Physics P7 Revision PowerPoint

Contents • • • • • • • • • •

Time, Eclipses and the Moon Astronomical Coordinates Refraction Lenses and Telescopes Astronomical Distances and Brightness The Scale of the Universe Gas Behaviour Fusion Stars Collaborative Astronomy

Sidereal Time • • •

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Sidereal day – time it takes for the Earth to spin once. 23 hours, 56 minutes.

If you look at the night sky for long enough you can see distant starts appearing to move east to west. The movement is apparent – it’s not the stars that move. It’s the Earth that spins. For a star to reach the same position in the sky, the Earth must spin 360 degrees. This is called a sidereal day.

Solar Time • • • • • • •

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A solar day is 24 hours long.

The Sun and the Moon also appear to move across the sky from east to west. A solar day is the time it takes for the Sun to appear at the same position in the sky. Solar and sidereal days are different because Earth orbits the sun as well as spinning on its axis. The Earth orbits the sun in the same direction as it spins. So, the Earth needs to spin slightly more than 360 degrees before the sun appears at the same position in the sky. The Moon appears to go slower than the Sun, taking about 25 hours to appear at the same position in the sky.

This is because the Moon orbits the Earth in the same direction as the Earth is rotating.

Which Stars Can You See? • The stars that you can see change depending on the time of the year. • As the Earth orbits the Sun, the direction we face changes slightly each day. • This means that we see a slightly different patch of the sky each night – so we see different stars.

• An Earth year is the time it takes the Earth to orbit the Sun once. • So, on the same day each year, you should be able to see the same stars in the night sky.

Moon Phases • • • • • •

The Moon reflects light from the sun. Only the half facing the Sun is lit up, leaving the other half in shadow. As the Moon orbits the Earth, we see different amounts of the Moon’s dark and litup surfaces. You see a Full Moon when the whole Moon is illuminated by the Sun. You see a New Moon when the illuminated half of the Moon is facing the Sun and the half that is in shadow is facing Earth. The rest of the phases are in between these two extremes.

Moon Phases Diagram 1 – New Moon 2 – Waxing Crescent 3 – First Quarter 4 – Waxing Gibbous 5 – Full Moon 6 – Waning Crescent 7 – Last Quarter 8 – Waning Gibbous

Lunar Eclipses • • • • •

Sometimes the Moon passes into the shadow of the Earth as it orbits. The Earth blocks sunlight from the Moon, so almost no light is reflected from the Moon and so it seems to just disappear. Total lunar eclipse – no direct sunlight can reach the Moon. Partial lunar eclipse - More often, the Moon isn’t fully in the Earth’s shadow – so only part of it appears dark. In both senses, the Moon is dark and harder to see.

Lunar Eclipse Diagram

Solar Eclipses • •

The Moon is (by chance) just the right size and distance away that when it passes between the Earth and the Sun, it can block out the Sun.

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Total solar eclipse – from some parts of the Earth the Sun is completely blocked out. Partial solar eclipse – again, from some parts of the Earth the Sun is not completely blocked out, but only partially.

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Partial solar eclipses are more common.

Solar Eclipse Diagram

Regularity of Eclipses •

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Eclipses are not very often occurrences. The Moon orbits the Sun at an angle to the Earth’s orbit around the Sun. So, most of the time, the Sun, Moon and Earth do not line up to cause eclipses. And when there is an eclipse, only small regions on Earth can see them.

Partial eclipses are more common than total eclipses because the three objects (Sun, Earth and Moon) do not have to line up perfectly for a partial eclipse.

Astronomical Coordinates • The position of stars are measured like distances on Earth – using a system like the longitude and latitude system.

• The sky appears to move as the Earth moves. There are two fixed positions that astronomers measure from:

• Pole Star – Doesn’t appear to move – nearly always above the North Pole. • Celestial Equator – In imaginary plane running across the sky – extending the equator of the Earth.

Angles in Astronomical Coordinates • Declination Angle – Celestial latitude, measured in degrees. • Right Ascension – Celestial longitude (‘how much across’) – measured in degrees or time.

