Surveying The Stars and Cosmology Flashcards
Define the luminosity of a star
Total energy emitted per second
Define the brightness/intensity of a star
The power received from it per unit area at earth, =L / 4 x pi x R^2
How does apparent magnitude describe a star?
Classifies stars according to their apparent brightness to the naked eye on earth, 1 is brightest 6 is dimmest
What is the astronomical unit for distance?
Mean distance between earth and sun
What is a parsec?
The distance at which 1 AU subtends an angle of of one arc second
Define absolute magnitude
A stars apparent magnitude if it was at a distance of 10 parsecs from earth
What is black body radiation?
A body which absorbs all wavelengths of EM radiation and can emit all wavelengths of EM radiation
What is Wien’s displacement law?
The wavelength at peak intensity is inversely proportional to the absolute temperature of the object
What is Stefan’s law?
The total energy per second emitted by a black body at absolute temperature T is proportional to its surface area and T^4
Give two problems which occur when observing stars?
Atmosphere only lets certain wavelengths of light through and man made light pollution
Define a continuous spectrum
Contains all possible wavelengths emitted by hot objects
Define an emission spectrum
Only specific wavelengths from excited electrons in hot gases
Explain how an absorption spectrum is formed
By shining white light through a cool gas, photons excite electrons to higher energy levels, which then de-excite and emit photons which appear black as they are against a complete set of colours
What is radiation pressure?
Pressure exerted on any surface exposed to EM radiation
For three stars with similar surface temperatures and known absolute magnitudes, which would be the largest?
One which has the brightest (lowest) value of absolute magnitude, as brighter absolute magnitude means greater power output, so must have greater area at similar temperatures
What is meant by a supernova?
A object that produces a rapid increase in brightness or rapid drop in absolute magnitude
State the defining properties of a neutron star
Extremely dense and made up of neutrons
Why are there concerns about a supernova occurring in a nearby part of the galaxy?
Collapsing star can produce gamma ray outbursts with energy similar to the total output of the sun, highly collimated meaning if in direction of earth, can cause extinction level event
Give two reasons why it’s difficult to directly observe an exoplanet
Light from star which it’s orbiting brighter than reflected light off planet and planet very small and distant so sub tends a very small angle compared to resolutions of telescopes
What is a standard candle?
Objects whose absolute magnitude is known and apparent magnitude can be measured
Describe the controversy caused by measurements of type 1A supernovae
Measurements of supernovae do not agree with predictions from Hubbles law, so expansion of universe accelerating, but no known energy source causing acceleration, suggests dark energy exists
How would you work out the radius of an exoplanet from the ‘dip’ in brightness of a star?
Fractional drop in intensity at each dip equal to the ratio of area of the exoplanet disc to that of the star disc
What are the minimum values on the absolute magnitude and temperature axis for the HR diagram?
Absolute magnitude - 15, temperature - 50000K
What are the maximum values on the absolute magnitude and temperature axis for the HR diagram?
Absolute magnitude - (-10), temperature - 2500K
What are two likely causes of short lived gamma ray bursts?
Neutron stars merging to form a black hole or a neutron star falling into a black hole
Why is it important to not only use the transit method to detect exoplanets?
Dips in light curves can be caused by other effects, planets further away from star have very long orbit periods and may take years of observation using only the transit method
Describe the link between galaxies, black holes and quasars
Supermassive black holes are found in the centre of galaxies and quasars are produced by supermassive black holes
Explain what is meant by cosmic microwave background radiation and how it’s existence supports Big Bang theory
Radiation which comes from all parts of the universe, can be interpreted as radiation leftover from Big Bang, spectrum has a peak in microwave region which corresponds to a temperature of 2.7K
Explain how the relative abundance of hydrogen and helium supports the Big Bang theory
Theory suggests that a brief period of fusion occurred when universe young resulting in production of helium from fusing hydrogen, fusion stopped when universe expanded and cooled, resulted in relative abundance of hydrogen and helium in ration 3:1
What are type 1 supernovae?
Supernovae which have no prominent hydrogen lines
What are type 1A supernovae?
Supernovae which show strong absorption lines due to silicon and have a peak luminosity at about x10^9 that of the sun’s
What are pulsars?
Rapidly rotating neutron stars which produce beams of radio waves
What is the event horizon?
Sphere surrounding a black hole from nothing including light can ever emerge
What is a likely cause of long lived gamma ray bursts?
Supernovae
