Why are different sized and therefore different temperatured Stars, Different colours?? What causes these different colours? All stars use hydrogen for fussion don’t they, shouldn’t the colours be the same or is it something to do with the amount of re-emitted energy from the atoms causing different colours??
I never found out the answer to this please enlighten me… 
PerfectDark
Well, while the experts come in, I would say this is like a fire which can burn hotter or cooler depending on the amount of fuel, the stage of the process and other factors.
They are different colors because they are different temperatures. Very hot ones are blue, average ones are yellow, cooler ones are red.
Why do they have different temperatures? Two reasons…size and age. Large stars have more gravity. More gravity means that hydrogen is squashed together more, so fusion takes place faster, meaning more heat. Red dwarf stars are very small, so the hydrogen isn’t compressed very much, so there isn’t much fusion, so they are low temperature.
New stars are hotter because they are fusing hydrogen, which yields the most energy when it is fused. But as a star gets older, fusion converts the hydrogen to helium. Helium fusion doesn’t yield as much energy, so the star starts to cool. As heavier and heavier elements are synthesized, less and less energy is produced. If a star is small, it will burn out. If it is larger it will contract to a black hole when the kinetic/heat energy isn’t enough to counteract the gravitational energy, or it will explode into a nova or supernova. All elements heavier than iron are synthesized in supernovas.
So, small old stars are reddish and cool, large new stars are blue-white and hot.
Well, blue photons have more energy than red photons. The hotter the surface, the larger the fraction of blue photons in the emitted radiation. We can see the effect around us too - warm objects (say, a human body) emit far infrared light, and can be detected by proper equipment. Hotter objects like burning charcoal and dim lightbulbs emit reddish glow. Even hotter objects, such as a bright lightbulb, emit more or less white light. Even hotter objects, like the flame of a gas burner, emit blue light.
There’s no such thing as a yellow star. The sun and other stars like it are white. The very coolest stars will have a reddish tinge while the very brightest will have a bluish tinge. But all stars in the middle look white to the human eye.
So, it’s not just God with a few buckets full of paint?
Shucks, cause my solution to global warming was going to be that we fill a few satellites with blue paint and shoot them to the sun, painting it blue, and blue stars are cooler, see, so that would reverse the global warming.
Lemur866 explained it very well.
dtilque - - are you talking about the way a star looks in the night sky? I think there are actual yellow stars (again, based on their temperature), but the human eye is not sensitive to color of dim light, so many stars in the night sky appear white.
That is, all elements heavier than iron found in nature are synthesized in supernovas. Many elements heavier than iron are synthesized by man. Most are transuranic (heavier than uranium), but two are not. One of them is technetium, atomic number 43, which is lighter than silver, heavier than zirconium. All of its isotopes are radioactive, one of the longest half-lives belonging to [sup]99[/sup]Tc, with a half-life of 213,000 years. Two other isotopes are on the order of millions of years, but all the rest are hours or days.
It has only been found in nature in African pitchblende (a uranium rich ore) in extremely minute quantities as a spontaneous fission product of [sup]238[/sup]U.
Yes, AWB, but it’s likely that technetium and promethium are created in supernovae as well. However, since we’ve never been close enough to a supernova to see them before their half-lives burn them to nothingness… and since the supernova from which the Earth was created happened a good 6 billion years or so before humans arrived on the scene… we don’t find technetium or promethium in nature. The only radioactive elements found in nature are those, like uranium, that have half-lives in billions of years.
All elements heavier than iron are synthesized in supernovae. They just decay too quickly for us to see them.
(Note: there’s no verdict on extremely heavy transuranics (104 and above) being created in supernovae, so far as I know).
LL
dtilque:
The sun isn’t yellow? You need your eyes checked.
The star colors do a pretty good job of following the theoretical “black body” curve through the coodinates of the CIE Chromaticity diagram. It’sw relatively simple to calculate, and you can show how the color coordinates go from the extreme left-hand corner from red through orange to yellow (a very bright yellow, but clearly yellow) close to white and eventually to blue hot.
The sun has a blackbody curve that has its peak in the green part of the visible spectrum. However, that peak is not very sharp and the sun produces ample light throughout the visible spectrum. The human eye is not sensitive enough to detect the peak, so light from the sun looks white to the human eye.
I should qualify that a bit. It looks white to the unfiltered eye or with a neutral filter. But we look at the sun through an atmosphere which is not a neutral filter (can you say Rayleigh scattering), so it looks slightly yellowish when the sun is high in the sky and very yellowish, even reddish, at sunrise and sunset. If you were to go into orbit and look at the sun through a neutral filter, it would look pure white.
This is true of all stars except those at the extremes. Extremely hot stars have their peaks in the UV and give off plenty of blue light but not as much red. So they will have a bluish tinge. Very cool stars have their peaks in the IR and give off a lot of red but not as much blue, so they have a reddish tinge. But the blackbody curves are broad enough that any star with a peak in the visible spectrum will give off plenty of light throughout that spectrum. Hence the star will appear white to human eyes.
I’ve heard of red giants before, but never red dwarfs (except as a tv series).
The [url=“http://www.britannica.com/bcom/eb/article/8/0,5716,71238+3,00.html”]Encyclopedia Britannica[/url entry on stellar evolution seems to give more complex causes.
Dtilque:
There IS Raylegh scattering, but it does NOT provide a huge amount of filtering. The sun’s yellow because thats where it is on the blackbody curve. A tungsten filament acts like a very similar blackbody, and is similarly ellow-white.
Rayleigh scattering does play a significant role in affecting the color of sunlight near sunrise and sunset, where the blue-weighted scattering makes the sun appear red.
Incandescent light filaments heat up to around 3000K (I forget the exact temp), the sun’s surface is 5800K. That’s a big, big difference.
I’ll grant you that a star whose temperature is the same as an incandescent light filament will be close to the same color as the light. Not exactly the same color, as the star makes a better blackbody than the filament. A star with a temperature of 3000K is an M type star (rather cool). These stars will appear red to us.
The sun’s apparent yellowness is a function of how high it is in the sky. It’s all Rayleigh scattering.
There was an article about the true colors of stars in the Sept 1992 Sky & Telescope. Unfortunately, that issue is not on line or I’d give a link to it.
[slight hijack] So why can’t stars be green hot? Why red-orange-yellow-white-blue? [/slight hijack]
Green stars…
http://itss.raytheon.com/cafe/qadir/q72.html
Lame answer re: yellow sun
http://itss.raytheon.com/cafe/qadir/q819.html
re: atmospheric filtering of blue light & the yellow sun
http://itss.raytheon.com/cafe/qadir/q1014.html
General re: colors…
http://itss.raytheon.com/cafe/qadir/q619.html
Dtilque:
I draw your attention to Figure 6-1 in the Electro-Optics Handbook (formerly the RCA Electro-Optics Handbook, but now some other company has its name on it, and mine is an old copy) This overlays plots of a 5900 K blackbody, Solar Irradiance outside the atmosphere, and solar irradiance at sea level. The blackbody curve is very similar to the solar irradiance above the atmosphere. The solar irradiance at sea level curve differs from these chiefly by the chasms carved into it by water and carbon dioxide absorption, but there’s no visible ayleigh scattering effect, and the peak f the curve is not moved from the above-amosphere value. If anything, the large Ozone absorption even cuts down on the red side of the absorption, making blue even more prominent, the opposite of the Rayleigh scattering effect.