• Right ascension increases the further east you go.

• It is possible to have an angle measured in time because the Earth turns 360 degrees through 24 hours.

Celestial Sphere Diagram

Orbit and Speed of the Planets • All planets orbit the Sun in the same direction, but they each move at different speeds.

• The closer to the Sun, the faster the planet is orbiting. • You can see Mercury, Venus, Mars, Jupiter and Saturn without a telescope. • Planets appear to gradually move from west to east.

Retrograde Motion • • • • • •

Every so often, planets appear to change direction (relative to the stars) and go in the opposite direction – creating a loop. This is retrograde motion. Only happens with the outer planets – Mars-Neptune. We see this motion because the planet is moving relative to the Earth. The Earth orbits faster than any of the outer planets, so sometimes we see the outer planets in FRONT of Earth, and then once we’ve caught up we see it BEHIND Earth. Mars appears to change direction once every two years or so. Slower-moving planets further out ‘change direction’ less frequently. See the diagram HERE for a better explanation!

Waves and Refraction • • • •

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Refraction as the change of wave speed and direction. Speed of a wave is affected by the density or the substance of which it is travelling through.

Light waves travel more slowly in a denser medium (usually). Wave speed = frequency x wavelength

The frequency for each type of wave is fixed – so if the speed changes, the wavelength also changes. For example if the speed decreases, then so does the wavelength.

‘Face On’ Refraction • The change in speed and wavelength can cause the wave to change direction too – this is called refraction.

• If light hits the boundary face on, it slows down but carries on along the SAME direction.

• It now has a shorter wavelength but has the same frequency.

‘Angular’ Refraction

• If a wave meets a different medium at an angle, part of the wave hits the bottom first and then slows down.

• The other part will carry on at first, faster speed for a while. • The wave changes direction and so we say it has been REFRACTED.

Lenses and Refraction • • • •

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Lenses use refraction to focus light waves to form an image of an object. As a light ray hits the surface of the lens and passes from air to glass, it slows down. This means the light ray bends towards the ‘normal’ (the line at right angles to the boundary at the boundary at the point where the ray enters or leaves). When it hits the ‘glass to air’ boundary on the other side it speeds up and bends away from the normal. The curvature of the lens means that all the parallel rays hitting the lens are bent towards the same focal point, where an image is formed of whatever the light is coming from.

Prisms • • • • • • •

Triangular prisms can refract light to form a spectrum. Different wavelengths of light refract by different amounts. White light is a mixture of lots of wavelengths of coloured light. White light disperses into its separate colours as it enters a prism. Rectangular prisms have parallel boundaries so the rays bend one way as they enter, then bend back by the same amount as they leave. Light leaving the glass is parallel to the light entering – so white light emerges. But triangular prisms do not have parallel boundaries. Which means the different wavelengths don’t recombine – so you get a nice rainbow spectrum!

Converging Lenses • • • • • •

Converging lenses bring the light rays together. It causes rays of light to converge to a focus. All lenses have a principal axis – a line which passes straight through the middle of the lens. The focal length of a lens is the distance between the middle of the lens and the focal point. Focal length is related to power. The more powerful the lens, the more strongly it converges parallel rays of light, so the shorter the focal length. To make a more powerful lens from the same material, you make it with a strongly curved surface.

The Lens Power Equation •

Lens power is measured in dioptres (D).

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Power (D) = 1 / focal length (m)

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Simple equation – simply plug the numbers in.

For example, if you have a 200mm lens (0.2m), then you do 1 / 0.2 = 5.0 – so the lens power is 5 dioptres.

Converging Lens Ray Diagrams – Point Source • • • •

1 – Mark focal point on principle axis. Draw 3 parallel rays from star (or whatever the object is) – one to the centre of the lens, one towards the top and the bottom. Only draw rays as far as the middle of the lens. 2 – Middle ray doesn’t get refracted – extend this ray past the focal point of the lens. 3 – Draw in the rest of the top and bottom rays – making them meet the middle ray above or below the focal point. This is where the image is formed – it is a real image because this is where all of the rays meet.

