UT researchers find mass matters when it comes to star brightness

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Thought Jupiter was big? Turns out, stars have to weigh at least 70 Jupiters to shine at all.

Astronomers at UT and the University of Hawaii studied 31 faint binary systems — or pairs of stars — for 10 years to come to this conclusion, published in the Astrophysical Journal Supplement Series.

Stars are formed when large clouds of dust and gas condense due to gravitational force, according to Trent Dupuy, UT astronomy research fellow. They need to be hot enough and have enough mass to sustain nuclear fusion, which releases enormous amounts of energy as light and heat. Dupuy and the rest of the team team discovered that stars that weigh less than 70 Jupiters don’t shine and become brown dwarfs.

“Binaries are constrained by the basic law of gravity — the speed of their orbits around each other depend on their masses and the distance between them,” Dupuy said.

He added that by observing the speed of the orbits and the distance between the stars, their masses can be calculated.

The research team, including Michael Liu of the University of Hawaii, used the Keck Observatory, the Canada-France-Hawaii Telescope and the Hubble Space Telescope to obtain their data.

“The ten-meter telescopes on Mauna Kea (are) a combination of the largest telescopes in the world at the best site in the world and the best instrumentation to make those observations possible and successful,” said Randy Campbell, Keck Observatory science operations manager.

He said the telescopes have instrumentation that detects infrared light, which is important for studying objects in the universe that don’t emit visible light, and adaptive optics systems that remove the atmosphere’s blurring of images.

Campbell added that this study is a good example of how various types of telescopes and instruments can complement each other. Keck Observatory can take high spatial resolution images, the Canada-France-Hawaii Telescope takes wide angle pictures and Hubble can observe colors that the atmosphere otherwise absorbs.

Dupuy said he first started research on star brightness as a graduate student at the University of Hawaii.

“Of course I was interested in exoplanets,” Dupuy said. “But it was exciting do something people have never done before, using new technology and basic physics applied to things only recently discovered.”

In the past, mathematical models predicted that the cutoff mass is 75 to 80 Jupiters, which is higher than the 70 obtained from this study, Liu said.

“The result is too new for the theorists to go back and look at why the masses are different,” Liu said. “But this might motivate them to look more closely at the evolution of stars.”

The next step for the team is to improve their result with a much larger sample, Dupuy said. The team hasn’t recorded many mass measurements for very low mass coldest stars, since both have only been found recently, within the last five to 10 years.

Dupuy added that binaries that are very close together cannot yet be distinguished as two different stars.

“They won’t be detected until the next generation of telescopes,” Dupuy said.

So far, the study has taken 10 years to obtain data needed to measure orbits of binaries.

“It’s just the nature of the project,” Liu said. “The time it takes for these stars to orbit is usually a decade, sometimes a couple decades.”

Dupuy said that waiting is just part of astronomy, even with the biggest telescopes in the world.

“We knew that it would take a really long time,” Dupuy said. “But now it’s done!”