Stars

Question 1

Planet

Answer 1

Object in orbit around a star
It must have a mass large enough for its own gravity to give it a round shape, have no fusion reactions and have cleared its orbit of most other objects

Question 2

Planetary satellites

Answer 2

A body in orbit around a planet. This includes moons and man made satellites

Question 3

Comets

Answer 3

Small irregular bodies made up of ice, dust and small pieces of rock. They orbit around the sun in highly eccentric elliptical orbits. They range from a few hundred meters to tens of kilometers

Question 4

Solar system

Answer 4

A star and all the objects that orbit it

Question 5

Galaxy

Answer 5

A collection of stars and interstellar dust and gas

Question 6

How is a protostar formed

Answer 6

Nebulae are formed over millions of years as gravitational attraction pulls particles together.
As dust/gas gets closer together, gravitational collapse accelerates and denser regions begin to form
These regions get hotter and denser until a protostar forms

Question 7

Nebulae

Answer 7

Clouds of dust and gas

Question 8

How does a protostar become a main sequence star

Answer 8

Nuclear fusion needs to start, requiring extremely high pressures and temperatures to overcome electrostatic repulsion.
Once hydrogen to helium fusion starts, radiation pressure from emitted photons and gas pressure from nuclei in the core push outwards. Gravitational forces inwards are balanced

Question 9

Life cycle of stars with low mass

Answer 9

They remain in the main sequence for longer because temperature is lower so fusion happens slower
Stars with lower mass (between 0.5 and 10 solar masses) form red giants then white dwarfs

Question 10

Red giants

Answer 10

When fusion decreases in a low mass main sequence star, the star collapses inward.
As it shrinks, pressure increases enough to start fusion in a shell around the core. The core is inert as there is very little hydrogen.
Outer layers slowly move away from the star, cooling as they expand.q

Question 11

White dwarfs

Answer 11

Outer layers of red giant drift off leaving the hot core. This is very dense but no fusion takes place
The core is prevented from collapsing by the electron degeneracy pressure due to the Pauli exclusion principle

Question 12

Pauli exclusion principle

Answer 12

Two electrons cannot exist in the same energy state. This is only true up to the Chandrasekhar limit(1.44 solar masses - this is the maximum mass of a stable white dwarf)

Question 13

Life cycle of more massive stars

Answer 13

More massive stars become red supergiants. Then, a supernova occurs. Stars above the Chandrasekhar limit become neutron stars. Stars above 3 solar masses become black holes

Question 14

Red supergiants

Answer 14

Star collapses, but as it is larger it becomes much hotter so fusion of helium into heavier elements occurs.
A series of shells form inside the star, with lighter elements fusing in the outer shells and heavier elements in the inner shells
This continues until the star develops an inert iron core

Question 15

Neutron star

Answer 15

Gravitational collapse continues, forming a star made almost entirely out of neutrons

Question 16

Black hole

Answer 16

Gravitational collapse continues until gravitational field is so strong that light cannot escape

Question 17

The Hertzsprung-Russel diagram

Answer 17

Graph of luminosity against surface temperature(increasing right to left)

Question 18

Where do different stars fall on HR diagram

Answer 18

Main sequence stars form a curved line from top left to bottom right -hotter stars are more luminous
White dwarfs are very hot and very dim so they are bottom left
Red supergiants are very luminous because of their size but have relatively low temperature. They are found around the top right
Smaller red giants split off from the main sequence in the middle

Question 19

Energy levels in atoms

Answer 19

When electrons are bound to their atoms in a gas, they can only exist in discrete energy levels
Energy levels are negative because external energy is required to remove an electron from the atom
An electron with zero energy is free and maximum negative energy is known as the ground state

Question 20

Exciting an electron

Answer 20

When an electron moves from a lower to a higher energy level it is excited. This happens when it absorbed a photon of a specific frequency
E=hf

Question 21

Emission line spectra

Answer 21

Each element produces a unique emission line spectra because of its unique set of energy levels. Atoms in a gas are are excited, then when electrons drop back into lower energy levels they emit photons

Question 22

Absorption line spectra

Answer 22

Dark spectral line against the background of a continuous spectrum
Formed when light from a source that produces a continuous spectrum passes through a cooler gas
Some photons are absorbed by the gas, creating a pattern where specific wavelengths are missing from a continuous spectra
Photon will be reemitted but in a random direction so intensity is greatly reduced

Question 23

Diffraction grating

Answer 23

When light passes through a diffraction grating it is split into a series of narrow beams depending on wavelength. White light is therefore split up

Question 24

Diffraction grating equation

Answer 24

Nλ = d sin(θ)
Where N is the order of maxima, d is the grating spacing and θ is the angle between the zeroth order maxima and the Nth order maxima

Question 25

Wien’s displacement law

Answer 25

Peak wavelength is proportional to absolute temperature
For all black body emitters, peak wavelength x temperature = constant

Question 26

Black body radiation

Answer 26

Idealised object that absorbs all the EM radiation that hits it and emits distribution of wavelengths

Question 27

Stefan’s law

Answer 27

L = 4πr ^2σT^ 4

Question 28

How does intensity against wavelength graph change with temperature

Answer 28

As temperature increases, peak wavelength decreases and peak becomes sharper