Chapter 19 - Stars

Question 1

Birth of a star

Answer 1

1. Nebulae formed as gravitational attraction between particles of gas and dust pulls them together.
2. As dust and gas get closer, gravitational collapse accelerates, leading to hotter and denser regions. A protostar is formed.
3. Very high pressures and temperatures are needed for fusion of H nuclei to make He nuclei. A star is formed.

Question 2

What is the stable phase of a star called?

Answer 2

Main sequence.

Question 3

Why does a star have an almost constant size?

Answer 3

Gravitational forces (inwards) countered by radiation pressure (outwards).

Question 4

Planet

Answer 4

Object in orbit around a star. Mass large enough for its own gravity to give it a round shape.

Question 5

Planetary satellites

Answer 5

Body in orbit around a planet e.g. moons.

Question 6

Comets

Answer 6

Small, irregular bodies made up of ice, dust and rock. Highly elliptical orbits.

Question 7

Solar system

Answer 7

Contains the star (e.g. the Sun) and everything around it.

Question 8

Galaxy

Answer 8

Collection of stars and interstellar gas.

Question 9

How are red giants formed from stars with low mass?

Answer 9

• Gravitational fusion > radiation pressure.
• Core shrinks, pressure increases so fusion happens in shell around core.
• Core becomes inert as there is less H and low temperatures.
• Fusion continues in outer layers, which expand and cool.

Question 10

How are white dwarfs formed (from stars with low mass)?

Answer 10

Layers of red giant drift off into space, leaving behind a hot and dense core. No fusion takes place.

Question 11

Pauli exclusion principle

Answer 11

Two electrons cannot exist in the same energy state.

Question 12

Electron degeneracy pressure

Answer 12

Created when core of a star begins to collapse and electrons are squeezed together, preventing further collapse.

Question 13

Chandrasekhar limit

Answer 13

1.44 M☉ - maximum mass of a stable white dwarf.

Question 14

What happens to more massive stars?

Answer 14

• Form red supergiants and ejects core material into space through supernova.
• After supernove, a neutron star or black hole will be formed, depending on the mass of the remnant core.

Question 15

What is the Hertzsprung-Russell diagram?

Answer 15

Shows the relationship between luminosity and average surface temperature of stars in our galaxy.

Can be used to map out life of stars.

Question 16

Why are energy levels in gas atoms negative?

Answer 16

External energy is required to remove electron from atom.

Question 17

What does it mean when an atom is excited?

Answer 17

Electron moved from lower to higher energy level.

Question 18

Why is a diffraction grating used in place of a double slit?

Answer 18

More line - clearer and brighter interference pattern.

Easier to determine the centre of maxima.

Question 19

How would you find the wavelength of monochromatic light?

Answer 19

Measure angle between several different maxima and the zero order maximum.

Plot sinθ against n (order of maxima), gradient is λ/d (grating spacing).

Question 20

What is a black body?

Answer 20

Idealised object that absorbs all the EM radiation incident on it and when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature.

Question 21

Wien’s displacement law

Answer 21

Peak wavelength of radiation emitted by a black body is inversely proportional to temperature.

Question 22

Stefan’s law

Answer 22

Total radiant heat energy emitted from a surface (luminosity) is proportional to the fourth power of its temperature.