Stellar Physics

Question 1

What is a black body?

Answer 1

An ideal black body can absorb and emit all frequencies perfectly. (Stars are NOT ideal black bodies, but we treat them as if they are.)

Question 2

How is the surface temperature of a star related to its peak wavelength?

Answer 2

The shorter the peak wavelength, the higher the surface temperature of the star.

Question 3

What are the 7 spectral classes of stars?

Answer 3

The spectral classes are: O, B, A, F, G, K and M.

Question 4

How does the colour vary between the spectral classes?

Answer 4

(From class O to M) Blue, blue-white, white, yellow-white, yellow, orange, red.

Question 5

How does the mass / radius vary between the spectral classes?

Answer 5

From class O to M, the mass / radius decreases.

Question 6

How does the luminosity / surface temperature vary between the spectral classes?

Answer 6

From class O to M, the luminosity / surface temperature decreases.

Question 7

How does the lifetime vary between the spectral classes?

Answer 7

From class O to M, the lifetime increases.

Question 8

What unit does surface temperature have?

Answer 8

Surface temperature is measured in Kelvin, K.

Question 9

What is another word for the luminosity of a star?

Answer 9

The luminosity is the power output of a star. (Energy emitted per second)

Question 10

What affects the apparent brightness of a star?

Answer 10

The apparent brightness of a star is affected by the luminosity, distance and wavelength distribution of the star.

Question 11

What are Hertzsprung-Russell diagrams?

Answer 11

Hertzsprung-Russell diagrams plot the temperature and luminosity of stars.

Question 12

What is the diagonal line on H-R diagrams called?

Answer 12

The diagonal line is the Main Sequence.

Question 13

What is the Main Sequence of stars?

Answer 13

The Main Sequence contains most of the stars in the universe. These stars are still fusing H -> He. Our sun is in the Main Sequence.

Question 14

What groups are in the top-right of H-R diagrams?

Answer 14

Red giants and Red supergiants are found in the top-right of H-R diagrams.

Question 15

What group is in the bottom-left of H-R diagrams?

Answer 15

White dwarfs are found in the bottom-left of H-R diagrams.

Question 16

What are red giants/supergiants?

Answer 16

Red giants/supergiants are the brightest of all stars, but are colder than most stars. They fuse heavier elements.

Question 17

What are white dwarfs?

Answer 17

White dwarfs are some of the hottest stars, but are dimmer than most stars. The don’t fuse anymore elements because they are at the end of the star’s life.

Question 18

How are stars made?

Answer 18

Stars are born in huge clouds of dust, called nebula. Gravity pulls gas and dust together, increasing the density. Gravitational potential energy is converted into heat energy, increasing the temperature of the dust and gas. Eventually, the hydrogen has enough energy to undergo fusion, creating the star.

Question 19

What is nebula?

Answer 19

Nebula is a huge clouds of spacedust that makes up stars. Nebula is very cold, usually between 10 and 20 Kelvin.

Question 20

What are the 3 stages of a proton-proton chain reaction?

Hint: fusion happens 3 times

Answer 20

Stage 1: Two hydrogen nuclei (hydrogen-1) undergo fusion, producing a deuterium nucleus (hydrogen-2), a position and a neutrino. The positron is eliminated by an electron, producing energy in the form of gamma ray photons.

Stage 2: The deuterium nucleus fuses with a proton, producing a helium-3 nucleus and a gamma ray photon.

Stage 3: Two helium-3 nuclei fuse to form a helium-4 nucleus and two free protons, which can go on to start more fusion reactions. This is why it’s a chain reaction.

Summary:
A proton fuses with another proton to form deuterium.
Then deuterium fuses with a proton to form a helium - 3 nucleus.
Two helium - 3 nuclei then fuse to form a helium - 4 nucleus.

Question 21

What happens when a red supergiant stops fusing elements?

Answer 21

A supernova is created - a large explosion. This leaves behind either a black hole, or a neutron star (a high density small star, made almost entirely of neutrons)

Question 22

What happens when a red giant stops fusing elements?

Answer 22

The outer layers of the star are left behind in space as a planetary nebula. This leaves a dense core - a white dwarf.

Question 23

What happens when a white dwarf cool down?

Answer 23

When a white dwarf cools down, it will fade away, simply becoming a black dwarf.

Question 24

What do large mass stars become?

Answer 24

Large mass stars become red supergiants.

Question 25

What do low/medium mass stars become?

Answer 25

Low/medium mass stars become red giants.