When stars like our sun die, they tend to go out with a whine, not a bang, unless they’re part of a binary (two) star system that can cause a supernova explosion. Now, for the first time, astronomers have observed the radio signal of such an event in a galaxy 400 million light-years away. The finding was published on May 17. Naturecontains tempting hints of what the companion star should be like.

explosive death of a star

Stars with eight times the mass of our sun explode their outer layers when they begin to deplete the nuclear fuel in their cores. This process produces clouds of colorful gas, mistakenly called planetary nebulae, and leaves behind a dense, compact, hot core known as a white dwarf.

Our own sun will pass through this transit in about 5 billion years before it slowly cools and dies. However, if a white dwarf somehow gains mass, its self-destruct mechanism is triggered when it becomes about 1.4 times the mass of our Sun. The ensuing thermonuclear explosion destroys the star in a special type of explosion called a Type Ia supernova.

But where did the extra mass come from to fuel such an explosion?

Previously, we thought it might be gas emitted by a larger companion star in a close orbit. But the stars tend to eat in a messy way, spewing gas everywhere. A supernova explosion would hit any spilled gas and cause it to glow at radio wavelengths. Despite decades of research, radio telescopes have not detected any young Type Ia supernovae. Instead, we began to think that Type Ia supernovae should be pairs of white dwarfs that spiral inward and coalesce relatively cleanly, leaving no shocking gas or radio signals.

A rare type of supernova

Supernova 2020eyj was discovered by a telescope in Hawaii on March 23, 2020. For the first seven weeks or so, it behaved like any other Type Ia supernova.

But over the next five months it stopped fading. Around the same time, it began to exhibit the properties of a gas extremely rich in helium. We began to suspect that supernova 2020eyj belonged to a rare subclass of Type Ia supernovae, in which a blast wave traveling at more than 10,000 kilometers per second passed gas that may have only been collected from the outer layers of a surviving co-star.

To try to verify our hypothesis, we decided to test whether the churning gas was sufficient to generate a radio signal. Because the supernova was too far north to be observed by telescopes like the Australian Telescope Compact Array near Narrabri, we instead used a range of radio telescopes located in the UK to observe the supernova about 20 months after it exploded.

To our great surprise, we made the first clear detection of a Type Ia supernova baby at radio wavelengths, confirmed by a second observation about five months later. Could it be a “ball” that not all Type Ia supernovae are caused by the merger of two white dwarfs?

One of the most striking features of Type Ia supernovae is that they seem to have reached nearly the same luminosity. This is consistent with all of them reaching a similar critical mass before they explode.

It was this quality that led astronomer Brian Schmidt and his colleagues in the late 1990s to conclude that the Nobel laureate: The expansion of the universe after the Big Bang was not slowing down (as everyone expected) due to gravity, but was slowing down. now accelerating due to the effects of what we call dark energy.

So Type la supernovae are important cosmic objects, and it worries astronomers that we still don’t know exactly how and when these stellar explosions occur, or what makes them so consistent. In particular, if the merger of pairs of white dwarfs can have a total mass of about three times our Sun, why would they all emit roughly the same amount of energy?

Our hypothesis (and radio confirmation) that supernova 2020eyj occurs when enough helium gas is ejected from the companion star onto the surface of the white dwarf to slightly exceed the mass limit provides a natural explanation for this consistency.

The question is why we haven’t seen this radio signal in another Type Ia supernova before. Maybe we tried to spot them right after the explosion and gave up too easily. Or maybe not all companion stars are that helium-rich and don’t shed their gaseous outer layers well. But as our research has shown, patience and perseverance sometimes pay off in ways we never expected, allowing us to hear the dying whisper of a distant star.