SC7-Astronomy (Triple only topic)

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SC7-Astronomy (Triple only topic)

SP7a-the solar system

Many years ago, in the 1st century, Ptolemy (a Greek astronomer), developed a theory of the geocentric universe. This theory suggests that everything orbited the Earth in perfect circles. People didn't have telescopes and saw that the sun and moon were travelling across the sky in the same way every day and night. Therefore this model was accepted for a long time.

However, in the 1500s, Nicolaus Copernicus suggested that the sun was at the centre of the universe. He suggested that the Earth and all the other planets orbited the sun in perfect circles. This is called the heliocentric model. Galileo Galilei was one of the first to use a telescope to observe the sky. He was looking at Jupiter when he saw four aligned objects that appeared to be orbiting Jupiter. These were in fact four moons of Jupiter known as the Galilean moons or as Callisto, Europa, Ganymede and Io. This showed that not everything orbited around the Earth and that the Geocentric model was wrong. Also, Galileo observed the phases of Venus which suggested that the Venus passes the sun as it has phases like the moon does.

Gradually, as there were more technological advances, more evidence has been collected for the Heliocentric model.

The current model states that the Planets in our solar system orbit the sun, but these orbits are actually elliptical rather than circular. The sun is not actually at the centre of the universe either.

Objects in our Solar system include:

Planets: these are large objects that orbit a star. There are 8 planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Dwarf planets: These are planet-like objects that do not meet all the criteria to be an actual planet. They either may not be big enough or have not cleared its orbit.

Moons: These are thought to have been either captured or formed and they orbit planets with almost circular orbits. They are natural satellites (not artificial).

Artificial satellites: These usually orbit Earth in fairly circular orbits and humans have built them.

Asteroids: these are lumps of metal and rock that orbit the sun. They are usually found in the asteroid belt which orbits between Mars and Jupiter.

Comets: lumps of ice and dust that orbit the sun. As they get closer to the sun, they develop a coma (a type of atmosphere around the nucleus), and they develop an ion tail formed by charged particles from solar wind. The tails always point away from the sun. The other tail is the dust tail which is curved as each individual particle starts its own orbit around the sun.

Telescopes are used to observe the universe. They help distant objects to be seen clearly. The most common one is an optical telescope which detects visible light.

Telescopes use both refraction and reflection to allow distant objects to be seen.

In order to improve the quality of the image that is seen you can:

Increase the diameter of the objective lens (the aperture)-the big lens at the end o the telescope where light from the distant object enters the telescope. he bigger the telescope, the more light that can enter.

Use a higher quality objective lens.

Space telescopes have a clearer view than those on Earth. The atmosphere blocks a lot of light from being seen (hence why stars twinkle). To observe the frequencies absorbed, telescopes must be sent beyond the atmosphere.

Light pollution makes it hard to see fain objects in the sky. It reflects and absorbs light coming from space.

If a telescope is placed on Earth, the best possible place to put it to get the best view is on top of a mountain. This is because the atmosphere is scattered more and they are usually in dark places away from cities.

To avoid problems to do with the atmosphere and the weather, telescopes can be sent into space.

Different telescopes detect different types of the EM spectrum. The first telescopes ever used were optical. Many objects in the universe are not detectable using visible light-so other types of EM waves are used to observe them.

X-ray telescopes are a good way of viewing violent, high temperature events in space, like supernovas, the accretion disc of a black hole, the corona etc.

Radio telescopes are what discovered CMB radiation and help scientists to learn more about the universe.

Bigger telescopes give better resolution and can gather more light. Improved magnification means that we are able to look farther into space. Therefore more galaxies are being discovered.

Modern telescopes send the data to a computer, which would create a clearer image and make it easier to analyse and compare later on.

SP7b-Gravity and orbits

Gravity provides force that creates orbits. The planets move around the sun in almost circular orbits. An object at a constant speed is constantly accelerating. The planet is constantly changing velocity but the gravitational pull of the sun is pulling it inwards, making the planet move in almost circular orbits. The object keeps acceleration to what it is orbiting but the instantaneous velocity keeps it travelling in a circle. The force that makes this happen is provided by the gravitational force between the sun and the planet.

The force due to gravity depends on mass and distance. Weight on any object depends on the gravitational field strength. The gravitational field strength depends on the mass of the body creating the field. The larger the mass of the body, the stronger the gravitational field.

