The Nearest Stars

Within 5 light years (ly) of the Earth, there are 4 stars known (just the Sun and the Alpha Centauri system).  Within 10 ly, there are 14.  Within 15 ly, there are 60 stars.  The number goes up—rapidly!  Undoubtedly, more stars will be discovered within 15 light years of the Sun.

And, cool is the rule when it comes to nearby stars.  Of the 60 known stars within 15 ly of Earth, an amazing 40 (two-thirds) are class M stars.  The remaining one-third include one A star, one F star, three G stars, six K stars, one L infrared dwarf, five very cool T infrared dwarfs, and three white dwarfs.

The hottest (and bluest) star within 15 light years of the Sun is none other than Sirius (α Canis Majoris)—the brightest star in the night sky—just 8.58 light years distant.  Sirius A is an A1V (main-sequence) star, twice as massive as our Sun, 71% wider, 25 times more luminous, and only 225 to 250 million years old—just a single orbit around the galactic center.  Sirius rotates much faster than the Sun, too, spinning around once on its axis every 5.4 days.  Think about all these things the next time you look up and see Sirius chasing Orion across the meridian these late-winter eves.  And that Sirius has a white dwarf companion that orbits it once every 50 years, too.

All but two of the nearest 57 stars that are not white dwarfs have a luminosity class of V, meaning they are dwarf or main-sequence stars.  The first exception is Procyon (α CMi A).  Its luminosity class of IV-V indicates it is bright for its temperature and spectral type (F5) and beginning to evolve into a subgiant star on its way towards becoming a giant star.  The other exception is Kapteyn’s Star, a red subdwarf star of spectral type and luminosity class M2VI.  A subdwarf star is underluminous for its temperature and spectral type.  This is caused by low metallicity.  The scarcity of elements other than hydrogen and helium in the star results in a more transparent stellar photosphere and thus a star that is a little smaller than it normally would be.  Incidentally, the fact that we have three white dwarf stars within just 15 light years of us suggests that white dwarfs are copious throughout our galaxy.

You might be wondering how many planets have been discovered orbiting these 60 nearest stars.  Beyond the eight planets orbiting our Sun we find another eleven confirmed planets, plus several more unconfirmed planets.  This is a rapidly advancing field and no doubt many more planets will be added to the list in the coming decade.

The masses of the confirmed planets include one a little over three times the mass of Jupiter, one a little more massive than Neptune, one a little less massive than Uranus, six super-Earths, and two just a third more massive than Earth.  Their orbital periods range from 4.7 up to 121.5 terrestrial days, and then one planet (the super-Jupiter) orbiting once every 6.9 years.  Orbital eccentricities range from circular (0.00) to 0.32, with the super-Jupiter in a very elliptical orbit having an eccentricity of 0.702.  The super-Jupiter is orbiting Epsilon Eridani (K2V, 10.48 ly), with all the rest of the confirmed exoplanets orbiting M-dwarf stars.

“The Nearest Stars” by Todd J. Henry, Observer’s Handbook 2017, RASC, pp. 286-290.

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