Tag: supernova

Is Betelgeuse About to Blow? Maybe… Maybe Not

The famous supergiant on Orion’s shoulder has rapidly dimmed, stoking excitement that a supernova may be in the offing.

Artist’s impression of the tortured, bubbling photosphere of a dying Betelgeuse. [ESO/L. Calçada]

Do you hear that ticking? Doesn’t it sound like a stellar timer is counting down to the inevitable demise of a massive star? While the excitement may be the amplified construct of social media predictions of the death of Betelgeuse, our stellar neighbor really is close to going supernova.

“Close”, however, is relative. It could be as “human close” as blowing up any minute now… to “galactic close” as blowing up in a hundred thousand years, maybe more.

So, what’s all the fuss about? In a nutshell, the brightest star in the famous constellation of Orion is bright no more. In the past few weeks, Betelgeuse has dimmed noticeably, stoking predictions that it could be about to spectacularly erupt at any time, becoming as bright as a full Moon and casting its own shadows at night.

While this may sound ominous, a Betelgeuse supernova poses no threat to life on Earth. It’s located a safe 600 light-years away, so if it did explode, we’d be treated to a historic cosmic firework display and not doomsday. Any energetic particles spewing from the explosion may reach the solar system in 100,000 years, but would have a minimal impact; the heliosphere (our Sun’s extended magnetic “bubble” that encompasses all the planets) would be more than powerful enough to deflect the tenuous gases.

The constellation of Orion, with the ruddy Betelgeuse in the upper left-hand corner on Orion’s “shoulder.” [Photo by Frank Cone from Pexels]

There has always been excitement over Betelgeuse and its explosive potential. It’s a massive star, with a mass 12-times that of our Sun, which has reached the end of its life. But with a lifespan of only eight million years or so, it may sound odd that it’s dying of old age. As a comparison, our Sun—an “average” yellow dwarf star—sounds geriatric in comparison; it’s approximately five billion years old. But the strange physics of stellar evolution dictates that the more massive the star, the shorter its lifespan. Betelgeuse is on borrowed time, whereas our Sun is only middle-aged. In other words, Betelgeuse has lived fast and it will die young.

As a star that’s about to die, Betelgeuse is experiencing the final throes of violent processes that signify the conclusion of stellar evolution—a phase that sees a massive star puff up into a red supergiant. In the case of Betelgeuse, while it is 12-times more massive than our Sun, it has expanded into a grotesque, bubbling mess of superheated plasma, puffed up to nearly 1,000-times wider than our Sun. If Betelgeuse were transplanted into the middle of our solar system, it would swallow all the planets out to Saturn. Yes, even Jupiter would be ingested.

A precision observation of Betelgeuse’s asymmetric photosphere, highlighting bright spots and a non-spherical shape, as captured by the Atacama Large Millimeter/submillimeter Array (ALMA). 

After guzzling all of its hydrogen fuel long ago, it’s now fusing heavier elements inside its tortured interior to the point where iron is being created. For any massive star, the fusion of iron is the death knell; energy is being absorbed, and soon, its immense gravity will cause the whole mess to collapse, generating an almighty shockwave that will, ultimately, rip Betelgeuse apart as a supernova.

As reported by astronomers before Christmas, the observed dimming could be interpreted as a precursor to the anticipated supernova, and for good reason. But Betelgeuse is known to regularly vary in brightness, so astronomers suspect that, while this is an unprecedented dimming event, the famous star will soon return to its “regular” brightness once more, reclaiming its rank as ninth brightest star in the sky.

In short, don’t place any serious money on Betelgeuse exploding soon. While there is a tiny chance that it might have already exploded, the light from the supernova currently galloping across the 600 light-year interstellar divide between us and Betelgeuse, it’s way more likely that it’s just Betelgeuse being Betelgeuse and keeping variable star astronomers on their toes.

That’s not to say the dimming event isn’t exciting, on the contrary. Seeing a prominent star in the night sky fade with your own eyes is something to behold, so when you get clear skies, look for Orion and ponder The Hunter’s missing shoulder.

Whatever Happened to Hyper-Velocity Star HD 271791?

One scenario: Exploding star flings binary parter away at high velocity (Max Planck Institute for Astrophysics)
One scenario: Exploding star flings binary parter away at high velocity (Max Planck Institute for Astrophysics)

HC 271791 is a star with a problem, it’s moving so fast through our galaxy that it will eventually escape from the Milky Way all together. However, there is a growing question mark hanging over the reasons as to why HD 271791 is travelling faster than the galactic escape velocity.

