19 : Stars

Question 1

Star birth

Answer 1

• Nebulae are formed as the tiny gravatational attraction between particles of dust and gas pulls particles together forming vast clouds.
• as dust and gas get closer this gravitational collapse accelerates.
• due to tiny variations denser regions form which pull more dust and gas gaining mass and getting denser and getting hotter ad gravatational energy transferred to thermal energy
• a protostar forms

Question 2

protostar

Answer 2

A vert hot dense sphere of dust and gas

Question 3

Protostar becomes a star

Answer 3

Nuclear fusion - produce kinetic energy
- extremely high pressures and temperatures inside the core affect needed in order to overcome the electrostatic repulsion between hydrogen nuclei

Question 4

Star life

Answer 4

Once star is formed it remains in stable equilibrium
- gravatational forces act to compress star
- radiation pressure from photons emitted during fusion and gas pressure from nuclei push outwards
How long star remains stable depends on size and mass of its core

Question 5

Planets

Answer 5

Is in orbit around a star
- mass large enough for its own gravity to five it a large shape
- no fusion reactions
- cleared its orbit of most other objects (asteroids)

Question 6

Planetary satellites

Answer 6

A body on orbit around a planet
- man-made satellites
- moons

Question 7

Comets

Answer 7

Range from a few hundred metre to tens of kilometres
- irregular bodies made up of ice, dust and small pieces of rock. All comets orbit the sun, hugely eccentric elliptical orbits (around a sun)
- as they approach the sun they have tails

Question 8

Solar system

Answer 8

Contains the sun and all objects that orbit it

Question 9

Galaxies

Answer 9

Collection of stars and interstellar dust and gas

Question 10

Red giants

Answer 10

• stars between 0.5M and 10M will evolve into red giants
• at start of phase the reduction in energy released by fusion in the core means that the gravatational force is now greater than the reduced force from radiation and gas pressure
• core of star begins to collapse and as it shrinks pressure increases enough to start fusion in a shell around the core
• inert cores where fusion no longer takes place as there is very little hydrogen left and temperature isn’t high enough for helium nuclei to overcome electrostatic repulsion
• it continues in the shell around the core this causes the periphery of star expands as layers move away from the core. Layers expand and collapse giving star a red colour

Question 11

White dwarfs

Answer 11

• most of layers of red giant around the core drift into space as a planetary nebula
• leading behind hot core as a white dwarfs
• very dense
• no fusion takes place inside white dwarf

Question 12

Electron degeneracy pressure

Answer 12

• Pauli exclusion principle - two electrons cannot exsist in the same energy state
• when core of star begins to collapse under the force of gravity the electrons squeeze together creating pressure that prevents core from further collapse

Question 13

Chandrasekhar limit

Answer 13

• electron degeneracy pressure is only sufficient to prevent collapse if core has mass less than 1.44M

Question 14

Red supergiants

Answer 14

• cores of massive stars are hotter and helium nuclei amused by fusion are moving fat enough to overcome electrostatic repulsion so fusion of helium nuclei into heavier elements occurs
• the star expands firing red supergiant
• inside temperatures and pressures are high enough to fuse massive nuclei together forming series of shells inside the star
• star develops an iron core - iron nuclei cannot fuse because such reactions cannot produce energy
• star is very unstable and leads to death of star in a catastrophic implosion of layers bounce off the solid core, leading to s shockwave that ejects all the core materials into space (supernova)

Question 15

Neutron star

Answer 15

If the mass of the core id greater than Chandrasekhar limit, the gravitational collapse continues forming neutron star

Question 16

Black hole

Answer 16

Core has a mass greater than 3M the gravatational collapse continues to compress the core. The result is a gravatational field s throng that in order to to escape and object needs to escape with velocity greater than speed of light.

Question 17

Hertzsprung-Russell diagram

Answer 17

Graph of stars in our galaxy showing the relationship between the luminosity and their average surface temperature

Question 18

Luminosity

Answer 18

Total radiant power output of the star
- greater luminosity the brighter the star

Question 19

Life cycle of stars

Answer 19

• low mass stars evolve not red giants moving away from that min-sequence. They gradually loose their cooler outer layers and end up as white dwarfs
• Higher mass stars starts as supergiants before rapidly consuming their fuel and swelling into red supergiants before they go supernova

Question 20

Energy levels in gas atoms

Answer 20

• eoletron cannot have quantity of energy between two levels
• energy levels are negative because external energy is required to remove and electron from the atom. Te negative velours also indicate that the electrons are trapped within the atom or bond to the positive nuclei
• an electron with zero energy is free from the atom
• energy level with the most negative value is the ground level

Question 21

Electron moves from lower to higher level

Answer 21

Atom is said to be excited
- raising an electron require external energy

Question 22

Electron moves from higher to lower level

Answer 22

• it looses energy
• energy is conserved so as electron makes transition a photo is emitted from the atom
• de-excitation

Question 23

Emission line spectra

Answer 23

Each element produces a unique emission line spectrum because of its unique set of energy levels
- black background with colourful lines

Question 24

Continuous spectra

Answer 24

All visible frequencies of vaelengths are present
- atoms of a heated solid metal

Question 25

Absorption line spectra

Answer 25

Series of dark spectral lines against the background of a continuous spectrum. Dark lines have the same wavelength as the bright emission spectral lines for the same gas atoms

Question 26

Forming emission line spectrum

Answer 26

If the atoms in a gas are existed when the electrons drop back into lower levels they emit photons with a set of discrete frequencies specific to that element - each coloured lines represent a unique wavelength of photon emitted when an electron moves between two specific energy levels

Question 27

Forming an absorption line spectrum

Answer 27

Light rom a source that produced continuous spectrum passes through a cooler gas. The photons pass through the agas some are absorbed by the gas atoms raining electrons into gusher levels and so ecciting atoms - only photons with energy exactly equal to the difference between the different energy levels are absorbed creating dark lines in the spectrum - photons are re-emitted when they electron drops back down to a lower energy level they’re are emitted in al possible directions so intensity in the original direction is greatly reduced

Question 28

Detecting elements within stars

Answer 28

Wen light from a star is analysed it is found to be an absorption line spectrum some wavelengths of light are missing - the photons have been absorbed by atoms of cooler gas in the outer layers of the stars

Question 29

Diffraction grating

Answer 29

An optical comment with regularly spaced slits or lines that diffract and slit light into beams of different colour travelling in different directions. These beams can be analysed to determine the wavelengths of spectral lines in t laboratory of from starlight

Question 30

Forming maxima

Answer 30

Light is diffracted art each slit and the interference pattern is the result of superposition of the diffracted waves - the formation of maximum at a particular point depends on the path difference and base difference of the waves from all the slits

Question 31

Black-body radiation

Answer 31

At any given temperature above absolute aero and object emits readitaion of different wavelengths and different internsities - a black body is an idealised object that absorbs all the electromagnetic radiation that shines onto it and, when in thermal equilibrium exist a characteristic distribution of wavelengths at a specific temperature

Question 32

Wien’s displacement law

Answer 32

Relates absolute temperature to the peak wavelength - wavelength max is inversely proportional to the temperature

Question 33

Stefans law

Answer 33

Total power radiated per unit surface area of a black body is directly proportional to the fourth power of the absolute temperature of the black body