Brown Dwarfs vs. Stars: What Makes a Star a Star?

At what point does a clump of gas ignite, turning into a star? Astronomers now have an answer to what makes a star — and what makes a brown dwarf.

Brown dwarf

Illustration of a brown dwarf.
Douglas Pierce-Price, Joint Astronomy Centre, Hawai'i

When you go outside at night and look up at the sky, regardless whether you’re in a bright city or the dark countryside, there’s a lot more than meets the eye. For every 100 stars you see, astronomers think there could be dozens of “failed stars” — also known as brown dwarfs — that you don’t.

The dividing line between stars and their cooler cousins has been an open question in astronomy for some time, remaining mostly in the realm of theory and simulations. At the 230th meeting of the American Astronomical Meeting in Austin, Texas, a pair of researchers announced that they have an answer — based not on computer simulations but on real observations. The study will appear in the Astrophysical Journal Supplement Series (full text here).

Though brown dwarfs are often dubbed “failed stars,” the name is a bit of a misnomer: These objects were never slated to become stars in the first place. Stars are born when large pockets of gas and dust cool and collapse in on themselves. But if there’s less than the critical amount of gas and dust, the resulting body won’t be hot or dense enough to ignite hydrogen fusion. These brown dwarfs are cooler than the lowest-mass stars we know but still hot enough to radiate at infrared wavelengths.

Even nearby brown dwarfs are so faint, they weren’t discovered as a class until 1994, and their dimness continues to make them difficult to study. That’s why, until recently, only a handful of brown dwarfs had even had their mass measured. Trent Dupuy (University of Texas at Austin) and Michael Liu (University of Hawai'i) set out to change that.

Defining Stardom

Coolest brown dwarf ever found?

Brown dwarf binary imaged using the Laser Guide Star (LGS) Adaptive Optics system on the Keck II telescope.
Michael Liu / Univ. of Hawai'i

Almost a decade ago, Dupuy and Liu began observing the orbits of 31 low-mass binary systems, all within 130 light-years of Earth. Each pair consists of two brown dwarfs or extremely low-mass stars — in other words, objects on both sides of the defining line of stardom. An object's color determines its class (brown dwarf or bona fide star).

By measuring the period and size of each pair’s mutual orbit, astronomers could calculate each object’s mass. The researchers targeted systems close enough together that they would complete more than a third of their orbits between 2008 and 2010, a requirement to ensure accurate measurements of the systems’ orbital parameters.

Since dwarf stars are quite small and faint, Dupuy and Liu made use of two of the most powerful telescopes available: the ground-based Keck Observatory and the space-based Hubble Space Telescope. By observing each binary with both telescopes, they nailed down the object's positions as well as their motions through space. They then used the Canada-France-Hawaii Telescope to take images with a wider field of view, so they could determine the center of mass around which each system orbits. Their final data set, visualized below, yielded masses for 38 brown dwarfs — increasing the number of brown dwarfs with known masses by an order of magnitude.


This animation shows several of the binaries from Dupuy and Liu's study, each orbiting around its center of mass (marked by an ‘x’). Each binary is shown roughly where it is located in the night sky, and its color indicates its surface temperatures (from warmest to coolest: gold, red, magenta, or blue). The background image is a map of the entire sky visible from Hawaii and a silhouette of Mauna Kea (home of Keck Observatory and the Canada-France-Hawaii Telescope, where this study was conducted over the past decade). The actual sizes of these orbits on the sky are very small (about one billionth the area covered by each ‘x’), but the orbit sizes shown in the animation are accurate relative to each other. The animation is also in extreme fast-forward; each second corresponds to approximately two years of real time.

Based on these masses, Dupuy and Liu have determined that a gaseous ball must contain a minimum of 70 times Jupiter’s mass to ignite nuclear fusion and give birth to a star. Anything less than that will produce an object fated to brown dwarf status. The lower the mass, the lower the object’s temperature, and according to Dupuy and Liu, the objects in this sample with a surface temperature cooler than about 1600K (2400°F) were all brown dwarfs.

Dupuy and Liu’s finding matches what’s expected from theoretical models, and it’s an exciting example of our capability to begin observationally exploring the question of what makes a star a star.

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