Stars

Question 1

What is the H-R diagram?

Answer 1

A plot of the Abs magnitude of a star against its colour (B-V)

Question 2

What are the stages on the H-R diagram?

Answer 2

Main sequence
Red Giants
White Dwarfs
Supergiants

Question 3

What is the theoretician’s H-R diagram?

Answer 3

A plot of the log of luminosity against the log of the temperature

Question 4

What is the main sequence?

Answer 4

Where a star is fusing hydrogen in its core
Makes up majority of the star’s lifetime (Myrs to 1000 Gyrs)
Rs ranges from 0.1 to 10 Solar Radii
Lowest mass are 0.05 Solar Masses highest of 100

Question 5

What is a red giant?

Answer 5

Hydrogen in the core has depleted
Helium fusion begins
Rs ranges from 10 to 1500 Solar Radii
Not much time is spent in this place
Similar masses to main sequence

Question 6

What is a white dwarf?

Answer 6

These are the end states of most sequence stars
Temperatures range from 3000 to 25000 kelvin
Rwd of about 0.01 Solar radii
In an isolated state they stay here

Question 7

What is a brown dwarf?

Answer 7

An object intermediate between a planet and a star
Radius of 0.013 to 0.0 Solar Radii
Initiate deuterium fusion
Most energy comes from gravitational with heat released as they contract

Question 8

What is a neutron star?

Answer 8

Remnants of stars more massive than 5 to 8 solar masses
Only 10km in radius but have a mass of 1.4 Solar Masses

Question 9

What happens to very massive stars?

Answer 9

They turn into black holes

Question 10

What is an open star cluster?

Answer 10

A cluster of 100 to 1000 stars
Found in the disc of our galaxy
All stars are very young (1Myr to 1Gyr)

Question 11

What is a global star cluster?

Answer 11

A cluster of 10^5 to 10^6 stars
Distributed all over the galaxy
10^2 to 10^3 stars per pc^3
Very old stars (12 Gyrs)

Question 12

What is a H-R cluster diagram?

Answer 12

A plot of the absolute magnitude on the left and the apparent on the right against the colour on the bottom

Question 13

What is the equation for hydrostatic equilibirum?

Answer 13

dP/dr= -ρg
With P pressure and ρ density

Question 14

What is the central pressure of a star?

Answer 14

P_c=GMρ_av/R=3GM^2/(4πR^4)

Question 15

How does number density relate to pressure?

Answer 15

P=nk_BT

Question 16

How does the density relate to the pressure?

Answer 16

P=ρk_BT/µ
With µ the mean mass per particle
.

Question 17

What is the central temperature of a star?

Answer 17

k_B T_C=GMµ/R

Question 18

What is the lifetime of a star?

Answer 18

t=E/L
Where E is the total energy supply and L is the luminosity

Question 19

How much energy is gotten from fusion?

Answer 19

7MeV per Hydrogen atom

Question 20

What is the main source of stellar energy?

Answer 20

Nuclear energy, the fusion of hydrogen atoms

Question 21

What are the proportionalities with mass?

Answer 21

t_MS =E_FL∝M/M^4
= M^−3
So the lifetime of a main sequence star is proportional to then inverse of mass cubed .

Question 22

What are the possible outcomes for star?

Answer 22

For M<0.08 Solar Masses become brown dwarfs
For 0.08< M< 0.8 Solar Masses become white dwarfs but will never leave main sequence during life of the universe
For 0.8< M< 5 to 8 Solar Masses become white dwarfs
For 5 to 8 Solar Masses< M < 25 to 40 Solar Masses become neutron stars
For any mass greater the star becomes a black hole

Question 23

What is the Schwarzschild radius?

Answer 23

Rs=2GM/c^2=3(M/1 Solar Mass) km

Question 24

For what radii are accretion disks stable?

Answer 24

R is roughly 6GM/c^2
The Egrav is 17%
These are called quasars