Converging Lens Ray Diagrams – Extended Source • Extended sources are things that you cannot treat easily as dots – for example a planet or a moon in our Solar System or a galaxy.

• 1 – Treat two opposite edges of the object as point sources. • 2 – As before, the parallel rays will meet in line with (or at) the focal point of the length.

• 3 – A real image is formed between the two points where the rays meet.

Refracting Telescopes • A simple refracting telescope is made up of two convex lenses, each with different powers.

• There is an objective lens and an eyepiece lens. • Objective lens collects light from the object that is being observed and forms an image of it. The eyepiece lens then magnifies this image so that we can view it.

How Refracting Telescopes Work • • • •

The lenses are aligned to have the same principal axis and are placed so that their focal points are in the same place. Many objects in space are so far away from Earth that by the time their light arrives on Earth, the light rays are effectively parallel. The objective lens converges these parallel rays to form a real image between the two lenses. The eyepiece lens is much more powerful than the objective lens because it is much more curved. It acts as a magnifying glass on the real image and makes a virtual image – this is where the light entering the eye lens appears to have come from.

The Magnification Equation •

The angular magnification of a telescope can be calculated from the focal lengths of the eyepiece and objective lenses.

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Magnification = focal length of objective lens / focal length of eye lens

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Or if you are given the powers of the lenses rather than their focal length:

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Magnification = power of eye lens / power of objective lens.

Astronomical Mirror Telescopes • • • • • • •

Most astronomical telescopes actually use a slightly concave mirror instead of a convex objective lens. Concave mirrors are shiny on the inside of the curve. Parallel rays of light shining on a concave mirror reflect and converge. They are like a portion of a sphere. The centre of the sphere is the centre of curvature. The centre of the mirror’s surface is called the Vertex. Halfway between the centre of curvature and the vertex is the focal point F. These points all lie on an axis. Rays parallel to the axis of the mirror, for example those from a distant star, reflect and meet at the focal point (as with lenses). By putting a lens near the focal point of the mirror to as an eyepiece, you can form a magnified image – just like the simple refracting telescope.

Diffraction • This is when waves spread out as they pass through a gap or past an object. • The amount of diffraction depends on the size of the gap relative to the wavelength of the wave.

• The narrower the gap, the more the wavelength and the more the waves will spread out.

• A narrow gap is about the same size as the wavelength of the wave.

Apertures •

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Some objects in the sky are so distant and faint that only a small amount of radiation from them can reach us.

To collect enough radiation to see these objects, you need a telescope with a huge objective lens or mirror. The diameter of the objective lens is called the aperture. The wider the aperture, the more radiation can get into the telescope and the image formed will be better.

Making large lenses is difficult because they are so heavy and they are very expensive – so instead a lot of big telescopes use mirrors. Big mirrors are much bigger to make more accurately than big lenses.

Aperture Size • • • • •

The size of the aperture must be larger than the wavelength. You get blurry images when radiation entering a telescope spreads out at the edges of the aperture. Diffraction can cause rings on your image because the light rays are spreading out. So, the way to resolve this is to have an aperture which is wider than the wavelength of radiation that you want to look at.. This way, radiation passes into through the aperture and into your telescope with very little diffraction – and so you will get a sharp image.

Diffraction Grating and Spectrum • A diffraction grating has very narrow slits – small enough to diffract light. • When white light passes through the gaps in a diffraction grating, the different wavelengths of coloured light are all diffracted by different amounts.

• The creates a spectrum of coloured light. • Astronomers can use these spectra to analyse the light coming from stars.

Computerised Telescopes • • • • • • • • •

The advantages: Astronomers don’t need to be at the telescope. They can program the telescope to track an object in the sky. Computerised telescopes can scan large objects in the sky – usually the telescope would have to be constantly repositioned – but the computers can do this. The telescopes can be precisely controlled by computers. Telescopes can be remotely placed at the top of hilltops to avoid light pollution. People can access these telescopes via the internet. Astronomers can work from home. Computers can control information from multiple telescopes or radio dishes. Using multiple telescopes means that observations can carry on throughout the day because astronomers can look at telescopes that are on the other side of the world in their daytime. Computers can record and process data from telescopes. And, computers means we can have telescopes in space – bypassing the atmosphere!

