Cosmology

The Light Year

• The “light year” is a measurement of distance.
• 1 light year is the distance that light travels in 1 year.

 Example

(a) Calculate the number of metres in 1 light year.

 (b) Proxima Centauri, the next closest star after the Sun, is 4.3 light years from the Earth. Calculate this distance in metres.

 Solution

 

Proxima Centauri is 4.07×10¹⁶ m from Earth

 Age of the universe

Current thinking dates the age of the universe as approximately 14 billion years.

 Observable universe

• The universe consists of many galaxies separated by empty space.
• A galaxy is a large cluster of stars (e.g. theMilky Way).
• A star is a massive ball of matter that is undergoing nuclear fusion and emitting light and heat. The Sun is a star.
• The sun and many other stars have a solar system. A solar system consists of a central star orbited by planets.
• A planet is a large ball of matter that orbits a star (e.g. Earth or Jupiter). Planets do not emit light themselves.
• Many planets have moons. A moon is a lump of matter that orbits a planet (e.g. the Moon orbits the Earth. Deimos and Phobos orbit Mars.

These are features of the observable universe. The universe also contains matter that cannot be observed directly, e.g. dark matter.

Radiation from space

The main types of radiation received from space (being emitted from stars, galaxies, etc.) are

• Electromagnetic radiation (across the entire spectrum from radio to gamma)
• Cosmic rays (mainly energetic protons)
• Neutrinos (tiny elementary particles with no charge)

Detectors of electromagnetic radiation

 

 

Spectra

• White light is made up of a range of colours.
• These colours can be separated by splitting white light with a prism to obtain a spectrum.

 

• A spectrum can also be produced using a diffraction grating.
• A line absorption spectrum consists of a complete (continuous) spectrum with certain colours missing which appear as black lines in the spectrum.

• A line emission spectrum consists of lines of light of distinct colours rather than a continuous spectrum.

 

• Every element produces a unique line spectrum. Studying line spectra allows the elements present in a light source (e.g. a star)to be identified
• This can help to identify the type, distance, age or speed of a star.