GCSE Physics P7

The Earth fully rotates west-east on its axis one in just under 24 hours. We cannot feel the Earth's rotation, but due to this rotation the stars appear to move east-west across the sky once in just under 24 hours.

Stars appear to travel east-west accross the sky in 23 hours 56 minutes.

The Sun, Planets and Moon also appear to travel east to west across the sky. Their motion, and the time they take to cross the sky, is affectd by their relevant orbits. In the case of the Sun, it appears to travel accross the sky once every 24 hours.

A sidereal day is the time it takes for the Earth to rotate 360' on its axis.

A solar day is the time from noon on one day to noon on the next day i.e. 24 hours.

As the Earth rotates once on its axis, it is also orbiting the Sun. It is this orbiting movement that results in a sidereal day being shorter than an solar day.

A sidereal day is 4 minutes shorter than a solar day.

Due to tte orbiting movement of the Earth around the Sun, an observer looking at the nightsky from the Earth can see diffrent stars at different times of year, depending on the Earth's posistion in relation to the Sun's position.

When astronomers look into space, they describe the position of object in terms of the angles of decination and ascension. These angles describe the position of the stars relative to a fixed point on the equator.

A star with a positive decination will be visible from the northern hemisphere. A star with a negitive declination will be visible from the southern hemisphere.

Mercury, Venus, Mars, Saturn and Jupiter are all planets that can be seen from Earth with the naked eye.

These planets look similar to the stars, but they change thier positions in complicated patterns when compared to the background of fixed stars.

It is these complicated patterns that oivided some of the first evidence that the planets, including the Earth orbit the Sun.

We can use observations of Venus and an example to show how the planets change there positions against the background of the stars.

Venus is closer to the Sun than the Earth, so Venus' orbit of the sun takes less time than the Earths orbit of the sun. If Venus is observed over a long enough peiod (i.e. One Month), it can be seen to move compared to the background of stars.

Whilst the Earth is rotating on its axis, the Moon is orbiting Earth in the same direction. Due to this orbiting movement, the Moon appears to travel east-west across the sky in little over 24 hours.

For example, imagine you saw the Moon directly above you at a certain time of night. If you looked up again after one compleet rotation of the Earth, the Moon would not be directly above you because the Earths rotation would not yet have caught up with the Moons new orbital position.

The Moon appears to travel east-west accross the sky in 24 hours 49 minutes.

The lunar cycle describes the Moons appearance during its 28 day orbit of Earth.

The Moons shape has nothing to do with the shadow of the Earth, but due to the part of the Moon that is visible from the Earth.

The moon is visible from the Earth because we can see the light from the Sun reflected from it.

This means that the side of the Moon facing away from the Sun appears dark.

During the Moons orbit around the Earth, we can see the different faces of the moon:

Dark face - New moon

Light face - Full Moon

A solar eclipse occurs when the Moon passes between the Earth and Sun. This can occur during a new Moon and it results in the Moon casting a shadow on the Earth.

A total solar eclipse occurs when the moon is directly in front of the Sun it completely obscures the Earth's view of the Sun.

A lunar eclipse occurs when the Earth is between he Sun and the Moon. This results in the Earth casting a shadow on the moon.

Eclipses do not occur every month because the Moon doesn't orbit the Earth in the same plane as the Earth orbits the Sun. The Moons orbit is inclined by 5' to that of the Earths. Therefore, an elcipse can only occur when the Moon passes though the ecliptic.

This is more likely to occur when the Moons is the side of the Earth rather tan between the Earth and the Sun.

There are between 2 and 5 solar eclipses every year, but a total eclipse will only occur roughly every 18 months.

In a convex lens (also called a converging lens) rays of light are bent inwards as they pass though the lens. If the rays of light entering the lens are parallel, the rays will be brought to a focus  at the focal point.

A more powerful lens has a more curved surface and a shorter focal length than a weaker lens.

The power of a lens is measured in dioptres and can be calculated using the following formula.

Power (dioptre) = 1/ Focal length (metre)

You need to be able to draw ray diagrams for the formation of a real image from a distant point and a distant extended source.

