Small Stars & New Spectral Types

Probably the single most informative piece of information about a star that can be directly observed is it’s spectral class. And what is that, you ask? It’s a letter designation that was given to a star based on a simple characteristic – the strength of the hydrogen absorption lines in it’s spectrum. A type stars have the deepest, most broad lines, B stars the next most, all the way, historically to O stars, which show no lines at all in their spectra.

Astronomers soon figured out that this A through O scheme had missed a very important point that forced them to reconsider. Remember your high school chemistry? Very cool stars show weak hydrogen line absorption because they don’t put out enough energy to raise up the electron in hydrogen to the next electron shell. The energy in the photon isn’t absorbed at all. Very hot stars, on the other hand, put out so much energy that they totally strip the electrons from the hydrogen nuclei, so there’s nothing left to absorb the energy. Stars in the middle temperature range show the strongest lines in their spectra.

But that wasn’t known until later. So rather than reclassify about 10 bazillian stars that already had had their spectra classified, they changed the scheme to make more sense. From then on, every poor, freshman slob that took astronomy was forced, on pain of flunking, to learn the ridiculous order O B A F G K M, which orders stellar spectra, not by absorption line strength, but by stellar temperature.

Of course, the only thing the student was going to remember six weeks after graduation (and forever, it seems) would be the mnemonic device: “Oh Be A Fine Girl, Kiss Me”. Awful, but useful. Some will remember the oddball R, N and S classes that were added later. They were absorbed into an even more oddball “Carbon Star” class, which itself wrecks a concept that the whole scheme was trying to highlight – the relation between the stars’ temperature and its mass. It, and they, were dropped, because the R, N and S classes were describing something else.

Astronomers always knew there would be cooler, smaller stars that would need designations beyond M, but they would be dim and tough to spot. And where’s the cutoff before you’re talking about really large planets anyway? Active stars can’t be much smaller than 80 Jupiters in mass, or they can’t fuse hydrogen into helium (the working definition of a star). But some normal stars can, in the course of things, blow off their outer layers, leaving only their hot cores, which can be well below that 80 Jupiter mass limit. They can only cool down over time, leaving behind some interesting objects known as brown dwarfs. And the cooler we find them, the older they are. For no good reason, astronomers designated relatively warm brown dwarfs spectral class L and relatively cool ones spectral class T.

Now there is a new record setter for pint size stars. From Sky and Telescope:

The new discovery (named CFBDS J005910.83–011401.3 for its celestial coordinates, or CFBDS 0059 for short), is about 40 light-years away in Cetus. Its discoverers estimate that is has a mass of 15 to 30 Jupiters, based on spectral indicators of its surface gravity, which means it’s certainly no record-setter in the low-mass department. But if it’s that cool, it must be pretty old: 1 to 5 billion years for a brown dwarf of that mass, the researchers say, based on evolutionary cooling models.

The researchers are also calling it a link between brown dwarfs and giant planets.

The missing link, that is. Call it spectral class Y.

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