For the first time, scientists have captured images of a nova during its early fireball stage – when it ejects material, and gases expand and cool. They have observed the expanding fireball from a violent explosion called Nova Delphinus 2013.
Astronomers have reported near-infrared interferometric measurements of the angular size of Nova Delphini 2013, starting one day after the explosion and continuing with extensive time coverage during the first 43 days. The image shows Tycho’s supernova remnant. Image credit: X-ray – NASA / CXC / SAO; infrared – NASA / JPL-Caltech; optical – MPIA / Calar Alto / O.Krause et al.
“This is the first time astronomers have been able to witness an expanding fireball with such great detail, rather than as a tiny point of light way out in the galaxy. It was amazing to see the material expanding outward each day after the explosion,” said Dr Gail Schaefer of Georgia State University, who is the first author of a paper published in the journal Nature.
The term nova was coined when the famous 16th century Danish astronomer Tycho Brahe first realized that on rare occasions the unchangeable patterns of the fixed stars could be suddenly joined by bright interlopers that took days or weeks to gradually fade from sight.
Astronomers eventually traced the culprit responsible for these stellar conflagrations – an exotic, compact star called a white dwarf whose intense gravitational field is able to strip matter from a larger nearby companion star.
“The white dwarf continually sucks hydrogen from its partner, forming an ocean on its surface. After drawing about as much mass as the entire planet Saturn, the pressure reaches a critical point, then boom! The stellar surface turns into one titanic hydrogen bomb hurling a fireball out into space and propelling a formerly dim, obscure star system into prominence as a nova in our night skies. The ferocity of the expansion is breathtaking, engulfing a region the size of the Earth’s orbit within a day, and passing Jupiter’s orbit in less than two weeks,” explained study co-author Prof Peter Tuthill of the University of Sydney’s Institute for Astronomy.
The exploding nova in question, Nova Delphinus 2013 (also known as V339 Delphini or PNV J20233073+2046041), was first detected on 14 August 2013 by the Japanese amateur astronomer Koichi Itagaki.
Within 15 hours of discovery and within 24 hours of actual explosion, the astronomers pointed array telescopes, located at historic Mount Wilson Observatory in the San Gabriel Mountains of Southern California, toward the nova to image the fireball and measure it.
They measured Nova Delphinus 2013 on a total of 27 nights over two months; the first measurement represents the earliest size yet obtained for a nova event.
The measurement of angular expansion rate of the nova, combined with measurements of the expansion velocity from independent spectroscopic observations, allowed the astronomers to determine distance to the star – 14,800 light-years from our Sun.
Knowing the nova’s distance along with its angular size allowed the team to determine the fireball’s physical size at different times of observation.
Time evolution of the two-component model of Nova Delphini 2013; the time since the explosion (in days) is indicated in each panel; intensity refers to the flux per unit area. Image credit: G. H. Schaefer et al.
During the first observation on 15 August 2013, the fireball was roughly the size of Earth’s orbit.
Two days later, it was already the size of Mars’ orbit, and by day 12, the fireball surface would extend out to Jupiter’s orbit.
When last measured 43 days after detonation, it had expanded nearly 20-fold to nearly the size of Neptune’s orbit.
But it was the thermonuclear explosion back on the white dwarf’s surface that fueled this remarkable expansion rate of more than 600 km-per-second.
“The significance is that these new data allow us to study in detail exactly how the fireball evolves as the gas expands and cools. It seems like the ride is a lot more complicated and bumpy for the gas than the simple models used previously would have predicted,” said co-author Dr Theo ten Brummelaar of Georgia State University.
Perhaps most surprising of all, despite the fury of the detonation on the white dwarf’s surface, the star itself escapes relatively unscathed and continues to buzz around its host like a persistent mosquito accumulating more matter for a repeat performance at a future date.
“For Nova Delphinus 2013, an encore is unlikely to occur in our lifetimes; but the galaxy is rich with other systems like this one, awaiting their moment to shine anew in the night sky,” the scientists said.
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G. H. Schaefer et al. The expanding fireball of Nova Delphini 2013. Nature, published online October 26, 2014; doi: 10.1038/nature13834
