ASTROPHYSICS Flashcards
Red giant
A giant star with a relatively low surface temperature. That fuses helium nuclei in its core to form carbon
Nebula
A cloud of gas or dust in interstellar space
White dwarf
High surface temperature and low luminosity very small radius so small surface area remnant of a low mass star
Black dwarf
The theoretical end point of the cycle of a low - mass star
Red supergiant
Very hot, massive, luminous star with a short life span. iron is the heaviest element produced in the core as it has the highest binding energy per nucleon and cannot be fused together
Supernova
The explosion that occurs when a massive star’s core no longer undergoes fusion and the core collapses under gravity and the outer layers explode in a supernova
Neutron star
A dense ball of neutrons that remains at the core of a star after a supernova explosion. A star’s core becomes a neutron star if its between 1.4 -2.5 solar masses.
Pulsar
A rotating neutron star that emits energy in the form of radio waves at regular intervals
Black hole
A remnant of a massive star formed when its matter collapses to a point (singularity). A star’s core only becomes a black hole if its is more than 2.5 solar masses
Binary star
A system of two stars orbiting around their common centre of gravity
Black body
An object that is a perfect emitter and absorber of radiation, a black body absorbs all wavelengths that fall upon it and radiates all wavelength, the spectrum of wavelengths emitted only depends on its temperature.
Black body radiation
The radiation emitted from an object due to its temperature.
Hertzsprung Russell diagram
Logarithmic scaled graph of luminosity of stars against their surface temperature, on the temperature scale at each interval temperature halves and high temperatures are on the left
Stefan Boltzmann law
L= 4pi sigma r^2 T^4
where:
L= Luminosity
r= radius of star
T= temperature of star
Boltzmann constant,K
Relates the average energy of a molecule to its absolute temperature
Wien’s law
lambda peak x T= 2.898 x 10^-3 mk
Emission spectra
Characteristic colours (frequencies) of light given out by an element when electrons are excited.
Main sequence
Main group of stars on a Hertzsprung: Russell diagram, these are starts that fuse hydrogen nuclei to helium in their core and remain at a constant diameter.
Light year
Distance travelled by light in one year
Astronomical unit
The radius of the orbit of the Earth around the sun, 1Au = 1.5 X 10^11m
Trigonometric parallax
The apparent displacement of an object when viewed along two different lines of sight, relative to a fixed background of stars
Stellar parallax
The apparent shifting of stars against a fixed background when observed from different positions in the Earth’s orbit around the sun
Parsec
Distance that gives a parallax angle of 1 arc second, 3.26 light years
Standard candles
Astronomical objects of known luminosity
Luminosity
Rate at which a source radiates energy
Big bang theory
The universe evolved from a single hot explosion
Red shift
EDIT
Closed universe
The universe will expand until a point where the universe’s density is so high that the high gravitational forces cause the universe to contract ‘Big Crunch.’ This happens when the density of the universe is greater than the critical density.
Big Crunch
A possible fate for the universe where it collapses back on itself to a singularity
flat universe
The universe will continue to expand until an infinite amount of time . The Universe has a density equal to the critical density
Critical density
The density of the universe that just allows it to stop the expansion of space after infinite time
Dark matter
Matter in the universe that can’t be detected by emission or absorption of radiation
Hubble’s law
The speed of a receding galaxy is proportional to its distance from the observer
Hubble’s constant
Ratio of the speed v of a receding galaxy to its distance d from an observer
Using parallax to determine distance:
- Record the angular displacement of a nearby star relative to a fixed background of distant stars
- Over a period of 6 months
- We know the distance from the earth to the sun is 1AU
- Use trigonometry to determine the distance to the star
Using standard candles to determine distance
- Identify a standard candle in the star cluster/galaxy and know its luminosity.
- Measure the intensity of the star on earth (I).
- Rearrange I = L/ 4πd2 to d = √𝐿4𝜋𝐼
Explain how Hubble’s law predicts an expanding universe
- Hubble’s law states that the velocity of a galaxy is directly proportional to the distance of the galaxy
- So the most distant galaxies are receding the fastest
- This means that at the start of the universe, all matter must have been concentrated in a single point and has been expanding ever since the big bang
- So all of space has been expanding since the beginning of time
Using Hubble’s law and redshift determine how to calculate distance:
- Measure the wavelength of spectral lines from distant galaxies and compare it to laboratory values of the spectral lines λ
- Calculate apparent shift in wavelength Δλ of spectral lines of distant galaxies and determine redshift z = Δλ /λ
- Use the equation z = v/c to determine the recessional velocity v of the galaxy (c is the speed of light)
- Rearrange Hubble’s law d = v/H0 to calculate the distance to the galaxy
Explain what happens to a star like our sun at the end of its main sequence
- When hydrogen fusion stops in core (main sequence ends), Temperature cools down gravity is greater than the outwards pressure due to nuclear fusion .
- Core contracts
- Temperature and density increases enough for fusion of helium nuclei in core to start
- Helium fusion stops the core collapses again under gravity
- For stars the same size as our sun: the outer layers of the sun are ejected into space
- The temperature does not rise enough for further fusion to begin
- The core becomes a white dwarf
The Doppler effect:
An increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other.
Calculating the age of the universe
- Using speed= distance / time ( V=d/t)
- v= H0d
- Equating them so : d/t = H0d
- Cancelling d
- 1/t = H0
- t= 1/H0
Explain why the exact age of the universe is uncertain
- Hubble’s constant is determined from measurements of the distance to distant galaxies and redshift.
- Measurements of distance have high uncertainties.
- So Hubble’s constant is uncertain.
- So age of universe = 1/H0 is uncertain.
- Supernovae observations indicated dark energy causing universe to accelerate in its expansion.
- Assumption in derivation of t=1/H0 is that universe has expanded at a constant rate (if it has been accelerating due to dark energy this is not true)
Explain why the ultimate fate of the universe is uncertain
- We cannot observe dark matter directly – we are uncertain about how much there is.
- So calculating the average density of the universe has uncertainty.
- We do not know the nature of dark energy and how it interacts with gravity.
- So we cannot predict how gravity/dark energy will affect the ultimate fate.
