For decades, astronomers searched for the lower size limit for stars. How much mass does an object need to have before it can create its own energy — the hallmark of being a star? Astronomers Trent Dupuy from the University
For decades, astronomers searched for the lower size limit for stars. How much mass does an object need to have before it can create its own energy — the hallmark of being a star? Astronomers Trent Dupuy from the University of Texas Austin and Michael Liu from the University of Hawaii used the Canada-France-Hawaii Telescope, W. M. Keck Observatory and the Hubble Space Telescope for a decade to answer that very question.
Stars form in the heart of nebulas when a cloud of gas and dust collapses due to gravity. As the cloud collapses, the particles in the cloud begin to move faster, increasing the temperature of the cloud. In the center, the resulting ball of matter becomes hot enough and dense enough to sustain nuclear fusion at its core. Fusion produces huge amounts of energy that makes stars shine. In the sun’s case, it’s what makes most life on Earth possible.
But not all collapsing gas clouds are created equal. Sometimes, the collapsing cloud makes a ball that isn’t dense enough to ignite fusion. These “failed stars” are known as brown dwarfs.
This simple division between stars and brown dwarfs has been used for a long time. In fact, astronomers have had theories about how massive the collapsing ball has to be in order to form a star, or not, for more than 50 years. However, the dividing line in mass had never been confirmed by experiment.
Astronomers Dupuy and Liu have done just that. 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.
How did they reach that conclusion? For a decade, the two studied 31 faint brown dwarf binaries, pairs of these objects that orbit each other, using two powerful telescopes in Hawaii — the Keck Observatory and Canada-France-Hawaii telescopes — as well as data from the Hubble Space Telescope.
Their goal was to measure the masses of the objects in these binaries, since mass defines the boundary between stars and brown dwarfs. Astronomers have been using binaries to measure masses of stars 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.” Because brown dwarfs are dimmer than stars, they can only be well studied with the world’s most powerful telescopes.
To measure masses, Dupuy and Liu collected images of the brown dwarf binaries over several years, tracking their orbital motions using high-precision observations. They used the 10-meter Keck Observatory telescope, along with its laser guide star adaptive optics system and the Hubble Space Telescope, to obtain the extremely sharp images needed to distinguish the light from each object in the pair.
However, zoomed in, high-resolution images have no reference frame to identify the center of mass. Wide-field images from the Canada-France-Hawaii Telescope containing hundreds of stars provided the reference grid needed to measure the center of mass for every binary. The precise positions needed to make these measurements are one of the specialties of WIRCam, the wide field infrared camera at CFHT.
Seventy Jupiters is the critical mass below which objects are fated to be brown dwarfs. This minimum mass is somewhat lower than theories had predicted but still consistent with the latest models of brown dwarf evolution. This new work will help astronomers understand the conditions under which stars form and evolve — or sometimes fail. In turn, the success or failure of star formation has an impact on how, where, and why solar systems form.
