Date: December 25th, 2012

Title: Astronomy Word of the Week : Wolf-Rayet

Podcaster: Dr. Christopher Crockett

Organization: United States Naval Observatory


Description:Numbering only in the hundreds, the most massive and brightest stars known are also among the most rare.  And they make our sun look like a candle next to a searchlight.  The astronomy word of the week is “Wolf-Rayet”!

Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

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Some stars just defy imagination.

Our own sun sounds pretty impressive to us puny humans: 1.3 million times the volume of the 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 don’t really have a well-defined surface.  They’re actually quite fluffy.  That’s because they have trouble holding themselves together.  Pressure from the intense light alone is enough to tear the star apart.

Which leads us to one of the defining characteristics of Wolf-Rayet stars.  They exhibit 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.  Put another way, that’s like spitting three Earths into space annually!

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.  The only reason Wolf and Rayet knew something was different about these stars was the presence of strange features in their spectra.

Most stars, when you break up the starlight into its component colors or frequencies, show gaps in their spectra.  Specific colors are missing from the star light.  They are missing because they are absorbed by atoms and molecules in the star’s atmosphere.  Studying these missing colors—or absorption lines—is how astronomers determine the chemical makeup of everything in the Universe.

Wolf and Rayet noticed three stars in the constellation Cygnus without absorption lines.  These stars showed emission lines instead.  Rather than missing colors, there were colors that were much brighter than all the others.

This can happen when a cloud of gas is heated by something nearby.  Usually, astronomers find these in nebulae surrounding hot, young stars.  In this case, intense light from these blazing stars was irradiating plumes of escaping gas.  The ultraviolet radiation caused helium in the discarded material to fluoresce.

One of the reasons that Wolf-Rayet stars are so rare is because it is difficult to form very massive stars.  But more than that, Wolf-Rayet stars are ephemeral.  They are just a phase in the evolution of stellar heavyweights.  After quickly burning through the available hydrogen in their cores, they rapidly start shedding their outer layers into space.  From here, they will go on to a violent death: a supernova explosion.

Only one such star 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 1000 light-years away, it is actually part of a six member star system.  Gamma 2, while appearing like a single star to the naked eye, is actually a very close binary star.  They are separated by the same distance as the Earth and the Sun. One is a blue supergiant, about thirty times heavier than the sun.  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!

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.

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 annihilations heat up the core, which produces more gamma rays, which produces pairs, which leads to more annihilations, which produces more gamma rays…..

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.  The entire star is obliterated. Nothing is left behind.

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 explosion ever recorded. If a nearby star were to undergo such an explosion, like the 8000 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.  They show us just how extreme the Universe can become.  They also give us a peek at what astronomers think the very first stars might have been like.  The early Universe was most likely a place where 300 solar mass stars were common and pair-instability supernovae seeded galaxies with the first attempts at elements like carbon, oxygen, and nitrogen.

Some of those atoms may have found there way to a collapsing cloud in some backwater corner of the Milky Way five billion years ago.  When the Sun and Earth formed, they incorporated atoms from all the stellar generations that preceded them.  Who knows? Maybe some part of you started off in a Wolf-Rayet star that helped get the whole Universe going.

End of podcast:

365 Days of Astronomy
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