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 15.  Within 15 ly, there are 58 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 58 known stars within 15 ly of Earth, an amazing 37 (64%) are class M stars.  The remaining 36% include one A star, one F star, three G stars, six K stars, one L infrared dwarf, five very cool T infrared dwarfs, one extremely cool Y infrared dwarf, 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 8.65 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 237 to 247 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 48 stars that are not white dwarfs or infrared 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 58 nearest stars.  Beyond the eight planets orbiting our Sun we find another eighteen confirmed planets, plus at least three 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 55% more massive than Jupiter, one a little more massive than Neptune, one a little less massive than Uranus, thirteen super-Earths (1.14 M up to 7.7 M), and two less massive than Earth (0.75 M and 0.98 M).  Their orbital periods range from 2 up to 636 terrestrial days, and then one planet (the super-Jupiter) orbiting once every 6.9 years.  Orbital eccentricities range from circular (0.00) to 0.55, 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 except for the four close-in planets orbiting Tau Ceti (G8.5V, 11.89 ly).

Color is indicative of the spectral type of the star, or the primary in a multiple star system. Circles indicate single stars and diamonds indicate multiple star systems.
Spectral types of the nearest stars
The number of single star systems outnumbers multiple star systems in the solar neighborhood.
However, the number of stars in multiple star systems exceeds the number of stars in single star systems in the solar neighborhood.
The solar neighborhood is dominated by main sequence stars.
The number of stars in the solar neighborhood known to have planets has been increasing each year, but is still relatively small.

References
NASA Exoplanet Archive https://exoplanetarchive.ipac.caltech.edu.
“The Nearest Stars” by Todd J. Henry, Observer’s Handbook 2019, RASC, pp. 286-290.

WISEA J045921.21+154059.2: A Bright, Nearby Infrared Dwarf

Are you up for an observing challenge?  If you have a telescope equipped with a CCD camera and an infrared filter, you might be able to detect the brightest known L-type infrared dwarf star WISEA J045921.21+154059.2.  (This author prefers to use the term “infrared dwarf” rather than the more popular caconym brown dwarf.)

WISEA J045921.21+154059.2, also known as WISE J045921.20+154059.4 and 2MASS J04592088+1541054, is a high-proper-motion star located not far from Aldebaran in the constellation Taurus at α2000 = 4h 59m 20.89s, δ2000 = +15° 41′ 05.42″.  This cool star has a spectral classification of sdL0.  Though its parallax has not yet been measured, its high proper motion may indicate it’s a star just a few light years away.  The “sd” classification on the L0 spectral type indicates that it is a subdwarf star—underluminous in comparison with a “normal” L0 star.

Infrared dwarfs—as their name implies—radiate mostly in the infrared portion of the spectrum rather than at visible wavelengths.  You can see this in the apparent magnitudes listed below.  Remember, the lower the number the brighter the star is at that wavelength/passband.

g = 20.08 ± 0.03
(PS1 g magnitude: center wavelength 4866 Å; green light)

r = 18.70 ± 0.01
(PS1 r magnitude: center wavelength 6215 Å; red light)

i = 17.14 ± 0.01
(PS1 i magnitude: center wavelength 7545 Å; infrared)

z = 16.49 ± 0.01
(PS1 z magnitude: center wavelength 8679 Å; infrared)

y = 16.19 ± 0.01
(PS1 y magnitude: center wavelength 9633 Å; infrared)

J = 14.96 ± 0.03
(2MASS J magnitude: center wavelength 12,350 Å; infrared)

H = 14.61 ± 0.06
(2MASS H magnitude: center wavelength 16,620 Å; infrared)

Ks = 14.30 ± 0.06
(2MASS Ks magnitude: center wavelength 21,590 Å; infrared)

W1 = 14.09 ± 0.03
(AllWISE W1 magnitude: center wavelength 34,000 Å; infrared)

W2 = 13.85 ± 0.04
(AllWISE W2 magnitude: center wavelength 46,000 Å; infrared)

W3 = 11.86 or brighter
(AllWISE W3 magnitude: center wavelength 120,000 Å; infrared)

W4 = 8.99 or brighter
(AllWISE W4 magnitude: center wavelength 230,000 Å; infrared)

References
Best, W. M. J., Magnier, E. A., et al. 2017, arXiv:1701.00490 [astro-ph.SR]