This week we’re looking for a clear night, bundling up in layers, and heading out into the yard to see a rainbow of star colors. The color of a star is almost always an indication of the phase of its life. The middle months of winter are the best time of the year to easily find examples of stars from one end of the spectrum to the other – from the youngest stars to the great giants that most stars become at the very end of their existence.
For all of the stars we are going to visit tonight, the eye may be enough to distinguish color differences, but a pair of binoculars may make the differences more apparent. You may also want to follow my example and listen to She’s A Rainbow (Rolling Stones) during this nocturnal outing.
The spectral types of stars (scientific term for their colors) are traditionally labeled by astronomers using a series of letters. How this particular series
evolved into its current apparently arbitrary order is a story in itself, though
honestly not a very interesting one (and you know it must be boring if even I’m
not willing to go rambling on about it!). Nonetheless, the Harvard spectral class letters are the standard which every student of astronomy will eventually encounter. Thankfully there are catchy phrases to help memorize the order.
(O)h. We will start in the constellation Orion, which you should have little trouble locating. Facing south at about 8pm, the great hunter is displayed about ½ way up in the sky, with the three stars of his belt laid out in a line running diagonally from lower left to upper right – Alnitak, Alnilam, and Mintaka. The stars at both ends of the belt, Alnitak and Mintaka are O stars, which have their peak visible emission in the blue region of the spectrum. O stars are in their infancy, with ages of a few million years at most. Having just formed
the temperature in their outer atmospheres is 30,000 degrees or higher, and at
these temperatures the majority of their emission is in the ultraviolet; we see
these stars as bluish.
(B)e. As the stars emerge from their cradles and begin to stabilize their output of energy, they cool slightly and move into the B spectral class. These stars appear white with a bluish cast. Staying within Orion, the central star of the belt, Alnilam, is a B class star, as are the corner stars of Orion in the lower left (Saiph), lower right (Rigel), and upper right (Bellatrix). These stars are from 10 to 100 million years in age, and in Orion these are all huge stars with masses far greater than that of our Sun.
(A). The maturing star reaches adolescence as an A star. Follow the line of Orion’s belt down in the direction of the horizon to find the brightest star in the sky, Sirius, the Dog Star, is a beautiful example of this spectral class. The structure of the star is now stable, its temperature in the outer atmosphere has cooled off to a balmy 10,000 degrees. These are simply white stars. Planets may be forming from the protoplanetary disk that surrounded the star in its infancy. Depending on their mass, class A stars may be from a few hundred millions years to a couple billion years in age.
(F)ine. As the star heads for mid-life, the whiteness of its younger days gives way to a yellow tinge. An F star appears as a yellow-white object. To find a bright example, look above Sirius and further east by about the same distance you travelled from Orion’s belt to Sirius. You will find a rather bright yellow-white star, which is the head of the “Little Dog”, Canis Minor. F stars run a bit hotter than our Sun, with ages of 1-3 billion years.
(G)irl or (G)uy. The color of G stars is very familiar, for our Sun is a G star. The best example in the current night sky is a bit of a farther jump from Orion than we’ve made so far. Let’s see if you’re up for it. Between 7-9pm, look almost directly overhead to find a bright yellowish star. (In my experience directly overhead is not usually where people look when I say directly overhead – I mean straight up folks – if you’re standing up while doing this it should not feel comfortable if you’re over 40). This is the star Capella in the constellation
Auriga. If you’ve had difficulty telling the colors apart so far, Capella may be the first star that looks definitely different from the others we’ve visited. A G star is in the middle of its life – our sun, at 4.7 billion years old, is expected to remain stable for about another 5 billion years.
(K)iss. Spectral class K brings us to the orange colored stars. These stars are approaching the end of their hydrogen fuel supply, and are beginning to swell in size, with their atmosphere temperatures dropping to around 4000 degrees. Still facing south, look for Jupiter and the Pleiades if you have been following me for the last couple of weeks, and draw a line from the Pleiades on the upper right through Jupiter and come to the next bright star. If you can’t find these easily, look for the brightest white “star” overhead, a bit less overhead than Capella was – much easier on the neck. This will be Jupiter. The star we’re shooting for is slightly to the lower left of Jupiter, and lies at the upper left of a V pattern pointing southwest. This is the monstrous star Aldebaran in Taurus the bull (the V is his head and horns). The beast has a diameter about 40 times that
of our Sun.
(M)e. The final stop on our survey of star colors is the M spectral class. These stars are very close to the end of their active lifetimes, with at most a few million years to go before their violent end. These are giant and supergiant stars. REeturning to Orion, we an easily observe perhaps the most famous of these, Betelgeuse, in the upper left corner of the constellation. If placed at the center of our solar system, Betelgeuse would extend nearly to the orbit of Jupiter. This is a truly colossal object. Located approximately 500 light years from Earth, Betelgeuse is the closest supergiant and has been extensively studied. Recent observations have shown material being ejected from the star, as well as its rapid approach toward a wall of interstellar gas. When
Betelgeuse inevitably becomes a supernova (sometime in the next million years), it will outshine the Moon for several months, and be easily visible in broad daylight.
So there you have it – a brief tour of the February evening sky visiting all of the colors of the astrophysical rainbow, and a phrase to help remember the spectral classes, which I, at least, will never forget: Oh Be a Fine Girl Kiss Me.