All Stars are White
Bad Astronomy: All stars that you can see at night are white.
Good Astronomy: Stars actually emit all the colors of the rainbow.
Literally!
I have had a couple of people email me and suggest this as a Bad Astronomy
topic. I don't think it's Bad, so much as Not Really Thought About. Most
people don't really think about stars having colors, so they tacitly
assume all stars are white. But look at the Sun! It looks yellow to me,
and
it's a star!
This one is easy to disprove by yourself. Go outside on a clear night and
look at the stars. The best ones to look at are the brightest. In the Summer,
(for the Northern Hemisphere) Vega is a bright star high overhead, and
is clearly blue. Antares is another summer star and is also clearly red
(or orange). In the Winter, you can see Betelgeuse in the constellation of
Orion, which is very red. Aldebaren, a star in Taurus (near Orion) is
also very red.
But most of the dimmer stars really do look white. What's going on here?
First off, stars really do have all different colors. Back in the 19
th
century (and before) it was known that when you heat an object up, it glows,
and furthermore the color of the glow depends on the temperature of the object.
A man named Wien (pronounced "Veen"; he was German) was even able to apply
some math to this and calculate the temperature of an object given
its color, although just
why this worked was unknown. It wasn't until
the 20
th century that this was understood, when another
scientist named Planck helped develop quantum mechanics.
Mind you, this only works for objects that are
glowing, and therefore
giving off their own light,
and not objects that are simply reflecting light.
Color is just another word for
wavelength;
light behaves like a wave, and the color of the light depends on the
wavelength. Planck worked out a mathematical relation of temperature,
color and brightness of a freely glowing object. He found, as Wien did,
that an object at a given temperature will emit most of its light at a
certain wavelength, and less at all other wavelengths. When you plot up
the brightness of an object versus the wavelength (or color), you get
a curve now called a "Planck curve", or a "blackbody curve", because it
represents a black object heated up. The plot below shows three such curves
scaled so that the peak brightness are the same. See how the hottest one
(at 7000 degrees Kelvin, a temperature scale much like Centigrade) peaks
at a wavelength that is blue, and the cooler one (at 3000 K) at red? The
one in between (5500 K) peaks in the green. [
Note(added September
29, 2000): the graphic below is a bit confusing. The original colors of
the plots matched the stars I discuss in the text, but somehow got messed
up when put on the page. I'll fix this when I get a chance. Sorry about
the confusion!]
This is why stars are different colors: they have different temperatures!
Vega is a very hot star, and so it glows blue. Betelgeuse is much cooler, and
so it looks red. I will point out here that stars are really not
blackbodies, and some deviate substantially from being so. They
absorb light, taking light from one part of the spectrum and re-radiate
it at another. These curves are only approximations. The Sun, for example,
should peak at about 5000 Angstroms or so, having a surface temperature
of 5500 K. However, due to complicated processes, it actually peaks
bluer than that, around 4800 Angstroms (a solar spectrum plot
can be found here).
Oddly, when you mix all the colors of sunlight, you get white. It may peak
in the blue, but the combination of colors (and the way our eye
interprets them) makes us perceive this light as white. Many people
claim the Sun looks yellow to them. I am not sure why, and have never
found an adequate explanation. Is it contrast with the blue sky? I don't
know.
So, back to the original question: why do so many stars
look white? Are most of them like the Sun?
Nope. Oddly,
the vast majority of stars in the sky are cool, red stars,
usually too dim to see. The reason most stars appear white to us is because
we have two different kind of light sensors in our eyes. Sensors called
"rods" detect brightness, while sensors called "cones" detect color. The
cones are not very sensitive, so if a light is too dim they are not activated,
and we perceive the color as white. So even a red star looks white if it is
dim, and only brighter stars look like they have color to us!
If you have a pair of binoculars, look at some stars that are bright
but still look white to your naked eye. You'll find that lots of them
through the binoculars suddenly have color! The binoculars focus more
light into your eye, and for brighter stars there will be enough light
to activate the cones in your eye. A telescope will show even more stars
with colors. There is a star, named Albireo, that to the naked eye looks
like one star, but is actually two in close orbit around each other. One
of the stars is a striking red, while the other a brilliant blue. Through
even a modest telescope this is one of the most beautiful sights in the
sky.
The Milky Way is one of the many galaxies in the universe and the Sun is
one of the 100 billion stars it has in it. The Milky Way is so big in
size that it takes light about 100 thousand years to travel from end to
end. It tends to look patchy because in some parts of the galaxy the
stars are closer to each other, as a result of
which those parts tend to shine out brighter than the others, in other
parts dark nebulae block out the light thus giving a patchy effect.
Galaxies are of elliptical, irregular and spiral types, the Milky Way is
a spiral type of galaxy. It has a curved arm of stars which radiate
from a glowing center. The brightest part of the Milky Way is in the
constellation called Sagittarius. The Sun and its family are
approximately 30,000 light years from the center of the Milky Way. The
nearest galaxy to our own is the Magellanic cloud which is about
1,50,000 light years away.