Podcaster: Sabrina Stierwalt

Title: Everyday Einstein: Betelgeuse, Betelgeuse, Betelgeuse! Is a Familiar Star About to Explode

Organization: Quick and Dirty Tips


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Description: Betelgeuse, a massive star in the Orion constellation, is dimming. Could it be about to explode in a spectacular supernova? What would that look like here on Earth?

Bio:When not writing and recording podcasts for the Everyday Einstein show, Dr. Sabrina Stierwalt is an extragalactic astrophysicist at the California Institute of Technology and Adjunct Faculty at the University of Virginia. Before moving to Los Angeles, Sabrina received her PhD in Astronomy and Astrophysics from Cornell University. Sabrina earned a B.A. in Physics and Astronomy from UC Berkeley. She studies star formation and gas kinematics in interacting galaxies to better understand how galaxies form and evolve. She travels all over the world to observe the sky with world-class telescopes in Australia, India, Chile, and even on top of volcanoes in Hawaii.

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We think of the night sky as reliable and unchanging. The sun sets every evening revealing the same stars found in familiar patterns throughout the year. Those patterns are so predictable, in fact, that we use them to orient ourselves here on Earth. This steadiness is a result of the timescales required for any particular step in the stellar evolution process. It takes millions, even billions, of years for stars to live out their lives. In our lifetimes, we see only a snapshot of their life story. From the star’s perspective, we are but a tiny blip in the timeline.

So when a star changes on timescales we can actually observe, this change is big news. And that’s why right now, all eyes are on Betelgeuse. I’ll tell you more in a moment.

What is Betelgeuse?

Betelgeuse is what is known as a red super giant star. This is what most stars, including our Sun, will eventually become as they near the ends of their lives. Betelgeuse is only 8 million years young, which makes it a relative baby compared to our 4.5 billion-year-old sun. So why is it facing down stellar death?

Stars spend most of their lives fighting back against gravitational collapse by converting mass into energy via nuclear fusion. Bigger, more massive stars have more fuel for this fusion but they also burn through that fuel much faster. Think of a Chevy Silverado with its 36-gallon gas tank compared to a Toyota Prius that holds 12 gallons. The Chevy has more fuel to start off, but you’ll still have to fuel up sooner in the Chevy because it uses that fuel faster. Betelgeuse is over ten times the mass of our sun so it has ten times the fuel to start with, but it also has a luminosity of 140,000 the times of our sun, which means it burns through that fuel 140,000 times faster.

When a star uses up all of its fuel for nuclear fusion, its outer layers begin to expand. The star becomes larger (hence the “giant” in its name). Betelgeuse, in particular, is 1,400 times larger than the sun. If we were to plunk Betelgeuse down in our solar system in place of our sun, the giant star would swallow Mercury, Venus, Earth, and Mars. The outer edges of its surface would even tickle Jupiter in its orbit.

Why is Betelgeuse known as a red super giant? As a result of this expansion, the heat generated by the star is now spread out over a larger area. That causes the star to be cooler overall and thus emit more light at longer, redder wavelengths.

Where can we find Betelgeuse in the night sky?

Betelgeuse is actually fairly easy to pick out in the night sky, and not just because it’s bright. The star has some pretty famous neighbors. The red giant marks the right shoulder of Orion the Hunter.

The many stories of the Orion constellation are among my favorites when I teach our night sky as seen by different cultures. According to Greek mythology, Orion got into trouble with Gaia the Earth goddess for boasting that he was such a talented hunter that he could rid the planet of all wild animals. She sent the scorpion to battle him, and when Orion found he couldn’t take the scorpion down with his bow, he fled to the sea. There his friend and former hunting companion, Artemis, accidentally killed him. She was so devastated that she placed his picture in the sky.

Even here in Los Angeles, Orion is easy to spot. He is most visible in the winter in the northern hemisphere and in the summer from the southern hemisphere (January to March). Sky gazers can usually first pick out the three stars grouped close together on the sky that form his belt and then the five points that mark his feet, shoulders, and head. There are also his weapons, a club traced by stars above his head and a shield (or a bow depending on whom you ask) stretched out in front of him. The bright star marking his right shoulder, the one holding the club, is noticeably redder than the others. That’s Betelgeuse.

Why are astronomers intrigued by it right now?

In the last few months of 2019, all eyes turned to Betelgeuse. Astronomers noticed the star dimming significantly and even changing shape. The European Southern Observatory posted a video showing images of the star from January of 2019 to December of 2019 and the change is striking.

Dimming, for Betelgeuse and other red giant stars like it, is nothing new. It’s expected that stars nearing the end of their lives will throw off their outer layers as they struggle to find that balance between energy generation through fusion and gravitational collapse. But the star has never dimmed this much. Previously ranked as the 10th or 11th brightest star in the night sky, Betelguese now ranks somewhere near 20th.

What does the dimming of Betelgeuse mean?

When news of the dimming first broke, rumors circulated that the star might be about to go supernova. Once stars run entirely out of fuel and ultimately give in to the collapse of gravity, those outer, expanded layers of the red giant star collapse down onto the star’s core and then rebound in a violent, impressive explosion of light and stellar material known as a supernova.

Some suggested that the significant dimming was a result of those outer dusty layers being ejected from the star and then blocking our view of the star’s light. But most astronomers are less convinced that the epic bounce of those outer layers is imminent on any timescales relevant to us earthlings. We all know it’s coming, but the explosion could happen tomorrow or a million years from tomorrow.

Likely, what we are seeing is an extreme case of the normal dimming cycles we expect from stars like Betelgeuse. Red giant stars are prone to brightening and dimming much like an old engine sputtering out its last efforts before it goes kaput. What we are seeing could be the combination of two less extreme dimming cycles that happen to overlap in time right now.

What will happen on Earth if Betelgeuse goes supernova?

Catching a star in the act of going supernova would be a huge deal. We are able to see these fantastic light displays after they go off, which means we’ve studied the events post-supernova in detail. The last supernova to occur close enough to be visible to the naked eye was SN1987a, which happened in the year, you guessed it, 1987. Before that, the next-to-last supernova visible here from Earth erupted in German astrologer Johannes Kepler’s time in 1604.

Astronomers have noted that a nearby supernova explosion could devastate life here on Earth, given the incredible energies involved. The radiation would damage our ozone layer allowing in more harmful ultraviolet rays from the sun and would devastate phytoplankton in the ocean, the foundation of our food chain. But that devastation only happens if the supernova is within 50 lightyears. Luckily for us, we’re tucked a safe 600 lightyears away from Betelgeuse, so none of its radiation will affect us.

But this vast distance means that Betelgeuse may actually have already exploded in a supernova. The star we see in the night sky may no longer exist! Light takes a finite time to travel—it doesn’t instantly appear in one place after being emitted in another. At a distance of 600 lightyears, it takes light 600 years to travel to us from Betelgeuse, meaning we are only seeing the star as it appeared 600 years ago.

How long would we be able to see it?

A single supernova can shine brighter than an entire galaxy, lighting up the sky for months. Given how close Betelgeuse is to us, its last hurrah as a supernova could rival the full moon in brightness and be visible in broad daylight.

But once the light from this fantastic stellar explosion goes out, Betelgeuse will be lost to naked eye observers for good. We will still be able to see it with our telescopes, particularly those that can detect light at longer wavelengths than our eyes can see, but it will no longer be easily spotted as the red shining beacon marking Orion’s shoulder. Instead, it will be left to live out its life as the black hole it is eventually fated to be.

End of podcast:

365 Days of Astronomy

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