Star Colors and Temperatures
by Dr Jamie Love
© 1997 - 2011
© 1997 - 2011
We are spending another month with ORION because he is such a great pointer to other stars and constellations. By now you should be well acquainted with "The Hunter" and his "Dogs". We will use ORION to help us identify some of the obvious stars and constellations that hover above him.
You will recall that ORION's belt is along the Celestial Equator
with Rigel in the Southern Hemisphere and Betelgeuse
in the Northern Hemisphere. Notice that ORION's belt is along the Celestial
Equator but NOT parallel to it. Specifically, the "belt star"
on ORION's left hip is on the equator but the other two stars,
running across his belt from his left to his right, are slightly
below it so they are in the Southern Hemisphere with Rigel.
Today
we will concentrate our efforts on the Northern Hemisphere. (So
all these stars and constellations will have a declination that is positive, right?)
What's that big, bright star above ORION's left shoulder that looks like Betelgeuse?
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That's Aldebaran and we don't use ORION's shoulder to find
it - we use his belt.
Imagine a line from ORION's right hip star
(which is slightly below the Celestial Equator) to the opposite
star on his belt (which is right on the Equator). Now extend that
line about 20o further (one "handful of sky") and you
land on Aldebaran. You can't miss it. Aldebaran
has a magnitude of 0.9 so it is about as bright as Betelgeuse.
They are also similar in color although Aldebaran is more
orange (less red) than Betelgeuse.
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Aldebaran is the brightest star in the constellation of
TAURUS the Bull.
TAURUS is one of the Zodiac constellations
and we will discuss the Zodiac in a subsequent lesson. (By the end of this course you will be able to identify all the Zodiac constellations.)
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Here I've
drawn some "constellation lines" that show the image
of a bull's head, or simply the letter "V"!. Many books
will also add various lines on TAURUS' left side (your right)
to show his shoulders and thus give you the "complete"
constellation. But I've chosen to leave them out because they
involve dimmer stars and no one agrees on how they should be connected
anyway.
Hopefully your imagination can see that Aldebaran makes the right eye of TAURUS with other bright stars making the rest of his face and horns. You may have guessed (correctly) that another name for Aldebaran is alpha-TAURUS. The second brightest star in TAURUS is called beta-TAURUS using Bayer's system, or Alnath if you prefer the ancient name. | 
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Try to remember that Aldebaran is the orange star in the right eye of the bull and Alnath is the bluish-white star at the tip of his left horn.
Why is Aldebaran orange? And, for that matter, why is Betelgeuse red? Why is Rigel white? (Or blue-white?)
Stars are balls of gas with a nuclear furnace below the surface. The energy from the nuclear furnace moves its way "up" and heats the stellar materials as it rises. Eventually that energy reaches the surface of the star and heats it. The starlight that we actually see is really the light given off by the surface of the star.
It's important to understand that there is a big difference in the physics between the center and the surface of a star. Energy is created below the star's surface. Next month I will tell you all about that physics. For now all you need to know is that the center of a star is incredibly hot. Its temperature is measured in millions of degrees (Kelvin or Celsius - it doesn't really matter). As that energy works towards the surface it spreads out and cools so the star's surface is much cooler than its core. A star's surface temperature is measured in thousands of degrees - a lot cooler.
What's temperature have to do with color?
How can temperature affect color? I thought color had to do with which parts of the light spectrum were absorbed or reflected?
Well, that's true of reflected light and most of the things you
see are seen by reflected light. Light shines on something, the
reflected light bounces off the object and some
of it reaches your eye.
Emitted light works differently
because it's the light that is produced by the object.
Examples of emitted light include the light directly from flames,
lamps, your computer screen and stars including the Sun.
The Sun produces and emits light, some of which
is reflected off the Moon. The Moon produces no light of its own.
Therefore, sunlight is emitted light but moonlight (indeed the very image
of the Moon) is reflected (sun)light. Hopefully you now understand
the difference between the reflected light from the Moon and planets
versus the emitted light from the Sun and other stars.
Emitted light can be produced in several ways. For example, your computer screen uses electrons to cause chemicals on the screen to glow and thus emit light in different colors. The color produced depends upon which blob of chemical is excited by the electron beam. Some curious astronomical phenomena produce color by electron stimulation but the colors of stars are caused by heat energy being emitted, as light, from the star's surface.
As materials get hotter they emit more light in different colors.
Conversely, as they cool they emit less light and do it using
different colors.
| Here you see a diagram of a bar of metal being heated from the left. It's hottest at the source of heat - "blue-white hot". It radiates away some of that heat so that a little further along the bar it is less hot, only "white hot", and it emits that color. | 
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Further along and the bar is cooler and
now only yellowish-white light is emitted. This is followed by
yellow, then orange and finally red. The bar still
emits heat and light beyond the red, but we can't see that heat because it is
infrared ("below the red"). However, we can certainly "feel"
it as heat if we were to touch it! 
