Sun
- Medium sized star, consisting of hydrogen and helium
- It radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum
Stars
- Powered by nuclear reactions that release energy.
- In stable stars, nuclear reactions involve the fusion of hydrogen into helium.
Galaxies
- Made up of many billions of stars.
- The Sun is a star in the galaxy known as the Milky Way.
- Other stars that make up the Milky Way are much further away from the Earth than the Sun.
- Astronomical distances can be measured in light-years, where one light-year is the distance travelled in space by light in one year. One light-year is equal to 9.5 × 1015m.
Life cycle of a star
- A star is formed from interstellar clouds of gas and dust that contain hydrogen.
- A protostar is an interstellar cloud collapsing and increasing in temperature as a result of its internal gravitational attraction.
- A protostar becomes a stable star when the inward force of gravitational attraction is balanced by an outward force due to the high temperature in the centre of the star.
- All stars will run out of hydrogen as fuel for the nuclear reaction.
- Most stars expand to form red giant. Bigger stars expand to form red supergiants when most of the hydrogen in the centre of the star has been converted to helium.
- A red giant forms a planetary nebula with a white dwarf star at its centre.
- A red supergiant explodes as a supernova, forming a nebula containing hydrogen and new heavier elements, leaving behind a neutron star or a black hole at its centre.
- The nebula from a supernova may form new stars with orbiting planets.
The Universe
The Milky Way is one of many billions of galaxies making up the Universe. The diameter of the Milky Way is approximately 100000 light-years.
- Redshift — an increase in observed wavelength of electromagnetic radiation emitted from receding stars and galaxies.
The light emitted from distant galaxies appears redshifted in comparison with light emitted on the Earth. Redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory.
Microwave radiation of a specific frequency is observed at all points in space around us and is known as cosmic microwave background radiation (CMBR). CMBR was produced shortly after the Universe was formed. This radiation has been expanded into the microwave region of the electromagnetic spectrum as the Universe expanded.
The speed at which a galaxy is moving away from the Earth can be found from the change in wavelength of the galaxy's starlight due to redshift.
The distance of a far galaxy can be determined using the brightness of a supernova in that galaxy.
The Hubble constant H0 is the ratio of the speed the galaxy is moving away from the earth to its distance from the earth.
The current estimate for H0 is 2.2 x 10-18 per second.
Represents an estimate for the age of the Universe and that this is evidence for the idea that all matter in the Universe was present at a single point.