Betelgeuse has been the focus of media attention lately. The red supergiant is nearing the end of its life, and when a star 10 times the mass of the Sun dies, it extinguishes in spectacular fashion.
With brightness recently dropped to its lowest point in a century, many space enthusiasts are thrilled that Betelgeuse could soon go supernova, exploding with dazzling fireworks that can be seen even in daylight.
While the famous star on Orion's shoulder is likely to die over the next million years – practically a couple of days in cosmic time – scientists argue that its darkening is due to the pulsation of the star. This phenomenon is relatively common among red supergiants, and Betelgeuse is known to have been in this group for decades.
Coincidentally, researchers at the University of California, Santa Barbara have already made predictions about the supernova brightness that could occur when a pulsating star like Betelgeuse explodes.
Physics PhD student Jared Goldberg published a study with Lars Buildsten, director of the P.I. Kavli (KITP) and Gluck's physics professor and KITP senior scientist Bill Paxton, which details how the pulsation of a star will affect the subsequent explosion when it does. The article appears in the Astrophysical Journal.
“We wanted to know what it would look like if a pulsating star exploded at different pulsation phases,” said Goldberg, a researcher at the National Science Foundation. 'The earlier models are simpler because they do not include time-dependent ripple effects.'
When a star the size of Betelgeuse finally runs out of material to coalesce at its center, it loses the external pressure that kept it from collapsing under its own enormous weight. The resulting core collapse occurs in half a second, much faster than it takes to notice the star's surface and plump outer layers.
When the iron core collapses, the atoms dissociate into electrons and protons. They combine to form neutrons and in the process release high-energy particles called neutrinos. Usually neutrinos barely interact with other matter – 100 trillion of them pass through your body every second without a single collision.
However, supernovae are some of the most powerful phenomena in the universe. The numbers and energies of neutrinos produced in core collapse are so great that even though only a small fraction collides with stellar material, it is usually more than enough to launch a shock wave that could explode a star.
The resulting explosion hits the outer layers of the star with staggering energy, creating an explosion that can briefly outshine the light of the entire galaxy. The explosion remains bright for about 100 days, as the radiation can only escape after the ionized hydrogen reunites with the lost electrons and becomes neutral again.
The characteristics of a supernova vary with the mass of the star, the total energy of the explosion and, importantly, its radius. This means that Betelgeuse's ripple makes predicting how it will explode much more difficult.
The researchers found that if the entire star pulsates in unison – breathing in and out if you like – the supernova will behave as if Betelgeuse were a static star with a given radius. However, different layers of a star can oscillate against each other: the outer layers expand, and the middle layers contract, and vice versa.
“The light from the compressed portion of the star is weaker,” Goldberg explained, “just as we would expect from a more compact, non-pulsating star.” Meanwhile, light from parts of the star that were expanding at the time would appear brighter, as if it were coming from a large, non-pulsing star.
Goldberg plans to present a paper with physics professor Andy Howell and KITP researcher Evan Bauer in the American Astronomical Society's Notes to Research, summarizing the results of the simulations they did specifically for Betelgeuse.
Sources: Photo: ALMA (ESO / NAOJ / NRAO)
