In Its Last Days Kepler Captured A Supernova Doing Something Rare

An artist's impression of a single-degeneracy Type Ia supernova of the sort the Kepler space telescope captured towards the end of its life, with a white dwarf drawing material off a red giant star until it explodes. NASA/JPL-Caltech

Johannes Kepler was the last person credited with discovering a supernova within our galaxy. In its last days, the space telescope named after him observed something unusual in a similar explosion, helping us understand these events.

After the Kepler telescope's reaction wheels failed, astronomers used a series of remarkable engineering feats to prop it up using the pressure of sunlight so it could add to its astonishing haul of exoplanet discoveries.

For half its time, the Kepler 2 mission looked towards thickly clustered stars for the best chance of finding planets, as its first mission had done. The rest of the time it had to look outwards. With fewer galactic stars to study, astronomers, including Dr Brad Tucker of the Australian National University, seized the opportunity to put Kepler to a different use. By targeting patches of sky filled with 10,000 galaxies, they hoped to find supernovae before they became bright enough to be noticed on Earth.

On February 4, Kepler saw the first signs of a supernova in a galaxy 170 million light-years away. Most Type Ia supernova, the sort Kepler himself saw, take three weeks to reach peak brightness, but this one, named SN 2018oh, underwent brightening three times as fast early on in its cycle.

Type Ia supernovae are caused by interactions between white dwarfs and companion stars. “It’s possible in the case of SN 2018oh that the shock wave from the exploding white dwarf ran into the companion star, creating an extremely hot and bright halo that accounts for the added brightness and heat we observed,” Tucker said in a statement.

Astronomers have been on the lookout for this sort of reaction in previous Kepler observations. Tucker told IFLScience we would only see it if the companion star was not another white dwarf, something we think Kepler's seen before and would change how we view Ia supernovae. Moreover, visibility depends on the two stars being suitably aligned from Earth, and the companion being quite large.

Tucker told IFLScience Kepler's data is only downloaded every three months, but ground-based telescopes checked the same parts of the sky. The combination of Kepler's tracking of the early brightening at 30-minute intervals, with color measurements from telescopes in Chile and Hawaii, led to three papers in The Astrophysical Journal and the Astrophysical Journal Letters.

All supernovae are useful in helping us understand the universe, but Type Ia events are by far the most significant. The consistency of their brightness at peak allows us to measure the distance to their host galaxies, essential for grasping the scale of the universe. SN 2018oh will help us understand these events better, and refine our estimates of their distances.

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