Black Body Radiation
A Black Body is an idealized body that reflects no radiation incident on it, i.e. it absorbs all radiation incident on it. It is capable of radiating on 'its own account' by virtue of its temperature, which is different from reflecting radiation.
The nature of an individual Black Body curve is indicative of the temperature of the emitting body.
Although Black Bodies are an idealized model, Black Body Radiation proves to be a good approximation to radiation actually emitted by Stars.
Light radiated by condensed matter is continuous and approximates to black-body radiation.
The diagram shows Black Body curves for various temperatures (with their 'Rock of Gibraltar' profile). The area under each curve is indicative of the power emitted at a particular temperature, which obviously increases with temperature. The peak of each curve displays the characteristic that, as temperature increases, the peak moves to lower and lower wavelengths (or equivalently, higher and higher frequencies).
We experience this in real-life when we switch on something like an electric cooker. As it heats up, we will detect Infra-Red as the peak shifts up to higher frequencies. Warning up even more, we will perceive a red color. This is as far as we normally get with domestic heating appliances.
However, the diagram shows higher temperatures (the Visible Spectrum is shown explicitly). At 6000K, which is similar to the temperature of the Sun, the paek of radiation will peak in the blue. This star (or our Sun, if you like, will not actually look blue for the reason that a significant amount of radiation at other colors will also be emitted, and a mixture of colors will produce different colors as far as the eye is concerned, just like in a color television set where all colors are produced by a mixture of just three colors. The tendency of a mixture of all colors will be to produce a white color. The color of the Sun is not immediately obvious to us because the atmosphere will strip off the higher frequency radiation preferentially (to produce a blue sky), the extreme example being a red Sun at sundown.
Hotter stars can however appear blue or blue-white.
Light radiated by condensed matter (which includes gas or plasma at high pressure)is continuous and approximates to black-body radiation.
A gas which is not at high pressure will emit a spectra consisting of individual discrete wavelengths. Further it will not only emit at these discrete wavelengths, but also absorb radiation at these discrete wavelengths (absorption lines).
Hydrogen Alpha (Hα) line lie in the middle of the red part of the visible spectrum. It is produced by a transition from Hydrogen's second excited state down to its first excited state. Emission will peak at about 10,000K. Above this, Hydrogen will tend to become ionised, thereby reducing electronic transitions from a mass of Hydrogen. At too low a temperature, little (if any) Hydrogen would be excited up to second excited state. Hα filters are commonly used to view the Sun.
Helium is very resistant to ionization. Neutral helium can exist up to 30,000K.
Spectra are analysed using a spectrometer. A beam of light from a star is collimated and then either passed thru a prism or bounced off a diffraction grating to separate the spectrum out.
The diagram at top shows a spectrometer using a prism, the prism being rotated to eventually record the entire spectrum. The lower diagram shows a static diffraction grating diffracting the light ont a CCD device, recording the spectrum all in one go.
Scientifically, temperature is a measure of the average kinetic energy of the substance/material involved.
This is different than measuring the actual amount of heat contained within a substance/material, and understanding of this distinction is more necessary in Astronomy than in 'normal' life. For example, the corona around the Sun has a temperture measured in millions of degrees whereas the 'surface' of the Sun (i.e. the surface of the photosphere) has a temperature of about 5800K. The corona however is very thin, and although its constituent particles are moving extremely fast such that we can talk of a high temperature, the actual heat energy in the corona is less than might be imagined if you were unaware of the distinction I have just mentioned.
There are examples of common experience which display very high temperatures but fairly low 'amounts' of heat, e.g. a sparkler firework which can be safely held in the hand, sparks from an electric saw or similar, etc. .
A 'problem question' related to these properties runs like this:- whereas you normally cook an egg in boiling water, what would happen if when you got up the water was off. Could you switch the oven on to 100 degrees and then put an egg inside and get it to cook in the same time. In order to start answering this question you would have to consider this: - would I be happy to put my hand into boiling water at 100 degrees and would I be happy to put my hand in the oven at 100 degrees?
One feature I have used already is the Kelvin Temperature Scale (denoted K) which is graduated the same as the Centigrade/Celsius scale but starts at Absolute Zero. So in round numbers, 0° Centigrade will be 273K and 100° Centigrade will be 373K.
Why is the Sky Blue?
Protons, electrons, positrons and Helium nuclei.
Various speeds but the fastest travel at the speed of light.
Those that strike the upper atmosphere produce a shower of secondary particles. Only the most energetic can be detected at ground level.
low energy from Sun, but high-energy a mystery a mystery, possibly supernovas.