ET Life •

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It’s possible life exists outside of the Solar System. Our planet can’t be the only one that supports life.

Scientists believe that nutrients and things like water may be available on planets outside of the Solar System. There is evidence of planets orbiting around hundreds of nearby stars – it is likely there are lots more given the number of stars out there. Even if only a small proportion of stars have planets orbiting them, many scientists think there is probably life somewhere else in the universe. But so far, there is no concrete evidence that ET life exists now, or ever has done.

The Atmosphere and Observations • • • • • •

The atmosphere can interfere with astronomical observations. The atmosphere only lets certain bits of the EM spectrum through and blocks some others. Radio waves pass every easily through the atmosphere, but visible light can be badly affected. Light can get refracted by water in the atmosphere which blurs images. Light can also be absorbed by dust particles in the air – light pollution. Sites for astronomical observations on Earth are picked very carefully to try and minimise all of these problems. Or, we can take measurements above the atmosphere…

Space Telescopes • • • • • • • •

If you want to want to view EM radiation that is blocked by the Earth’s atmosphere, the best thing to do is put a telescope in space. The first space telescope, the Hubble Telescope, was launched by NASA in 1990. It can see objects that are about A BILLION TIMES fainter than you can see by standing outside and looking up on a cloudy night. But, getting a telescope into space is hard. And they can be difficult to repair. The first pictures from Hubble were blurry because the mirrors were the wrong shape. So they had to be changed. Space telescopes are expensive – Hubble cost over £3 billion to build, maintain and repair. Most astronomy is still done using Earth-based telescopes and astronomers have developed good techniques to remove the effects of the atmosphere. There are more telescopes on Earth, so it is easier to get to one because there is less demand.

Space Programmes • • • • •

These are projects that send people, machinery, telescopes and so on into space. They are expensive to undertake. The Apollo programme cost $135 billion in today’s money. Governments have to balance paying these sums of money for space exploration with other priorities such as defence, healthcare and coping with natural disasters. Many countries’ space programmes are linked – so cut-backs in one country can have a knock-on effect on the others.

Parallax • •

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Parallax is how we measure the distance to nearby stars. Parallax is the apparent change in position of an object against a dark background. It makes closer stars appear to move relative to the distant ones over the course of a year. The parallax angle is HALF the angle moved against distant background stars over 6 months (at opposite ends of the Earth’s orbit). The nearer an object is to you, the greater the angle.

Measuring Parallax

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The angle is often measured in arcseconds ‘seconds of arc’ rather than degrees.

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1 arcsecond = 1” = (1 / 3600) degrees.

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Parallax is useful for calculating distances to nearby stars. The smaller the parallax angle, the more distance the star is.

Astronomers use a unit of degree called the parsec . 1 parsec = about 3 light years.

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A parsec (pc) is the distance to a star with a parallax angle of 1 arcsecond.

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You can calculate the distance to a star in parsecs using this equation: Distance (pc) = (1 / angle (arcseconds))

Observed Intensity • • • • •

The luminosity of a star depends on its size and temperature. The bigger and hotter it is, the more energy it gives it. The brighter it is. As you move away from a star, it looks dimmer – this is because the energy that is reaching the star is reduced because it spreads out through space. The observed intensity of the light of star as seen on Earth depends on its luminosity and how far it is away from Earth. If you looked at two stars with the same luminosity, but one was further away, the one that was further would look dimmer.

Cepheid Variables • • • •

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A group of stars called Cepheid Variables pulse in brightness. How quickly they pulse is linked to their luminosity. The greater the luminosity of the star, the longer the time between pulses is (the pulse period). If you see two Cepheid Variable stars with the same observed brightness that pulse at different rates, you know that the luminosity of the star with the longer pulse period has the greater luminosity. Astronomers can work out the distance to a Cepheid Variable by comparing the luminosity (calculated from the pulse period) and the observed brightness of the star.

