We know that stars have different colours because they have different energy. So blue stars have a lot of energy because the blues's frequency is very high $E = h\nu$. The colour of the star is its electromagnetic radiation which is influenced by the elements of the star. All stars issue white light, but we know that the absorption spectrum isn't a continue spectrum because some elements absorb some radiations. So does the blue star contain elements which absorb low radiations and so there are only high radiations which color the star with blu/violet? And what are these elements? How can we know if an element absorb radiations with low or high frequency?
[Physics] Energy and colours of the stars
astrophysicsfrequencystarsthermal-radiationvisible-light
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You have asked too many questions. Here are the answers First you need to understand that light is nothing but electromagnetic wave, there is nothing like color associated with any light. Color is completely a phenomena of your eye. When you see objects you just receives light reflected or refracted from those objects which is perceived by your eye as color. Eye contains three types cells (cones) red,blue, green, these are activated by different wavelengths of light in different way and generate different colors you see.
Your second question I think every object in universe in thermal equilibrium both absorbs and emits radiation continuously. When you see a black object, that object is actually absorbing radiation in visible spectrum but must be emitting radiation of some other wavelengths. It is not just absorbing all the radiation, if it were, its temperature will go on increasing and it will never attain thermal equilibrium. This process of absorbing and emitting radiation occurs because of charged matter present (mainly electrons). Electromagnetic field of electromagnetic radiation accelerates these charged particles and accelerated charged particles emit electromagnetic radiation. You might want to read more about black body radiation to understand more about it.
The tricky bit is that there are other effects happening in stars that lead to the spectrum being "incomplete". Quoting the last paragraph of the link mentioned by OP,
When the light leaves the surface of the Sun, it is very nearly a continuous spectrum. However, as it passes through the Sun's atmosphere, gasses present in that atmosphere absorb specific wavelengths of light, leaving the pattern seen in the spectrum above. By studying those lines and matching them to known lines from elements, astronomers are able to determine what gasses are present in the Sun's atmosphere.
Light will leave the surface of the Sun at pretty much a black body spectrum, but the Sun contains multitudes of hydrogen and helium, for example. These elements (just like any other) have quite specific spectral lines: the differences in energies between their energy levels are quite specific and they can absorb only photons with those precise energies. Due to this effect, the frequencies corresponding to these precise energies will end up deprecated on the spectrum we can detect on Earth. Essentially, when we look at sunlight, we see there are a few frequencies missing.
These effects were first noticed, if I recall correctly, by Fraunhofer. By cataloguing the spectral lines of different elements, one becomes able to identify the composition of bodies by looking at their spectra.
If we had a different star, the spectrum could be different, since it depends on the specific chemical composition of the star's atmosphere.
Best Answer
Is your question not a contradiction in itself? You say that because of different star energies the colors occur, and then you switch this thesis with stars emitting white light, and elements absorbing the nonblue-parts.
Essentially this is not completely correct (unless I understand you wrongly). Saying that stars have different energy is not precise and I think uncorrect in this case.
What you should look into is black body radiation and Plancks law of radiation. Essentially this topic states that a higher temperature responds to different wavelengths, that are emitted off the star. Take a look at this image.
As you can see this shows the intensity dependent on wavelenght. The different curves correspond to different temperatures and higher temperatures cause more energetic radiation, thus radiation with lower wavelength and higher frequency. This so called black body radiation describes the different colors of objects at different temperatures. Think of metal that doesn't (or almost) emit visible light on its own when it is at room temperature, but heated up to sufficient temperatures it will start glowing. So yes the temperatures of stars correlate with their colors, the more detailed understanding of this comes from nuclear fusion and the different types of stars. This is represented in the Hertzsprung-Russel diagram.
The diagram represents a correlation between stellar color and size, which has to do with the nuclear processes involved in the star. I think you should research on this topic as well, as writing it out here would take some time.
But as to your final statement, yes indeed there are elements absorbing light in stars, especially in the stars outer sphere. I am sure you have heard of the Fraunhofer lines, and won't give an explanation here, but as to our current understanding of stars the color is not influenced due to complete absorption of gigantic parts of the electromagnetic spectrum, the line spectra give us clues about the elements that the star consists of, which again also has some relation to the size of the star (and its age). To understand this even better, I think the following image will help.
You see the exact correlation of stellar spectra as predicted by the black body radiation (the first image). Resonance absorption due to stellar elements cause the discrete absorption lights, the temperature however is the main factor governing the color.