In a galaxy far away, an old star exploded and became a supernova. About 170 million years later on Feb. 4, 2018, the light emanating from the explosion was received by an arsenal of high-powered telescopes.
NASA’s Kepler space telescope detected the unfurling light of SN 2018oh, as it has been labeled. The first ground-based facility to identify the signal was with the All-Sky Automated Survey for Supernova and soon observatories around the globe were monitoring the supernova as part of a unique scientific experiment designed to help solve the mystery of how stars explode.
NASA retired the Kepler space telescope on October 30, following the exhaustion of fuel supplies after nine and a half years of ground-breaking operations. But from December to May, while there was still fuel left, the Kepler team oriented the spacecraft toward two distinct patches of sky that were simultaneously observable from Earth by ground-based observatories. The telescopes were able to view both patches of sky teeming with galaxies. Each of these thousands of galaxies has billions of stars.
While the telescopes watched, a few of those stars ended their long lives in dramatic explosions. With its unique capabilities, Kepler observed the minute changes in brightness of these explosions from their very beginnings while the ground-based telescopes tracked changes in color and the atomic composition of these dying stars.
With the combined data from these telescopes, astronomers achieved what they had hoped for — an unprecedented observation of the onset of a supernova. Three research papers by 130 scientists attempt to explain the unusual data revealed in the details of SN 2018oh, which was caught in the spiral galaxy UGC 4780 in the Cancer constellation. One of the papers has been accepted for publication in The Astrophysical Journal Letters, while the other two have been accepted to The Astrophysical Journal.
A hot, bright burn
SN 2018oh is an example of a Type Ia supernova — the kind that astronomers use to track the expansion of the universe and probe the nature of the invisible “dark energy” that glues together the cosmos.
A typical Type Ia supernova brightens over the course of three weeks before gradually fading away. But Kepler observed this particular supernova brightening rapidly a few days after the initial explosion — about three times faster than a typical supernova at this time period — before reaching peak brightness. Meanwhile, color details obtained by the Dark Energy Camera at Cerro Tololo Inter-American Observatory in Chile, and the Panoramic Survey Telescope and Rapid Response System at Haleakala Observatory in Hawaii, showed this supernova gleaming blue during this period of intensity, an indication of high temperatures.
For nearly a decade, scientists have been in search of a signal of a supernova similar to this one. Because Kepler was already staring at this patch of sky before the supernova went off, it was able to detect its early signals and measure it continuously for weeks.
The scenarios giving rise to Type Ia supernovae have been long-debated. So far, most evidence points to the merging of two white dwarfs, the compact corpses of stars, as the source of these explosions. Yet theoretical models have held out the possibility of an alternative scenario, in which a single degenerate white dwarf siphons off so much material from its companion star that it can no longer sustain its own weight and blows up.
Some of the scientists examining SN 2018oh’s peculiar data believe it is a compelling example of this alternative scenario. They explain that the shock wave from the exploding white dwarf ran into the companion star, creating an extremely hot and bright gaseous material that accounts for the added brightness and heat observed.
Another group of scientists favor a different mechanism to explain the excess flux of light and temperature. Type Ia supernovae produce radioactive nickel during the explosion. The radioactive decay of this heavy metal produces much of the light we see from Type Ia supernovae. If a large amount of nickel was located in the outer layers of the exploding material it would produce the observed early bump in the light.
Refining the models
If the single degenerate white dwarf theory holds true for SN 2018oh, the next step is to figure out the frequency of this kind of Type Ia supernova. If, however, the theory of nickel in the outer layers prevails, we will glean details about the inner workings of supernova explosions. Either way, understanding the details of Type Ia supernovae are important for refining the models used in cosmology to estimate the expansion rate of the universe.
The team of astronomers detected more than 40 supernova candidates during this experiment with Kepler, including several others that are also proving scientifically interesting. Though Kepler’s fuel has run out and cannot be replaced, the data it has collected on supernovae, exoplanets and other astronomical phenomena will be studied for many years to come.
The authors of these papers include scientists from dozens of institutions, including members of the Kepler team. Additional observatories providing valuable data to support the experiment include Las Cumbres Observatory, a global network of robotic telescopes based in Goleta, California; Tsinghua-NAOC and Lijiang Telescopes in China; Konkoly Observatory in Hungary; Lick Observatory on Mount Hamilton in California; Las Campanas Observatory in Chile, and others.
Date: Dec 6, 2018