Exploded star could help measure the Universe

Using a galaxy located two billion lightyears from Earth as a sort of cosmic magnifying class, astronomers been able to observe an exploded star that could help measure the expansion rate of the Universe.

A composite showing gravitationally lensed type Ia supernova iPTF16geu. The background is a view of the night sky by the Palomar Observatory. The left image was captured with the Sloan Digital Sky Survey. The centre image was taken by the Hubble Space Telescope and shows the lensing galaxy SDSS J210415.89-062024.7. The righ image was also taken with Hubble and shows the four lensed images of the supernova explosion, surrounding the foreground lensing galaxy.
Credit: ESA/Hubble, NASA, Sloan Digital Sky Survey, Palomar Observatory/California Institute of Technology

Type Ia supernova are a variety of exploded star that always shine with the same intrinsic brightness. As a result, astronomers can measure how bright they appear from Earth, compare it with their true brightness and use these values to calculate how far away they are.

Using a new technique, astronomers have been able to observe a gravitationally lensed type Ia supernova for the first time. Gravitational lensing is based on Einstein’s general theory of relativity, which says that mass can bend space and time. Large objects like galaxies can be used to bend the light of more distant objects and enable astronomers on Earth to see them. In this way, the galaxy acts as a magnifying glass of sorts.


Read more about gravitational lensing from BBC Sky at Night Magazine:

iPTF16geu is a type Ia supernova located 4.3 billion lightyears away. Light from the exploded star was magnified using gravitational lensing via a galaxy two billion lightyears away, splitting it into four separate images, observed using the Hubble Space Telescope. The four images were observed on a disc with a radius of 3,000 lightyears around the foreground galaxy, making it one of the smallest extragalactic gravitational lensed discovered so far.

iPTF16geu can now be used to learn more about the technique of gravitational lensing and, as it is a type 1a supernova, to help calculate the rate of expansion of the Universe.

The four images of the gravitationally lensed supernova, with the lensing foreground galaxy in the middle.
Credit: ESA/Hubble, NASA

“Resolving, for the first time, multiple images of a strongly lensed standard candle supernova is a major breakthrough. We can measure the light-focusing power of gravity more accurately than ever before, and probe physical scales that may have seemed out of reach until now,” says Ariel Goobar, Professor at the Oskar Klein Centre at Stockholm University and lead author of the study.

“If you measure the arrival times of the different images, that turns out to be a good way to measure the expansion rate of the universe,” says Goobar.

The discovery of the supernova was made possible by the Intermediate Palomar Transient Factory (iPTF), a new survey exploring the sky for events such as these. 30 years ago, the discovery rate of supernovae was about two per month, but now they are detected daily using the Palomar Observatory in Southern California. iPTF16geu was initially identified as a supernova on 5 September 2016, and thought to be a type Ia about one billion lightyears away. Follow up investigations revealed it to be four billion lightyears away, and that a foreground galaxy was acting as a gravitational lens.

“There are billions of galaxies in the observable Universe and it takes a tremendous effort to look in a very small patch of the sky to find these kind of events. It would be impossible to find an event like this without a magnified supernova directing you where to look,” says Goobar. “We got very lucky with this discovery because we can see the small scale structures in these galaxies, but we won’t know how lucky we are until we find more of these events and confirm that what we are seeing isn’t an anomaly.”

“It is extremely difficult to find a gravitationally lensed supernova, let alone a lensed Type Ia. Statistically, we suspect that there may be approximately one of these in every 50,000 supernovae that we identify,” says Peter Nugent, an astrophysicist in Berkeley Lab’s Computational Research Division. “But since the discovery of iPTF16geu, we now have some thoughts on how to improve our pipeline to identify more of these events.”

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