Use this method to draw a ray diagram:

1)Draw a ray line(ray 1) that runs from the top of a object parallel to the principle axis. At the middle lens, bend this ray inwards so it passes though the focal point.

2)Draw a secondray (ray 2) that runs from the top of the objust straight though the centre of the lens as it crosses the principle axis.

3)Draw a third ray that runs from the top of a object though the focal point on the same side as the object. When the ray hits the centre of the lens, bend it to travel parallel to the principle axis.

4)The image is formed where these rays cross.

With a distant extended source, the images formed are real, inverted and diminished (smaller). If the object crosses the principle axis, draw another ray going from the bottom of the object.

When looking into space, the objects are so far away that rays of light fro them are effectively parallel. Therefore, we draw the rays of light entering telescopes parallel rays.

A simple refracting telescope is made from two converging lenses of different powers. The eyepiece lens is a higher power lens than the objective lens.

Astronomical telescopes will normally use concave mirrors for the objective lens instead of convex lenses. This allows them to be bigger and therefore, collect more light.

If a distant object is magnified, the image appears to be closer than the object. Therefore, the angle made by ray lines entering the eye is greater. This increase in angle is called the angular magnification and makes the image appear bigger/ closer.

The angular magnification of a telescope can be found if you know the focal length of the two lenses being used.

Parallax can be thought of as the apparent motion of any object against a background. However, it is actually the motion of the observer that causes the parallax motion of an object.

It looks as though the star has moved but it is actually the movement of the Earth's orbit around the Sun that causes the observer to see this change position.

The distance to the staes is so great that we cannot observe the parallax motion with the naked eye.

However, a simple way to observe the parallax is if you hold out your hand in front of you and alternately close one eye then the other. Although it appears to move your just looking at it from a different angle.

The parallax angle of a star is half the angle moved against a background of distant stars in 6 months

An object that is further away from the Earth will have a smaller parallax angle than a closer object.

Astronomers use parallex to measure interstellar distances using the unit parsec. A parsec is of a similar magnitude to a light year with 1 parsec equalling roughly about 3 1/4 light years.

Angles are measured in degrees, minutes and seconds. A star that is one parsec away has a parallax angle of one second of an arc.

Distance (parsecs) = 1/ Parallax angle (arcseconds0

Another method that astronomers use to measure the distance to the stars is to observe how bright any stars are. This method sounds very simple. i.e. a close star will appear brighter than a more distant star.

Unfortunately, not all star have the same intrinsic brightness. The intrinsic brightness is how much energy the star is emitting, and it depends on the star's size and temperature.

A larger or hotter star will emit more light than a smaller or cooler star, so it may appear brighter even though it is further away.

The star Antares is 500 light years from the Earth. There are over 100000s tars nearer to Earth than Antares, but Antares has an intrinsic brightness 10000 times greater than that of the sun and it is the 15th brightest star visible from the Earth.

The observed brightness of a star depends on its intrinsic brightness and its distance from the Earth

A star with a very high intrinsic brightness may appear dim if it is very far away.

For example Sirius appears as the brightest star in the night sky. It is relatively close to the Earth and has an intrinsic brightness 23 times greater than the Sun.

A Cepheid variable star does not have a constant intrinsic brightness. It pulses and the frequency of the pulses is related to the brightness. By measuring this frequency, astronomers can estimate the intrinsic brightness. If we know how bright the star really is and can see how bright it appears, we can work out how far away it is. 

in 1920, a great debate about the scale of the universe took place between the two prominent astronomers - Heber Curtis and Harlow Shapley.

Telescopes had revealed that the milky way contained lots of stars and this observation led to the realisation that the Sun was a star in the milky way galaxy. Telescopes had also revealed many fuzzy objects in the night sky. These objects were originally called neblae and they played a major role in the debate.

Curtis blieved that the univese consisted of many galaxies like our own, and the fuzzy objects were distant galaxies. Shapley believed that the universe contained only one big galaxy and the nebulae were nearby gas clouds with in the milky way.

