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Science
by Jerry Oltion

WHEN BETELGEUSE BLOWS

 

Whenever I go outside in the winter, I look for Orion, the constellation that's supposed to be a hunter but looks to me like a big butterfly. I admire the whole thing because it's beautiful, but I always take an extra moment to examine the upper left wingtip, the bright red star called Betelgeuse. Why that star in particular? Because one of these days it's going to blow up, and I want to witness it from the very start.

Blow up?! That's right. One of the brightest stars in our sky is poised to become even more spectacular. It could happen any day now. And when it does, it'll be the event of a lifetime.

 

Novae

When a star explodes just a little bit, we call it a "nova." When one really goes all out, we call it, not surprisingly, a "supernova." The difference is dramatic.

A nova happens when a small, dense stellar remnant called a white dwarf gathers ("accretes") hydrogen from a normal companion star. The hydrogen piles up on the surface of the white dwarf, getting hotter and denser as more hydrogen falls down on top of it, until it reaches the point where it can undergo nuclear fusion. That normally happens in a continuous, more or less gentle reaction deep inside stars, and the heat and light take time (years) to percolate outward to the surface. That's what makes stars shine hot and bright and steadily for millions, even billions of years.

With a white dwarf covered in hydrogen, however, the fusion happens on the surface and it happens all at once. It's essentially a big hydrogen bomb going off, and it flings its heat and light (and the entire hydrogen shell) outward into space for all to see.

That's why it's called a "nova." That's Latin for "new," which is what we see when one blows up: a new star in the sky. Depending on its distance, a nova can be one of the brightest stars in the sky for a few weeks, slowly dimming as the ejecta from the explosion cools off.

A nova explosion doesn't destroy the white dwarf, nor its companion star. After the flash, they begin exchanging hydrogen again, and when enough of it accumulates on the surface of the white dwarf, the white dwarf will go nova again. This can take anywhere from decades to millennia, depending on the mass of the white dwarf.

 

Supernovae

Supernovae are a different kettle of chaos. Supernovae happen in two ways. One is when a white dwarf accretes enough matter from a companion star to squeeze its core tight enough to initiate a runaway fusion reaction down inside. When that happens, it blows the white dwarf apart.

The other, more common, way is for a massive star to run out of fuel and collapse. Under normal operation, the pressure from the heat and light generated by nuclear fusion keeps gravity at bay, but when fusion stops, gravity takes over and the star collapses inward, fast. There comes a point where it can't collapse any further. A force called electron degeneracy pressure keeps it from growing any denser, and the infalling matter meets an impenetrable wall and it bounces. It rebounds outward at a substantial fraction of the speed of light, undergoing runaway fusion as it does so, and the combination of forces rips the star apart.

The explosion can often overcome the electron degeneracy pressure and squeeze the core past the limit, turning normal matter into nothing but neutrons, leaving behind a rapidly rotating neutron star. If the neutron star is too massive, it can collapse further into a black hole.

And all that stuff that went flying out into space glows like a bazillion Las Vegases. It's not just incandescent gas, but the material that was created during that moment of intense fusion. During their normal lives, stars only create elements up to the atomic weight of iron, but the supernova blast creates temperatures and pressures high enough to forge the remaining heavy elements. One element in particular, nickel, is produced in abundance. The form of nickel that's created is also radioactive, and the energy of its radioactive decay (and the decay of its decay product, cobalt) keeps the rest of the ejecta glowing brightly for months after the explosion.

 

Too Close for Comfort

How bright is bright? Really bright. A supernova can outshine all the rest of the stars in a galaxy combined. That's something like 200 billion stars, all put to shame by one star going out in a blaze of glory.

That leads to an obvious question, often asked nervously with a wary eye on the sky: What happens to Earth if one blows up nearby? Or even not so nearby?

Cosmic distances are vast. The closest star to our own Sun is over four light-years away, meaning that it takes light itself, the fastest thing in the universe, four years to travel from there to here. That's a long, long way (24 trillion miles, give or take), but not far enough. If a supernova went off that close, we would be toast. No questions asked, no quarter given, not even on the night side of the Earth. It wouldn't be just the intensity of the heat and light, which would be about the same as having a second Sun in the sky, but the gamma radiation would hit us like a trillion malfunctioning X-ray machines. It would cook everything on Earth. It might even be strong enough to rip away our atmosphere.

Radiation intensity drops off as the inverse square of distance, so at twice the distance we would only get 1/4 as much radiation. Opinions vary on just how far would be far enough to not do us significant harm, but it's on the order of 50-100 light-years.

But then there are gamma ray bursts. The initial flash spreads radiation out in all directions, but down inside the supernova there's a spinning core that's being compressed into either a neutron star or a black hole, depending on its mass. If it's a neutron star we're okay. It'll sit there and spin, casting a beam of light and radio waves around like a lighthouse and creating a pulsar, but the beam intensity won't be dangerous. If the core collapses all the way to a black hole, however, there's a strong chance that additional material falling into the black hole will create what's callled an accretion disk, in which it orbits the black hole in a tight spiral that winds up the star's magnetic field until it creates a tubular particle accelerator along the spin axis. That particle accelerator sends a beam of gamma rays outward that's thousands of times stronger than the initial flash from the explosion itself.

