Astronomers know that when a star runs out of fuel and reaches the end of its life, it may go through an epic final stage in which it explodes in a tremendous output of energy called a supernova.
But there’s much we still don’t know much about how supernovae occur or the many different types that can exist.
Recently, astronomers from Florida State University discovered a one-of-a-kind supernova that’s challenging what we know about these dramatic events — and that could have a profound impact on other fields like the search for dark energy. We spoke to the study’s lead author, Eric Hsiao, an assistant professor of physics, to learn more.
How to categorize supernovae
As astronomers aren’t exactly sure about the details of what causes a supernova, they categorize them by observing them and using these observations to split them into two main types: A class called Type Ia, which is a thermonuclear explosion of a low-mass star, and almost all other types of supernova, which occur when the core of a massive star collapses.
This distinction between thermonuclear explosions and core collapse is key to understanding the nature of supernovae.
“Textbooks have been written on these two main divisions,” Hsiao said. “That we’re sure about. But the details of how they explode exactly, that is still a topic of active research.”
When it comes to Type Ia supernova, normally a low-mass star wouldn’t go supernova, so there needs to be a companion star to give it a kick. Our sun, for example, is a low-mass star without enough mass to trigger a supernova. So it won’t go supernova — instead, when it approaches the end of its life, it will swell up to become a red giant, encompassing the Earth.
But in a binary system, where a low-mass star and a companion star orbit each other, a supernova can occur and it can be staggeringly bright — brighter even than a massive star undergoing core collapse.
“This type of supernova is interesting because even though it’s from a low-mass star, they turn out to be the brighter of the two categories,” Hsiao said.
An astronomical oddity
Astronomers are still debating what particular configurations of stars cause a Type Ia supernova, but they know what they usually look like — something that gets very bright, and which reaches peak brightness within a few weeks.
But Hsiao and his colleagues spotted something unique: A Type Ia that is extremely bright but increases in brightness more slowly than all these others, over the course of a month. It’s one of the slowest-brightening Type Ias observed.
To explain this oddity, they ran a bunch of models to find out what configuration of stars would give that result. Their data lined up with one particular hypothesis called the core-degenerate scenario, in which a white dwarf actually orbits inside the puffy outer layers of a red giant. When the white dwarf merges with the red giant, it triggers the supernova.
“We think this is what’s happening: We’ve got this huge red giant star, and the companion is a white dwarf that’s orbiting inside of that star,” Hsiao explained. When this pair merged, it produced the bright supernova. “So that’s why my object is interesting, because we think it exploded not as a white dwarf, but as perhaps a pair of the core of the red giant star and a white dwarf.”
The scenario of a red giant with a white dwarf inside it going supernova has been proposed as a theory, but it’s never actually been seen in action before. So finding this object opens up a new way of studying supernovae.
Blowing smoke rings in space
When a low-mass star like our sun approaches the final phase of its life, its core becomes denser and it begins to lose its outer layers. As the star loses its grip on its outer layers, it lets go of mass in chunks that form smoke-like rings, Hsiao explained.
This creates a central star with a ring of matter around it, forming exquisitely beautiful but short-lived structures called planetary nebulae. And planetary nebulae can have a number of rings, which develop over time.
“We think our object is just about to become a planetary nebula, if it’s not already,” Hsiao said. “I love planetary nebulae — that’s why I’m very excited about this explanation.”
If their theory is correct, then the object should spawn the next ring as little as eight years after the explosion. The team intends to keep observing the object to see if they can spot another ring. If there’s another brightening event, that suggests there is another ring and their model is right.
Using supernovae to study dark energy
Not only is this research interesting for those seeking to understand the final life stage of stars, it may also have an impact on an entirely different field: The study of dark energy.
One reason that Type Ia supernovae have been the target of a lot of research is that they are very bright, and that brightness can be accurately calibrated.
“We can use them as mile markers,” Hsiao explained.
That makes them useful for measuring distance, which in turn can be used to measure the expansion of the universe. “So Type Ia are credited for the discovery of dark energy,” he said.
We know that the universe is expanding, and that expansion is accelerating. But we don’t know why. Astronomers theorize that there must be some kind of previously unknown energy which is affecting the universe’s expansion, and that this unknown energy makes up around 68% of the universe. They call this unknown energy dark energy.
The problem is, if some Type Ia supernovae are like Hsiao’s object, that could throw off the dark energy measurements, which assume that all Type Ia supernovae brighten in the same way. And complicating things further, this object belongs to a group of Type Ia that tend to explode in particularly faint, distant galaxies. So even a small number of these supernovae could throw off dark energy calculations considerably.
“We don’t believe it changes the conclusion that dark energy is here,” Hsiao said, “but certainly this doesn’t help for us to study the properties of dark energy.”
It’s not all bad news though. To study dark energy in more detail, it would help if we had more accurate information about how supernovae behave over time. Currently, this is represented in calculations by an error figure, because we don’t have all the information about how supernovae evolve.
Studies like this one help to constrain what we do know about the formation of Type Ia supernovae, and give more information about how their brightness may vary over time. By understanding more about specific types of supernovae, we can organize them more precisely and use that information to make more accurate calculations in the future.
The future of supernovae is bright
All of these new insights into supernovae behavior have come from studying just one unusual object. But Hsiao and his colleagues want to find out if there are other oddities out there, so their next task is to take on the handful of other strangely bright Type Ia supernovae that have been discovered and see if they can explain those as well.
“It’s a big task!” Hsiao said. But this work could allow us to see supernovae in a whole new way.
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