So-called hyper-velocity stars were first predicted to exist back in 1988 when astrophysicist Jack Hills at Los Alamos National Laboratories pondered what would happen if a binary star system should stray too close to the supermassive black hole lurking in the galactic nucleus. Hills calculated that should one of the stars get swallowed by the black hole, the binary partner would be instantly released from the gravitational bind, flinging it away from the black hole.

This would be analogous to a hammer thrower spinning around, accelerating the ball of the hammer rapidly in a circle around his body. When the thrower releases the hammer at just the right moment, the weight is launched into the air, travelling tens of meters across the stadium. The faster the hammer thrower spins the ball, the greater the rotational velocity; when he releases the hammer, rotational velocity is converted to translational velocity, launching the ball away from him. Gold medals all ’round.

So, considering Hills’ model, when one of the stars are lost through black hole death, the other star is launched, hammer-style, at high velocity away from the galactic core. The fast rotational velocity is converted into a hyper-velocity star blasting through interstellar (and eventually intergalactic) space.

Hills actually took his theory and instructed the astronomical community to keep an eye open for speeding stellar objects, and sure enough they were out there. HD 271791 is one of these stars, travelling at a whopping 2.2 million kilometres per hour, a speed far in excess of the galactic escape velocity.

However, the 11 solar mass star didn’t originate from the Milky Way’s supermassive black hole (inside the radio source Sgr. A*), it was propelled from the outermost edge of the galactic disk. There is absolutely no evidence of a supermassive black hole out there, so what could have accelerated HD 271791 to such a high velocity? After all, stars aren’t exactly easy objects to throw around.

If HD 271791 used to be part of a binary pair, its partner would have had to suddenly disappear, releasing its gravitational grip rapidly. One idea is that HD 271791’s sibling exploded as a supernova. This should have provided the sudden loss in a gravitational field — the rapidly expanding supernova plasma will have dispersed the gravitational influence of the star.

However, according to Vasilii Gvaramadze at Moscow State University, the supernova theory may not be sound either; by his calculations a binary pair simply cannot produce such a large velocity. Gvaramadze thinks that a far more complex interaction between two binary pairs (four stars total) or one binary pair and another single star some 300 solar masses. Somehow, this “strong dynamical encounter” caused HD 271791 to be catapulted out of the system, propelling it at a galactic escape velocity.

Although this complex slingshot theory sounds pretty interesting, the supernova theory still sounds like the most plausible answer. But how could a sufficient rotational velocity be attained? As Gvaramadze points out, even an extreme rapidly orbiting binary pair cannot produce a star speeding at 530-920km/s.

This is in contrast to research carried out by scientists at the Max Planck Institute for Astrophysics and the University of Erlangen-Nuremberg. In a January 2009 press release, Maria Fernanda Nieva points out that this hyper-velocity star possesses the chemical fingerprint of having been in the locality of a supernova explosion. This leads Nieva to conclude that HD 271791 was ejected after its binary partner exploded. What’s more, a Wolf-Rayet may have been the culprit.

Up to now such a scenario has been dismissed for hyper-velocity stars, because the supernova precursor usually is a super-giant star and any companion has to be at large distance in order to orbit the star. Hence the orbital velocities are fairly modest. The most massive stars in the Galaxy, however, end their lives as quite compact so-called Wolf-Rayet stars rather than as super-giants. The compactness of the primary leaves room for a companion to move rapidly on a close orbit of about 1 day-period. When the Wolf-Rayet-star exploded its companion HD 271791 was released at very high speed. In addition, HD 271791 made use of the Milky Way rotation to finally achieve escape velocity. —Maria Fernanda Nieva

Even though Gvaramadze’s stellar pinball theory sounds pretty compelling, the fact that HD 271791 contains a hint of supernova remnant in its atmosphere, the supernova-triggered event sounds more likely. But there is the fact that just because this 11 solar mass star was near a supernova some time in its past, it certainly doesn’t indicate that a supernova was the cause of it’s high speed.

For now I suppose, the jury is still out…

Publication: On the origin of the hypervelocity runaway star HD271791, V.V.Gvaramadze, 2009. arXiv:0909.4928v1 [astro-ph.SR]

Original source: arXiv blog