Lecture 25

Question 1

What type of stars form black holes?

Answer 1

The most massive stars that are over 20Msun)

Question 2

What causes black holes?

Answer 2

Black holes are caused when gravity is stronger than any pressure and all of the core mass is concentrated into a single point.

Question 3

What do black holes do to the space time continum?

Answer 3

They tear it and create a singularity

Question 4

Why are black holes black?

Answer 4

Gravity is so strong that light cannot escape

Question 5

How does escape velocity prevent light from escaping black holes?

Answer 5

escape velocity= sqrt(2GM/R). If escape velocity > c then nothing can escape.

Question 6

What is the Schwarzschild Radius?

Answer 6

It is the distance from a black hole where nothing can escape

Question 7

What is the event horizon?

Answer 7

The spherical surface defined by the Schwarzschild radius.

Question 8

Where do black holes start to “suck” things in?

Answer 8

Only near the event horizon. Other orbiting objects will orbit normally.

Question 9

What is tidal force (spaghettification)?

Answer 9

Spaghettification occurs near the event horizon and causes objects to be stretched and then shredded.

Question 10

How does the Schwarzschild radius of a supermassive black hole differ from a smaller black hole?

Answer 10

The event horizon is much larger so the force is gentler and stars fall in without being shredded apart

Question 11

What is the gravity felt by objects near a black hole (equation)?

Answer 11

g= GM/D^2

Question 12

How is time affected near the event horizon?

Answer 12

Time is dilated near the event horizon due to the the strong gravitational field (general relativity)

Question 13

How can black holes be observed?

Answer 13

• you can watch things move near them
• you can watch matter fall into them
• you can use gravitational lensing of background sources
• you can measure gravitational waves produced by merging black hole

Question 14

What are black hole binaries?

Answer 14

• gas from a companion star orbits a black hole and forms an accretion disk
• the very hot accretion disk emits mostly UV and x-rays
• there are large Doppler shifts as material approaches the black hole

Question 15

What happens to orbital speeds in the centre of galaxies?

Answer 15

They become very fast which suggests that there is something small but very massive being orbited