The Milky Way • • • • • •

They showed that the Sun is a star and is in the Milky Way. You can see about 1500 stars on a clear night. If you look through a small telescope, you can probably see about 500,000. Most of the stars appear to be in one bright, straight line – the Milky Way. Away from this line, the number of visible stars is much smaller.

The Milky Way is actually a spiral galaxy – but because we are a part of it, we see it edge on as a bright strip in the sky.

The Curtis-Shapley Debate • In the 1920s there was a debate about the size and structure of the Universe. • Harrow Shapley and Heber Curtis were famous American astronomers. • Through telescopes some people had seen some faint, fuzzy objects that they called nebulae.

• Some of these objects looked spiral-shaped but some were just blobs. • Shapley and Curtis argued about these nebulae were and where they were.

Shapely vs Curtis • • • •

Shapley believed that the universe was one gigantic galaxy – about 100,000 parsecs across. He believed that the Sun and Solar System were far from the centre of the galaxy. He believed that nebulae were huge clouds of gas and dust. These clouds were relatively nearby and actually part of the Milky Way.

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Curtis thought that the universe was made up of many different galaxies. He thought our galaxy was smaller than Shapley suggested. About 10,000 parsecs across with the Sun at the centre. The spiral nebulae were other very distant galaxies completely separate from the Milky Way.

Shapely vs Curtis – Who Won? • • • • • •

Both. Shapley was correct in saying that the Solar System is far from the centre of our galaxy.

Curtis was right in saying that there are many galaxies – about 100 million of them! Curtis was right about the spiral nebulae. Hubble used Cepheid Variable stars that they’re really far away. The debate wasn’t over until the 1930s when better telescopes meant that we could see the nebulae much more clearly, rather than just blurry blobs.

Hubble • • • • • • • •

Hubble showed that there were objects outside of our galaxy. He helped to solve the debate by showing observations of the Andromeda nebula. Using the latest telescopes which were available at the time he proved that this spiral-shaped fuzzy blob actually contained many stars, some of which were Cepheid Variables. He calculated the distance to the Andromeda nebula by working out the distance to the Cepheid Variables within it, using their relationship between their brightness and pulse period. He found that it was about 2.5 million light years away. Which is much further than any stars in our galaxy! He studied other spiral nebulae – and found that they were too far away to be in the Milky Way and must be in separate spiral galaxies themselves. Astronomers tend to use units of Megaparsecs when talking about distances to objects that are outside of the Milky Way. 1 Megaparsec = about 3x10^19 km. The distance to the nearest spiral galaxy is about 0.8 Mpc.

Redshift •

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When a galaxy is moving away from us the wavelength of light from it changes – the light becomes redder. This is redshift. By seeing how much the light has been red-shifted, you can work out the recession velocity of the galaxy – which is how quickly it is moving away. The greater the redshift, the greater the speed of recession.

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The more distant the galaxy, the faster it moves away from us.

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The Big Bang Theory • Redshift suggests that the universe is expanding from a single point. • This could be explained by an initial explosion millions of years ago that started off the expansion.

• This theory is accepted by most scientists. • Basically TBBT says that all the matter and energy in the universe was compressed into a very small space and then it exploded 14,000 million years ago and has been expanding ever since.

Calculating Recession Speed •

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Redshift is fairly easy to measure – so the galaxy’s recession velocity can be calculated easily.

Speed of recession (km/s) = Hubble constant (s^-1) x distance (km) Or: Speed of recession (km/s) = Hubble constant (km/s per Mpc) x distance (Mpc) The Hubble constant can have the units s^-1 or km/s per Mpc.

The value of the constant is roughly 2x10^-18 s^-1 or 70km/s per Mpc. But the value is still being researched and there are a lot of different ways to calculate it. But the different methods come up with slightly different answers. Using data on Cepheid variable stars from distant galaxies has given us better values of Hubble’s constant.

The Kinetic Theory • • • •

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The Kinetic Theory says that gases are randomly moving particles. Oxygen consists of oxygen molecules, neon consists of neon atoms and so on. These particles are constantly moving in random directions. They are constantly colliding with one another and with the walls of their container.

