Wolf 359

ESO, Digitized Sky Survey, U.K. Schmidt Telescope
© PPARC and the Association of Universities
for Research in Astronomy, Inc. (AURA).

Larger image

Motion of Wolf 359 (CN Leonis) over several years
in blue and red images (more).

The Star

A very cool, main sequence red dwarf (M5.8Ve), Wolf 359 is our Sun's dimmest stellar neighbor within 10 ly, with less than 2/100,000th of Sol's visual luminosity. If our Sun, Sol, were replaced by Wolf 359, then an observer on Earth would need a telescope to see its round shape clearly, and daylight would be very dim with only ten times the brightness of full moonlight with Sol.

NASA -- larger image



Wolf 359 is a dim red dwarf star, as dim as Gliese
623 B (M5.8Ve) at lower right of Gl 623 A (M2.5V).

On the other hand, Wolf 359 is also a "flare star." Designated with the variable star name CN Leonis, the star can brighten dramatically from time to time. Observations in 2006 found night-to-night variations in its magnetic flux, "on time scales as low as 6 hours in line with chromospheric variability" (Reiners et al, 2007). By comparison, however, flares on Wolf 359 are rarer and not as violent as those observed on Proxima Centauri, Kruger 60 B, or UV Ceti.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

Wolf 359 is a flare star, like UV Ceti (Luyten 726-8 B)
shown flaring at left. UV Ceti is an extreme example
of a flare star that can boost its brightness by five times
in less than a minute, then fall somewhat slower back
down to normal luminosity within two or three minutes
before flaring suddenly again after several hours.

The star has a mass only around 9.2 to 13 percent of Sol's (RECONS estimate; Caillault and Patterson, 1990, page 826; and NASA Star and Exoplanet Database, derived from Henry and McCarthy, 1980), about 16 to 19 percent of Sol's diameter (Doyle and Butler, 1990, page 336; and Caillault and Patterson, 1990, page 826), and only around 2/100,000th of its visual luminosity. With less than 20 percent of Sol's mass, Wolf 359 is so small that it can transport core heat to its surface only through convection, unlike larger red dwarf stars like Gliese 752 A -- also known as Wolf 1055 A or Van Biesbroeck's Star (more). It may be less than 10 billion years old. Some alternative designations and useful star catalogue numbers are: CN Leo, Gl 406, G 45-20, LFT 750, LTT 12923, LHS 36, and 2MASS J10562886+0700527.

Jeffrey L. Linsky, JILA, STScI, NASA




Larger image.




Like Gliese 752 B, Wolf 359 is so small,
with less than 20 percent of Sol's mass,
that it can transport core heat only
through convection, unlike larger larger
red dwarf stars like Gliese 752 A (more).

Habitable Zone

With a spectral type of M5.5, Proxima Centauri can be used as a rough proxy for Wolf 359 (M5.8). Accounting for infrared radiation, the distance from Proxima where an Earth-type planet could have liquid water on its surface is around 0.022 to 0.054 AU (Endl and Kürster, 2008; and Endl et al, 2003, in pdf) -- much closer than Mercury's orbital distance of about 0.4 AU from Sol -- with a corresponding orbital period of 3.6 to 13.8 days (Endl and Kürster, 2008), while the NASA Star and Exoplanet Database has calculated a slightly farther out habitable zone between 0.033 and 0.064 AUs around Proxima. At those close distances, the rotation of the planet would be tidally locked with the star so that one side would have eternal daylight and the other would be in eternal darkness.

Hunt for Substellar Companions

The Hubble Space Telescope was recently used to search for faint companions about Wolf 359. No large orbiting body (stellar or substellar, such as a brown dwarf) were found as close as the distance from the Earth to the Sun -- i.e., one AU -- from Wolf 359 (Schroeder et al, 2000). Previous searches using ground-based, photographic astrometry and infrared speckle methods had already failed to find a large orbiting body at greater distances from the star. In addition, a cold debris or dust disk has not been detected (Lestrade et al, 2010; and Gautier et al, 2007).

Life Around a Flare Star

Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (such as Wolf 359) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf itself, doubling its brightness or more.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.0042 AU for Wolf 359 with an orbital period of around 8 hours), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.

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