The Brightest Red Dwarf

by Ken Croswell

Published in Sky and Telescope (July 2002, page 38).

I have a soft spot for red dwarfs--I guess I feel sorry for them. Small, cool, faint, and feeble, most of them struggle for centuries to generate the same amount of visible light that the Sun throws off in a year. Although red dwarfs outnumber all their brighter brethren put together, not a single one glows brightly enough to stir the unaided eye.

Still, one of these dim red suns must be the brightest in Earth's sky. But which one? Those in the know might pick Proxima Centauri, the closest star to the solar system. It's a good choice; the little red star that tags beside the bright suns of Alpha Centauri is only 4.2 light-years from Earth. Another educated guess might be Barnard's Star, our next closest neighbor at 5.9 light-years.

But no, these well-known stars don't even come close. The actual red-dwarf champion resides three times farther than Proxima Centauri. Although easily visible through binoculars, it's an obscure pinpoint of light in an obscure constellation, both of which share a common legacy. The star's name--Lacaille 8760--honors a modest but hardworking eighteenth-century French astronomer so devoted to science that he died at the age of 49 from the rigors he suffered while observing the sky.

The Celestial Attic

During his working life, Nicolas Louis de Lacaille made more observations and calculations than all of his peers put together. "Lacaille's deeply sincere modesty, his profound honesty, and his sustained devotion to his science impressed all who knew him," wrote astronomy historian Owen Gingerich (Harvard-Smithsonian Center for Astrophysics). David S. Evans (University of Texas, Austin), author of a book on Lacaille, wrote that the French astronomer "lived for science and nothing else. In none of the accounts does he ever appear as a definite personality; he has few friends and no emotions. He seems a man without a private life who appears to pour forth the flood of his researches and disappears into an obscurity in which those researches are at once the only light and the only memorial."

Unfortunately, to the extent that Lacaille is known today, it is primarily for the 14 constellations he added to the southern sky--constellations so dim and pointless that astronomers and casual stargazers alike have complained about them ever since. Consider, for example, the reaction of Lick Observatory's Heber Curtis when he saw the far-southern sky: "It looked like somebody's attic!"

Lacaille invented his constellations while at the Cape of Good Hope in South Africa, where he observed from 1751 to 1753. Living in the age of reason, Lacaille shunned mythological figures and instead honored inanimate objects, especially scientific instruments. His constellations read like a Who's Not Who of the southern sky: Antlia Pneumatica, the Air Pump; Caelum, the Engraving Tool; Circinus, the Geometer's Compasses; Fornax Chemica, the Chemist's Furnace; Horologium Oscillatorium, the Pendulum Clock; Mons Mensae, Table Mountain, which overlooked Lacaille's observatory; Microscopium, the Microscope; Norma et Regula, the Level and Square; Octans, the Octant; Pictor, the Painter's Easel; Pyxis Nautica, the Ship's Compass; Reticulum Rhomboidalis, the eyepiece reticle with which Lacaille measured star positions; Sculptor, the Sculptor's workshops; and Telescopium, the Telescope. Had Lacaille lived today, no doubt there'd be a forgettable southern-sky constellation called Computerium, the Computer.

Lacaille did more than label asterisms. Using a mere half-inch refractor, he charted the skies invisible from his native France, measuring and mapping the positions of 9,766 southern stars. Along the way he compiled a catalog of nebulae and star clusters that predated the famous work of his fellow Frenchman, Charles Messier. Lacaille's 42 deep-sky objects included the Lagoon Nebula, the Eta Carinae Nebula, and the globular clusters Omega Centauri and 47 Tucanae (S&T: February 1960, page 206).

Catch a Moving Star

Lacaille's full star catalog wasn't published until 1847, nearly a century after his death. Star number 8760 was faint and red, seemingly nothing special. It came to the attention of the astronomical community in 1875, when German astronomer Carlos Moesta, who had been the first director of Chile's National Astronomical Observatory, compared its position in various star catalogs and noticed that the star had moved. Despite the star's dimness, this high proper motion suggested that Lacaille 8760 was a close neighbor of the Sun.

And indeed it is. The best modern parallax measurements--courtesy of the Hipparcos satellite--give Lacaille 8760 a distance from Earth of only 12.9 light-years, making it the 24th closest star system to the Sun. It is also the brightest red dwarf (spectral class M) in the sky, with an apparent magnitude of 6.69, fainter than the conventional naked-eye threshold of 6.0. Even though red dwarfs account for 80 percent of the galaxy's stars, no red dwarf is visible to the unaided eye. Bright yellow stars like the Sun are clearly rarities.

Appropriately, Lacaille 8760 graces a constellation that Lacaille invented, and one that's dim even by his standards: Microscopium. Even among hard-core observers, few have ever seen Microscopium; its brightest star is only 5th magnitude. Microscopium is west-southwest of Piscis Austrinus, the well-known home of Fomalhaut, the solitary white star that lights up a relatively featureless autumn sky.

