Tikalon Blog by Dev Gualtieri

Brown Dwarfs

September 22, 2016

When we think of stars, and of our Sun especially, we think of light. The phrase, "Let there be light," occurs early in the Bible, in Genesis 1:3. "Let there be light" is fiat lux in Latin, and the lux is the SI unit of illuminance. In Greek, "Let there be light" is γενηθητω φως (genetheto phos), and our word photon is derived from φως (phos).

The Starry Night, an 1889 oil on canvas painting by Vincent van Gogh (1853-1890), presently at the Museum of Modern Art.

(Via Wikimedia Commons.)

Not all stars are bright in visible light. Barnard's Star, named after American astronomer, Edward Emerson Barnard is a good example of this. While Barnard didn't discover this star, he found that it has the largest proper motion of any star (10.3 arcsec/year). This large proper motion is a consequence of its proximity to the Earth, just six light years.

Although it's just six light years away, Barnard's Star is invisible to the unaided eye, while Alpha Centauri, a triple star system at 4.37 light years distant, shines brightly in the constellation, Centaurus. The Alpha Centauri star system has gotten a lot of recent press, with the discovery of the potentially habitable planet, Proxima Centauri b.

The invisibility of Barnard's Star is a consequence of its being a red dwarf (spectral type M). Proxima Centauri, the host star of Proxima Centauri b, is also a red dwarf. Such stars are cool stars of low mass with a surface temperature of less than 4,000 K. In comparison, our Sun has a surface temperature of about 5,800 K. While Barnard's Star emits very little visible light, its emission in the short-wavelength infrared is considerable, as can be seen in the graph.

Brightness of Barnard's Star, relative to visible light, as a function of wavelength

Brightness of Barnard's Star as a function of wavelength.

The letters are the photometric system designations.

(Graphed using Gnumeric.)

Red dwarfs are common in our Milky Way Galaxy, and they may constitute three-quarters of its stars. There are cooler dwarf stars, the brown dwarfs, that are even less visible. These stars are about fifty times heavier than our gas giant planet, Jupiter, and they are not massive enough to fuse hydrogen to helium in their cores, the principal energy reaction of the main sequence stars. They generate some heat through other, minor nuclear reactions.

Size comparison of a small star, a brown dwarf, and our gas giant planet, Jupiter. The radii of the spheres is not that different, but it must be remembered that the volume scales with the radius cubed, and the cores become more compressed as the radius becomes larger. (Modified Carnegie Institution for Science image.)

Brown dwarfs are failed stars in that they are too small to sustain hydrogen fusion. After forming, these failed stars slowly cool, contract, and dim over time.[2] Depending on its initial size, the temperature of a brown dwarf will range from as cool as a gas giant planet to nearly as hot as a star.[2] Knowledge of the distribution of brown dwarfs is important to the understanding whether they form in isolation, or whether they are ejected from planetary systems.[2]

Brown dwarfs are so dim that the first such star, GD 165B, wasn't discovered until 1988. Many more brown dwarfs have been discovered since the advent of infrared orbital observatories, such as the Two Micron All Sky Survey (2MASS). As reported in a recent arXiv article, the Sondage Infrarouge de Mouvement Propre (SIMP, not to be confused with strongly interacting massive particles), a ground-based all-sky survey in the infrared "J" band (1.220 μm wavelength), has discovered 165 brown dwarfs in close proximity to the Earth.[1-2]

The SIMP project consists of a southern hemisphere telescope at the Cerro Tololo Inter-American Observatory, and a northern hemisphere telescope at the Observatoire du Mont-Mégantic.[1] About a third of the 165 sources discovered by SIMP had unusual compositions or other peculiarities.[2] There were six unusually red M and L dwarfs and twenty-five unusually blue M and L dwarfs.[1]

J magnitude sources in 2MASS and SIMP in a 6.7 square degree area.

(Fig. 1 of Ref. 1.[1])

Says Jonathan Gagné, an astronomer at the Carnegie Institution of Washington and co-author of the study,

"The search for ultracool brown dwarfs in the neighborhood of our own Solar System is far from over... Our findings indicate that many more are hiding in existing surveys."[2]

This work was supported by the Fonds de Recherche Québécois-Nature et Technologie and the Natural Science and Engineering Research Council of Canada.[2]

Brown Dwarfs

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