If we're in the path of that beam, we're pretty much hosed (literally!) even if we're ten thousand light-years away. The burst only lasts a few minutes, but that would be enough to wipe out our ozone layer and leave us vulnerable to the Sun's ultraviolet radiation.

Fortunately, the odds of being in the beam are pretty low. A star has to be massive enough to collapse all the way into a black hole, and its spin axis has to be pointing right at us in order to beam its gamma ray burst at us.

 

Back to Betelgeuse

So how about Betelgeuse? It's the closest star that's likely to go supernova anytime soon. Astronomically speaking, that means sometime within the next 100,000 years or so, but it could happen tonight. When it blows, how dangerous will it be?

Estimates of its distance vary from 400 to 1000 light-years. Our best measurements put it somewhere about 500-600. That's far enough away that the flash won't harm us. It doesn't seem massive enough to form a black hole when it explodes, so the gamma ray burst danger seems low. And even if it did produce a gamma ray burst, its rotational axis is angled about 20 degrees away from us, so we wouldn't get the brunt of the beam anyway.

So the most likely scenario is that when Betelgeuse blows, we'll see the star grow dim as it shrinks, then it will slowly brighten again as it explodes. It'll happen slowly because it takes time for that much mass to move outward, even at a fair fraction of the speed of light. Remember that it takes light eight minutes to reach Earth from the Sun, and Betelgeuse is far larger than Earth's orbit. It takes light over an hour to cross from one side of Betelgeuse to the other, and the explosion happens slower than light, so it will play out slowly.

It will be one of those ridiculous events, though, that just doesn't stop. It will get brighter and brighter and brighter until we think it can't possibly get any brighter, then it will get brighter still. By the time it peaks out, it will be about as bright as a full Moon, which is the second-brightest object in our sky after the Sun itself.

And it'll stay that way for months. It will be visible in the daytime, and not just as a bright spot in the sky. It'll be an insanely bright spot, as hard to miss as the reflection of sunlight off a windshield. It could be bright enough to cause retinal damage if you stare at it, because all that light will be concentrated in a single point source. (At its immense distance, even Betelgeuse's explosion will be too small to show any dimension to the naked eye.)

 

Neutrinos

Oddly enough, the first thing we detect won't be the flash. As I mentioned above, the actual explosion takes hours to break out. But that incredible nuclear blast at the core of the star produces one byproduct that flees the star at the speed of light: neutrinos.

Neutrinos are quite possibly the universe's smallest, lightest, and most elusive particles. They're so tiny and so nearly massless that they hardly ever interact with anything. It's often said that a neutrino could pass through a light-year of lead and never hit an atom. That's a good thing, because the universe is awash in neutrinos. Our own Sun showers us with about 65 billion of them every second per square centimeter, which means about 2 quadrillion of them just went through your body as you read this sentence. Odds are, none of them hit anything.

But when an entire star blows through its entire mass of fuel all at once, the neutrino emission gets truly phenomenal. About 99% of the gravitational collapse energy is converted to neutrinos, which means something like 1058 of them are produced in maybe 10 seconds. Even at Betelgeuse's enormous distance, that means we'd receive about 3,000 times as many neutrinos for those 10 seconds as we receive from the Sun. Odds are, none of them will hit anything within our bodies, either.

But there are several neutrino detectors on Earth, each with thousands of times the mass of a human body, so they will receive plenty of hits. Estimates vary, but there could be as many as 2 million neutrino detections in those few seconds. And those will arrive long before we see the flash of light, so we should have an hour or two of warning to go outside and look up.

 

The Aftermath

It has been estimated that a galaxy the size of the Milky Way should have about 1-3 supernovae per century, on average. Indeed there have been about 20 recorded in the last 2000 years, but none since 1604, ironically six years before the invention of the telescope. We've been waiting a long time for another one, although we see them popping off all the time in distant galaxies.

We do see the remnants of past explosions, though. The Crab Nebula is probably the most spectacular. It's what's left of the explosion of 1054. That was nearly a thousand years ago, yet the expanding shell of debris is still glowing brightly enough to see in even a small telescope.

When Betelguse blows, its ejecta will create a nebula that will grow and glow for thousands of years, becoming one of the most spectacular telescope objects visible anywhere in the sky. Long after the brilliant "new star" fades from naked-eye sight, it will continue to expand and to excite wonder for generations.

So next time you go outside in the wintertime, have a look at the upper left star in Orion. It's the one that's obviously redder than the others. The one that's destined to blow up someday, maybe someday soon.
 

__________________________________

Jerry Oltion has been a science nut since he was old enough to spell "curious." He has written science fiction almost as long, and has done astronomy somewhat less. He writes a regular column on amateur telescope making for Sky & Telescope magazine, and spends many, many nights a year out under the stars.
 

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