When they collide, they bounce off each other and the container. The particles take up hardly any space. Most of the gas is empty space.

Gases and Temperatures • Increasing the temperature gives the particles more kinetic energy. They move around and/or vibrate more quickly.

• The coldest temperature possible is -273C. Absolute Zero. • At absolute zero atoms have the smallest amount of kinetic energy possible. • Absolute zero is 0 on the Kelvin scale. • Kelvin and Celsius are very similar but the only difference is where they begin.

Kelvin-Celsius Conversion

• To go from Celsius to Kelvin, add 273. • To go from Kelvin to Celsius, subtract 273. Absolute zero

Freezing point of water

Boiling point of water

Celsius

-273C

0C

100C

Kelvin

0K

273K

373K

Kinetic Energy and Temperature • Kinetic energy is proportional to temperature. • Anything that is moving has kinetic energy. • Increasing the temperature of the gas gives its particles more energy – as you heat up the gas, the average speed of the particles increases.

• If you double the absolute temperature (Kelvin), you will double the kinetic energy of the particles.

• The relationship is proportional.

Gas and Volume • • • • •

Decreasing the volume means increasing the pressure. Gas particles have the same mass, so when they collide they exert a force onto something. In a sealed container, gas particles collide with the container walls, exerting a pressure on that. If you put the same amount of gas in a bigger container, the pressure of the gas will decrease. Fewer collisions between the particles and the container walls. When the volume is reduced, the particles get more squashed up, so they hit rhe walls more often and as a result the pressure goes up!

Gas Pressure and Volume Equation • •

The volume of the gas is inversely proportional to its pressure at a constant temperature.

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At constant temperature: Pressure x volume = constant

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For example – halve the volume = double the temperature.

Example: ‘A gas a constant temperature in a 50ml container has a pressure of 1.2 atmospheres. Find the new pressure if the container is reduced to 40ml.’ P x V=C gives 1.2x50 = 60 – so 60 is our constant. Then do 60/40 = 1.5 – so 1.5 atmospheres is our new pressure.

Gas Pressure and Temperature • • •

Increasing the temperature increases the gas pressure. If you heat the gas the particles are moving faster because they have more kinetic energy. Increased kinetic energy means the particles are hitting the walls of the container and each other more other, increasing the pressure.

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Pressure is proportional to absolute temperature. At a constant volume: Constant = pressure / temperature (K)

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Pressure can be measured in Bars, Pa or Atmospheres. 1 bar = ~1 atmosphere.

Gas ‘Expansion’ • • •

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If a gas stays at a constant pressure, heating it up will also increase the volume. The gas will ‘expand’. The molecules are further apart so the collisions are not as frequent – but with more force because they have more kinetic energy.

At constant pressure: Constant = volume / temperature (K)

The Discovery of Fusion •

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It was only discovered in the early 20th century. Before that, scientists used to say that the Sun burned up its own material. But then it was realised that the Sun would need an infinite amount of fuel to keep burning up its own material indefinitely. It was then suggested that hydrogen was turning into helium inside the Sun and that when this happens the mass gets ‘lost’. They measured the mass of helium and hydrogen atoms to figure that out. The missing mass was changed into energy and powered the sun. Hans Bethe got the Nobel Prize for explaining fusion.

Nuclear Fusion • • • • • •

Nuclei need to be brought close together in order for fusion to work. In essence, these nuclei join (‘fuse’) together to form a bigger nucleus. This is nuclear fusion. In stars, hydrogen nuclei fuse together to make helium nuclei. Energy is liberated when lighter nuclei fuse together to make heavier nuclei up to the size of an iron nucleus. Nuclei can only fuse like this if they are brought close together. For that to happen you need a lot of energy. High temperaturees and pressures are required.

Converting Mass into Energy •

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Nuclear fusion converts mass into energy.

E=mc^2 E= amount of energy released. M= amount of mass lost. (Energy = mass x (speed of light in a vacuum)^2) You can calculate how much energy is released during fusion or fission using the equation above.

