The Lake County Astronomical Society

The "Coolest" Stars

Jack Kramer

In a star-filled sky, our reaction to all those heavenly lights is usually something like "You've seen one star, you've seen 'em all!" Star-hoppers merely use them as steppingstones to find more interesting deep sky objects. The fact is, seldom do most of us set out to look specifically at a star, unless it's a particularly challenging or colorful double, or perhaps one that shows dramatic swings in brightness. However, there is another class of star that is worth a glance -- these are the ones that are particularly red in color. They are generally placed in spectral class N, and because they don't appear to follow a normal stellar evolutionary path, they're generally not considered part of the Main Sequence classification. Note that although they're referred-to as "red", they are really more of an orange-colored hue. Beyond an admiration of their vivid color, what makes these interesting is a sort of contemplation of what is going on up there. A red star is near the end of its life. The nuclear furnace is exhausting its fuel and cooling down. Heavier elements, particularly the metals, are now forming. In some cases they have become large, bloated giants; in other cases, they're red dwarfs.

The carbon star is an additional class that has been recognized just within the past twenty years or so. It's shown as spectral class C, often with a subscript denoting the amount of carbon detected. These are some of the coolest and reddest stars known and are producing prodigious amounts of carbon, to the point that their spectra show absorption bands for carbon. This has led to the conclusion that these stars are embedded in a halo of carbon that has been blown off from them. Were it not for this absorbing cloud, we would probably see carbon emission lines instead.

A common characteristic of most red stars is that their brightness is variable, with periods ranging from days to years. In some cases, the periods are not consistent, showing recurrent cycles of different length. And in still other cases, no consistency has definitely been shown. Something is definitely going on out there!

As you observe, you might imagine how that star appears close-up, from a doomed planet circling it. Will the star soon end its life in a violent explosion? Will it simply wink out, its fuel exhausted, to become one of those enigmatic brown dwarfs? If you watch these stars regularly, you at least have a shot at being an eyewitness to whatever may be their fate.

To help you identify these stars, the following table shows a number of the more prominent ones. The list has been extracted from the Saguaro database of deep sky objects, with some additional data added. The letter designation of the star is followed by the abbreviated name of the constellation where it resides. You'll notice that in some cases the magnitude swings are quite large; the periods from maximum to minimum brightness are shown in days. The Notes column indicates which of them are classified as carbon stars, plus any other noteworthy features. These are some "way cool" stars!

Very Red Stars (Mostly N Type, Mostly Variable)

Newly-Discovered Cool Stars Add to the Spectral Class Alphabet Soup

Now, for the first time in more than 100 years, astronomers are considering the addition of a new spectral type to the main sequence stellar classes of OBAFGKM. These letters denote the different surface temperatures for hydrogen-burning stars along the so-called main sequence: from hot, blue-white, massive, luminous O stars to cool, red, lightweight, and dim M stars. Our Sun is a middle-of-the-road, yellow-white type-G star. You may have heard the memory-jogger phrase (mnemonic) that goes: "Oh, Be A Fine Girl/Guy, Kiss Me", with the first letters of each word standing for each of the spectral classes.

Recently, Caltech astronomers proposed extending the main sequence one step further to type L, to account for a new population of objects turning up in the 2-Micron All-Sky Survey, which is mapping the near-infrared sky from sites in Arizona and Chile. L dwarfs are very cool, with surface temperatures of 1,500 to 2,000 degrees Kelvin and they emit most of their light at infrared wavelengths. Spectral analysis reveals that some of them are substellar motes called brown dwarfs, while others seem just barely capable of sustaining hydrogen fusion in their cores, which thus makes them "true" stars. It is estimated that there are as many L dwarfs as stars of all the other spectral types put together, but their masses are so small (on the order of 30 to 80 Jupiters) that they don't contribute much to the overall dynamics of the Milky Way. If the professional astronomical community accepts the new spectral classification, the familiar mnemonic will have to be modified, perhaps to "Oh, Be A Fine Girl/Guy, Kiss My Lips."

Added to this alphabet soup are the more unusual stars not generally considered part of the main sequence. There are cool stars of classes N, R, S, and the newer C class, for carbon stars. At the other end are the hot blue-giant type W stars -- a type known as Wolf-Rayet stars that are distinguished by broad emission features and very turbulent atmospheres.

In case you're desperate for something to do and really feel compelled to memorize all the spectral classes of stars (both on and off the main sequence), then you may want to create a mnemonic that somehow encompasses OBAFGKMNRSCW!