P7 OCR 21st Century Science

The Earth fully rotates west-east on its axis once in under 24 hours. We cannot feel the Earth spinning, but it is due to this rotation that the stars APPEAR to move east-west across the sky in 23 hours 56 minutes. 

A sidereal day is the time it takes for for the Earth to rotate 360 degrees on its axis - 23 hours and 56 minutes. 

A solar day is the time from noon on one day, to noon the next day - 24 hours.

A solar day is longer because the Earth is orbiting the Sun and rotating at the same time once it has done a full rotation, the Sun is no longer directly overhead. The Earth then takes 4 minutes to catch up. 

• A solar eclipse occurs when the Moon passes between the Earth and the 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 the Sun and the Moon. This results in the Earth casting a shadow on the Moon.

Eclipses do not occur every month because the Moon does not orbit the Earth in the same plane as the Earth orbits the Sun. The Moon's orbit is inclined 5 degrees to that of the Earth's. Therefore, an eclipse can only occur when the Moon passes through the ecliptic. This is more likely to occur when the Moon is to the side of the Earth, rather than between the Earth and the Sun.

There are between 2 and 5 solar eclipses every year. 

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

The observed brightness of a star depends on its intrinsic brightness and distance from the Earth. A star with a very high intrinsic brightness may appear dim if it is very far away.

A Cepheid variable star does not have a constant intrinsic brightness. It pulses and the frequency of the pulses is related to its brightness. By measuring this frequency, astronomers can estimate the intrinsic brighness. 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 two astronomers - Heber Curtis and Harlow Shapely. 

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 nebulae and they played a major role in the debate. 

Curtis believed that the Universe consisted of many galaxies like our own, and the fuzzy objects were distant galaxies.Shapely believed that the Universe contained only one big galaxy and the nebulae were nearby gas clouds within the Milky Way.

The Edwin Hubble came along in the mid 1920s, were he had observed Cepheid variables in one nebula and found that it was much further away than any star in the Milky Way. This observation provided evidence that there are separate galaxies in the Universe. 

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

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

Hotter objects emit radiation of a...

• Higher temperature 
• Higher peak frequency (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 - which is colder than blue. 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 are absorbed).

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