The Colour of Stars

You might think that the stars that we see in the night sky are all white but that is only because our eyes cannot easily distinguish colours when objects are very faint. In reality, the night skies are ablaze with colour, with stars ranging from cool blue hues to fiery reds. Take a look at the image of the constellation Orion below. In particular, look at the top left star Betelgeuse and the bottom right star Rigel. They might look white initially but even on a fast 2-second exposure without the aid of a telescope, closer inspection of the picture can reveal a glimpse of reds and blues.

Zoom in with a telescope and Betelgeuse can now clearly be seen below as a fiery red star. That's because Betelgeuse is classified as a Class M red supergiant, one of the largest stars known. Strange as this may seem, fiery red usually indicates a cool star that is losing its radiation and dying out, while cool blue stars are the hottest, youngest stars. Betelgeuse is in fact in the last stages of its life and is expected to go supernova any time in the next million years (which is a very short time in stellar terms).

You can go deeper into the mysteries of star colours with spectroscopy - this is the science of determining the properties of a star by analysing the electromagnetic light radiation from it. By attaching a special optical filter called the Star Analyser 100 to my telescope eyepiece, I am now able to see the the colour spectrum that is unique to light radiation produced by Betelgeuse. The Star Analyser is basically a diffraction grating (as you would see on the surface of a CD or DVD) that is mounted in a standard 1.25 inch optical filter cell.

The image of the spectrum produced by the Star Analyser can be further analsyed using special software to reveal the spectral profile of the star that will tell you a lot about the star you are observing. The broad shape of the spectrum gives information about temperature - for example, this spectrum of cool Betelgeuse is quite different to that of hot Sirius. Narrow absorption (dark) or emission(bright) lines on the spectrum tell us about the chemical elements that are present in the star and how they are behaving - for example the spectrum of Betelgeuse reveals the tell-tale signature of molecules (and not just hydrogen atoms) in its atmosphere. The software I used to produce this spectral profile is a free product called Visual Spec - see http://astrosurf.com/vdesnoux/

Now let's take a look at the star in the opposite end of Orion, Rigel. Seen through the telescope below, you can clearly see that this is a pale blue star. Rigel is in fact a Class B blue supergiant. While blue supergiants are much smaller than their red counterparts (like Betelgeuse), they are amongst the hottest and brightest stars in the known Universe.

 

Attach the Star Analyser to the front of the CCD imager eyepiece and you'll see the spectrum produced by Rigel is quite different from that of Betelgeuse - with the bands in the blue end of the spectrum much stronger and the red band being significantly weaker.

Analysis of this spectrum with Visual Spec shows a presence of neutral helium and moderate hydrogen lines. Ionized metal lines include Magnesium II and Silicon II.

Let's now take a look at Sirius - the Dog Star. Sirius is the brightest star in the night sky and that is primarily because of its intrinsic luminosity and its proximity to our Earth. At a distance of just 8.6 light years away, the Sirius system is one of Earth's near neighbors. It is a Class A star - amongst the more common naked eye stars - and these are white or bluish-white, as you can see from the image below.

The spectrum produced by Sirius:

Analysis of Sirius' spectral band reveals strong hydrogen lines and also lines of ionized metals (Iron II, Magnesium II, Silicon II) and the presence of Calcium II lines.

And I can't talk about the colour of stars without of course mentioning our very own star - the sun. The sun is sometimes called a yellow star and it does indeed appear yellow to us on Earth (viewed through our atmosphere) but the Sun's own intrinsic color is white (aside from sunspots), with no trace of color. And if you want to do a spectral analysis of the sun, there's no need for a Star Analyser 100 to produce an image of its spectrum bands - just look out for the rainbow in the sky just after it rains!