Object Survives Being Swallowed by a Star

Long before the Bible's tale of Jonah being swallowed by awhale, a small wannabe star emergedintact after being engulfed by a neighboring giant star, scientists say.

The victim was a brown dwarf, a failedstar too small to sustain the nuclear reactions that ignites regular stars.The purpetrator was a red giant, an ancient star thatonce resembled our Sun but which puffed up to enormous size after its hydrogenfuel was depleted. The red giant has since expelled most of its gas into spaceand transformed into a dense, Earth-sized star called a white dwarfs.

Using the European Southern Observatory's Very Large Telescope,astronomers spied the binary system that remains: the brown and white dwarfs.The brown dwarf is thought to have survived being swallowed by its companionduring the white dwarf's redgiant phase.

The discovery, detailed in the Aug. 3 issue of the journal Nature, provides the first solid evidencethat an object as small as a brown dwarf—which is just one step up from giant planet mass—can surviveanother star's red giant phase. Previously, only reddwarfs, stars with masses about a third that of our sun, have been known towithstand such events.

Two dwarfs

Called WD 0137-349, the system is located about 300 light-yearsfrom Earth. Its two dwarfs are separated by only a few thousandths the distancebetween Earth and the Sun and the objects rotate around in each other in about2 hours.

In the past, the two objects were farther part, but thetemporary engulfment by the red giant's gas envelope is thought to have sloweddown the orbital speed of the brown dwarf, causing it to spiral inwards towardsthe center of its larger neighbor.

Although too small to become a star, the brown dwarf was stillbig enough to avoid vaporization when it was engulfed.

Had it been less than 20 Jupiter masses, "it would haveevaporated during this phase," said lead author Pierre Maxtedof Keele Universityin England.

Cosmic catalyst

But there's another reason the brown dwarf survived. Scientiststhink the failed star sped up its companion's red giant phase, the way enzymesspeed up biological reactions while remaining unharmed. When it was engulfed,the brown dwarf amassed matter from the red giant's gas envelope, which it thenradiated off into space. By doing so, it shortened its companion's red giantphase dramatically.

"Normal single red giants that don't swallow anythingprobably last about 100 million years, but in this system, it may have onlylasted a few decades," study team member Matt Burleigh of the University of Leicester in Englandtold SPACE.com.

The brown dwarf's reprieve from destruction is only temporary,however. Its orbit is slowly shrinking, and in about 1.4 billion years, it willbe close enough for the white dwarf to siphon gas from surface. When thishappens, the brown dwarf will slowly shrink in mass, while the accumulatingmatter on the white dwarf will trigger massive thermonuclear explosions called novasevery few years.

History repeats

In about 5 or 6 billion years, what happened in WD 0137-349 willrepeat in our solar system. Our sun will run out of hydrogen and become ared giant, expanding until its diameter is about the size of Earth's orbit.Unlike the brown dwarf, however, our planet is notexpected to survive—at least not in its present form.

"It's an ongoing debate whether the Earth will be swallowedup or not," Burleigh said. "But what's for certain to happen isthat the Earth's atmosphere and seas will be boiled off. Even if it doesn'tquite get engulfed, Earth will be pretty much lifeless."

Several million years after the red giant phase, our Sun willshrink and become a white dwarf. At this point, the orbits of Jupiter and Saturn will double or even triple sincethe new white dwarf anchoring our solar system will be much less massive thanour Sun is now.

Planets farther out might not be so lucky; they could become untethered and float off into interstellar space, Burleighsaid.

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