Lecture 22

Question 1

What is the main composition of interstellar matter (ISM)?

Answer 1

Hydrogen, helium and ~2% metals (heavier elements than helium)

Question 2

What are the two different temperatures of ISM clouds?

Answer 2

1. Warm clouds- mostly atoms and ions
2. Cold clouds- mostly atoms, molecules and dust. Denser than warm clouds by factor of 100.

Question 3

How does a star form from a gas cloud (simply)?

Answer 3

• interstellar gas cloud contracts
• becomes smaller and denser
• becomes so dense and hot that hydrogen fusion begins (core must reach at least 5 million K)
• a star is born

Question 4

How does pressure occur?

Answer 4

Pressure of a gas comes from the gas particles bouncing off a surface

Question 5

How do gravity and pressure interact in a star?

Answer 5

• gas atoms are pulled together via gravity
• gas atoms are pushed apart due to thermal pressure
• as gravity contracts the cloud, density and pressure increase

Question 6

What is the “Jeans Mass”?

Answer 6

The relationship between mass, temperature and density that determines whether a cloud will collapse into a star
- if Mcloud<Mjeans then pressure wins, no collapse
- if Mcloud>Mjeans then gravity wins, collapse

Question 7

What does the Jeans Mass tell us?

Answer 7

That cold, dense clouds collapse more easily

Question 8

What starts the collapse of a star?

Answer 8

A supernova shockwave, colliding clouds, turbulence or spontaneous cooling

Question 9

What are the stages of collapse of a star?

Answer 9

• cloud collapses under its own gravity
• spins faster (conservation of angular momentum)
• heats up as it collapses (conservation of energy)
• flattens into a disk (conservation of momentum)

Question 10

What happens in the densest parts of the cloud?

Answer 10

The centre becomes very hot, dense and opaque- light cannot escape and heat is trapped

Question 11

When does a protostar form?

Answer 11

When the cloud becomes dense, opaque and begins to heat up

Question 12

How does a gas sphere differ from a regular solid object?

Answer 12

A gas sphere held together by its own gravity contracts as it radiates, growing hotter

Question 13

How does a star travel along the H-R diagram during its formation?

Answer 13

• as the cloud collapses to a protostar it moves up and left
• as the collapse continues but the temperature stabilises it moves downwards
• as the core reaches millions of K, fusion begins and the star moves left
• eventually the stellar thermostat keeps luminosity and temperature stable as hydrogen fuses to helium. The star becomes part of the main sequence.

Question 14

How long does it take for stars to reach the main sequence on the H-R diagram?

Answer 14

It takes different amount of time depending on the mass of the star

Question 15

What stars form fastest and first?

Answer 15

Massive stars

Question 16

How do massive stars affect the formation of smaller stars?

Answer 16

• radiation winds affect the formation of smaller stars
• supernovae can trigger a collapse of adjacent areas in the molecular cloud

Question 17

What does the interstellar mass function describe?

Answer 17

The amount of mass of a star forming cloud that goes into forming different types of stars

Question 18

Why does the disk that forms a star flatten?

Answer 18

Collisions between particles average out motions and orbits