mountain profile Institute for Astronomy University of Hawaii

Astronomers Prove What Separates True Stars from Wannabes

5 JUNE 2017


Dr. Trent Dupuy, UT Austin
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Dr. Mike Liu
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This animation shows several of the binaries from this study, each orbiting around its center of mass, which is marked by an x. Colors indicate 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 Maunakea, home to Keck Observatory and the Canada-France-Hawaii Telescope where this study was conducted over the past decade. Each binary is shown roughly where it is located on the night sky. The actual sizes of these orbits on the sky are very small (about one billionth the area covered by an "x"), but the orbit sizes shown in the animation are accurate relative to each other. The animation is also in extreme fast-forward, where every one second in the animation corresponds to approximately 2 years of real time.
Credit: Trent Dupuy, Karen Teramura, PS1SC

Astronomers have shown what separates real stars from the wannabes. Not in Hollywood, but out in the universe.

"When we look up and see the stars shining at night, we are seeing only part of the story," said Trent Dupuy of the University of Texas at Austin and a graduate of the Institute for Astronomy at the University of Hawaii at Manoa. "Not everything that could be a star 'makes it,' and figuring out why this process sometimes fails is just as important as understanding when it succeeds."

Dupuy is the lead author of the study and will present his research today in a news conference at the semi-annual meeting of the American Astronomical Society in Austin.

Stars form when a cloud of gas and dust collapses due to gravity, and the resulting ball of matter becomes hot enough and dense enough to sustain nuclear fusion at its core. Fusion produces huge amounts of energy - it's what 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 over 50 years. However, the dividing line in mass has never been confirmed by experiment.

Now, astronomers Dupuy and Michael Liu of the University of Hawaii 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 W. M. 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). However, binary brown dwarfs orbit much more slowly than binary stars, due to their lower masses. And 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, the price of such zoomed-in, high-resolution images is that there is 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 result of the decade-long observing program is the first large sample of brown dwarf masses.

"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, who is the co-author of the study.

The information they have assembled has allowed them to draw a number of conclusions about what distinguishes stars from brown dwarfs.

Objects heavier than 70 Jupiter masses are not cold enough to be brown dwarfs, implying that they are all stars powered by nuclear fusion. Therefore 70 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.

In addition to the mass cutoff, they discovered a surface temperature cutoff. Any object cooler than 1,600 Kelvin (about 2,400 degrees Fahrenheit) is not a star, but a brown dwarf.

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.

This research will be published in the next issue of The Astrophysical Journal Supplement, and a preprint can be found online at

Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakalā and Maunakea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.