Observing the Sky
- A sidereal day is the time taken for a star to return to the same position in the sky. This takes about 23 hours and 56 minutes.
- A solar day is the time taken for the sun to appear at the same position in the sky. This is 24 hours.
- A sidereal day is 4 minutes shorter than a solar day.
- Solar and sidereal days are different because the Earth orbits the sun as well as spinning on its axis.
- The moon seems to go slower than the sun, taking about 24 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.
- As the Earth moves around the sun, the direction we face changes slightly each day, therefore we can see a slightly different patch of the sky each night.
- 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.
Eclipses and the Moon
- The Moon doesn't glow itself, it only reflects the light from the sun.
- As the moon orbits the Earth we see different amounts of the Moons dark and lit-up surfaces.
- As the moon orbits the Earth it sometimes passes into the Earth's shadow. The Earth blocks sunlight from the moon, so almost no light is reflected from the moon and it just seems to disappear.
- A total lunar eclipse is where no direct sunlight can reach the moon, otherwise there is a partial lunar eclipse or no eclipse at all.
- By chance, the moon is just the right size and distance away from the sun, that when it passes between the sun and the Earth, it can lock out the sun. This is a lunar eclipse.
- From some parts of the Earth the sun is completely blocked out, this is a total solar eclipse.
- in other parts there is some light shining through, this is a partial solar eclipse.
- however in most parts of the world the sun won't be blocked out at all.
- Eclipses of any type do not happen very often.
- This is because the moon orbits the Earth at a slight angle to the Earth's orbit around the sun.
- For an eclipse to happen there needs to be a clear alignment, and because of the way that the moon and Earth are tilted, this does not happen very often.
- Partial eclipses happen a bit more often as they don't need to line up perfectly.
- Converging lenses are fatter towards the middle and they cause rays of light to converge to a focus.
- All lenses have a principle axis going through the middle of the lens.
- The power of a lens can be calculated by:
power= 1 / focal length
- Simple refracting telescopes use two converging lenses, an eye lens and an objective lens.
- The lenses are aligned to have the same principal axis and are placed so that their focal points are in the same place.
- The objective lens converges parallel rays to form a real image between the two lenses.
- The eye lens is much more powerful than the objective lens. It acts as a magnifying glass on the real image and makes a virtual image.
- The angular magnification of a telescope can be calculated:
-magnification = focal length of objective lens / focal length of eye lens
- Astronomical telescopes use a concave mirror instead of a convex objective lens.
- Concave mirrors are shiny on the inside of a cvurve.
- Parallel rays of light shining on a concave mirror reflect and converge.
Astronomical Distances and Brightness
- The distance to nearby stars can be measured by Parallax.
- The parallax angle is half the angle moved against background stars over 6 months. The nearer the object you are the greater the angle.
- This angle is often measured in arcseconds.
- Observed brightness depends on the distance to the star.
- The further away you are the less energy reaches you, so the star seems less bright.
- The intrinsic brightness of a star depends on its size and how hot it is.
- The bigger and hotter it is the more energy it gives out and so it is brighter.
- So the observed brightness of a star as seen from earth depends on its intrinsic brightness and how far away it is.
- Cepheid variable star's pulse depends on its brightness.
- A group of stars called Cepheid variables pule in brightness.
- How quickly they pulse is directly linked to their intrinsic brightness. The brighter the star, the longer the time between bulses.
- So if you see two Cepheid variable stars with the same apparent brightness, that pulse at different rates, you know that the star with the longer pulse period must have the higher intrinsic brightnes.
- So the star with the longer pulse period must be further away.
The Scale of the Universe
- universe one gigantic galaxy about 100000 parsecs across
- thought that our sun and solar system were far from the centre of the galaxy
- believed that nebulae were huge clouds of gas and dust
- universe made up of many galaxies
- thought that out galaxy was about 10000 pc across, with the sun near the centre
- the spiral nebulae were other very distant galaxies, separate from the MilkyWay
- Neither was both completely wrong ow right.
- Shapley was right that the solar system is far from the centre of out galaxy
- Curtis was right about the spiral nebulae and that there are many galaxies in the universe.
- Hubble solved the Curtis-Shapley debate with his observations of the andromeda nebula.
- Using images taken with a large telescope, he found that this spiral-shaped fuzzy blob contained many stars, some of which were Cepheid variables.
- Hubble calculated the distance to the andromeda nebula by working out the distance to the Cepheid variables within it, using the relationship between their brightness and pulse frequency.
- He found that it was 2.5 million light years away, much further than any stars in out galaxy.
- He studied other spiral nebulae and found similar results.
- They were all too far away to be part of the Milky Way, and so must be separate spiral galaxies themselves.
- Distant galaxies are moving away from us.
- When a galaxy is moving away from us the wavelength of the light from it change, it becomes redder. This is called red shift.
- By seeing how much light has been red-shifted, you can work out the recession velocity of the galaxy.
- The greater the red shift, the greater the speed of recession.
- Using red shift, Hubble compared the speed and the distance for many distant galaxies and found a pattern - the more distant the galaxy, the faster it moves away from us.
- Red shift is fairly easy to measure, so a galaxy's recession velocity can be found easily enough.
- The distance to a distant galaxy can be found from its recession velocity, using Hubble's law:
-speed of recession (km/s) = Hubble Constant (s-1) X Distance (km)
- The value of Hubble's constant is still being researched
- When you increase temperature you give its particles more kinetic energy and vice versa.
- The coldest that anything can ever get is -273°C - this is absolute zero (absolute zero is actually -273.15).
- At absolute zero atoms have as little kinetic energy as possible.
