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Astro Bob: Astronomers discover way to predict when a star will go supernova

We know which stars may end their lives as supernovae, but the question of when has always proved elusive. That may be changing.

Cas A supernova
This is a false color image of the supernova remnant Cassiopeia A (Cas A) using observations from the Hubble, Spitzer and Chandra X-ray telescopes. Cas A is the expanding shell of debris, now 10 light-years across, formed after the explosion of a massive supergiant star located 11,000 light-years away.
Contributed / NASA, JPL-Caltech
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Wouldn't it be nice to know when a star is going to blow up? We have a pretty good handle on what types of stars could explode as supernovae, but timing's another thing. Astronomers are being honest when they say "it could be tonight or 10,000 years from now." Still, the answer leaves us wanting.

Orion Dec 22_2019_S arrow_Fea.jpg
Betelgeuse is a bright red star located in the constellation Orion the Hunter.
Contributed / Bob King

Betelgeuse is a great example of a star we're confident will explode as a supernova. This famous red supergiant is about 764 times bigger than the sun and shines 14,000 times brighter at peak light. Subbed in place of the sun, its gaseous surface would extend nearly to Jupiter.

Betelgeuse and its kin burn hot compared to the sun because they're so much more massive. All that internal pressure cranks up the temperature, causing the star to devour its nuclear fuel faster than a gas-guzzling Chevy Suburban. Smaller, cooler stars are more frugal. With luck, Betelgeuse might live for 10 million years or so. Stars like the sun on the other hand faithfully produce energy for 10 billion years or more.

One of the side benefits of a long stellar life is the additional time afforded any planets orbiting a star to evolve life. If Betelgeuse has such worlds they're likely to be barren.

Fusion diagram
Making energy by crushing hydrogen atoms together happens in several steps. First, two hydrogen atoms join to make deuterium. In step 2, deuterium takes on another hydrogen to form helium-3, liberating energy in the process. Two helium-3s join to form a stable helium atom plus two hydrogen atoms that are now available to start the process all over again.
Contributed / Bob King

Stars creates energy by combining elements through the process of nuclear fusion, starting with the simplest and most abundant element of all: hydrogen. In the 30-million-degree (F) heat of the sun's core, hydrogen atoms fuse to form helium, releasing energy in the process. That energy reaches its surface and showers us with sunshine and warmth.

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Eventually, a star's hydrogen supply runs dry. Then it turns to burning its own ash, fusing helium into carbon. The more massive the star, the more elements it can fuse. After carbon comes oxygen, followed by neon, silicon and others until it arrives at iron.

Supergiant cutaway
A supergiant star is big and hot enough to forge elements all the way up to iron in its core. Other lighter elements, also created by the star, burn in nested shells around the core. Unfortunately, once iron is made, fusion stops, the core collapses and a supernova explosion ensues.
Contributed / NASA, CXC, S. Lee

Iron's a star-killer. You can't fuse it to release more energy like the others — the star suddenly reaches a dead end. Without continued heat and outward pressure to fight back its own self-gravity, it collapses inward. This initiates a shock wave that reverberates back out through the star, tearing it apart in a catastrophic explosion called a supernova. In child-speak — "star go boom!"

Veil Nebula
The aptly named Veil Nebula in Cygnus the Swan is a magnificent supernova remnant visible in smaller telescopes. It formed when a star about 20 times larger than the sun self-destructed some 8,000 years ago and continues to expand to this day.
Contributed / Doug Neverman

The sun is too small to cook up the really heavy stuff like silicon let alone iron. Its end will be a more peaceful affair. Instead of exploding it will expel its outer layers, while the core will persist as a white dwarf star the size of the Earth. Things get really quiet after that. Like an ember, the former sun will slowly cool over the next few trillion years until reaching it final state as a black dwarf.

A close look at Betelgeuse
This is the sharpest ever image of Betelgeuse (bright pink at center) with a resolution equivalent of seeing a tennis ball on the space station from the ground. It was made with the European Southern Observatory's Very Large Telescope. Around it you can see large plumes of gas and dust extending into space.
Contributed / ESO and P. Kervella

What if a star could send a signal to alert us to its imminent collapse? Maybe they are. A team of astronomers in the UK and France discovered that when it comes Betelgeuse-like red supergiants, they can tip us off. To appreciate their result, first know that stars release material into space all the time as stellar winds. Even the sun blows a steady breeze of particles into space. Giant stars like Betelgeuse do it with greater strength and volume!

The outward pressure from Betelgeuse's own radiation pushes against dust and gases in the star's outer envelope, lofting them into space. Because the star's surface is so far from its core, gravity holds it only weakly, so even a gently push sloughs off considerable mass.

Supergiants can shuck off up to half their total "possessions" in a lifetime, something that would impress even Marie Kondo . They can lose that material gradually or through sudden, massive outbursts caused by instabilities deep within the star. Either way, a shroud of dust blocks the light of the star and dims it.

Supernova artist view
Once a star stops fusing elements in its core, a fate expected for Betelgeuse in Orion and Antares in Scorpius, it collapses, producing a shockwave that rips it apart.
Contributed / ESO

The group examined photographs of several dozen red supergiants taken within 10 years of their supernovae blow-ups. They discovered that shortly before the stars met their demise, they became very faint and red, an indication of a rapid and massive release of dust and gas.

If the fading were due to a steady wind, the stars would be red and faint for many decades before exploding. Whatever caused the sudden dimming "must do it on a very rapid timescale," the authors write. "Within a year of core-collapse."

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They concluded that the sign of an imminent explosion of a red supergiant would be a radical change in the star's visible and infrared light on timescales of less than a month. When Betelgeuse faded suddenly in 2019 we all wondered if the end was near. Now that we have a new yardstick, I look forward to a Betelgeuse-goes-kablooey forecast. Soon, please, if possible.

Read more from Astro Bob
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Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at nightsky55@gmail.com.
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