In the mid 1920's, Edwin Hubble observed Cepheid variables in one nebula and found that it was much futher away than any star in the milky way. this observation provided the evidence that the observed nebulae was a seperate galaxy, supporting Curtis' idea that the universe contains many different galaxies.

Observations of many Cepheid variables have shown that most nebulae are distant galaxies and have allowed astronomers to measure the distance to these galaxies, and hence determine the scale of the universe.

By observing Cepheid variable stars in distant galaxies, Edwin Hubble discovered that the universe was expanding, i.e. the further away a star was thr faster it was moving away. The speed of recession can be calculated by using the following formula:

=Hubble Constant     X Distance

Speed Of Recession 

 =Hubble Constant X Distance

Cepheid variable stars are used to accurately calculate the Hubble constant because we know how far away the are. So we can use redshift to find out how fast they are moving away.

Astronomers can now use the Hubble constant and red shift data to calculate the distance of other galaxies.

Fluid pressure is caused by particles in a fluid moving about. When a partical collides with an object it exerts a force. This force is felt as pressure.

The amount of pressure depends on..

-The number of collisions per second

-the momentum of particals

As the volume is reduced, the particles have less room to move about and so they collide with piston more often increasing the pressure.

If the fluid is heated up the particles will move around faster. This increases their momentum and the force they exert when they collide with the piston.

As the temperature of a gas is reduced, the particals in the gas move slower and the presure falls.

The particles eventually stop moving all together. At this point the particles have no more energy to lose and the temperature cannot become any lower. This occurs at -273'c otherwise known as absolute zero.

Absolute temperature is a measure of temperature starting at absolute zero and is measure in kelvins (k)

To concert from kelvin into degrees celsius subtract 273

To convert from degrees Celsius into kelvin add 273.

A star has three main parts.

The core is the hottest part of the star where the fusion takes place. The convective zone is where the energy is transported to the surface by convection currents. The photosphere is where the energy is radiated into space.

Like all hot objects, the stars emit a continuous range of electromagnetic radiation.

Hotter objects emit radiation of a 

-Higher temerature 

- Higher peak frequency (i.e. frequency where most energy is emitted) than colder objects.

An object that is red hot emits most of its energy in the red frequency range. The frequency of light given off from a star provides evidence of how hot it is.

The removal of electrons from a star is called ionisation. The movement of electrons within the atom causes it to emit radiation of specific frequencies called line spectra. Different elements have characteristic line spectra.

Due to its high temperature, the spectrum from a star is a continuous spectrum apart from the spectral lines of the elements it contains (these lines are missing because they have been absorbed). 

By comparing a stars spectrum to emission spectra from elements, we can find which chemical elements the star contains.

The Sun's spectrum is complex, indicating that it contains more than one element. However, by comparing the spectra we can see that the sun contains hydrogen as well as some other elements. In the Sun's case we know that this other element is helium.

Stars begin as clouds of gas (mainly hydrogen). As gravity brings these gas clouds together, that become denser.

The force of gravity pulls the gas inwards, causing the pressure and temperature to increase. As more gas is drawn in, the force of gravity increases. This compresses the gas so that it becomes hotter and denser, and forms a protostar.

Eventually, the temperature and pressure become so high that the hydrogen nuclei fuse into helium nuclei. Energy is released in this fusion process.

The Star is now a main sequence star and is stable.

Towards the end of a stars life, its 'fuel' begins to run out (there is insufficient hydrogen remaining in the core for fusion to continue). The star then under goes several changes, depending on the stars size.

When the core hydrogen has been depleted, the star becomes cooler. Small stars like the Sun become red giants, while larger stars become red super giants.

Red Giants and red super giants continue to release energy by fusing helium into larger nuclei such as carbon, nitrogen, oxygen.

One helium has been used up, red giants do not have enough mass to compress the core and continue fusion, so they shrink in to hot white dwarfs which gradually cool.

Red supergiants have much greater mass and higher core pressures, so fusion continues to produce larger nuclei such as iron.

Once the core is mostly iron, the star explodes in a supernova, leaving behind a dense neutron star or a black hole.

At the beginning of the 20th century, discoveries about the nature of the atom and nuclear processes began to help answer the mystery of where the Sun's energy came from.

