Binaries from this study, each orbiting around its center of mass, which is marked by an x. Image courtesy of Trent Dupuy, Karen Teramura, PS1SC

Astronomers have shown what separates real stars from the wannabes. Not in Hollywood, but out in the universe, according to a news release from the University of Hawai‘i this week.

“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 Hawaiʻi at Mānoa. “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 presented his research June 5 in a news conference at the semi-annual meeting of the American Astronomical Society in Austin. The research will be published in The Astrophysical Journal Supplement, and a preprint can be found online.

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. 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, Dupuy and IfA Astronomer Michael 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 Hawaiʻi — the W. M. Keck Observatory and Canada-France-Hawaiʻi telescopes’as well as data from the Hubble Space Telescope.

Read more at the Institute for Astronomy news release.

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