It was August 14 last year when suddenly an explosion lit up a corner of the universe 14,800 light years from the sun.
Like an interstellar hydrogen bomb, the fiery mass unfurled into the dark sky from its origin in a dying star. Over the span of 43 days, it grew from the size of Earth’s orbit to the size of Neptune’s orbit, expanding at a velocity of more than 600 kilometers per second.
Back on Earth, Japanese astronomer Koichi Itagaki was the first to train his telescope on the expanding fireball, which to the naked eye would have looked like a new star suddenly twinkling in the sky. Word quickly spread and scientists in California began observing the newly named Nova Delphini 2013, too.
Thanks to advancements in technology and a bit of pure luck, the astronomers were able to capture the thermonuclear explosion earlier than ever before, enabling them to gather new information about how the material is bursting into space and how fast.
The astronomers’ conclusions were published in this month’s issue of Nature and the author list includes Flagstaff resident and Lowell astronomer Gerard van Belle.
Alhough he didn’t observe this fireball, van Belle helped with the editing and analysis involved in writing the paper. He’s an expert on the topic because the Navy Precision Optical Interferometer at Lowell’s Anderson Mesa site is incredibly similar to the telescope that observed Nova Delphini. The system combines light captured from six telescopes positioned across an area the size of a football field, allowing it to produce the same resolution as a telescope hundreds of meters in diameter. According to Georgia State University, a telescope like this located in Los Angeles would have the power to resolve an object the size of a nickel sitting atop Paris’ Eiffel tower.
Van Belle is one of just a few hundred astronomers in the world who can operate this type of telescope. In an interview last week, he explained the complex process of the expanding fireball with the ease of a seasoned teacher.
The process begins with an old star known as a white dwarf that is about the size of Earth and has the mass of the sun. In other words, a single teaspoon of its dense matter would weigh more than five tons. White dwarfs commonly exist with a companion star that is going through its own aging process that involves swelling up and dumping hydrogen-rich gas that the denser, older star drags onto its surface.
When this layer of hydrogen gets a few hundred meters deep, it triggers a thermonuclear explosion that scientists liken to an interstellar H-bomb.
“The material that got exploded off the surface of the star goes gushing out into interstellar space,” van Belle said. “You could think of it as polluting the interstellar medium.”
The fireball rapidly expands but eventually the dust and gas get cooler, condense and begin to decline in brightness and the star fades back into blackness. The white dwarf isn't freed from its hydrogen gas blanket for long, though, because after the explosion subsides the companion star begins spilling gas onto its smaller neighbor again.
The entire fireball process takes place over a few dozen days, which is “intrinsically interesting” to scientists because it’s a rare event in a death of a star that happens on a human time scale versus millions of years, van Belle said.
Lowell Observatory director Jeff Hall echoed those thoughts.
“You look up at the sky at night and see the same constellations night after night and it seems like stars are always in the same place when in fact the universe is a very dynamic place,” he said. “This is one of those things that is cool because you can see it expanding.”
It was also significant that scientists were able to observe such a large part of the fireball process because they don’t come along very often. In our entire galaxy, which is what astronomers can currently cover, these fireballs only happen about once a year, van Belle said.
And while they hardly affect life on Earth, these explosions are actually tiny sources of the interstellar material that eventually forms into new stars and planets, he said.
“You want to know how the interstellar medium is being polluted by these things and how it forms the material going into stars and planets,” van Belle said. “We’re putting together the details about the galactic ecology.”
Soon enough Lowell will be able to do more boasting about its interferometer. The Navy, which owns the telescope and contracts out its operations to Lowell, will be installing four telescopes with bigger, 72-inch mirrors that will allow detection of even fainter objects in the sky.
At that point, Hall said, the Navy Precision Optical Interferometer will be the most powerful instrument of its kind in the world.