greater than 25 solar masses become black holes.

Why are black holes only formed from the most massive stars?

• Only the most massive stars make big enough supernovae

• Only the heaviest elements made in stars can make black holes

• No other stars evolve fast enough

• Only the most massive stars can overcome neutron degeneracy

Answer explanation

Stars over 25 solar masses are so massive that when they collapse, not even neutron degeneracy pressure (the same pressure that holds neutron stars up) can't resist the collapse, and so the star collapses infinitely until it becomes a black hole.

The escape velocity of a black hole at the event horizon is...

• The speed of sound

• Twice the speed of sound

• The speed of light

• Twice the speed of light

If the sun were to suddenly become a black hole of the same mass as the sun, what would happen to the Earth's orbit?

• Nothing

• It would decay and Earth would fall in

• Earth would orbit farther away

• Earth would fly off

Answer explanation

Orbital parameters are decided almost entirely by the mass of the central body (if that central body is much more massive than what is orbiting it). So, if the sun becomes a black hole the same mass as the sun, the Earth's orbit shouldn't change.

How come light is affected by gravity?

• Light has a very tiny mass

• It's not affected by gravity

• Gravity distorts spacetime

Answer explanation

Light rides on spacetime, and objects with mass distort spacetime. So, when light encounters those distortions, its path of travel is affected.

There is a GPS satellite in orbit around Earth and a model one in a museum on Earth's surface. After one hundred years, which is older?

• The one in orbit

• The one on the surface

• They age the same

• It depends on how many devices ping the satellite

Answer explanation

According to the lecture slides, clocks in Earth's orbit tick 45 millionths of a second faster than on Earth.

Which of these will NOT happen to an astronaut that falls into a black hole?

• They'll turn redder

• They'll age more slowly

• They'll relay what the see inside the black hole

• They'll get stretched out

Answer explanation

No information can escape a black hole. Once the astronaut falls in, there's no way they can communicate with the outside again.

How do we detect black holes orbiting each other?

• They pulse as they orbit each other

• We'll only see the explosion when they merge

• We can see the light from background stars ripple from the black holes' gravity

• We detect their gravitational waves

Answer explanation

Black holes emit no light, BUT black holes orbiting each other emit gravitational waves, which we can detect with instruments like LIGO.