19.1 - 19.7 Flashcards
what are nebulae
clouds of dust and gas (mostly hydrogen). often larger than our solar system
how are nebulae formed?
over millions of years, the tiny gravitational attraction between dust and gas pulls the particles towards each other
how is a protostar formed
tiny variations in nebulae cause denser regions. these regions pull in more dust/gas getting even denser. the GPE turns into thermal energy forming a protostar
what is a protostar
a very hot and dense sphere of dust and gas.
how is a star formed?
when the core of a protostar becomes hot/dense enough to cause nuclear fusion. high temp/pressure is needed
why do stars not collapse under their own gravity?
the compressing force of gravity is equal to the radiation pressure and gas pressure pushing outwards
define ‘main sequence star’
the main period of a stars life during which it is stable
what characterises a planet
- its mass is large enough that its own gravity gives it a round shape
- it has no fusion reactions
- it cleared its orbit of most other objects
what’s the difference between a dwarf planet and a planet
the main difference is that dwarf planets have not cleared their orbits of other objects. (e.g. pluto has many similar sized objects near its orbit)
what are asteroids
objects that are too small and uneven to be planets. usually in near-circular orbits around the sun and without the ice that comets have
what are satellites
an object in orbit around a planet (natural or man-made)
what are comets
small, very irregular objects made of dust, rock and ice. many orbit the sun in highly eccentric, elliptical orbits
what are galaxies
a collection of stars and interstellar dust/gas. on average they contain 100Billion stars
what is a ‘red giant’ star
an expanding star at the end of its life. its core is inert and fusion only takes place in a shell around the core
what happens when a star runs out of hydrogen
when a star runs low on hydrogen the energy released from fusion is much less. gravity is stronger so the star shrinks.
what happens when a star shrinks at the end of its ‘main sequence’
The increase in pressure is enough to start fusion in the shell around the core. this causes the periphery around the star to expand as layers slowly move away
why is a red giant red
as the star expands its outer layers cool so they glow red
how is a white dwarf formed
the outer layers of a red giant drift off into space forming a planetary nebula. this leaves the hot core as a small white dwarf
what is electron degeneracy pressure
a quantum-mechanical pressure created by electrons in the core of a collapsing star due to the Pauli exclusion principle
why do white dwarfs not collapse under their own gravity
electron degeneracy pressure
what is the Chandrasekhar limit
the mass of a star’s core, beneath which the electron degeneracy pressure is strong enough to prevent gravitational collapse
what happens when massive stars run out of hydrogen
the energy released from fission decreases so it shrinks. the core is so hot that the fusion of helium starts. this causes it to expand into a red supergiant.
what happens to a red supergiant
it fuses elements into iron, forming a stable core. Iron nuclei cant fuse so the star implodes due to its own gravity. the outer layers bounce off the core ejecting them into space (a type II supernova)
whats a neutron star
the remanent core of a massive star after it goes supernova. the core collapsed under its own gravity causing a very high density. It’s made almost entirely of neutrons
whats a black hole
the remanent core of a massive star after it goes supernova. the core has collapsed so far that the escape velocity is greater than the speed of light
what’s a Hertzsprung-Russel diagram
a graph showing the relationship between the luminosity and average surface temperature of stars in our galaxy
what is the luminosity of a star
the total radiant power output, measured in Watts
energy levels in atoms
an electron cant have a quantity of energy between two levels
energy levels are negative since energy is needed to remove them from the atom
electrons with zero energy are free from the atom
each element has unique energy levels
define ‘ground state’
the energy level with the most negative value possible for an electron. its the most stable energy state
what is meant by an ‘excited atom’
an atom containing an electron or electrons that have absorbed energy, moving to a higher energy level
what is de-excitation
when an electron moves from a higher energy level to a lower one. it emits energy in the form of a photon
define ‘spectroscopy’
a technique in which spectral lines are measured to identify elements within stars
define ‘spectral lines’
a line in an emission or absorption line spectrum at a specific wavelength
what’s an emission line spectrum
a set of specific frequencies of EM waves emitted as excited electrons move back to lower energy levels
what’s a continuous spectrum
a spectrum in which all visible frequencies/wavelengths are present
what’s an absorption line spectrum
a set of specific frequencies shown as dark lines on a continuous spectrum. they were absorbed by atoms to excite electrons
how can elements in a star be analysed
when the light they emit is analysed it looks like an absorption line spectrum. this is checked against elements emission line spectrum to see if they are present in the star
what’s a black body
an ideal object that absorbs all of the EM radiation incidents on it. when in thermal equilibrium it then emits a distribution of wavelengths for a specific temperature
what is Wiens displacement law
the maximum wavelength at which the intensity of radiation emitted by a black body is at a maximum is inversely proportional to the absolute temperature of the body
