06 March 2013

The 100 Stars within 20 light years

A sphere centered on the Sun with a radius of just under (actually almost exactly) 20 light years has 100 stars in it. (Source: RECONS (Research Consortium on Nearby Stars). ---Unfortunately, this data includes some brown dwarfs and Red subdwarfs, which complicates things, because these things are not stars, really, and, worse, there are probably lots more of them than this data shows, but the essential points are correct anyway).

  • The brightest star, type A, is Sirius, Alpha Canis Majoris. 8.58 ly distant.
  • The second brightest is Procyon, Alpha Canis Minoris. Type F-5, just edging off the main sequence. 11.4 ly distant.
  • The third brightest is Altair, Alpha Aquilae. Type A7, 16.7 ly.
  • The fourth brightest star is THE SUN. Type G main sequence star.
  • The fifth brightest is Alpha Centauri A, 4.37 ly distant. Main sequence type G2, similar to the Sun but a bit older.
So our star is in the 96th percentile. Not exactly the "garden variety" we're told about in popular treatments. Stars like the Sun are common, but not nearly as common as dimmer stars.

Of the remaining  95 stars, there are no F dwarfs. (stars of the next brighter class relative to the Sun). (Only Procyon, see above). Stars in this class are also relatively common. We have two A dwarfs (Sirius and Altair) and one F dwarf (Procyon) in our actual Sun-centered population. Probably more typically it would be two or three F type in 100, probably on average there should only be one A dwarf. The very bright classes O and B are only found in star forming regions (because they don't live very long), and are exceedingly rare compared to these other classes. Which is why they're often referred to as giants and supergiants.

Among the remaining 95, then there are five more G dwarfs (same class as the Sun), all quite a bit dimmer than the Sun (in addition to Alpha Centauri A and the Sun, above, so 7 total, including the Sun). (Tau Ceti, Sigma Draconis, Eta Cassiopiae A, 82 Eridani, and Delta Pavonis).

There are 17 K dwarfs. (Next dimmer class). These include Alpha Centauri B, Epsilon Eridani, 61 Cygni A & B, Epsilon Indi A, AX Microscopii, Omicron-2 Eridani, 70 Ophiuchi A & B, Eta Cassiopiae B, and 36 Ophiuchi A, B & C, plus several stars that only have catalog numbers. When you get to 20 light years or so, the mid-to late-K dwarfs are very inconspicuous in the sky.

That leaves 78 stars. Of these 78, (skipping the most numerous category), seven are White Dwarfs, which are stars which were once bright G or brighter stars but which have ended their lives as burnt out little white stars, no longer undergoing nuclear fusion in their cores. These include Sirius-B and Procyon-B; and van Maanen's star. The rest just have catalog numbers. All white dwarfs are very inconspicuous. Sirius at 8+ ly is far and away the brightest star in Earth's sky, but its white dwarf companion, besides being drowned out by close proximity to Sirius, would not be a naked eye star in its own right even if Sirius A weren't there... despite being one of the six or seven nearest stars to the Sun.

That leaves 71. Of those, another 7 are T-dwarfs, or methane dwarfs, better known as brown dwarfs. These are failed stars that never did undergo nucleosynthesis. These stars, if they are stars at all, are practically invisible, almost no matter how close they are. The only ones with 'names' are Epsilon Indi B & C, and the only reason they have those designations is that they're in the Epsilon Indi system (although A is a relatively dim star in its own right; see above). [In fact, as noted, it's likely that there are even more of these, so if you count them as stars, the percentiles are off but the meaning is clear. There may well be over 100 more of these. If you don't count them as stars, you could extrapolate percentiles but just multiplying by ~108%].

So, disregarding the brown dwarf issue, we're left with 63. The majority. A landslide in a presidential election.

All 63 of the remaining stars are red dwarfs. Class M.*  Red dwarfs are little stars that burn hydrogen to helium, slowly and steadily, produce on average just a percent to a few percent of the light of the Sun, and live up to a trillion years without evolving off the "main sequence" (so we're told... the Universe is only 13.7 billion years old, so you have to believe the theory).

Over half of the individual stars in this population are in multiple systems, which translates, if you think about it, to something less than half of the systems being multiple. (If you have two systems, one single and one binary, 2/3 of the stars are in the binary system).

There's every reason to believe in disk populations in typical spiral galaxies, these proportions ought not to be too far from the norm.


To me, the takeaway is confirmation of something I already knew, which is that most stars are red dwarfs, and the brighter stars become increasingly rarer, more or less in direct proportion to increase in their mass, which directly correlates to increasing brightness, with minor adjustments for age and composition.

The Sun is a relatively bright star, not a typical star.

But on the other hand, there is a selection effect when you look at the night sky. Very very bright stars are SO MUCH brighter than dimmer stars that they dominate the night sky in visible light. MOST of the naked eye stars are exceptionally bright and very rare stars. Whereas, in the actual population, most of the stars are very dim dwarfs. And if you count the bodies that never really started shining with thermonuclear energy, there are even more of them. They're everywhere.

For  the future of humanity, the brown dwarfs are less interesting, because it's hard to imagine how future Homo stellaviator (Man the Starfarer) will be interested in systems that only contain them. These stars, if they even are stars, glow only with the heat of their own collapse and
are so cool they don't really emit enough light to drive photosynthesis or solar power generation. It's hard to see how it would be worth the cost and time to travel to such systems.

The abundance and energetics of thermonuclear fusion core Red Dwarfs, though, may be another story. If we someday literally cross the great voids of space in person, these are what's out there in huge numbers, and they are engines of energy production, like the Sun but smaller and weaker, and their systems are already known to typically have planets and probably other material in abundance. I'm enough of a dreamer to think of all that as real estate.

*(One is Class L, a special class of even lower temperature Red Dwarfs, which may also be under-represented even in this population study of the very nearest stars because they're so dim some are being missed. But this particular star is probably in thermonuclear fusion. The dimmer L's should be classed with the Ts. Look up spectral class T and L on Wikipedia if interested in this subject).

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