Better Red Than Dead

By red-dwarf standards Lacaille 8760 is fairly luminous, having a spectral type of M0 and an absolute magnitude of 8.71, meaning it emits 1/36 as much visible light as the Sun. Its temperature and luminosity imply a mass about 60 percent that of the Sun. In contrast, the dimmest red dwarfs, such as Proxima Centauri, have masses close to the hydrogen-burning limit, 8 percent of the Sun, and put out less than a ten-thousandth of the Sun's light.

Despite its feeble glow, Lacaille 8760 generates energy the same way as other main-sequence stars, by converting hydrogen into helium at its center. However, Lacaille 8760's core is less hot, slowing the nuclear reactions and lessening the production of energy. On the positive side, by hoarding its fuel, the star will far outlive our star. Lacaille 8760 will still be shining some 70 billion years after our more profligate Sun has exhausted its fuel and shrunk to a white dwarf.

Lacaille 8760 is a single star, so it may easily entertain orbiting planets. Astronomers already know of two giant planets that revolve around another nearby red dwarf, Gliese 876 in Aquarius (S&T: April 2001, page 20). Until recently, however, planets orbiting red dwarfs were thought to be unlikely abodes of life. The trouble stems from the star's dimness. In order to have a climate mild enough to allow for liquid water, a red dwarf's planet would have to huddle close to its star. For example, an Earth-temperature planet circling Lacaille 8760 would have to park itself about half as far from the star as Mercury is from the Sun.

Unfortunately, a planet this close to the star would have its rotation tidally locked. One side would permanently face the star, and the other side would face away. This same effect makes one side of the Moon constantly face Earth. As a result, the day side of the red-dwarf planet would fry and the night side freeze. Worse yet, the planet's atmosphere would vanish, because air passing over the frigid night side would freeze out onto the surface.

During the 1990s, though, Robert Haberle and Manoj Joshi of NASA's Ames Research Center in Moffett Field, California, suggested that this scenario is too pessimistic. Their calculations showed that the planet's atmosphere would protect itself. If the air was at least 15 percent as thick as Earth's, it would ferry enough heat from the day side to the night side to prevent an atmospheric freeze-out. Blessed with light, warmth, water, and air, the planet's eternal day side could give rise to life--which wouldn't face Earthly tribulations like night and seasons.

So far, however, Lacaille 8760 has given no sign that it has any planets, let alone any life. If life does exist there, it may be older and wiser than we are--because the star itself probably is. One sign of Lacaille 8760's maturity is its quietness. Many youthful red dwarfs are cantankerous, spewing large flares that outshine the rest of the star. These red dwarfs, such as those in the Pleiades Cluster, spin fast. Rapid rotation intensifies a star's magnetic field, so young dwarfs often sport many spots. The starlight we see from Earth varies as the spots rotate into and out of view, resulting in a characteristic light curve. As cool stars age, their spin slows down. When the Sun itself was young it probably spun once every few days; today it spins once a month. Lacaille 8760 must be a slow spinner, since it has too few starspots to alter its light. Its spectrum also indicates low magnetic activity and hence slow rotation and a moderate or old age.

Still, Lacaille 8760 isn't ready for retirement just yet. In 1979 Patrick Byrne (Armagh Observatory, Northern Ireland) detected a small flare from the dwarf. As a result, Lacaille 8760 now bears a variable-star designation, AX Microscopii. Nevertheless, Byrne observed Lacaille 8760 for 26 hours and detected only one flare. He concluded that its "flare activity may be unusually low when compared with that of other flare stars of similar type."

The pronounced elliptical shape of Lacaille 8760's orbit around the galaxy also suggests that the star is middle-aged or old. Milky Way stars are born in places like the Orion Nebula, clouds of gas and dust that line the galactic plane. These clouds have circular orbits around the galaxy, which the newborn stars inherit. As the stars circle the galaxy, however, they get perturbed by the very interstellar clouds that gave them birth. The largest interstellar clouds weigh hundreds of thousands of times more than the Sun, and their gravity can yank passing stars from their pristine circular orbits. As a result, the older a star and the more it has traveled around the galaxy, the more elliptical its orbit will probably become.

The 4.6-billion-year-old Sun has completed 20 trips around the galaxy. Its orbital eccentricity is only 6 percent, and its distance from the galactic center varies from about 27,000 light-years--its present distance--to about 30,000 light-years.

In contrast, Lacaille 8760 has an orbital eccentricity of 23 percent, suggesting a longer tenure. Of course, like the Sun, Lacaille 8760 is currently about 27,000 light-years from the galaxy's center. But it's sidling closer to the galaxy's heart. At its closest approach it will pass within 21,000 light-years of the galactic center, then journey outward to a maximum distance of 34,000 light-years.

As they orbit the Milky Way, stars bob up and down through the galactic plane like horses on a merry-go-round. At the moment the Sun is a bit north of the galactic plane and moving northward at 7 kilometers per second. This velocity carries the Sun--and us--upward 1.5 astronomical units each year. In 15 million years, the Sun and our remote descendants will attain a height of 200 to 250 light-years above the galactic plane. After that, we will plunge back down. Lacaille 8760 is also moving up through the galactic plane, but nearly five times faster. It will ultimately climb 1,500 light-years above the galactic plane.