Nuclear Equations • • • • • • • • • • • • •

Two hydrogen nuclei combine to make a larger hydrogen nucleus: 1-1H + 1-1H -> 2-1H + 0-1e + energy Top left: 1+1 = 2 Top right: 2+0 = 2 Bottom left: 1+1=2 Bottom right: 1+1=2

A small hydrogen nucleus and a larger hydrogen nucleus combine to make a helium nucleus: 1-1H + 2-1H -> 3-2He + energy Top left: 1+2=3 Top right: 3 Bottom left: 1+1=2 Top right: 2

Two helium nuclei combine to make a larger helium nucleus: 3-2He + 3-2He -> 4-2He + 1-1H + 1-1H + energy Top left: 3+3=6 Top right: 4+1+1=6 Bottom left: 2+2=4 Bottom right: 2+1+1=4

Balanced!

Continuous Spectra • • • • • • • •

Continuous spectra contain all possibly frequencies. This makes a continuous spectrum (one without any gaps). Hot objects like stars emit radiation. Hot objects ALWAYS emit more of one frequency than the others. This wavelength is called the peak frequency. The peak frequency emitted by an object depends on its temperature. The higher the temperature, the more energy the photons radiated will have – and so the peak frequency is higher. Red = lower frequency = cool object (let’s say a star). Blue = high frequency = hot star. You can tell how hot a star is by looking at its colour. ^

Line Spectra • The theory of electrons moving between energy levels. • Electrons can only be in certain energy levels/shells around the nucleus. • The lowest energy level is right by the nucleus. • Electrons can move between energy levels if they gain or lose electrons – ionisation.

Absorption Spectra • At high temperatures electrons can become excited and jump into higher energy levels by absorbing radiation.

• Only certain levels can occupy electrons, so electrons absorb a particular frequency of radiation to get to a higher energy level.

• Electrons in gas atoms absorb a particular frequency of light, making gaps in the otherwise continuous spectra.

• These gaps appear as dark lines.

Emission Spectra

• Electrons are unstable in the higher energy levels so they tend to fall from higher to lower levels.

• They lose energy by emitting radiation of a particular frequency. • This gives a series of bright lines formed by the emitted frequencies.

Using Spectra • • • •

Spectra can be used by astronomers to work out what stars are made out of. Energy levels in atoms are different for each element. So each element has its own line spectrum. The photosphere (surface) of a star emits a continuous spectrum of radiation. This radiation produces emission and absorption lines in the spectrum. By looking at the position of these lines in the star’s spectrum, you can work out the chemical elements that are present in the star’s atmosphere – by comparing it with known spectra in the lab.

The Beginning of a Star • • • • •

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They are born in clouds of dust and gas – hydrogen and helium. Gravity causes the denser regions of the cloud to contract very slowly into clumps. When these clumps get dense enough, the cloud breaks up into protostars. Photostats continue to collapse under gravity – reducing in volume. This means the particles are more squashed up, increasing the pressure and the temperature. Eventually the temperature at the temperature at the centre of the protostar reaches a few million degrees and hydrogen nuclei begin to fuse to make helium nuclei.

This releases a huge amount of energy and creates enough outward pressure/radiation pressure to stop gravitational collapse. Now the star has reached the main sequence stage. It will stay like that relatively unchanging whilst it fuses hydrogen into helium in the core.

The Composition of Stars •

The closer to the core/centre, the hotter the temperature in the star is.

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The Core – This is the centre of the star where most of the fusion takes place.

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This means that the nuclei are close enough to fuse together.

Pressure from the weight of the rest of the star makes the core very hot and denser than the rest of the star.

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The Photosphere – The surface of the star. The outer region where energy is radiated into space.

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This is the part of the Sun we can see from Earth.

Energy released from fusion is transported by photons of radiation and convection currents to the surface of the star.

Main Sequence Stars • • • •

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A star stops being in the main sequence when it runs out of hydrogen in the core. It then starts to fuse things like helium nuclei, carbon, neon, silicon and stuff like that together. This is what most of the stuff in the universe is made out of. It swells up to become a red giant or a supergiant star. In the process of swelling up, the star’s photosphere cools down.

