# Astrophysics and cosmology

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• Created by: emma
• Created on: 23-04-13 16:25
the gravitational force which attracts two masses is an...
inverse square law
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what is the gravitational field strength?
the force acting on unit mass placed at a point in the field
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what is the flux/intensity of a star?
the power recieved per unit area of detector on earth
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what are the two possible methods of finding the distance,d, to a star?
the parallax method and the standard candle method
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what is the parallax method?
observe the star from opposite sides of the earths orbit round the sun. make measurments of its apparent position. use trigonometry to determine a distance.
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what laws does the radiation from a star follow?
the laws of black body radiation
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in wien's law, what is LAMBDAmax?
the wavelength at which the energy output peaks
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stars are placed in a hertzsprung-russell diagram according to their...
luminosity and temperature
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how big must the initial mass of a star be in order for a supernova explosion to occur when it dies?
8 times the mass of our sun
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the greater the distance of a galaxy from us...
the greater the red shift
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how can we tell that the universe is expanding?
absorption lines in light from distant galaxys are shifted to the red end of the spectrum relative to those in light from galaxies closer to us. this is the doppler effect.
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what name is given to the proportional relationship between recesion speed and distance?
Hubble's law
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how can we use hubbles law to deduce the age of the universe?
assuming that the universe has always been expanding at its current rate: age = D%speed = 1%hubble constant (inverse of the gradient of hubble's speed against distance graph)
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what is an open universe?
it will keep expanding forever, even though the expansion will carry on slowing down.
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what is a closed universe?
it will stop expanding and start to contract.
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what is a flat/ critical universe?
one which will stop expanding but only at an infinite time in the future.
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why might the expansion be speeding up?
dark energy? - scientists don't know what this is yet.
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why do hotter stars tend to have more mass and be more luminous?
at the centre of a massive star, there is larger gravitational pressure. this makes the nuclear fusion reactions run very fast producing lots of energy. more mass = larger--> greater sa. large sa and high temperature. greater luminosity.
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according to the staffan-boltzmann law, as the temperature of a body increases...
it emits more energy
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weins law tells us that...
temperature determines the spectral output of the stars
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why are values for luminosity not as accurate as we would like?
luminosity involves using values for distances to the stars which we cannot measure exactly.
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what is the main sequence?
the diagonal line across the HR diagram. most stars exist in this state for the majority of their lives.
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why are their fewer stars in other parts of the HR diagram?
stars exist in these stages for much briefer periods, so fewer are currently in these stages.
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why don't we see many blue super giants on the HR diagram?
blue supergiants burn out in just afew million years and most have already done so.
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what is a protostar?
an accreting collection of gasses.
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what determines the life cycle of a star?
its initial mass
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describe the life cycle of a solar mass star.
protostar --> sun-like star --> red giant --> planetary nebula --> white dwarf
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describe the life style of a massive star.
protostar--> blue supergiant --> red giant --> type II supernova --> black hole/ neutron star
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if we observed a star at each phase of its life...
it would appear at different points on the HR diagram. we can plot its life as a path arround the diagram.
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what is the astronomical unit?
1.5 x 10 ^(11) (the radius of the earths orbit around the sun)
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## Other cards in this set

### Card 2

#### Front

what is the gravitational field strength?

#### Back

the force acting on unit mass placed at a point in the field

### Card 3

#### Front

what is the flux/intensity of a star?

### Card 4

#### Front

what are the two possible methods of finding the distance,d, to a star?

### Card 5

#### Front

what is the parallax method?