Scientists use Kepler telescope to study supernovae
A new study describes the most extreme known example of a “fast-evolving luminous transient” or FELT supernova. Image Credit: NASA/JPL-Caltech
NASA’s Kepler space telescope was designed to discover exoplanets, but throughout its initial and extended missions, scientists have used its precision and ability to continuously collect data to study supernovae, the explosions produced by dying massive stars.
Used to measure the expansion of the universe, supernovae are difficult to detect because scientists have no way of knowing when or where one will occur, meaning they usually miss the critical first few moments of the event. Supernovae produce and scatter the heavy elements that go into new generations of stars, planetary systems, and life itself.
When Kepler launched in 2009, astronomers realized its ability to look at single patches of the sky for long periods of time made it ideal for observing objects other than exoplanets, including active galactic nuclei and supernovae.
“Kepler opened up a new way of looking at the sky,” said Kepler project scientist Jessie Dotson of NASA’s Ames Research Center in Silicon Valley, California, in a NASA news release. “It was designed to do one thing really well, which was to find planets around other stars. In order to do that, it had to deliver high-precision, continuous data, which has been valuable for other areas of astronomy.”
Active galactic nuclei are supermassive black holes at the centers of galaxies that are extremely bright due to their active consumption of surrounding hot gas. Because supernovae were thought to be extremely rare, the initial proposal to use Kepler for something other than planet hunting referred only to searching for active galactic nuclei.
However, in 2012, astronomer Ed Shaya of the University of Maryland, while studying Kepler data, observed a sudden 10 percent brightening in the light coming from a galaxy. Though it could have come from a supernova, the brightening could also have been the result of a computer error, NASA said.
To find its source, Shaya and Robert Olling, also of the University of Maryland, developed software to improve their measurement of Kepler data based on temperature variations and and precise pointing of the telescope. With the new software, they confirmed the initial brightening, which persisted, to be a supernova signal, and discovered five more supernovae from a sample of over 400 galaxies.
Joining with scientists in Australia and Chile, Shaya and Olling began an international collaboration project known as the Kepler Extra-Galactic Survey (KEGS). To date, they have found more than 20 supernovae, including an unusual, not well understood type known as a Fast Evolving Luminous Transit or FELT.
When Kepler was re-purposed in 2009 after a failure of two reaction wheels that controlled its orientation, mission scientists arranged for it to study individual areas of the sky for three months, then rotate it to another area. The three-month periods were labeled “campaigns.”
An infographic on types of supernovae. Image Credit: NASA
Types of supernovae vary based on the circumstances of the precursor star. Binary systems with at least one massive star produce Type 1a supernovae, which occur when one of the system’s stars is a dense stellar remnant known as a white dwarf. This dead star steals mass from its companion until it reaches 1.4 solar masses, at which point it explodes, unable to sustain its own weight.
Scientists use Type 1a supernovae as “standard candles” to measure great distances and the expansion of the universe. While this type has the same intrinsic brightness, the most distant ones were found to be less bright because its light is stretched by expanding space. Observing this phenomenon led scientists to realize the universe is expanding, and its rate of expansion is accelerating.
Two merging white dwarfs in a binary system can also produced a Type 1a supernova. Astronomers remain uncertain as to whether this process produces a brighter explosion. If the answer is yes, this will need to be taken into account when using these supernovae to measure the universe’s expansion.
Type II supernovae occur when a single massive star undergoes a core collapse, producing a shockwave. In 2011, members of KEGS successfully, in optical light, observed a shockwave produced by the explosion of a star roughly 500 times the size of the Sun. However, according to NASA, they detected no shockwave when a star 300 times the size of the Sun exploded, indicating there may be more than one kind of Type II supernova.
During the extended K2 mission, the researchers observed a FELT, or variation of a Type 1a explosion. This type of supernova brightens very quickly, about 10 times faster than the standard Type 1a. One theory attributes this to the star ejecting a shell of gas about a year before the explosion. When the explosion does occur, its energy hits the shell, causing the rapid brightening.
By observing with ground-based telescopes, scientists can determine a supernova’s color and changes over time, enabling identification of the explosion’s chemical composition and the nature of the precursor star. However, these telescopes are limited by weather and their inability to observe during daylight hours.
Combining Earth- and space-based telescopic observations of individual supernovae is giving scientists unprecedented insight into these phenomena. K2‘s 16th campaign, which ran from December 7, 2017, to February 25, 2018, observed 9,000 galaxies, and its 17th campaign, which just started, is looking at 14,000 galaxies.
While Kepler will soon run out of fuel, scientists hope to observe supernovae with its successor, the Transiting Exoplanet Survey Satellite (TESS), scheduled for launch this spring.
Correction: An earlier version of the story incorrectly stated members of KEGS observed Type II supernovae shockwaves produced by stars of 500 and 300 solar masses. This should have been stars 500 and 300 times the size of the Sun, respectively.
Video courtesy of JPL
Laurel Kornfeld
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.
A correction: the progenitor stars of the two Kepler supernovae were not 300 and 500 times more massive than the Sun, but had radii 300 and 500 times larger than that of the Sun (to be more exact, 280 +/- 20 and 490 +/- 20 times the solar radius). The progenitor masses are not well-determined, but likely were in the range of 10-20 solar masses.
Aside from hypothetical “population III” stars in the very early universe, stars don’t get to be as large as 500 solar masses. It is thought that stellar masses top out around 100 or 150 M_sun.