This artist’s concept illustrates how the most extreme “pumpkin star” found by Kepler and Swift compares with the sun. Both stars are shown to scale. KSw 71 is larger, cooler and redder than the sun and rotates four times faster. Rapid spin causes the star to flatten into a pumpkin shape, which results in brighter poles and a darker equator. Rapid rotation also drives increased levels of stellar activity such as starspots, flares and prominences, producing X-ray emission over 4,000 times more intense than the peak emission from the sun. KSw 71 is thought to have recently formed following the merger of two sun-like stars in a close binary system. (Credit: NASA’s Goddard Space Flight Center/Francis Reddy)
Earth will not be the only place with pumpkins this Halloween.
Recently, astronomers using observations from NASA’s Kepler and Swift missions have unveiled a batch of 18 rapidly spinning stars that spin so fast they’ve been squashed into “pumpkin-like” shapes.
The stars produce X-rays at more than 100 times the peak levels ever seen from the sun and are thought to be the result of close binary systems where two sun-like stars’ merge.
Steve Howell, a senior research scientist at NASA’s Ames Research Center in Moffett Field, California and leader of the team, explained the behavior of the stars.
“These 18 stars rotate in just a few days on average, while the sun takes nearly a month,” he said in a statement. “The rapid rotation amplifies the same kind of activity we see on the sun, such as sunspots and solar flares and essentially sends it into overdrive.”
Astronomers dubbed the most extreme member of the group KSw 71, a K-type orange giant that is more than 10 times larger than the sun, rotates in just 5.5 days and produces X-ray emission 4,000 times greater than the sun does at solar maximum.
The rare stars were found as part of the Kepler–Swift Active Galaxies and Stars Survey (KSwAGS ) of the original Kepler field of view, which measured the brightness of more than 150,000 stars between 2009 and 2013 to detect the regular dimming from planets passing in front of their host stars.
“A side benefit of the Kepler mission is that its initial field of view is now one of the best-studied parts of the sky,” team member Padi Boyd, a researcher at NASA’s Goddard Space Flight Center in Greenbelt, Md., who designed the Swift survey, said in a statement. “Our group was looking for variable X-ray sources with optical counterparts seen by Kepler, especially active galaxies, where a central black hole drives the emissions.”
Using X-ray and ultraviolet/optical telescopes aboard Swift, the researchers conducted the survey, imaging about 6 square degrees or 12 times the apparent size of a full moon, in the Kepler field.
“With KSwAGS we found 93 new X-ray sources, about evenly split between active galaxies and various types of X-ray stars,” team member Krista Lynne Smith, a graduate student at the University of Maryland, who led the analysis of Swift data, said in a statement. “Many of these sources have never been observed before in X-rays or ultraviolet light.”
The researchers determined the rotational periods and size of 10 of the stars, which range from 2.9 to 10.5 times larger than the sun, using Kepler measurements.
Ames manages the Kepler and K2 missions for NASA’s Science Mission Directorate. NASA’s Jet Propulsion Laboratory in Pasadena, Calif., managed Kepler mission development. Ball Aerospace & Technologies Corp. operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.
Goddard manages the Swift mission in collaboration with Pennsylvania State University in University Park, the Los Alamos National Laboratory in New Mexico and Orbital Sciences Corp. in Dulles, Va. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, with additional collaborators in Germany and Japan.
A paper detailing the findings will be published in the Nov. 1 edition of the Astrophysical Journal and is now available online.
