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Wolf Rayet Stars


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

Wolf Rayet Stars

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!

Wolf-Rayet stars are hot (25-50,000+ degrees K), massive stars (20+ solar mass) with a high rate of mass loss. Strong, broad emission lines (with equivalent widths up to 1000Å!) arise from the winds of material being blown off the stars.

Wolf-Rayets stars are divided into 3 classes based on their spectra, the WN stars (nitrogen dominant, some carbon), WC stars (carbon dominant, no nitrogen), and the rare WO stars with C/O < 1. The WN stars optical spectra show emission lines from H, NIII (4640Å), NIV, NV, HeI, HeII, and from CIV at 5808Å. In the UV, there are strong emission features from NII, NIII, NIV, NV, CIII, CIV, HeII, OIV, OV, and SiV.

The WC stars optical spectra show emission lines from H, CII, CIII (5696Å), CIV (5805Å), OV (5592Å), HeI, and HeII. No nitrogen lines are seen in the WC stars. In the UV, there are strong emission features from CII, CIII, CIV, OIV, OV, SiIV, HeII, FeIII, FeIV, and FeV.

Wolf-Rayet stars represent a final burst of activity before a huge star begins to die. These stars, which are at least 20 times more massive than the Sun, “live fast and die hard”, according to NASA.

Their endstate is more famous; it’s when they explode as supernova and seed the universe with cosmic elements that they get the most attention. But looking at how the star gets to that explosive stage is also important.

When you look at a star like the Sun, what you are seeing is a delicate equilibrium of the star’s gravity pulling stuff in, and nuclear fusion inside pushing pressure out. When the forces are about equal, you get a stable mass of fusing elements. For planets like ours lucky enough to live near a stable star, this period can go on for billions upon billions of years.

Being near a massive star is like playing with fire, however. They grow up quickly and thus die earlier in their lives than the Sun. And in the case of a Wolf-Rayet star, it’s run out of lighter elements to fuse inside its core. The Sun is happily churning hydrogen into helium, but Wolf-Rayets are ploughing through elements such as oxygen to try to keep equilibrium.
Because these elements have more atoms per unit, this creates more energy — specifically, heat and radiation, NASA says. The star begins to blow out winds reaching 2.2 million to 5.4 million miles per hour (3.6 million to 9 million kilometers per hour). Over time, the winds strip away the outer layers of the Wolf-Rayet. This eliminates much of its mass, while at the same time freeing its elements to be used elsewhere in the Universe.

Eventually, the star runs out of elements to fuse (the process can go no further than iron). When the fusion stops, the pressure inside the star ceases and there’s nothing to stop gravity from pushing in. Big stars explode as supernova. Bigger ones see their gravity warped so much that not even light can escape, creating a black hole.

We still have a lot to learn about stellar evolution, but a few studies over the years have provided insights. In 2004, for example, NASA issued reassuring news saying these stars don’t “die alone.” Most of them have a stellar companion, according to Hubble Space Telescope observations.
While at first glance this appears as just a simple observation, cosmologists said that it could help us figure out how these stars get so big and bright. For example: Maybe the bigger star (the one that turns into a Wolf-Rayet) feeds off its companion over time, gathering mass until it becomes stupendously big. With more fuel, the big stars burn out faster. Other things the smaller star could influence could be the bigger star’s rotation or orbit.


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