Gravitational field strength also varies with distance. The closer you get to a star or a planet, the stronger the gravitational force is. The stronger the force, the larger the instantaneous velocity needed to balance it. The closer you are to a planet, the faster you need to go to remain in orbit. For an object in stable orbit, if the speed of the object changes, so must the size of the orbit. Faster moving objects will move in a stable orbit with a smaller orbit than slower moving ones.

Planets have almost circular orbits. Most moons have almost circular orbits around planets. Comets have highly elliptical orbits.

SP7c-Life cycles of stars

Stars Initially form from a nebula-interstellar clouds of gas and dust (mainly hydrogen).

The dust and gas are pulled together by gravity to form a protostar. As the temperature increases, the star gets denser and more particles collide with each other. When temperatures get high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei. This gives off huge amounts of energy, which keeps the core of a star hot. A star is then born.

The star enters a long stable period called the main sequence. During this period, the outward pressure caused by thermal expansion balances the force of gravity, pulling everything inwards. This stage lasts several billion years. The heavier the star, the shorter its time on the main sequence.

The hydrogen core will begin to run out and the force due to gravity is larger than the pressure of thermal expansion. The star is compressed until it is dense and hot enough that energy created makes the outer layers of the star expand. This becomes either a red giant or supergiant depending on the size. It becomes red as the surface cools.

A star of similar size to our sun will then become unstable and eject its outer layer of dust and gas. This leaves behind a hot, dense solid core-a white dwarf.

Big stars glow brightly again as they undergo fusion to make heavier elements-this can go up to iron. They expand and contract several times, as the balance shifts between gravity and thermal expansion. Eventually they explode into a supernova.

The supernova throws outer layers of gas and dust into space, leaving a very dense core called a neutron star. If the star is massive enough, it will collapse to become a black hole in which not even light can escape from.

SP7d-Red-shift

The Doppler effect is the apparent change in the frequency of a wave relative to the movement of the source and the observer.

Most galaxies are moving away from each other. There is good evidence for this:

Different elements absorb different frequencies (or wavelengths) of light.

Each element produces a specific pattern of dark lines at the frequencies that it absorbs in the visible part of the EM spectrum.

When we look at distant galaxies, they have the same pattern but are at slightly lower frequencies than they should be.

There is an observed increase in the wavelength of light coming from the galaxies and the patterns have been shifted towards the red end of the spectrum.

Red shift is the increase of wavelength and decrease of frequency which is how galaxies are known to be moving away. When galaxies move to the red end of the spectrum, this is redshift. Measurements of redshift suggest that all distant galaxies are moving away from us very quickly, and its the same for every direction. More distant galaxies have greater shifts than nearer ones-they show a bigger observed increase in wavelength. More distant galaxies are moving away faster than nearer ones. This provides evidence that the universe is expanding.

SP7e-Origin of the universe

There are two theories of the origin of the universe, the Steady State and the Big Bang.

The steady State:

This is a later theory, suggested in 1948.

It is a theory that, as the universe expands, matter is continuously created so that the universe remains at the same density.

This theory believes that the universe didn't have a beginning and has always been here. The universe appears pretty much the same everywhere.

The big bang:

The universe originally started as one small, hot, dense point. It then exploded and started the expansion.

Since then it has been expanding.

This theory was suggested in the 1920's and the age of the universe is thought to have been 13.8 billion years old.

As it expands, gravity clumps mass together to form stars.

The universe is getting less dense.

Evidence for both theories is redshift. This is evidence as it shows that galaxies are moving away from up and the further away they are, the faster they are moving. If the galaxies are moving away faster, they must be going somewhere, meaning that the universe is expanding. This supports both theories as it shows that the universe is getting bigger as more objects are moving away.

Evidence for the big bang theory ONLY is called CMB radiation. This was discovered accidentally by two radio astronomers Penzias and Wilson who were studying radio waves but found a source of interference instead. CMB radiation is low frequency electromagnetic radiation that is coming from all parts of the sky at all times. When the universe exploded in the big bang, huge amounts of hot radiation was released. As it expanded, these waves would have increased, going from high frequency EM waves, to lower ones. This shows that the universe is expanding and that it has cooled from the originally hot gamma/x rays to what is now longer length cosmic microwave background radiation. This is evidence for the big back as the temperature of CMB is 2.7 K which suggests that the universe was hot and is now cooling.