How to Remember the Spectral Types of Stars when Seeking Life in Space

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This article is based upon the astronomer mantra for remembering the spectral types of stars. It is a tool in the quest for life in space.

“Oh Be A Fine Girl, Kiss Me” is a mantra used by Astronomers. It is a mnemonic to remind them of the spectral types, or (put simply) “colours” of main sequence stars. Indeed most stars form the “main sequence”, and they all follow the same pattern. O,B,A,F,G,K and M are the “spectral types”. Searching for life in space, you need to know which stars are most likely to be suitable to support a stable solar system of planets.

The Brightest Stars

So, starting from the top, O,B,A stars are the brightest: hot blue, blue and blue cum white in turn. Such stars also tend to be the largest: typically many times the size of the sun. They are also usually the shortest lived! Such a star might last only a few million years – a mere flicker in cosmic terms – before going nova and dying! There are also Supergiant and Giant stars outside the main sequence which behave in much the same way. When they die, the greatest stars end up producing the heaviest of metals as they degrade into white dwarves or even black holes. Most famous A type star is Sirius, the Dog Star: brightest in our sky and 8.5 light years away (the closest A type to us). One of the nearer O types is Zeta Ophiuchi, some 460 light years away. A nearer B type is Regulus (77 light years away).

Middle Range Stars

Next come the F types (white), G (yellow), and K (orange cum yellow). Better mention at this point that each spectral type is subdivided from 0-9 also. Our own Sun is a G2. Our nearest F type is Procyon at 11.4 light years away. The nearest G type to Earth is of course our own Sun: a good trick quiz question. Next nearest G type is Alpha Centauri A (G2), 4 light years away and part of a binary system. Next out is Tau Ceti (G8) at 11 light years.The nearest K type to us is Alpha Centauri B. Next K type out is Epsilon Eridani (10.5 light years). Needless to say, such stars have been very popular in our quest for earth-like planets and life.

Red Dwarves

Finally we have the red dwarves: the M types. The title red dwarf says it all really. Forget about the “Star Trek” M type (habitable) planets: that’s pure fiction! These M types are indeed tiny red stars: and by far the most numerous! There are indeed red giants around too: our own Sun will become one during its early death throes. Betelgeuse (M2 type) is a fine example of a “runaway” non main-sequence red giant that is about to go supernova after only 10 million years of life to date. Being over 180 light years away, Betelgeuse may have gone supernova already without us knowing!

However, the down side of the mainstream red dwarves is that they are not very hot. That means your typical red dwarf usually has a relatively shallow “Goldilocks Zone”: the ideal area for a planet to be warmed to an optimum temperature for life. (Earth lies at the centre of Our Goldilocks Zone, Venus at the near edge, Mars at the outer boundary). The “Zone” of a red dwarf will be very near to the actual star, and There planets are liable to be “tidally locked” (with one face always towards its sun). Planet Gliese 581 g – our best hope for life to date – is a fine example of such a world. Gliese 581 is an M3 star, about 20 light years from us.

As Professor Brian Cox said recently, on “Wonders of the Universe” (BBC1), the good news is that these red dwarves can last for trillions of years before dying. He cited the example of our nearest star (other than the Sun): Proxima Centauri (M 5.5): 4 light years away and somewhere we might go after our own Sun has burnt itself out. The next M type is Barnard’s Star at 5.9 light years. Wikipedia do a neat list of our nearest stars if you wish to check these out in more detail. Bottom line is that red dwarves are the most stable and have by far the greatest longevity. That’s if you really want to plan long-term!

Paul Butters


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