More massive stars can fuse heavier nuclei. The more massive the star is, the hotter the core is. The hotter the core, the heavier it can fuse together – so it can create even heavier nuclei. Really massive stars can create iron.

The Life Cycle of Stars • • • • • •

Big Stars:

Main Sequence Star Red Supergiant (Mostly iron core) Supernova (explosion) Neutron Star OR a Black Hole

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Small Stars:

Main Sequence Star Red Giant White Dwarf (outer layers lost and leave a white dwarf)

The Life Cycle of Stars – Stages 1-4 • •

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Stage 1: All stars change when there is not enough hydrogen in the core for hydrogen fuse to carry on. Core shrinks, rest of star expands, photosphere cools. Low mass stars (eg the Sun) become red giants whilst high mass stars become red supergiants.

Stage 2: Core gets compressed by surrounding matter of the star and shrinks until the pressure and temperature of the core is high enough for helium fusion to begin. The star releases energy by fusing helium into larger nuclei such as carbon, nitrogen and oxygen. Stage 3: The core becomes unstable and gets compressed by the rest of the star when there is too little helium in the core for helium fusion to begin. Stage 4: Red giants don’t have enough mass to compress the core – so there is no more nuclear fusion. Outer layers of star are thrown off into space and the core shrinks to become a hot white dwarf. There is no nuclear fusion in hot white dwarf stars, so the star gradually cools down and fades.

The Life Cycle of Stars – Stages 5-7 • • •

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Stage 5: Red supergiants do have enough mass to increase the pressure and temperature of the core to fuse larger nuclei. Each time an element in the core becomes depleted the core shrinks until it is hot enough and at a high enough pressure for fusion to occur. This happens until the most of the core has been fused into iron.

Stage 6: Even red supergiants can’t fuse into iron – the core collapses and the star explodes as a supernova. This creates nuclei with greater masses than iron.

Stage 7: Core collapses to form a neutron star – or if there is enough matter a black hole from which even light cannot escape.

Luminosity vs Temperature • To the left is a Hertzsprung-Russell Diagram.

• Different types of stars group together in different groups if you plot luminosity against temperature.

• The different sections how the main stages of the life cycle of a star. The reason you see these different areas is because stars exist in the stable stages of their life cycle for long periods of time. You don’t see the unstable phases such as supernovas on the diagram because they happen too quickly.

Collaboration • • • • • •

Many space projects are too expensive for one country to undergo alone. Big projects are only possible if several countries collaborate together, for example the ISS, lead by the US by backed by 15 other countries. Each country has their own section of the space station. By working together you can get the best people and the best facilities for the job. For example the Americans and the Russians know about how to launch equipment into space. The European Extremely Large Telescope is a project involving astronomers from across the whole of the Europe, but it’s based in Chile. It is too complex and expensive for a single country to build and operate.

Observatory Locations • •

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Observatory locations are carefully chosen. Optical observatories are often put in remote locations, for example the Atacama Desert in Chile and the Roque de los Muchachos in the Canary Islands.

The idea is to avoid light pollution and dust particles which can interfere with observing. These are caused by street lighting and exhaust fumes. Astronomers want as little atmosphere between the observatory and the telescope as possible to minimise the distorting and blurring effects it has. So, observatories are built at high elevation where the atmosphere is thinner and has less effect on the light.

The Muna Kea Site in Hawaii is about 4,200 metres above sea level. Water can also cause problems by refracting light – so a dry location with low atmospheric pollution is a good location for a telescope. There are observatories in Australia and the Atacama Desert in Chile. Clouds obviously block the telescope’s light, so observatories are built in places where there are common cloudless nights.

Other Factors • • •

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Cost – There is the cost of building, running and then closing the observatory.

Access – Roads may need to be built and electricity and water and gas needs to be supplied. Some places are hard to access. Environment – Need to be careful about the environment and not disturbing wildlife and so on.

Social – Those who work need facilities such as accommodation and electricity and gas and water. Observatories do provide jobs, though.