# A-level Physics/Cosmology/Information from stellar observation
## Understand that stars and galaxies are detected by the electromagnetic radiation which they emit, whilst planets are detected by reflected sunlight

Stars, galaxies and planets are all visible to us here on Earth, but the reasons for our ability to see these stellar bodies differ:
* Stars and Galaxies - these themselves emit electromagnetic radiation, and can therefore be detected using this source
* Planets - these are not themselves sources of electromagnetic radiation, and are therefore only detectable via the sunlight which they reflect

## Sketch and interpret a graph to illustrate the variation with wavelength of the transparency of the Earth's atmosphere for the electromagnetic spectrum.

The ability of the different types of electromagnetic radiation to penetrate the Earth's atmosphere and therefore be detected on Earth varies within the spectrum. It can be broken down into three absorption categories: opaque (undetectable on the Earth's surface), partial absorption (some radiation makes it through, some doesn't), and transparent (radiation easily passes through the Earth's atmosphere).
* Opaque - includes: X-rays, Ultraviolet, and Long Wave Radio
* Partial Apsorption - includes: Gamma, Infrared, Radar Radio
* Transparent - includes: Visible, UHF Radio, Short Wave Radio
Follow this link for an example of this graph.

## Explain how the composition of stellar atmospheres may be obtained from stellar spectra

We can find out which chemical elements stars are made of from the radiation we don't receive from them. To explain this we need to consider the atoms of the emitting substance. an atoms comprises a very small, massive nucleus surrounded by a much larger volume which is sparsely occupied by electrons. When an atom absorbs energy, one or more of these electrons may become 'excited', i.e., jump to a higher energy level. if an excited electron then returns to its original energy level, energy is released as radiation. The wavelength of the radiation emitted by a particular electrons depends on precisely the amount of energy it releases as it returns to its unexcited state. the larger the amount of energy released by an electron, the higher the frequency - and the shorter the wavelength - of the radiation it emits
Stellar spectra include: continuous spectra, emissions spectra, and absorption spectra.
* Continuous Spectra - radiation of all frequencies within a certain range. When atoms are very close together, as in a solid or the dense matter of a star, there are so many different interacting forces that the electrons in atoms make jumps of all sizes within a certain range.
* Emissions spectra - a set of individual lines from which individual elements can be identified by their particular lines. When atoms are well separated, as in a gas, each type of atom emits its own distinctive wavelengths of radiation, which can be separated using a diffraction grating.
* Absorption spectra - the spectrum produced by the radiation from a star, or more specifically, the radiation from the atoms in the atmosphere of a star. It is a continuous spectrum with dark lines missing - Fraunhofer lines. These lines are representative of the elements present in the atmosphere of the star. Of the radiation emitted from the stars surface, some is absorbed and re-emitted in all direction by atoms in the atmosphere, meaning much less of this wavelength radiation is travelling in the original direction of travel, and therefore much less is reaching us, producing a dark line, a negative version of the characteristic emission spectrum of the atmospheric element.
These chemical elements can be identified by comparing the dark lines in the absorption spectra with the emission spectra of the individual elements present in the stars atmosphere.

## Understand what is meant by the Doppler Effect

Doppler Effect - the change in wavelength of a source due to the relative motion between the source and an observer.

## Recall and use Δλ / λ = v / c

A source of wavelength λ emitted at speed c takes λ/c seconds to emit one complete wave. If the source is moving away from the observer at v ms$^{−1}$, the wavelength observed will have increased by Δλ, therefore:
       Δλ / λ = v / c

## Understand what is meant by red-shift and by blue-shift and appreciate simple differences between red-shift and terrestrial Doppler Effects

Red-Shift - the observed increase in wavelength (reduction in frequency) caused by an emitter of radiation and a detector moving away from each other
Blue-Shift - the observed decrease in wavelength (increase in frequency) caused by an emitter of radiation and a detector moving towards each other
Terrestrial Doppler Effects on light are so small that they are barely noticeable, and so are only observed for sound and water waves (for example, the sound of a motorbike). The speeds of recession for planetary Red-Shift are a great enough proportion of the speed of light, c, to produce noticeable effects on light waves.
"Red-Shift is the Doppler Effect for light."