Week 9

Question 1

What stars produce black holes?

Answer 1

The death of very massive stars (>20 Msun)

Question 2

What causes black holes?

Answer 2

when gravity is stronger than any pressure
Space time continuum is torn!
(singularity is created)

Question 3

Why are black holes black?

Answer 3

Because gravity is so strong even light cannot escape!

Question 4

Escape velocity

Answer 4

The velocity needed to escape the gravity of the orbitee entirely
v(escape) =squareroot(2GM/r)

Question 5

Escape velocity and black holes

Answer 5

Decrease radius at constant central mass increases escape velocity, decrease it enough and the escape velocity becomes greater than the speed of light and not even light can escape the hole

Question 6

event horizon

Answer 6

edge of black hole

Question 7

can our sun become a black hole?

Answer 7

No, Only much more massive stars
have enough gravity to collapse to BH after core fusion
ends.

Question 8

BH gravity field

Answer 8

If a star (or any object) became a black hole, its

gravity would be different only near the event horizon

Question 9

Supermassive black holes event horizon

Answer 9

gentler near the event horizon because Schwarschild radius is much bigger

Question 10

Black hole tidal force

Answer 10

Tidal force near event horizon for a stellar Black Hole is very strong, will shred/stretch stars due to more gravity being felt on 1 side of the object then the other

Question 11

small vs large black whole tidal force effect

Answer 11

• Small black holes shred their food

- Large black holes swallow whole

Question 12

Time dilation near event horizon

Answer 12

Time passes more slowly because of strong gravitational field

Question 13

How do you observe black holes if you can’t see them?

Answer 13

• watch things move near them (dynamical mass)
• watch matter fall into them (and emit light)
• Look for remaining star orbiting something invisible

Question 14

Gravitational lensing by Black Holes

Answer 14

BHs distort space-time
strongly near event horizon:
Extreme light bending!

Question 15

gravitational waves

Answer 15

• ‘Ripples’ in the fabric of space-time caused by violent and energetic processes (e.g. black hole collisions)
• Travel at the speed of light, carrying energy and angular momentum

Question 16

What do gravitational waves do?

Answer 16

Gravity wave stretch/compress space as they

travel through it

Question 17

easons we have to infer that a supermassive blackhole lurks in the centre of our galaxy?

Answer 17

Tight orbit of stars around an invisible companion & giant bubbles filled with gamma rays expelled from the galactic centre.

Question 18

Description of the milky way

Answer 18

A disk 100,000 lightyears across filled with gas and stars, with a bulge of older stars in the galaxy centre.

Question 19

Spiral galaxy

Answer 19

disk shaped, often have spiral arms

Question 20

Elliptical galaxy

Answer 20

balls (sometimes flattened) of mostly old stars

Question 21

Irregular galaxy

Answer 21

weird clumpy galaxies with no obvious symmetry, often small

Question 22

An important part of the lifecycle of galaxies like the MW is the self-regulation of formation of future generations of stars. Which statement best describes this process?

Answer 22

Massive stars explode as Supernovae, heating nearby gas which then can’t form stars, and even forcing the gas out of the galaxy in a superbubble.

Question 23

Sort based on mass contribution the following three components of the Milky Way, by most significant to least significant.

Answer 23

Dark Matter > Stars > Planets

Question 24

When we observe the most distant galaxies what are we looking at?

Answer 24

because of the finite speed of light, we are observing how galaxies looked like in the past

Question 25

The Milky Way grew through merging with many smaller galaxies. What are the observational signatures of this process?

Answer 25

The motion of old stars in the bulge and halo of our galaxy are randomly orientated, meaning they were formed from collisions of small, accreted, galaxies all on different paths.

Question 26

Different wavelengths (frequencies) of light let us see different aspects of the galaxy, which type of light is best for the following components of the MW?

Answer 26

Radio -> Cold gas; Visible -> Stars; X-rays -> Hot gas; Gamma-rays -> Cosmic rays