To fully understand why we see each of these colors you need to know more about the amount of each wavelength of light emitted. We don't need to know those details in order to appreciate the fact that very hot substances give off their heat as emitted light and the color of that light depends on the amount of heat.
But I want to know the details!
OK, here's a brief run down of it all.
When a population of "particles" (molecules, atoms or ions - but mostly ions when talking about a star's surface) are heated, they move around very quickly - especially if they are a gas, as they are on the surface of a star.
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The more energy they have, the faster the particles move.
The energy of each particle depends on something called the "Boltzmann constant" and the distribution of these speeds in a population of particles was predicted by a very important physicist named Maxwell so this is known as the "Maxwell-Boltzmann distribution". Most particles in the population move around at slightly different speeds because they have slightly different amounts of energy (heat). Indeed, due to collisions with other particles, any one particle changes its speed as time goes on. So we tend to think of all the particles as a population and most of them will be moving at different speeds. However, there will be a peak energy (speed) representing the greatest number of particles and we can imagine this as the "average" energy (speed). | 
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The Maxwell-Boltzmann distribution plots the energy of particles against their speed and the graph looks like a lopsided curve. The peak is the most important part of the curve because it represents the "average" speed. (I keep putting "average" in quotes because the word "average" means different things to different people - especially mathematicians.)
You should understand and try to imagine that the position of the curve, including its peak, changes as the temperature changes. If the population of particles in the diagram above were heated (energy added) their distribution would shift to higher energies, so the Maxwell-Boltzmann distribution (the curve) would shift to the right and so would the peak.
Particles lose energy and that energy is emitted as light. Indeed, the emission of light is the ONLY way a particle can lose energy in the vacuum of space. To help distinguish between the light we see and the light we cannot see (because that light is at a wavelength our eyes have not evolved to see) physicists like to call all light "radiation" but don't confuse it with alpha or beta radiation.
Objects at a particular temperature emit light (radiation) most strongly at a particular wavelength (color). Notice I said MOST! In fact, the radiation emitted follows the Maxwell-Boltzmann distribution and along that distribution will be a maximum (peak) of emitted light.
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The "color peak" is the color that dominates the emission so it's the one we tend to notice.
For example, this is the light emission of an average star with a surface temperature of 5500oC. As a matter of fact, that is the surface temperature of our Sun but this could just as well be the light from the surface of any star with a surface temperature of 5500oC. Notice that the peak emission is in the yellowish part of the energy spectrum. That is why our Sun looks yellow. Notice, however, that in this emission you can find every color of the rainbow (literally | 
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If the Sun grew hotter, the ions on its surface would move faster and the Maxwell-Boltzmann distribution would shift to the right, carrying the peak with it. Then the overall color of the Sun would shift towards the bluer end of the spectrum and our Sun would take on a more greenish hue.
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OK and now for the math nerds out there!
Wien's displacement law predicts that the peak color (wavelength of maximal emission) will be proportional to the temperature (in Kelvin), or put another way,
Phew! |
This effect is not limited to metal bars or the surface of a star. If you look very carefully at a candle flame you'll see there is a blue-white spot at its hottest point (near the wick) with a white tip slightly above it, then a series of yellow, orange and finally red parts to the flame. (Flames flicker a lot so it's particularly hard to distinguish the yellow-orange-red zones.) Each of these colored zones is caused by a different population of gas at different temperatures. Their temperatures change very quickly as they move away from the hottest part of the flame and as they do so they lose energy. That means each zone has a different Maxwell-Boltzmann distribution and it is constantly changing as the gas moves and cools. As the Maxwell-Boltzmann distributions move so too do the peak colors and we see a series of colored zones in the flame.
What's the temperature of some stars I now know?
Rigel is a blue-white star and, we know from experiments done in the lab, that its surface gases must be over 12,000oC to give it that (peak) color. At the other extreme is Betelgeuse. Its deep red color shows it has a surface temperature of only 3400oC. Aldebaran is slightly warmer than Betelgeuse, around 4000oC, so it is more orange than red. We can use this nice color scheme to estimate the surface temperature of stars. All this is "backed up" with experiments done in the lab using very hot gases.
This lesson may appear to be particularly short but that's because it covers one of the most important bits of physics used in astronomy. After all, the color and brightness of a star is all an astronomer has to work with. So, before going on, please be sure you understand this important concept. Then, and only then, you can learn more about colors and how astronomers use color to classify stars by continuing on to the next lesson.
) but that some of those colors are poorly represented.
max = a constant.