Summary:
Using gravitational lensing, Hubble captured the light from a star that exploded more than 11 billion years ago as it faded over a period of a week. But this is not the whole story, this result was discovered in an image taken in 2010, thanks to the Barbara A. Mikulski Archive for Space Telescopes (MAST), the trove of HST data holding mysteries yet to be discovered.
The story began when an international group of astronomers decided to search for supernovae in images taken with Hubble over the years. A member of the team, Dr. Wenlei Chen, wrote a Machine Learning algorithm to analyze images stored in the Mikulski Archive for Space Telescopes (MAST). Their goal was to quickly examine a large number of images and identify transient events such as supernovae. Of all the observations analyzed, this is the only supernova identified that appears several times in the same observation. But this is not the only interesting thing about this discovery - not only do we have three images of the supernova, but also each of them is showing us three different stages in its evolution.
To understand how this is possible, we will first talk about how the light from this supernova was amplified by the gravitational lensing of a massive cluster of galaxies that is much closer to us, Abell 370. Typically, a supernova produced by a star that exploded more than 11 billion years ago, when the universe was less than a fifth of its current age of 13.8 billion years, it would be impossible to observe with Hubble. However, the warping of space due to the gravitational effect of such a large mass acts like a cosmic lens that bends and magnifies the light from the supernova that is far behind this group of galaxies. Gravitational lensing was first predicted in Einstein's theory of general relativity, whereby the fabric of space and time is warped due to the gravity of a massive objects, thereby changing how light from these distant objects travels towards us. .
Similarly, the team deduced that each of these three images of the supernova shows us the explosion at different times of its life. When a star explodes as a supernova, the intense light produced by the explosion decreases considerably in just a few days. This change in intensity with time is well known and is normally used to determine the distance to these objects. The hotter the supernova is, the bluer the image of it will be. As the supernova cools, its light becomes redder. Because the observations were made using various filters, which perceive different types of light, this change in color was captured in the observation.
But we ask ourselves, How is it possible? The fact that such a distant supernova is observable due to gravitational lensing, makes this explanation easier. We need to take into account that this gravitational lensing is not produced by a single object, but by a group of galaxies whose mass distribution is not uniform throughout the cluster. In Abel 370 we see galaxies and valleys, indicating that the light we are observing could have taken different paths. In some of them the light was delayed because it took a longer route through "valleys" of warped space.
Another important aspect of this discovery is that it is the first detailed observation of a supernova at such an early age in the history of the universe. Wenlei Chen, of the University of Minnesota School of Physics and Astronomy, points out that it is quite rare to see the early stages of a supernova explosion, even when they are close to us, as they are very short-lived and can pass unnoticed. Patrick Kelly, assistant professor in the University of Minnesota College of Physics and Astronomy and principal investigator on this study, describes this process in a simple sequence of events — "You have the massive star, the core collapses, it crashes, it heats up and then you see it cool down for a week.”
The team was also able to measure the size of the star that produced this explosion, a red supergiant some 500 times larger than the Sun, making this the first time the size of a dying star in the early universe has been measured. For this, the team used the supernova's brightness and cooling rate, both of which depend on the size of the parent star.
The team plans to continue their search for distant supernovae, this time using NASA's James Webb Space Telescope. Its objective is to build a catalog of distant supernovae and thereby contribute to the study of how the formation and death of stars has evolved over time.
Original images and article can be found at https://hubblesite.org/contents/news-releases/2022/news-2022-054