A look into starbursts

When stars like our sun end their lives, they don’t exactly go out with a bang. Instead, they do something more like a soft sigh.

But sometimes when the star is in a pair (we call this a binary star system) a huge stellar explosion can occur. This is called a supernova.

It was only recently that scientists saw the radio waves from such an event for the first time. This happened in a galaxy far, far away – more than 400 million light years away from us.

They published their findings in Nature on May 17. This discovery gives us some clues about the other star in the pair.

The end of a star

Stars, which are up to eight times heavier than our Sun, undergo dramatic changes when they run out of fuel at their core. They blow off their outer layers, which become colorful clouds of gas.

We call them “planetary nebulae” even though they’re not actually about planets. What remains is a dense, hot core. This is called a white dwarf.

In about 5 billion years, our own sun will do the same. Then it will slowly cool and fade. But sometimes, when a white dwarf gets much heavier, it can explode.

When it reaches more than 1.4 times the mass of our Sun, a large explosion called a Type Ia supernova can occur.

The question is, where does this extra weight come from?

We used to think it was coming from a larger star nearby, as it might be losing some of its gas to the white dwarf. But stars are pretty chaotic and tend to spill gas everywhere. I

In an explosion, this gas would start to glow in the radio wavelength range. But although we have been looking for a long time, we have never seen this glow with our radio telescopes.

So our next idea was that these explosions might be from two white dwarfs coming together and merging. This would leave no glowing gas behind, which could explain why we didn’t see any radio signals.

An unusual starburst

Supernova 2020eyj was discovered by a telescope in Hawaii on March 23, 2020. For about seven weeks it looked like any other Type Ia supernova.

But then it started doing something else. It didn’t get any weaker for the next five months. It also showed signs of lots of helium.

We thought that this supernova might be a special kind, where the explosion knocks past gas that can only come from a companion star.

To check this we used a group of radio telescopes in the UK. We wanted to see if there was enough gas to send out radio signals. We observed the supernova about 20 months after it exploded.

To our surprise, we found the first clear radio signals from a young Type Ia supernova. This was confirmed by another observation five months later.

This could be evidence that not all Type Ia supernovae come from the merger of two white dwarfs.

Why this discovery is important

The really interesting thing about Type Ia supernovae is that they all reach about the same brightness. This suggests that they all explode when they reach a similar size.

This was key to a major discovery by astronomer Brian Schmidt and his team in the late 1990s.

They used this information to find out that the universe isn’t slowing down in expansion like we thought. In fact, development is accelerating thanks to something we now call “dark energy.”

So these stellar explosions are really important for understanding our universe. But it bothers scientists that we don’t really know how and when they happen or why they’re all so similar.

For example, if two merging white dwarfs can have nearly three times the mass of our Sun, why do they all explode with about the same amount of energy?

Our new theory suggests that supernova 2020eyj exploded just as the white dwarf was just getting heavy enough from the helium-rich gas it had ingested from its companion star. This could explain why they are all so similar.

The question remains, however, why have we not seen this radio signal from any other Type Ia supernova before? Maybe we looked for the explosion too soon, or maybe not all companion stars are created equal.

But as our study shows, patience and perseverance can pay off. In this case, hearing the last moments of a distant star helped us.

The study was published in Nature.

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