lucky! A team of researchers discovers a supernova forming shortly after a red supergiant explodes. The light from the event is 11.5 billion years on its way to Earth. Astronomers’ observations benefit from a cluster of galaxies.
Astronomers have for the first time observed a distant supernova just a few hours after the explosion. The international research team was helped by a fortunate circumstance: The explosion took place behind a galaxy cluster, which acts as a kind of magnifying lens with its gravitational pull – and also created three images of the supernova. The exploded star was a so-called red supergiant, which was about 530 times larger than our sun, as the scientists report in the journal “Nature”.
Stars generate their energy through the fusion of atomic nuclei: First, helium is formed from hydrogen, later heavier elements including iron. This is why a core of iron forms inside large stars. In stars more than eight times as dense as our Sun, this iron core eventually becomes unstable – it collapses under the weight of its own gravitational pull, creating a neutron star or even a black hole.
The collapse of the iron core results in a strong pressure wave running outwards – a shock or shock wave. “When this shock wave reaches the surface of the star, it leads to rapid heating there,” explain Wenlei Chen from the University of Minnesota in the USA and his colleagues. As seen from Earth, the star becomes extremely bright – it lights up as a supernova. “Due to the heating, the outer shell of the star then expands and cools down again,” the researchers continue.
From this early phase – the rapid lighting up and subsequent cooling – the sky researchers can determine the size of the exploded star. “However, such data exist only for a few nearby supernova events, and even there there are no observations within the first day of the explosion,” Chen and colleagues write. To change that, the team combed through archive data from the Hubble Space Telescope. The researchers paid particular attention to large galaxy clusters.
Because with their gravity, galaxy clusters deflect the light of more distant objects, they act as gravitational lenses and can thus make a distant supernova visible in the first place. Even more: Such gravitational lenses can image distant celestial objects several times. Since the different images correspond to different light paths through the cluster, the astronomers see an object in the individual images at slightly different times – another advantage for observing supernovas.
Chen and his colleagues found what they were looking for in the galaxy cluster Abell 370: A supernova appeared here in December 2010, which was imaged three times by the gravitational lens. The light from the exploded star took 11.5 billion years to reach Earth. One of the three images shows the supernova about six hours after the explosion. From the course of the brightness of the supernova, the diameter of the original star is 530 times that of the sun – in good agreement with the theoretical expectations for a red supergiant.
The team’s discovery shows that supernovas in the early Universe are not strikingly different from stellar explosions in the cosmos today. Chen and his colleagues also see their observations as an example of how gravitational lenses can be used to study stellar explosions in the early Universe. Many more such events may be hidden in the Hubble Telescope archive data.