In 1911, there was a ground-breaking experiment- the Rutherford-Geiger-Marsden alpha partial scattering experiment.

In this experiment, a thin gold foil was bombarded with alpha particles. The effect on the alpha particles was recorded and these observations provided the evidence for our current understanding of atoms.

Most alpha particles were seen to pass straight though the gold foil. This would indicate that gold atoms were composed of large amounts of open space.

However, some particles were deflected slightly and even a few bounced back towards the souce. This would indicate that the alpha particles passed close to somthing positivly charges within the atom and were repelled by it.

These observations brought Rutherford and Marsden to conclude that..

-Gold atoms, and there for all atoms, consist largely of empty space with a small dense core. They called this core the nucleus.

-The nucleus was positively charged

- The electrons are arranged around the nucleus with a great deal of space between them.

We know that the nucleus contains positive protons and neutral neutrons held together by the short-ranged strong nuclear force. Protons normally repel each other (because they have the same charge), but this nuclear force is much stronger that the repulsive electrical force. Therefore, when protons are close enough, this force takes over and protons fuse into a larger nucleus.

This process releases large amounts of energy and this is the source of the Suns power.

Radiation is diffracted by the aperture of a telescope. To produce a sharp image, the aperture must be much larger than the wavelength of the radiation.

Large radio telescopes that detect the weak radio wave radiations can be built, but because radio waves have long wavelength they are affected by diffraction. This means that the image produced is not very sharp.

Light has a very short wavelength. Optical telescopes have a much larger aperture than the lights wavelength. Therefore the telescopes are able to produce a sharp image

The largest refracting optical telescope in the UK is sited at the Royal Observertory in Greenwich. However, its aperture of 70cm seems tiny when comared to the largest optical telescopes in the world- the 10m aperture reflecting Keck Telescopes at the  Maune Kea Observatories, Hawaii.

Hawaii has proven an ideal location for ground based telescopes for several astronomical reasons:

-Its high altitude means that there is less atmosphere above it to absorb the ligh from distant objects.

-The lack of nearby cities means that there is less pollution (light and standard) to interfere with the recieved signal.

-Its equatorial location gives it the best view of solar eclipses.

There are other this that must be considered when deciding where to build an observatory. For example, cost, environmental and social impact near the observatory and working conditions for employees.

The most famous space based telescope is the Hubble space telescope, which was launched in 1990 and designed in collaboration with European Space Agency and NASA. Its original launch date was delayed by two years because of the explosion of the space shuttle Challenger shortly after launch resulted in the shuttle fleet being grounded for two years.

After launch it was found that there was a fault on the mirror that was required that required expensive repairs. Despite these problems, the Hubble telescope has been a great success and provided images of the universe that could not have been obtained any other way. 

Advantages

-Avoids the absorption and refraction effects of the earth atmosphere.  

-Can use parts if the electromagnetic spectrum that the atmosphere absorbs.             

Radio telescopes use a metal reflector to reflect radio waves onto a receiver. Radio waves are not blocked by clouds, therefore, radio telescopes are able to detect objects that are too cool to emit much visible or infra red light. However, compared to optical telescopes, radio telescopes need to be built much larger and the images they produce are not as clear.

Infrared telescopes work much like optical telescopes. They have a better resolution than radio telescopes and can observe objects too cool to give off visible light. However, infra red light is easily absorbed by the Earth atmosphere so this type of telescope needs to be build at high altitude or be space based.

Most of the big new telescopes are developed though international collaboration. There are several advantages to this kind of joint venture.

-The cost of manufacturing the telescopes can be shared

-Astronomers from around the world can book time on the telescopes in different countries, allowing them to see the stars on other sides of the Earth.

The telescopes can be accessed directly at the site. They can also be accessed though remote computer control, which can be an advantage because astronomers do not have to travel to each telescope to be able to use it. They can also use the telescope at convenient times.

Schools in the UK can access the Royal Observatory over the internet. This kind of sharing of cost and expertise is essential for many of the big expensive science projects. For example, the Gemini Observatory in Chile, which opened in 2002, was the result of collaboration between Austria and six other countries.