While a core-colllapse supernova is the phenomenon thought to turn most stars into black holes after their last gasp, there is a limit for how huge stars that go through this can be.
A star that keeps fusing more and more elements needs to keep generating heat somehow. It keeps fusing heavier elements until iron, because fusion would require too much energy after that.
This is why its core collapses it and tears it to shreds. Mao saw that instead of occurring in stars of solar masses as previously thought, core-collapse supernovas probably only happen in stars that are 9 to anywhere between 23 and 27 solar masses. Because supernovas vomit out huge amounts of matter as well as light, he and his team were able to figure this out by looking at the elements released, such as oxygen and iron. Measuring the ratios of these elements gave an idea of mass.
NBH1 was the first star that ever observed turning directly into a black hole. Now, led by Caltech graduate student Dillon Dong MS '18 , a team of astronomers has established that the bright radio flare was caused by a black hole or neutron star crashing into its companion star in a never-before-seen process.
A paper about the findings appears in the journal Science on September 3. Hallinan and his team look for so-called radio transients—short-lived sources of radio waves that flare brightly and burn out quickly like a match lit in a dark room.
Radio transients are an excellent way to identify unusual astronomical events, such as massive stars that explode and blast out energetic jets or the mergers of neutron stars. This source is tied for the brightest radio transient ever associated with a supernova. Dong determined that the bright radio energy was originally a star surrounded by a thick and dense shell of gas. This gas shell had been cast off the star a few hundred years before the present day.
Yet, the gas shell itself, and the timescale on which it was cast off from the star, were unusual, so Dong suspected that there might be more to the story of this explosion. Following Dong's discovery, Caltech graduate student Anna Ho PhD '20 suggested that this radio transient be compared with a different catalog of brief bright events in the X-ray spectrum. Some of these X-ray events were so short-lived that they were only present in the sky for a few seconds of Earth time.
Through careful analysis, Dong established that the X-rays and the radio waves were likely coming from the same event. These two events have never been associated with each other, and on their own they're very rare.
So, what happened? After careful modeling, the team determined the most likely explanation—an event that involved some of the same cosmic players that are known to generate gravitational waves. They speculated that a leftover compact remnant of a star that had previously exploded—that is, a black hole or a neutron star—had been closely orbiting around a star.
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