- To change degrees centigrade in to kelvin just add 273.
- A decrease in volume gives an increase in pressure.
- An increase in temperature also increases the pressure.
Structure of the Atom
- In 1909, Ernest Rutherford and his men tried firing alpha particles (positive) at thin gold foil.
- Most of the particles went straight through, however a few came straight back at them, this was not expected.
From this experiment Rutherford realised that
- Most of the mass of a gold atom was concentrated in a tiny nucleus. The rest of the atom must be mainly empty space (as most of the alpha particles went straight through the foil)
- The nucleus has to have a positive charge, otherwise the positively charged alpha particles wouldn't be repelled by the nucleus and wouldn't scatter
The nucleus is held together by a string force.
- This strong force only has a small range, so it can only hold protons and neutrons together when they are separated by tiny distances.
- At very small separations, the strong force must be repulsive, as otherwise there would be nothing stopping the nucleus from crushing to a point.
Fusion and Stellar Structure
- Two nuclei can combine to form a large nucleus, this is called nuclear fusion.
- In the core of the sun, hydrogen nuclei fuse into helium nuclei and release energy.
- All nuclei are positively charged, they only contain protons and neutrons.
- Like charges repel, so there is a repulsive force between the two nuclei, trying to stop them being brought together.
- Nuclei can only fuse if they overcome this electrical force and get close enough for the attractive strong force to hold them together.
- For this to happen lots of energy is needed, a high temperature.
- A star has different zones. These are:
- The core
- The convection zone
- The Photosphere
- All hot objects emit radiation.
- Hot objects emit a continuous range of frequencies - a continuous spectrum
- Hot objects always emit more of one frequency than another.
- Each wavelength has a peak frequency. The peak frequency emitted y an object depends on its temperature. The higher the temperature, the more energy the photons radiated will have, and so the higher peak frequency.
- The intensity or brightness also depends on temperature. The hotter it is the more it glows.
- An atom contains electrons moving around a positive nucleus.
- Electrons can only be in certain energy levels.
- At high temperatures, electrons become excited and jump into higher energy levels by absorbing radiation.
- Because there are only certain energy levels an electron can occupy, electrons absorb a particular frequency of radiation to get to a higher level.
- You can 'see' this if a continuous spectrum of visible light shines through a gas - the electrons in the gas atoms absorb certain frequencies of the light, making gaps in the otherwise continuous spectrum.
- These gaps appear as dark lines.
- Electrons are unstable in the higher energy levels so they tend to fall down from higher to lower levels, losing energy by emitting radiation of a particular frequency.
- This gives a series of bright lines formed by the emitted frequencies.
- Energy levels in atoms are different for each element, therefore each element has its own spectrum.
- The photosphere of a star emits a continuous spectrum of radiations. This radiation passes through the gases in a star's atmosphere, which produces emission and absorption lines in the spectrum.
The Birth and Death of Stars
- Stars begin as clouds of Dust and gas.
- The denser regions of the cloud contract very slowly into clumps under the force of gravity.
- When these clumps get dense enough, the cloud breaks up into protostars that continue to contract and heat up as the pressure increases.
- Eventually the temperature at the centre of the protostar reaches a few million degrees, and the hydrogen nuclei start to fuse together to form helium.
- This releases an enormous amount of energy and creates enough outward pressure to stop the gravitational collapse.
- The star has now reached a main sequence stage.
- It stays like this relatively unchanging, while it fuses hydrogen into helium.
There are two paths that a main sequence star may take from here:
=cold black dwarf
=neutron star or black whole
Observing with Telescopes
- To detect faint sources you need a wide aperture (diameter).
- This is because only a small amount of radiation from these sources can reach us, and to collect enough radiation for a clear image a telescope with a huge objective lens or mirror is needed.
- Making large lenses is difficult and expensive so concave mirrors are used as they are easier to make more accurately.
- Whenever radiation passes through a gap, it diffracts. Radiation entering a telescope diffracts at the edges of the aperture, causing the image to blur. Therefore aperture size must be much larger than the wavelength being collected.
- Astronomers use local and remote telescopes.
- Most telescopes are now computer controlled. This has many advantages:
-instead of an astronomer always needing to be there, they can now program the telescope to track an object in the sky
-when doing a survey the telescope needs to be constantly repositioned to look at different areas of the sky, as telescopes can be programmed this makes the job much easier
-many telescopes are in remote areas, computer control allows telescopes to be operated via the internet, making it much easier than taking a long trip to get to the telescope
-without computer control, we couldn't have space telescopes
- The atmosphere can mess up measurements.
- This is because our atmosphere only lets certain wavelengths of electromagnetic radiation through and blocks all the others.
- Radio waves can pass through the atmosphere without much difficulty but visible light can be badly affected.
- Light gets refracted by water in the atmosphere, which blurs the images. It can also be absorbed y dust particles in the air.
- Because of this telescopes are put in space.
- Space telescopes do not have the problem of the atmosphere and therefore they can produce better images.
- However these telescopes are very expensive to build, and then bring up to space.
Observatories and Cooperation
Astronomers need to work together for a variety of reasons:
- many science projects are very expensive and for them to be financed several countries need to cooperate
- by working together the projects can have the best workers and the best facilities
Locations for observatories are chosen for astronomical reasons as well as other factors:
- visible light observatories are usually placed in remote areas to avoid light pollution as well as dust and other particles affecting the observations
- astronomers want as little atmosphere between the observatory and the telescope to minimise the distorting and bluring effects it has (due to this telescopes are often built at high altitude)
- water in the atmosphere can cause problems by refracting light, therefore dry and low-humidity locations are good for a telescope