Wolf-Rayets are hot, massive, bright stars

Wolf-Rayet Star: starry sky with orange and blue nebula around a star.
View larger. | Here’s an example of a Wolf-Rayet star, called WR 134. It’s the brightest star below the center of the image. Like all Wolf-Rayets, this star is hot and massive. It’s blowing off its outer layers at high speeds (notice the blue arc in the upper left of the image). The wide-field Mosaic camera on the Mayall 4-meter telescope at Kitt Peak National Observatory captured this image. Image via T.A. Rector/ H. Schweiker/ NSF/ AURA).

What are Wolf-Rayets?

Our sun is 1.3 million times Earth’s volume and 330,000 times Earth’s mass. Our sun is 10,000 degrees F (5,500 degrees C) at its surface. But, for a star, our sun is relatively puny. The real stellar heavyweights are Wolf-Rayet stars, the most massive and brightest stars known.

First, some numbers. Over 20 times the mass of our sun. Millions of times brighter. Plus, temperatures starting at and exceeding 50,000 degrees F (30,000 degrees C). And as to their size, that’s hard to say. Because stars this massive have trouble holding themselves together, they don’t last very long. They burn up their fuel quickly and blast mass into space eventually tearing themselves apart. Meanwhile, this radiation drives phenomenally strong stellar winds. Blowing at over ten million miles per hour, the stars shed about 2 thousand billion billion tons of material every year. That’s like spitting three Earths into space annually!

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

Starry background with bright star in center surrounded by a gaseous blue ring.
The blue bubble encircling WR 31a is a Wolf–Rayet nebula, created when speedy stellar winds interact with the outer layers of gases ejected by Wolf–Rayet stars. The bubble – formed around 20,000 years ago – is expanding at a rate of around 136,700 miles (220,000 kilometers) per hour! The lifecycle of a Wolf–Rayet star is only a few hundred thousand years. Despite beginning life with a mass at least 20 times that of the sun, Wolf–Rayet stars typically lose half their mass in less than 100,000 years. And WR 31a is no exception to this. It will eventually end its life as a spectacular supernova. Image via ESA/Hubble & NASA/ Acknowledgement: Judy Schmidt.

Can you see Wolf-Rayet stars with the unaided eye?

Only two Wolf-Rayet stars can be seen with the unaided eye. First, 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. Located about 1,000 light-years away, it is part of a six-member star system shining at 4.27 magnitude. Although Gamma 2 appears like a single star to the unaided eye, it is a binary star system. Yet they are only separated by the same distance as the Earth and the sun. One is a blue supergiant, the other is the Wolf-Rayet star. While it is currently nine times our sun’s mass, Gamma 2 has lost a considerable amount of its bulk. Most likely, it started off with over 35 times the mass of the sun.

Another visible Wolf-Rayet star is Theta Muscae. This triple star system in the southern constellation Musca has an apparent magnitude of 5.5. However, that brightness comes from the combined light of the Wolf-Rayet star, a blue supergiant and a white main sequence star. The Theta Muscae system is about 7400 light years away.

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. Image via ESO.

The most massive and luminous star is a Wolf-Rayet star

But even a star like Gamma 2 Velorum looks wimpy when compared to R136a1, which is the most massive star known. It is in the Large Magellanic Cloud, located 163,000 light-years away. In fact, it’s part of the R136 super star cluster and weighs in at roughly the mass of 315 suns. Plus, it is nearly 9 million times brighter and about 35 times the diameter of the sun. Shining at a dim magnitude 12.23, this massive star requires a telescope to see.

Consequently, R136a1, and stars like it, are a mystery to astronomers. They defy what we think we know about how stars form. One hypothesis 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 eventually merge to form a stellar behemoth.

Size comparison of stars
The size of the sun (yellow) relative 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 315 suns. You could line up about 35 suns along its diameter. Image via ESO/ M. Kornmesser.
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 1 million kilometers per hour (over 620,000 mph). Image via Nathan Smith (University of California, Berkeley)/ NASA.

The eventual fate of massive stars

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. Instead, R136a1 could go out as a pair-instability supernova.

In this case, the cores of very massive stars are held up by gamma rays released in nuclear reactions. So as the star crushes down on its core, the reactions speed up and the gamma rays fly about with more energy. But past a certain energy threshold, gamma rays begin to interact with atomic nuclei to produce electron-positron pairs. This reduces how far the gamma rays travel. Therefore, the electron-positron pairs annihilate one another forming another gamma ray, which forms another pair, and so on. Thus, rather than hold up the star, the gamma rays produce particle pairs instead.

Therefore, the counterbalancing force disappears. The star collapses and 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.

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. Image via Y. Grosdidier/ A. Moffat/ G. Joncas/ A. Acker/ NASA.

Bottom line: As massive and powerful as our sun appears to us, it pales in comparison to Wolf-Rayet stars. 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.

August 3, 2018

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