Brown dwarfs are dull, but they shed light on planets

By Chris Oliver
Advertiser Staff Writer

LIU: Studied brown dwarf from Mauna Kea scopes Brown dwarfs and more "Worlds Around Other Stars: New Sharp Views From Mauna Kea"a lecture by Michael C. Liu. 7 tonight School of Architecture Auditorium, University of Hawai'i-Manoa Free; campus parking, $3

In a universe populated by blue stragglers, red supergiants, ultraviolet catastrophes and black holes, brown dwarfs are — well ... sooo 2 billion years ago.

Which is exactly what makes them fascinating to astronomers like Michael Liu, who will make them the subject of "Worlds Around Other Stars: New Sharp Views from Mauna Kea," a public lecture tonight at the University of Hawai'i-Manoa.

"A brown dwarf is an object between a star and a planet," said Liu, member of a team that recently discovered a brown dwarf orbiting a nearby star similar to the Sun.

"All stars are at least 80 times bigger than the planet Jupiter, and they generate their own energy by nuclear fusion," Liu explained. "Planets are less than 15 times the size of Jupiter and have no source of internal energy.

Brown dwarfs have masses between planets and stars and are sometimes called 'failed stars,'" he said.

"Failing to shine may not be a star quality, but in astronomy's search for earth-like planets, finding brown dwarfs is a big step toward understanding how stars and planets came to be formed, Liu said.

Using the Gemini and Keck telescopes at Mauna Kea Observatory, Liu discovered one particular brown dwarf that is separated from its parent star by less than the distance between the Sun and the planet Uranus. That raises interesting questions.

"This companion is too massive to have formed the way we believe that planets do, from a disk of gas and dust when the star was young," Liu said. The parent star is very similar to our sun, yet it has a brown dwarf companion whose mass is dozens of times the combined mass of all the planets in our solar system."

While there are lots of ideas on how it formed, Liu said, "We don't yet know for sure."

After tracking the brown dwarf and companion star known as HR7672 for several months, Liu's team concluded they were "moving through space together, proving they are in orbit around each other."

Liu says HR7672 and its brown dwarf — which is about 60 times the mass of Jupiter — are estimated at 1 to 3 billion years old. That's slightly younger than the sun.

Though the existence of brown dwarfs has been speculated on for decades, Liu said. They were not observed directly until 1995. Since then, hundreds of planets and brown dwarfs have been found far beyond our solar system.

"To find these objects requires a large telescope, which is why the largest and most advanced telescopes — Keck, Subaru and Gemini — are critical for this work," Liu said. "Observing conditions on Mauna Kea are among the best in the world in terms of weather, sky darkness and a stable atmosphere which allows us to make very sharp images, critical for studying very faint objects like brown dwarfs."

But why brown?

Stars of different temperatures have different colors with hotter stars appearing blue and cooler stars appearing red. "Without internal energy, brown dwarfs continually fade and cool over their lifetimes," Liu said. "Before brown dwarfs were found, their existence was thought of as objects cooler than the lowest mass stars known as red dwarfs."

And in the curious minds of astronomers, brown is more exciting than "dull red."