Cosmology and Astrophysics

Define doppler effect

the apparent change of wavelength as a result of relative movement between a receiver and source

What happens to the oberserved wavelength of a wave source moving away from the observer?

increases

Define red shift

the apparent shift of the wavelength of light emitted by a star towards the red end of the light spectrum as result of star moving away

describe how the relative speed value inputted into the redshift equation differs depending on whether the source and receiver are nearing or receding

if the source and receiver are approaching each other, the value should be negative, otherwise positive

negative z value represents blue shift

What are the two main conclusions that can be drawn from observations of red shift from distant galaxies?

• all galaxy show red shift and so all galaxies are moving away

• the more distant a galaxy is, the greater its red shift and so faster it is moving away

• provides evidence for the BigBang theory that the universe is moving away from us, universe is expanding

CMBR stands for…

Cosmic Microwave Background Radiation

Hubble’s law

the speed at which a galaxy is moving away is directly proportional to the distance it is at

State the conclusion that can be made from Hubble’s Law and observations

the rate of expansion of the universe is accelerating, since galaxies are further away than predicted

Give one explanation for the acceleration of the universe’s expansion

the existence of dark energy - this explains where the energy required for the expansion rate to constantly accelerate comes from

What is the Astronomical Unit

the mean distance between the Earth and the Sun

What can parallax be used for

to calculate distances in space, using observations and trigonometry

What is the Hertzsprung-Russell diagram

a plot of a star’s stellar luminosity against its surface temperature

Describe how astronomers could determine the luminosity of a star

measure intensity and determine the distance to stars

then calculate using L=4 pi r² I - Stefan-Boltzmann law, only applies to black body’s radiators (all stars are)

Describe the life cycle of Sun

• the sun fuses to burn hydrogen into helium in its core

• when fusion ceases, the core of the Sun cools

• the core collapses under gravitational pull

• the sun expands and becomes a red giant

• the core becomes hot enough for helium fusion to begin

• helium begins to run out and the core collapses again

• temperature doesn’t rise enough for further fusion to begin

• so the Sun becomes a white dwarf star

State and explain the conditions necessary for fusion to occur in a star

• very high temperature to overcome repulsive forces between protons in nuclei

• very high density to give a sufficient collision rate

Describe the characteristics of a main sequence star

stars are converting hydrogen to helium in their core

stars on main sequence maintain a constant luminosity

Outline the experimental difficulties in determining the critical density accurately, rho = kH²

• v=Hd

• d difficult to measure for distant galaxies (due to is the main source of uncertainty when in the value of H)

• hence H is inaccurate

what is meant by the critical density of the Universe

density is large enough to prevent universe expanding for ever but not too big to cause a collapse of the universe

Explain how the value of the average mass-density of the universe will determine whether the universe is open or closed

• if closed: high density/above the critical density

• then gravitational pull sufficient to stop expansion

• open: lower than critical density, not sufficient to stop expansion, universe expands

Discuss the ultimate fate of the Universe (relate to dark matter and why fate is uncertain)

• universe may continue to expand or collapse back on itself

• fate depends on mass of universe

• since mass determines force on moving stars

• so far, not enough mass has been found to stop expansion

• there may be dark matter present which is currently undetectable

• dark matter has mass but does not emit e-m radiaiton

• the existence of dark matter will increase the density of the universe

Explain how standard candles are used in astronomy

• standard candle’s flux is measured

• unise inverse square law I = L/4pid²

• to calculate the distance to standard candle

Explain why stars have to be within a certain distance from the Earth for trigonometric parallax to be useful

trigonometric parallax is the change in position of a star against the background of more distant stars

parallax angle is the angle subtended at the star by the radius of the Earth’s orbit

if star is too distant, the angle is too small to measure and this gives a very large percentage uncertainty in distance

Describe how the distance to nearby stars is determined

• star is viewed from opposite ends of its orbit diameter about the Sun (six months interval)

• the radius of the Earth’s orbit around the Sun should be known

• the change in angle of the star against background of fixed stars is measured

• trig is used to calculate the distance of the star

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

• measure the wavelength emitted from the star

• measure change in wavelength between from the star to the wavelength emitted in the lab

• determine relative velocity using redshift formula

• then apply v = H₀d

what is redshift

what is doppler effect

• an increase in wavelength received from a moving away source

• the change in frequency/wavelength of wave due to the relative motion of source and observer

Explain why the observable universe has a finite size

• the universe started from a small initial point

• the universe has a finite age

• the observable universe is finite because we can only see as far as the speed of light * age of the universe

Wien’s law

• max wavelength is proportional to temperature

• lamda max T = 2.9 × 10^-3 mK

• only applies to black body radiators(all stars are)

Why more massive stars have higher luminosity

• experiences greater gravitation forces

• so have the greatest core temperature and density

• there’s a higher rate of fusion

• so the surface temperature is greatest

• there is also a high surface area

• so according to Stefan’s law, luminosity greatest

Why you can’t use Hobble’s law or trigonometry parallax when determining the distance of a star orbiting the black hole

• Hobble’s law only for cosmological distances, but the star is in out galaxy

• trigonometry parallax is for local stars because the parallax angle produces are large enough ot measure distance accurately

How to determine the size of the star using HR diagram

• calculate the luminosity of the star by using Wien’s law

• work out the temperature using HR diagram of luminosity against temperature

• then use Stefan Boltzmann equation of L=sigma*A*T^4 to find out the surface area and the radius of the star

Explain relative movement of nearby star is determined

• The Earth orbits around the Sun

• trigonometry parallax is used to determine the distance of the nearby star

• the position of distant star is “fixed“