Stars have fun names. There are red giants, yellow supergiants, red dwarfs, white dwarfs, neutron stars, magnetars and more. To hear astronomers talk stars, you might think you’re at J.R.R. Tolkien convention. Stars form when a cloud of gas and dust collapses due to gravity, resulting in a dense ball of material hot enough for nuclear fusion to fire up in its core. Fusion takes simple hydrogen and pairs it with other hydrogen atoms to create helium, releasing energy in the process that we see as star shine. Or in the case of the sun, sunshine and the heat that sustains life and good coffee here on Earth.
But not every gassy cloud collapses to form a star. It has to be dense enough to stoke a nuclear fire. Less massive clouds form “failed stars” known as brown dwarfs. Astronomers discovered the first brown dwarf star, Gliese 229B, located 19 light years away in the constellation Lepus, in 1994. They’re difficult to find because they’re cool — they shine brightest in infrared light, which is invisible to the human eye but detectable as heat. Yet despite the few we know to date, it’s estimated there are 70 billion of them in the thin, flattened disk of the Milky Way alone.
Among several nagging questions about brown dwarfs, one of the biggest is how massive a collapsing ball of dust and gas must be in order to form a star … or not. Now, astronomers Trent J. Dupuy (Univ. of Texas -Austin) and Michael C. Liu (Univ. of Hawaii) have come up with an answer which they published in a recent paper. They found that an object must weigh at least 70 Jupiters in order to start hydrogen fusion. If it weighs less, the star does not ignite and becomes a brown dwarf instead.
What’s a Jupiter? That’s one Jupiter mass or 2.5 times the mass of all the other planets in the solar system combined. Massive but only 1/70th of what you need to initiate nuclear fusion and stardom. Liu and Dupuy reached their conclusion after studying 31 faint brown dwarf binary stars (orbiting pairs of stars) using two of the world’s most powerful earthbound telescopes: the Keck Observatory and Canada-France-Hawaii telescopes in Hawaii along with a little help from the spacey Hubble.
This animation shows several brown dwarf binary stars that were used to figure out the dividing line between true stars that burn using nuclear fusion from “failed stars” or brown dwarfs. The pairs orbit around their center of mass (marked by an x) and colors indicate surface temperatures, from warmest to coolest: gold, red, magenta and blue. The background image is a map of the entire sky visible from Hawaii with a silhouette of the observatories involved in the decade-long study. Each pair is shown roughly where it’s located on the night sky. The actual sizes of these orbits of the sky are very small, but the orbit sizes shown in the animation are accurate relative to each other. Every second corresponds to ~2 years of real time. Credit: Trent Dupuy, Karen Teramura, PS1SC
Since mass defines the boundary between stars and brown dwarfs, their goal was to measure the masses of the stars in the binaries to try and set a lower limit on what makes a star. Astronomers have been using binaries to measure stellar masses for more than a century. To determine the masses of a binary, one measures the size and speed of the stars’ orbits around an invisible point between them where the pull of gravity is equal (known as the center of mass). This No easy take, it was made more difficult because binary brown dwarfs orbit much more slowly than typical binary stars, due to their lower masses. They’re also really dim, so you need the world’s biggest telescopes for the job.
Dupuy and Liu amassed images of the brown-dwarf binaries over several years into the largest sample of brown dwarfs ever. Their study concluded that objects heavier than 70 Jupiter masses are too warm to be brown dwarfs, implying that they’re all stars powered by nuclear fusion. Therefore 70 Jupiters is the critical mass below which objects are fated to be brown dwarfs. So the next time you hear the rumor that Jupiter is a “failed star,” it’s hardly true. The planet’s only 1/70th the way to a failed star or brown dwarf.
In addition to the mass cutoff, the two discovered a surface temperature cutoff. Any object cooler than about 2,400° F is not a star, but a brown dwarf. Fresh lava can sizzle at 2,200° F, only 200° cooler that the surface of a brown dwarf. For comparison, the sun’s surface temperature is four times hotter at 10,000° F (5,600° C).
“As they say, good things come to those who wait. While we’ve had many interesting brown dwarf results over the past 10 years, this large sample of masses is the big payoff. These measurements will be fundamental to understanding both brown dwarfs and stars for a very long time,” said Liu.
And that’s the story of Dupuy and Liu and the 62 dwarfs.