Astronomers Capture First-Ever Image Of A Nova’s Exploding Fireball Stage

Chuck Bednar for redOrbit.com – Your Universe Online
Observations of the expanding thermonuclear fireball from a nova that erupted last year have resulted in the first ever images of an exploding star during this stage and revealed how the ejected material’s structure evolves as the gas cools and expands, researchers from Georgia State University reported on Sunday.
The observations, which were conducted by researchers working at GSU’s Center for High Angular Resolution Astronomy (CHARA), reveal that the expansion is more complex than previous models had predicted. Lead author Gail Schaefer, an astronomer working at Georgia State, and 37 colleagues representing 17 institutions report their findings in the latest edition of the journal Nature.
According to the university, an amateur astronomer by the name of Koichi Itagaki first discovered a “new” star on August 14, 2013. That star, which was named Nova Delphinus 2013, was “quickly confirmed as an optical transient by a number of amateur and professional observers, and also confirmed spectroscopically as a nova by amateurs and professionals who observed independently at nearly the same time,” according to the American Association of Variable Star Observers (AAVSO).
“A nova occurs following the buildup of a thin layer of hydrogen on the surface of a white dwarf, a highly evolved star with the diameter of the Earth and the mass of the sun,” the university said in a statement. “The hydrogen is provided by a close companion, which is a normal star in a binary star system, where the two stars orbit about their center of mass.”
When this so-called hydrogen ocean is approximately 650 feet, the white dwarf’s surface gravity produces enough pressure at the bottom of the hydrogen layer to trigger thermonuclear fusion, the researchers explained. The light from the resulting explosion will be far brighter than the star’s normal appearance and could make it so that the object is suddenly visible to the naked eye in a location not previously known to be home to a bright star.
“Within 15 hours of the discovery of Nova Del 2013 and within 24 hours of the actual explosion, astronomers pointed the telescopes of the CHARA Array toward the nova to image the fireball and measure its size and shape,” GSU said. “The size of Nova Del 2013 was measured on 27 nights over the course of two months. The first measurement represents the earliest size yet obtained for a nova event.”
The CHARA array, which is located on the grounds of Mount Wilson Observatory in California, uses the principles of optical interferometry to combine the light from six telescopes to create images in extremely high resolution, equal to a telescope with a diameter of over 300 meters. As a result, it can be used to observe details of objects the size of an American nickel on top of the Eiffel tower from as far away as Los Angeles, the study authors noted.
By measuring Nova Del 2013’s expansion, they were able to determine that it was located 14,800 light years from the sun, meaning that while the explosion was witnessed here on Earth last August, it actually occurred roughly 15,000 years ago. During the first CHARA observation, the fireball’s physical size was roughly as large as the Earth’s orbit, but measurements conducted 43 days after detonation revealed that it had grown nearly 20-fold.
At that time, the size of the fireball was roughly equal to the orbit of Neptune, and it had a velocity in excess of 600 km per second (over 370 miles per second). Pictures of the fireball were created from the interferometric measurements thanks to the University of Michigan’s Infrared Beam Combiner (MIRC), an instrument which combines all six telescopes of the CHARA Array simultaneously to create images.
“The observations reveal the explosion was not precisely spherical and the fireball had a slightly elliptical shape. This provides clues to understanding how material is ejected from the surface of the white dwarf during the explosion,” the university said. The researchers also reported that the outer layers “became more diffuse and transparent” as the fireball expanded, and 30 days later, they found evidence “for a brightening in the cooler, outer layers, potentially caused by the formation of dust grains that emit light at infrared wavelengths.”
“Thousands of novae have been discovered since the first one was recorded in 1670, but it has only become possible in the last decade or so to image the earliest stages of the explosion thanks to the high resolution achieved through interferometry,” GSU concluded. “Studying how the structure of novae changes at the earliest stages brings new insights to theoretical models of novae eruptions.”
Related Links:
> Animation of Fireball expansion
> Keep an eye on the cosmos with Telescopes from Amazon.com
—–
Follow redOrbit on Twitter, Facebook and Pinterest.
—