Wolf-Rayets are the most massive and brightest stars known

Our own sun sounds pretty impressive: 1.3 million times the volume of Earth, 330,000 times as massive, 10,000 degrees Celsius at its surface, and 400 million billion billion watts of power. But, for a star, the sun is pretty puny. The real stellar heavyweights are the Wolf-Rayet stars: the most massive and brightest stars known.

First, some numbers. Over twenty times the mass of our sun. Millions of times brighter. Temperatures exceeding 50,000 degrees. And as to their size, that’s hard to say. Stars this massive are actually rather fluffy. That’s because they have trouble holding themselves together. Pressure from the intense light is enough to tear the star apart. This radiation drives phenomenally strong stellar winds. Blowing at over ten million miles per hour, the stars shed about two thousand billion billion tons of material every year. That’s like spitting three Earths into space annually!

WR124 is a Wolf-Rayet star 8,000 light-years from Earth.  This Hubble Space Telescope image shows hot clumps of gas, each weighing 30 times more than Earth, being blown off into space at nearly 100,000 miles per hour.  Credit: Y. Grosdidier, A. Moffat, G. Joncas, A. Acker, and NASA

Named after French astronomers Charles Wolf and George Rayet, who discovered them at the Paris Observatory in 1867, Wolf-Rayet stars are exceedingly rare. We know of only 500 in the Milky Way, plus a few hundred in the surrounding galaxies.

Only one can be seen with the naked eye. Gamma 2 Velorum, in the southern constellation Vela, is not only the closest Wolf-Rayet star but one of the brightest stars in the sky. Sitting about 1,000 light-years away, it is part of a six-member star system. Gamma 2, while appearing like a single star to the naked eye, is actually a pair of stars. They are separated by the same distance as the Earth and the sun. One is a blue supergiant, the other is the Wolf-Rayet star. Though currently nine times our sun’s mass, it has lost a considerable amount of its bulk. Most likely, it started off with over 35 times the mass of the sun!

AB7 nebula
AB7 is a nebula, 200 light-years across, in the Large Magellenic Cloud. It is lit up by a binary star system in its core.  One of the stars is a Wolf-Rayet blasting the surrounding space at a temperature of 120,000 degrees.  Credit: ESO

But even a star like Gamma 2 Velorum looks wimpy when compared to the most massive star known. At 165,000 light-years from Earth, it sits in the Large Magellanic Cloud—a satellite galaxy of the Milky Way. Part of the R136 super star cluster, R136a1 weighs in it at roughly 265 suns! And it is nearly nine million times brighter.

R136a1, and stars like it, are a mystery to astronomers. They defy what we think we know about how stars form. One of the leading hypotheses is that R136a1 did not form directly from the collapse of a molecular hydrogen cloud, but rather is the result of two massive stars colliding. A very close pair of stars could spiral in towards one another and eventually merge to form a stellar behemoth.

Size comparison of stars
The size of the sun (yellow) relative to to other stars.  The tiny red ball to the left is a “red dwarf”.  To the right of the sun is a blue dwarf, about 8 times heavier than the sun.  In the background is R136a1 weighing in at 265 suns.  You could line up about 35 suns along its diameter!  Credit: ESO/M. Kornmesser

Astronomers speculate on how R136a1 will end its life. Some think it is a candidate for a hypernova. A regular supernova will outshine an entire galaxy. A hypernova goes off with the power of a hundred supernovae. This is, basically, a supernova on steroids.

Another possibility is equally intriguing. R136a1 could go out as a pair-instability supernova.

The cores of very massive stars are held up by gamma rays released in nuclear reactions. As the star crushes down on its core, the reactions speed up and the gamma rays fly about with more energy. But there’s a catch.

Past a certain energy threshold, gamma rays begin to interact with atomic nuclei to produce electron-positron pairs. This effectively cuts down on how far the gamma rays can travel. The electron-positron pairs immediately annihilate one another to form another gamma ray, which forms another pair, and so on. Rather than hold up the star, the gamma rays produce these particle pairs instead.

The counterbalancing force disappears. The star collapses. The core compression triggers a runaway thermonuclear explosion. But rather than creating a neutron star or black hole, a pair-instability supernova leads to total stellar destruction. Nothing is left behind.

Eta Carinae
At 8,000 light-years away, Eta Carinae is a 150-solar-mass star that is a prime candidate for an extreme stellar explosion such as a hypernova.  If this happened, the light would be bright enough to read by here on Earth.  This Hubble Space Telescope image shows blobs of gas and dust being blown into space at over one million kilometers per hour! Credit: Nathan Smith (University of California, Berkeley), and NASA

A couple of recent supernovae are candidates for such an explosion. In a galaxy 240 million light-years from Earth, SN 2006gy is one of the most energetic stellar explosions ever recorded. If a nearby star were to undergo such an explosion, like the 8,000-light-year-distant Eta Carinae, astronomers estimate that you could read at night by the light of the supernova. You could even see it during the day.

As massive and powerful as our sun appears to us, it pales in comparison to some of its stellar cousins. The Wolf-Rayet stars are just one example. Weighing in at anywhere from 30 to over 200 times the mass of the sun, and shining a million times brighter, they show us just how extreme the universe can be.

December 11, 2012

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