Stars

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  • Created by: CPev3
  • Created on: 20-09-20 10:51

What is the value for solar mass?

1.99 x 1030 kg (the mass of the Sun)

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Description of a planet

  • In orbit around a star

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  • Cleared its orbit of most other objects

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  • Mass large enough for its own gravity to give it a round shape

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  • No fusion reactions
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Description of a dwarf planet

  • In orbit around a star

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  • Has not cleared its orbit of other objects

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  • Mass large enough for its own gravity to give it a round shape

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  • No fusion reactions
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Description of an asteroid

  • In orbit around a star

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  • Near-circular orbit

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  • Small and uneven

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  • No ice present
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Description of a planetary satellite

  • In orbit around a planet

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  • Includes moons and man-made satellites
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Description of a comet

  • In orbit around a star

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  • Highly eccentric elliptical orbit

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  • Develops a tail as it approaches the Sun

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  • Small and uneven

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  • Ice, dust and small pieces of rock present
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How is a nebula formed?

Tiny gravitational attraction between particles of dust and gas

Particles pulled towards each other

Vast cloud formed

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How is a protostar formed?

Gravitational collapse accelerates as particles get closer together

Denser regions pull in more particles

Temperature increases as gravitational energy transferred to thermal energy

Protostar formed in one part of the cloud

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How is a star formed?

Size of protostar increases as more particles pulled in

Temperature of core increases

Kinetic energy of hydrogen nuclei increases

Electrostatic repulsion between hydrogen nuclei overcome

Hydrogen nuclei fuse together to form helium nuclei

Kinetic energy produced

Star formed

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What is happening in a star?

Outward radiation pressure from photons emitted during fusion

+ outward gas pressure from nuclei in core

= inward gravitational attraction

= equilibrium maintained

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How is a red giant formed?

0.5 M to 10 M

.

Less hydrogen nuclei available for fusion

Less kinetic energy produced

Outward radiation + gas pressure < inward gravitational attraction

Core begins to collapse

Pressure increases in shell around core...

...enough for hydrogen nuclei to fuse together to form helium nuclei

Layers cool as they slowly move away from core

Red giant formed

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Description of a white dwarf

  • Very dense

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  • Mass ≈ that of the Sun

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  • Volume ≈ that of the Earth

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  • No fusion reactions

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  • Leaks photons created in its earlier evolution

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  • High surface temperature
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What is the Chandrasekhar limit?

The mass of a star's core beneath which the electron degeneracy pressure is sufficient to prevent gravitational collapse, 1.44 M

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What is electron degeneracy pressure?

Two electrons cannot occupy same energy state

Stops any further collapse of matter involved in contraction

For stars < Chandrasekhar limit

Low-mass stars cannot collapse into anything < white dwarf

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What is neutron degeneracy pressure?

For stars > Chandrasekhar limit

Electron degeneracy pressure breaks down

Electrons in shells interact with protons in nucleus to form neutrons

Outward neutron degeneracy pressure > inward gravitational attraction

Neutron star formed

Electron degeneracy pressure breaks down if star very massive

Further gravitational collapse

Black hole formed

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Description of a neutron star

  • Density ≈ that of an atomic nucleus (~ 1017)

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  • Mass ≈ 2 M

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  • Diameter ≈ 10 km

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  • Almost entirely made up of neutrons
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Description of a black hole

  • Strong gravitational field

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  • Nothing can escape from it

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  • Escape velocity would need to be > speed of light (3 x 108 ms-1)
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How is a red supergiant formed?

Greater than 10 M

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Less hydrogen nuclei available for fusion

Less kinetic energy produced

Outward radiation + gas pressure < inward gravitational attraction

Core begins to collapse

High temperature in core...

...allows helium nuclei to fuse together to form heavier elements

Layers cool as they slowly move away from core

Red supergiant formed

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What is happening in a red supergiant?

High pressure and temperature in core

Massive nuclei fuse together

Series of shells formed inside the star

Process continues until iron core formed

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How is a supernova formed?

Iron nuclei cannot fuse together as no kinetic energy can be produced

Star becomes very unstable

Outward radiation + gas pressure << inward gravitational attraction

Layers implode and bounce off core

Shockwave ejects all core material into space

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What is a continuous spectrum?

  • All visible λs (or fs) present

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  • Produced by the atoms of a heated metal
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What is an emission line spectrum?

Atoms in a gas are excited

Electrons drop to lower energy levels

Photons with a set of discrete λs (or fs) specific to the element are emitted

Unique emission line spectrum produced

Each coloured spectral line corresponds to photons with a specific λ (or f)

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What is an absorption line spectrum?

Light from a source that produces a continuous spectrum passes through a cooler gas

Some photons passing through the gas are absorbed by the gas atoms

Electrons are raised to higher energy levels

Atoms are excited

Photons with energy = difference between the energy levels are absorbed

∴ Only specific λs (or fs) are absorbed

Produces dark spectral lines

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When an electron drops back down to a lower energy

Photon re-emitted in any possible direction

Intensity in original direction greatly reduced

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What are the different states?

Ground state

  • When an electron has the lowest value for energy possible
  • When an electron is at its stablest

Excited state

  • When an electron gains energy and thus is raised to a higher energy level

Ionisation state

  • When an electron gains enough energy to be completely removed from its atom
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Wien's displacement law

λmax x T = w (2.9 x 10-3 mK)

∴ λmax ∝ 1 / T

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Stefan-Boltzmann law

L = 4 x π x r2 x σ (5.67 x 10-8 Wm-2K-4) x T4

∴ L ∝ T4

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What is a black body?

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

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White light through a diffraction grating

White light = continuous spectrum

Split up into its component colours by diffraction grating

d x sinθ = n x λ

∴ Greater λ = greater θ = greater diffraction

n=>0 becomes a visible spectrum (red on outside, violet on inside)

n=0 stays white as all λs pass straight through

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