Lacaille 8760 skirted closest to the Sun 20,000 years ago, during the last ice age, at a minimum distance of 12.1 light-years. Ever since, the little red star has been creeping away. As it continues to recede from Earth, it will continue to fade while another dim sun moves to take its place as the brightest red dwarf. In time, Lacaille 8760 will grow dim and obscure--just as its modest, hardworking namesake did.

Ken Croswell admits it: he has never seen Microscopium. Nevertheless, he managed to earn his Ph.D. in astronomy at Harvard University and has authored several highly acclaimed astronomy books, including Magnificent Universe and The Universe at Midnight.

"Elegant and eloquent"--Washington Post. See all reviews of Magnificent Universe here.

The Three Brightest Red Dwarfs
NameConstellationDistance (light-years)Spectral typeVisual magnitudeOther names
Lacaille 8760Microscopium12.9M06.69Gliese 825, HD 202560, AX Microscopii
Lacaille 9352Piscis Austrinus10.7M0.57.34Gliese 887, HD 217987
Lalande 21185Ursa Major8.3M27.49Gliese 411, HD 95735


Owen Gingerich's quote is from the Lacaille entry in the Dictionary of Scientific Biography; David Evans' quote is from the last paragraph of

Evans, David S., 1951. La Caille: 10,000 Stars in Two Years. Discovery, 12, 315.

Other discussions of Lacaille's life appear by

Armitage, Angus, 1956. The Astronomical Work of Nicolas-Louis de Lacaille. Annals of Science, 12, 163,

Evans, David S., 1980. Nicolas de la Caille and the Southern Sky. Sky and Telescope, 60, 4 (July 1980),

-----, 1992. Lacaille: Astronomer, Traveler (Tucson: Pachart),

Gingerich, Owen, 1960. Abbe Lacaille's List of Clusters and Nebulae. Sky and Telescope, 19, 207 (February 1960),


Usher, Peter D., 1979-1980. LaCaille at the Cape. Astronomy Quarterly, 3, 185.

Heber Curtis's derogatory quote about Lacaille's constellations appears in Sky and Telescope, February 1971, page 100.

Lacaille 8760's large proper motion was discovered by

Moesta, C. W., 1875. Schreiben des Herrn Dr. C. W. Moesta an den Herausgeber. Astronomische Nachrichten, 86, 245,

and Lacaille 9352's by

Gould, B. A., 1881. Star with Large Proper Motion (Lacaille 9352). Astronomische Nachrichten, 100, 9.

Lacaille 8760 is a single star, according to

Young, Arthur, Sadjadi, Sina, and Harlan, Eugene, 1987. The Incidence of Close Binaries Among Stars of Low Mass: Astrophysical Binaries. Astrophysical Journal, 314, 272,


Leinert, Ch., Henry, T., Glindemann, A., and McCarthy, D. W., Jr., 1997. A Search for Companions to Nearby Southern M Dwarfs with Near-Infrared Speckle Interferometry. Astronomy and Astrophysics, 325, 159.

The possibility of life on red-dwarf planets is described by

Croswell, Ken, 2001. Red, Willing and Able. New Scientist, 169, No. 2275 (January 27, 2001), p. 28.

Lacaille 8760's flare was discovered by

Byrne, Patrick B., 1981. Gliese 825--A New Flare Star. Monthly Notices of the Royal Astronomical Society, 195, 143.

The following compare Lacaille 8760 with active flare stars:

Butler, C. J., Byrne, P. B., Andrews, A. D., and Doyle, J. G., 1981. Ultraviolet Spectra of Dwarf Solar Neighbourhood Stars--I. Monthly Notices of the Royal Astronomical Society, 197, 815,


Doyle, J. G., Byrne, P. B., and Menzies, J. W., 1986. Optical Photometry and Spectroscopy for Five Dwarf M Stars. Monthly Notices of the Royal Astronomical Society, 220, 223.

From its lack of activity,

Doyle, J. G., 1987. An Activity-Rotation Relationship in F-M Dwarfs from Mg II h and k Flux. Monthly Notices of the Royal Astronomical Society, 224, 1P,

predicts that Lacaille 8760 rotates once every 25 days, while

Byrne, P. B., and Doyle, J. G., 1989. Activity in Late-Type Dwarfs. III. Chromospheric and Transition Region Line Fluxes for Two dM Stars. Astronomy and Astrophysics, 208, 159,

predict that the rotation period should be about 40 days.

Lacaille 8760's orbit around the Galaxy was computed by

Allen, Christine, and Herrera, Miguel A., 1998. The Galactic Orbits of Nearby UV Ceti Stars. Revista Mexicana de Astronomia y Astrofisica, 34, 37,

and its nearest approach to the Sun was computed by

Garcia-Sanchez, J., Weissman, P. R., Preston, R. A., Jones, D.L., Lestrade, J.-F., Latham, D. W., Stefanik, R. P., and Paredes, J. M., 2001. Stellar Encounters with the Solar System. Astronomy and Astrophysics, 379, 634.