Appreciating the Little Things

Marlon Cowart

So much in astronomy is colossal--the distances between stars, the power of supernovae, the inscrutable nature of black holes, the size of galaxies and superclusters. There are almost countless objects of surpassing beauty, hundreds within reach of amateurs' telescopes and digital cameras and many more with large observatories like the Hubble and Spitzer orbiters. The colossal and the beautiful occupy the majority of professional and amateur astronomers' attention, and fascinate the public in print and on television. When I observe, I turn to the showcase objects like the Orion, Omega, or Veil nebulae, brilliant clusters like NGC869, M11, or M44, and edge-on galaxies like NGC 4665.

However, there is another pleasure in our science which lies in contemplating things less conspicuous, that deserve appreciation as much for what they represent as what they look like. One such class of object are the red dwarf stars, low mass M class. These comprise the great majority of stars in our galaxy, at least 75%. I think their ubiquity merits contemplation.

So what is known of these little stars? First, in our nearest stellar neighborhood (within 20 light years) there are 83 known stars, and though 78 are M class, not a one is visible to us by naked eye. Most of stars we see in the sky are actually of the more rare types (G class like our sun, or the even more powerful B, A, and O class and red supergiants) but prominently visible because they are intrinsically very bright (our sun is a G class). At masses only 10-40% that of the sun, red dwarfs have less hot and less compressed cores, so their fusion of hydrogen into helium is much slower, and they have luminosities only 0.01-4% that of our sun. Surface temperatures are low 2000-3500 Kelvin, and they emit mostly low energy red and infrared light. Burning hydrogen so slowly, they will live incredibly long lives. While our sun will spend around 10 billion years in the main sequence, M class stars will live more than 100 billion years for the largest, and up to 10 trillion years for the smallest. This is many many times the current age of the universe (about 14 billion years). I've often wondered: with so many of these stars, and their extreme longevity, what are the prospects for habitable planets or life on M dwarfs? Turns out, astronomers have recently begun to study this question.

Here is some thinking on their latest findings. Planets have been found circling M dwarfs, nearby Gliese 876 (0.35 solar mass M3 in Aquarius, Magnitude 10.2) has three planets, on an 8-earth mass body and the other two 0.5 and 1.5 times as massive as Jupiter. These are large, but current techniques are strongly biased toward detection of large planets, especially those that orbit their star closely. It is thought that newer more sophisticated techniques will find the more earth sized planets in the future. The habitable zone, the orbit allowed for water to be present in the liquid state--neither frozen nor heated to steam--is smaller for small stars than for sun-like G-class and larger stars, and of course is much closer to the star, calculated as 1/5 to 1/20 that of the sun. In such a close-in orbit, a planet will necessarily be gravitationally 'locked' due to tidal forces, to the same side will face the stellar surface. This would lead to one side always facing the red light of the star and the other side in perpetual darkness. Sounds like Mercury--blistered by heat on one side and frozen on the other? Maybe not. It is thought that with an atmosphere (I read 15% or more dense than earth) or deep ocean, heat might be convected around the planet. Such a situation would likely give rise to very dynamic weather and currents with the heat transference. The situation should be different for a Europa or Ganymede sized body with water that orbited a large planet; these should receive illumination over their entire surface, and potentially endow an even more habitable environment. The low energy red/infrared light from an M-class star presents a problem for photosynthesis, not just because it is dim, but because of the lower energy of the photons it has less ability to break of chemical bonds. Think of chlorophyll catalyzing the splitting of water into oxygen, protons, and usable electrons, a process that allows the fixing of carbon dioxide into carbohydrates, and you see why this is important. There are alternatives though, energy processes in deep ocean vents allow thermophilic organisms to power their metabolisms by non-photosynthetic pathways, using hydrogen sulfide as a source of electrons. So there are other xenobiotic pathways other than photosynthesis that can enable life to develop.

Where can you see such stars? Well there are many sites that show the nearest stars - for example:
You can see Barnard's star in Ophiuchus at magnitude 9.5, and WX Ursa Majoris A and B at magnitudes 8.7 and 14.4 as well as many others. You may be able to see recently discovered Teegarden's star (M15.4). M dwarfs are not appealing for their visual appearance, but are intellectually interesting nonetheless. Some things may be less conspicuous, but earn our appreciation for what they represent, examples of the majority of star systems in the universe that will shine for hundreds of billions of years

Published in the November 2008 issue of the NightTimes