How does radar work?
A short pulse of radio waves are sent from a radio telescope towards a distant object. When the pulse hits the surface of object , it's reflected back to Earth. The telescope picks up the reflected radio waves and records the time,t, taken for them to return. The distance of the planet can then be worked out using speed = distance/ time .
Explain how radar can be used to find the average speed of an object relative to Earth.
You send two pulses separated by a certain time interval, to give two separate measurements of the object's distance. The difference between the distances shows how far the object has moved in the time interval.
What are the assumptions are made when using radar to measure planet distances?
1) The speed of the radio waves is the same on the way to the object and the way back to the telescope.2) The time taken for the radio waves to reach the object is the same as the time taken to return.
What is a more accurate method of measuring planet distances?
Using Doppler shifts
Why are there no relativistic effects when measuring planet distances with radar?
Even though the observer and object are both moving and the object's speed must be much less than the speed of light, so that there are no relativistic effects.
What two things does the brightness of a star depend on?
1) Luminosity ( how much light energy it gives out in a given time.)2) Distance from Earth (if we ignore light pollution.)
Define apparent magnitude
How bright a star looks when seen from Earth.
What does the apparent magnitude of a star depend on?
Depends on absolute magnitude(how bright it really is.) and how far away it is.
How can you find the distance of the star?
1) You measure how bright it looks (apparent magnitude)2) You calculate how bright it really is (absolute magnitude)3) And then you put these two variables into an equation.
Give an example of the type of stars that can have their distance calculated using apparent magnitude and absolute magnitude.
This method works for objects that you can calculate the brightness of directly - called standard candles.Cepheid variable stars are examples of standard candles because their brightness changes in a certain pattern.
Define astronomical unit (AU)
One astronomical unit (AU) is defined as the mean distance between the Earth and the Sun.
Define light-year (ly)
The distance that electromagnetic waves travel through a vacuum in one year is called a light-year (ly).
One light-year is equivalent to what AU?
What is the approximate length in metres of an astronomical unit?
1.50 x 10^11 m
What is the approximate length in metres of a light-year?
9.46 x 10^15 m
How many light years is Sun to the next nearest star, observable universe and the Milky Way galaxy?
1) 4.2ly2) 14 billion ly3) 100 000 ly
How far can we look back in time? why?
When we look at the stars we're looking back in time, and we can only see as far back as the beginning of the Universe.
Explain the Doppler effect.
When a sound source is still you hear the same pitch. When the sound source is moving away from you, you hear a lower pitch. Because the movement of the sound source courses the waves to stretch and have a higher wave length and hence lower frequency.When the sound source is moving towards you, you will hear a higher pitch. This is because the waves will bunch up causing the waves to have shorter wavelengths and a higher frequency.
What decides how effective the Doppler effect is?
How much the sound waves change depends on how fast the sound source is moving. The greater the sound source is moving, the larger the change. Hence the Doppler effect can be used to measure the speed of moving objects.
Give some examples of where the Doppler effect is used?
Police radar guns measure the speed of cars using microwaves, and cosmologists measure the speed of distant stars using the light they emit.
What is the principle that the Doppler effect used to measure distances is based on?
The method is based on the principle that an atom will emit and absorb radiation with the same, characteristic spectrum wherever it is - i.e. Earth or in a distant star.
Give an example of how the Doppler effect is used to measure the distances a far away objects.
If you detect the radiation emitted by a hydrogen atom in a star and compare it with the radiation emitted by a hydrogen atom on Earth, you can calculate how much the radiation is "shifted" and so determine the velocity of the star relative to the Earth using an equation.
When object is moving away from Earth, wavelengths of radiation become longer, towards the red end of the spectrum.
When an object is moving towards Earth, wavelength of radiation become shorter, towards the blue end of the spectrum.
What is the equation for finding the velocity of an object from the radiation it emits and absorbs?
v/c = Change in wavelength / wavelength
What part of a star emits the radiation?
When recording the spectrum of a star, what do the black lines indicate?
The star emits a continuous spectrum of radiation, atoms in the stars atmosphere of the star absorb certain wavelengths of the radiation, producing dark absorption lines within the spectrum.
What can be calculated about the star using the absorption lines?
By looking at the absorption lines from a star, the composition of the stellar atmosphere can be worked out.
How can a stars spectra be used to calculate how much the stars spectra has been shifted by the movement of the star?
Once you've worked out which atoms make up the pattern of absorption lines, you can compare the position of the absorption lines for each atom in the star's spectrum with the same spectrum recorded in the lab.
What is relativistic time dilation?
Time runs at different speeds for two objects moving relative to each other - but it's only really noticeable close to the speed of light.
What is the relativistic factor?
When does the relativisitic factor equal 1?
When v << c, the time measured by both observers is almost the same, t=t'.
What did hubble notice about planets recessional velocities and distance from Earth?
The planets recessional velocities are proportional to their distance from Earth.
What is the value for Hubbles constant H° ?
Generally accepted to be between 50kms^-1 Mpc^-1 and 100 Kms-1Mpc-1
What is the SI unit for Ho?
What did Hubbles work show?
By showing that objects in the Universe are moving away from each other, Hubble's work is strong evidence that the Universe is expanding.
How do cosmologists tend to look at Hubbles discovery about the expansion of objects in the Universe?
The galaxies are not actually moving through space away from us. Instead, space itself is expanding and the light waves are being stretched along with it. This is called cosmological redshift to distinguish it from redshift produced by sources that are moving through space.
What is the illusion created by Hubbles discovery about the expansion of objects in the Univerise?
Since the Universe is expanding uniformly away from us it seems as though we're at the centre of the Universe, but this is an illusion. You would observe the same thing at any point of the Universe.
If the Universe has been expanding at the same rate for its whole life, then what is the age of the universe?
t = 1/ Ho
Is the size of the Universe known?
The absolute size of the Universe is unknown(and changing) but there is a limit on the size of the observable Universe.
What is the Hot Big Bang theory?
The Universe started off very hot and very dense(perhaps as an infinitely hot, infinitely dense singularity) and has been expanding ever since.
Explain what cosmic microwave background radiation is.
The Hot Big Bang model predicts that loads of electromagnetic radiation was produced in the very early Universe. This radiation should still be observed today (it hasn't had anywhere else to go.). Because the Universe has expanded, the wavelengths of this cosmic background radiation have been stretched and are now in the microwave region.
When the satellite the Cosmic Background Explorer was sent up to have a detailed look at the radiation, what did it find?
- It found a continous spectrum corresponding to a temparature of 2.73k.
- The radiation is largely isotropic and homogeneous - it's about the same intensity whichever direction you look.
- There are tiny fluctuations in temperature, which were at the limit of COBE's detection. These are due to tiny energy-density variations in the early Universe, and are needed for the initial 'seeding' of the galaxy formation.
- The background radiation also shows a Doppler shift, indicating the Earth's motion through space. It turns out that the Milky Way is rushing towards a unknown mass at over a million miles an hour.
Why does the HBB model explain the large abundance of helium in the Universe?
The early Universe had been very hot, and at some point it must have been hot enough for hydrogen fusion to happen. This means that, together with the theory of the synthesis of the heavier elements in stars, the relative abundances of all of the elements can be accounted for.