Chapter 12 - Our Place In The Universe

1. Radiation is emitted from a very hot region of a star (called the photosphere) in a continuous spectrum.

2. Atoms in the atmosphere of the star absorb certain wavelength of the radiation, producing dark absorption lines within the spectrum.

3. Different atoms absorb different parts of the spectrum, resulting in a characteristic pattern for each atom. By looking at the absorption lines from a star, the composition of the stellar atmosphere can be worked out.

4. 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 recorded in the lab. This shows how much the spectrum has shifted by the movement of the star.

You can tell if an ambulance or police car is moving towards or away from you by the pitch of the sirens. You receive a higher frequency sound when it is moving towards you than when it is moving away. This id due to the Doppler effect, the difference between the frequency of a wave emitted and that of the wave received when there is relative movement of the wave source and a observer.

· When the car is moving away from you the sound is travelling in the opposite direction from the car, so are stretched out – ie. They have a longer wavelength and lower frequency when they reach you.

· The opposite happens when the car is moving towards you – the sound waves bunch up, so have a shorter wavelength and higher frequency when they get to you.

The doppler shift happens with all waves, so has a wide range of applications, for example police radar guns, to measure the speed of cars using microwaves, and cosmologists measure speed of distant stars using the light they emit.

You can calculate the speed of distant objects relative to the Earth by measuring their movement affects radiation they emit. The methods is based on the principle that an atom will emit and absorb radiation with the same, characteristic spectrum whenever it is – i.e. on earth or in a distant star.

For example, if you detect the radiation emitted by a hydrogen atom in a star and compare it with the radiation emitted by a hydrogen atom here on Earth. You can compare how much the radiation has ‘shifted’ and so determine the velocity of the star relative to the Earth.

How the radiation is shifted depends on the movement of the object:

· When an object is moving away from the Earth, the wavelength of its radiation get longer and the frequencies get lower – i.e. they shift towards the red end of the spectrum, so this is red-shift.

· When an object is moving towards Earth, the opposite happens and the radiation undergoes blue-shift.

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The equation for this is:

v/c =∆λ/λ

As long as v is a lot less than c.

1. Distances between objects in the solar system are enormous- one way of measuring them is by radar!

2. A short pulse of radio wave is sent from a radio telescope towards a distant object, (e.g. asteroid). When the pulse hits the surface of the object, it is reflected back to Earth.

3. The telescope picks up the reflected radio waves, and records the time, t, taken for them to return.

4. Radio waves in space, like all electromagnetic waves, travel at the speed of light, c, so you can work out the distance, d, to the object from:

2d=ct

5. Like most physics this is made on assumptions:

· The speed of the radio waves is the same on the way to the object and the way back to the telescope.

· The time taken from the pulse to reach the object is the same as the time taken for it to return.

· For this to be true the speed of light needs to be constant.

· The brightness of a star in the nights sky depends on two things – its luminosity (i.e. how much light energy it gives out in a given time) and its distance from us. So the brightest stars will be close to us and have a high luminosity.

· How bright a star looks from Earth is called its apparent magnitude – this depends on it absolute magnitude (i.e. how bright it really is) and how far away it is.

· So, to find the distance, you need to measure how bright it looks (apparent magnitude) and calculate how bright it really is. Don’t need to know the equation!

· This method works for objects that you can calculate the brightness of directly – called standard candles. Cepheid variable stars are examples of standard candels because the brightness changes in a certain pattern. So if you can find a Cepheid variable within a galaxy, you can work out how far the galaxy is from us.

Parallax is an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

On astronomical unit (AU) is defined as the mean distance between the Earth and the sun.

1 AU = 1.50 X 10 ^11

The distance that electromagnetic waves travel through a vacuum in one year is called a light year (ly).

1 ly = 9.46 x10^15

1. A big assumption of the methods previously is that the speed of the object being studied is a lot less that the speed of light.

2. This matters because as odd as it seems, time runs at different speeds for two objects moving relative to each other- but it is only really noticeable close to the speed of light. So you can ignore it as long as the object isn’t travelling too fast.

3. The effect is called relativistic time dilation. It males simplest equations like the Doppler effect a lot more complicated.

We live in a expanding universe. Evidnence for this comes from the shift in frequency and wavelength of light received from galaxies. The light we are receiving from galaxies all shows red-shift. The amount of red-shift depends on the speed of the galaxy relative to the Earth, known as recession. Hubble discovered that:

The speed of recession of a galaxy is proportional to its distance from the Earth.

This is known as Hubble’s Law, and it gives rise to Hubble’s constant:

V = Hd

Where v is the speed of a galaxy relative to Earth, d is its distance from earth and H is the Hubble’s constant. H has units s^-1 but its value is not known precisely known.

Since the universe is expanding uniformly away from us it seems that we must be at the centre of the universe but this is an illusion. You would observe this everywhere in the universe.

1. If the universe has been expanding at the same rate for its whole life, the age of the universe is t = 1 /H (time = distance / speed).

2. Since no-one know the exact value of H, we can only guess the age of the universe, which scientist recon is 13 billion years.

3. The absolute size of the universe is unknown, however there is a limit on the size what we call the observable universe, this is with planet Earth at the centre and the maximum distance that light can travel during the age of the universe, the sphere of the observable universe has a radius of 13 billion light years across.

If the universe has been expanding, it makes sense to think sometime in the past it must have been a lot smaller. If you go further enough back in time, the whole universe must have come from a single point – which is the basis of the big bang theory!

The big bang theory – The universe started off very hot and very dense and has been expanding ever since.

It is the best explanation we have got! But there is other evidence that also supports the big bang theory. For example the big bang model predicted loads of electromagnetic radiation was around at the very early universe, this radiation is still around today. Because the universe is expanding, the wavelength of this cosmic radiation has been stretched and is now in the microwave region.

1. In the late 1980’s cosmic radiation was really looked at, it was found that it had a continuous temperature of 2.73 K.

2. The radiation is largely isotopic and homogeneous – it’s about the same intensity whichever direction you look.

3. The big bang theory also explans the large abundance of helium in the Universe.

4. The early Universe has been very hot, and at some point is must have been hot enough for hydrogen to fusion to happen. This means with the theory of the synthesis of the heavier elements in stars, the relative abundances of all of the elements can be accounted for.