Surveying The Stars and Cosmology

Question 1

Define the luminosity of a star

Answer 1

Total energy emitted per second

Question 2

Define the brightness/intensity of a star

Answer 2

The power received from it per unit area at earth, =L / 4 x pi x R^2

Question 3

How does apparent magnitude describe a star?

Answer 3

Classifies stars according to their apparent brightness to the naked eye on earth, 1 is brightest 6 is dimmest

Question 4

What is the astronomical unit for distance?

Answer 4

Mean distance between earth and sun

Question 5

What is a parsec?

Answer 5

The distance at which 1 AU subtends an angle of of one arc second

Question 6

Define absolute magnitude

Answer 6

A stars apparent magnitude if it was at a distance of 10 parsecs from earth

Question 7

What is black body radiation?

Answer 7

A body which absorbs all wavelengths of EM radiation and can emit all wavelengths of EM radiation

Question 8

What is Wien’s displacement law?

Answer 8

The wavelength at peak intensity is inversely proportional to the absolute temperature of the object

Question 9

What is Stefan’s law?

Answer 9

The total energy per second emitted by a black body at absolute temperature T is proportional to its surface area and T^4

Question 10

Give two problems which occur when observing stars?

Answer 10

Atmosphere only lets certain wavelengths of light through and man made light pollution

Question 11

Define a continuous spectrum

Answer 11

Contains all possible wavelengths emitted by hot objects

Question 12

Define an emission spectrum

Answer 12

Only specific wavelengths from excited electrons in hot gases

Question 13

Explain how an absorption spectrum is formed

Answer 13

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

Question 14

What is radiation pressure?

Answer 14

Pressure exerted on any surface exposed to EM radiation

Question 15

For three stars with similar surface temperatures and known absolute magnitudes, which would be the largest?

Answer 15

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

Question 16

What is meant by a supernova?

Answer 16

A object that produces a rapid increase in brightness or rapid drop in absolute magnitude

Question 17

State the defining properties of a neutron star

Answer 17

Extremely dense and made up of neutrons

Question 18

Why are there concerns about a supernova occurring in a nearby part of the galaxy?

Answer 18

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

Question 19

Give two reasons why it’s difficult to directly observe an exoplanet

Answer 19

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

Question 20

What is a standard candle?

Answer 20

Objects whose absolute magnitude is known and apparent magnitude can be measured

Question 21

Describe the controversy caused by measurements of type 1A supernovae

Answer 21

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

Question 22

How would you work out the radius of an exoplanet from the ‘dip’ in brightness of a star?

Answer 22

Fractional drop in intensity at each dip equal to the ratio of area of the exoplanet disc to that of the star disc

Question 23

What are the minimum values on the absolute magnitude and temperature axis for the HR diagram?

Answer 23

Absolute magnitude - 15, temperature - 50000K

Question 24

What are the maximum values on the absolute magnitude and temperature axis for the HR diagram?

Answer 24

Absolute magnitude - (-10), temperature - 2500K

Question 25

What are two likely causes of short lived gamma ray bursts?

Answer 25

Neutron stars merging to form a black hole or a neutron star falling into a black hole

Question 26

Why is it important to not only use the transit method to detect exoplanets?

Answer 26

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

Question 27

Describe the link between galaxies, black holes and quasars

Answer 27

Supermassive black holes are found in the centre of galaxies and quasars are produced by supermassive black holes

Question 28

Explain what is meant by cosmic microwave background radiation and how it’s existence supports Big Bang theory

Answer 28

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

Question 29

Explain how the relative abundance of hydrogen and helium supports the Big Bang theory

Answer 29

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

Question 30

What are type 1 supernovae?

Answer 30

Supernovae which have no prominent hydrogen lines

Question 31

What are type 1A supernovae?

Answer 31

Supernovae which show strong absorption lines due to silicon and have a peak luminosity at about x10^9 that of the sun’s

Question 32

What are pulsars?

Answer 32

Rapidly rotating neutron stars which produce beams of radio waves

Question 33

What is the event horizon?

Answer 33

Sphere surrounding a black hole from nothing including light can ever emerge

Question 34

What is a likely cause of long lived gamma ray bursts?

Answer 34

Supernovae