Astrophysics - A level Physics Edexcel

What is a black body?

A black body is a perfect radiator and absorber of EM radiation. It gives off energy across the entire EM spectrum (ie at all different wavelengths).

What is lambda max?

Wavelength where you get peak energy output

Which law tells you how bright a star is?

Stefan-Boltzmann law

Define luminosity

Power measured at the star / The rate at which energy is radiated in all directions by an object

What do you measure luminosity in?

Watts

What is the radiant energy flux/intensity?

Power per unit area at distance d

How do you work out the time period of a standard candle?

You need to take it twice and mean average it

How far is a light year in metres?

9.46 x 10^15m

What is one astronomical unit?

The radius of the Earth's orbit around the sun

Define parallax

The apparent motion of an object, relative to its background, when it is viewed from different positions.

How is the amount of parallax used?

To determine its parallax angle.

How do you increase the percentage certainty of measurements of nearby stars?

Use the diameter of the Earth's orbit as a baseline, taking measurements 6 months apart.

How many arc seconds are there in a degree?

3600

What is the equation for distance in parsecs?

Distance = 1/ angle (arcsec)

What is the doppler effect?

The relative change in the wavelength and frequency of a wave due to relative motion between a source and a observer.

What is the spectrum of absorption lines of elements observed from distant galaxies is similar to but not the same as?

The absorption lines as observed in the lab.

What does the letter Z mean?

The amount of redshift.

What does the positive and negative values of red shift mean?

Positive value means red shift as its moving away. Negative value means blue shift as moving towards.

What happened when Edwin Hubble observed the redshift of galaxies?

He put the results onto a graph to show the distance a galaxy is away with the recessional speed.

What did Hubble's results show?

The further a galaxy is away, the greater the velocity.

What is Hubble's law in words?

Recessional velocity = Hubble constant x distance the galaxy is from us

What is Hubble's constant?

71 km/s/Mpc

How does Hubble's constant tell us the age of the universe?

Using v=Hd you can rearrange it to find the time taken for the galaxy to move as t=d/v=1/H

What is the corresponding expansion of the universe for red shift z?

z + 1

Why is the x axis of a Hertzsprung-Russel diagram log T?

The range of temperatures of the surfaces of stars is large

Classify a neutron star

Core remnant

Classify a black hole

Core remnant > 2.5 solar masses

Classify a white dwarf

Core remnant < 1.4 solar masses

What is a main sequence star?

A star in the main stable phase where it fuses hydrogen into helium

What is a standard candle?

An object that has a known luminosity.

How does a main sequence star become a red giant?

Hydrogen burning ceases in core so the core collapses. The star expands and the hydrogen burning then takes place in the shell.

How can you show that a lambda max against temperature graph a black body radiator?

Two values to show lambda max T = constant

Why does the thermal motion of helium atoms cause the broadening of spectral atoms?

There is a change in wavelength of emitted light because the helium atoms are moving. For particles moving away from the observer there will be an increase in wavelength and for particles moving towards the observer there will be a decrease in wavelength.

How can distances too large for the use of standard candles be calculated?

Measure change in wavelength and determine relative velocity using redshift formula. Then apply hubble's law.

The more distant the galaxy, the greater the shift towards a larger wavelength. What conclusions can we draw from this?

The galaxies are moving away from us, the universe is expanding and the greater the distance the greater the velocity of the galaxy.

How is a standard candle used to measure cosmic distances?

You measure the stars intensity on Earth and then use the inverse square law to find the distance.

How were astronomers led to propose the existence of dark matter?

Dark matter has mass but does not emit EM radiation. It was proposed when observations of galaxies indicated that they must contain more matter than what could be seen.

What are the consequences of the existence of dark matter for the ultimate fate of the universe?

The existence of dark matter will increase the average density of the universe and this makes it more likely that the universe is closed.

Explain why the observable universe has a finite size

The universe started from a small initial point (Big Bang). Some parts of the universe are so distant that their light has not had time to reach us yet.

Outline the life story of a white dwarf star starting from when it was a main sequence star

It fuses helium and becomes a red giant. The red giant ceases fusion and becomes a white dwarf. The white dwarf becomes a black dwarf.

What determines whether a main sequence star becomes a white dwarf?

The mass of the star - it will become a white dwarf if it is less than 8 solar masses

Explain why the most massive main stars have the shortest lifetimes

Greater mass means greater luminosity and temperature so when it burns hydrogen it runs out of fuel quicker as it fuses it at a greater rate and leaves the main sequence after hydrogen burning ceases.

What is a similarity between the sun now and when it becomes a red giant?

Fusion in core

What is a difference (apart from size, age and colour) between the sun now and when it becomes a red giant?

Hydrogen vs Helium fusion

What is a white dwarf?

A small hot dead star that consists of products of fusion such as carbon, nitrogen and oxygen

How does the high temperature in stars arise?

Gravitational force does work on hydrogen which increases the internal energy of gas

Define intensity

Power per unit area measured at the Earth

Describe how the pulsations of a Cepheid variable can be used to estimate the distance to the galaxy in which it is found

Measure the period and hence work out luminosity. Then measure intensity and use I=L/piD^2

Briefly explain how the value of the average mass-energy density of the Universe will determine whether the Universe is open or closed

It will be closed if there is a high density as there would be a gravitational force sufficient to cause the big crunch.

Ultrasound is used to investigate the blood in an artery in a human body by detecting a Doppler shift. What is Doppler shift used to measure?

Velocity of the blood

How can astronomers determine the luminosity of a star?

The temperature and type of star was identified. The HR diagram is used to find the luminosity.