Astronomers have been waiting for this for a long time, and at some time in the not so distant future the brilliant red star in the constellation Orion will explode. What will it look like?
Betelgeuse is so big and close its surface can be resolved with the Hubble Space Telescope!
What we know.
Betelgeuse is a red supergiant star located about 590 light years from Earth. If you were to replace our sun with this star, its outer surface would be located at about the orbit of Jupiter, but it is a variable star and its diameter has been directly measured to vary from about the orbit of Earth to the orbit of Saturn. It has an average luminosity of about 120,000 times our sun's output. It's mass is between 8 and 17 times the mass of our sun. It is surrounded by ejected shells of gas that extend out to 1400 AU (1 AU = Earth's orbit distance from the sun), with a plasma gas shell extending to 200 AU. The dust shell indicates that Betelgeuse has been going through a period of mass loss that may amount to 3% of our sun's mass every 10,000 years. It is no longer supplying most of its energy by fusing hydrogen to helium in its core, but is now in an advanced stage of life where hydrogen fusion is occurring in a shell surrounding the core. The helium ash is forming a large dense core about the size of Earth which will eventually get hot enough to fuse helium into heavier elements. By that time it will be within a few hundred thousand years of its end-of-life. For a star of this mass, it will explode in what astronomers classify as a Type II supernova. It will leave behind a neutron star with a mass of 1.5 times our sun, which will become a pulsar. Computer models don't give us a very accurate prediction of when this is likely to happen, but it could be anytime between 100,000 years and several million years from now.
So what will it look like?
Some years ago, I did a few back of the envelope calculations and came up with a brief post-explosion scenario at my online site The Astronomy Café. Here's what I said back in 1997, with some added information here and there.
"If we just consider what could happen as a result of its expanding shell of gas, typical shell velocities are about 10,000 kilometers/sec. The shell would arrive here about 100,000 years after we see the star brightening. The shell would carry about half the mass of Betelgeuse or perhaps 10 times the mass of the Sun, which equals about 2 x 10^58 protons. The flow of these particles in a shell with a radius of 590 light years would be about 100,000 protons per second per square centimeter. The solar wind 'flux', by comparison is about 300 million protons/sec/square centimeter at the Earth's orbit. Ram pressure is defined as density time velocity-squared. The Betelgeuse flux is traveling at perhaps 10,000 kilometers/sec compared to the 450 kilometers/sec of the solar wind. Its density is thousands of times less than the solar wind, so the Betelgeuse flux has an effective pressure that is (1/1000)(10,000/450)^2 = 0.5 times the solar wind or less, and spread out over a region much larger than the size of the solar system. This pressure will probably not be enough to cause the heliopause of our sun at 120 AU to change noticeably and so our solar system will be protected from the Betelgeuse flow because we are literally living inside a magnetic bottle due to our sun's solar wind! There is also the x-ray flux to contend with. Inside this shell, there is a bubble of plasma consisting of very hot and energetic electrons and magnetic fields, which produce copious amounts of X-ray light. We would be subjected to this x-ray flux for 10s of thousands of years until the expanding supernova remnant has aged sufficiently to quench this production mechanism. These X-rays, of the 'soft' variety' would not get down to the Earth's surface thanks to atmospheric shielding, but travelers in interplanetary space would need some additional shielding from the secondary electrons generated as these x-rays strike the skin of their spacecraft and liberate additional electrons. "
Looking at the night sky, the surface brightness of this shell will be so small that within a few decades after the explosion, you will not be able to see this nebula approaching and enveloping our solar system unless you had the right equipment. It will not look like some ghostly green aurora filling the skies.
Not much has changed in the details of these predictions, except that we have studied many more of these Type II supernova explosions since then with the Hubble Space Telescope, Chandra and other resources.
The Neutrino Pulse.
The core detonation will most likely generate a brief pulse of neutrinos that will reach Earth hours before we visibly start to see the star brighten. Then over the course of several weeks it will brighten until it reaches a magnitude of about -12, which is about the same brightness as the full moon. But in a spot of light that your eye can't resolve, it will actually hurt to look at it and may even cause retinal damage.
The most recent Type-II supernova called SN1987A, was spotted in the Large Magellanic cloud in 1987 at a distance of 168,000 light years. At the distance of Betelgeuse, it would take the SN1987A expanding shock wave about 20,000 years to reach our solar system.
Supernova 1987A remnant viewed by the Hubble Space Telescope. The ring is a light year across, and was created by the detonation shock wave reaching the inner edge of a dust shell ejected over 20,000 years ago.
Learning from Example.
SN 1987A produced a neutrino pulse of 10^58 neutrinos, which was reduced to only 24 detected on Earth due to the inverse-square law at the distance to SN1987A, and the fact that neutrinos interact very weekly with our detector matter. If we were to move this to Betelgeuse, we would get a pulse of 24(168,000/590)^2 = 2 million detected neutrinos! This is utterly harmless to humans, but will alert astronomers that Betelgeuse is about to blow.
The Crab Nebula, some 960 years after detonation, is surrounded by an expanding cloud of plasma that is now 6 light years in radius. It is the strongest x-ray and radio source in the sky after our sun. The cloud is expanding at about 1,500 km/sec or 3.5 million miles per hour. It is located 6,500 light years from Earth. At the distance of Betelgeuse, by 960 years this shell would be 14 times the diameter of the full moon! At about 95,000 years after detonation, this slow-moving plasma will finally reach the solar system. From then-on, we will be inside the volume of intense x-rays and high-energy particles.
Crab nebula. (Credit:NASA/Hubble)
But the good news is that the supernova shell of the famous and much older Cygnus Loop supernova remnant is at a distance of about 1,400 light years and has a diameter of 150 light years 8,000 years after the explosion. Its shell has already broken up into numerous filaments and a lot of empty gaps in-between. At the distance of Betelgeuse, this shell would reach our solar system after about 63,000 years; not much different than our estimate for the Crab nebula-type shell. It would occupy about 2/3 of the entire sky centered on Betelgeuse.
The Cygnus Loop supernova remnant showing the hot and cold regions as seen in x-ray light (Credit:NASA/ROSAT)
So, we can expect that about 100,000 years after Betelgeuse detonates, and after watching the expanding cloud get bigger for millennia and then seeing it fade away from view, we will have a new reminder of its presence. The plasma cloud and x-rays will enter our solar system and force us to adapt to this new interplanetary situation for the next hundred millennia or more, not because of the particles but because of the x-rays. The high-energy particles will only increase the cosmic ray background in our solar system, and will be of too-low an energy to penetrate our Earth's magnetic field...so no cancer risk unless you are an astronaut.
The bottom line...
But who knows what we will be doing by then! We might have abandoned space travel millennia before the Betelgeuse supernova, or we may not even care because our technology and biology is now utterly immune from such trivial space hazards!
Either way, the Betelgeuse Supernova will find us a dramatically different species than what we are now.