Astronomers have recorded the fastest explosion of a nova star ever seen.
They watched a white dwarf star “steal” gas from a nearby red giant and trigger an explosion bright enough that it could be seen from Earth with binoculars.
The nova explosion, called V1674 Hercules, occurred 100 light-years away on June 12 last year but lasted only a day – three times faster than any previous explosion.
A nova is a sudden outburst of bright light from a two star system. Each nova is created by a white dwarf – the very dense remnant of the star – and a nearby companion star.
Experts from Arizona State University hope their observation will help answer larger questions about the chemistry of our solar system, the death of stars and the evolution of the universe.
Astronomers have recorded the fastest explosion of a nova star ever seen. This illustration shows the type of two-star system that the research team believes V1674 Hercules to belong to
What is a white shell?
A white dwarf is the remnant of a smaller star that has run out of nuclear fuel.
While large stars—those with a mass more than ten times the mass of our sun—experience astonishingly violent climaxes like a supernova explosion at the end of their lives, smaller stars have escaped such dramatic fates.
When stars like the Sun reach the end of their lives, they run out of fuel, expand as red giants and later expel their outer layers into space.
All that remains is the very hot and dense core of the former star – the white dwarf.
White dwarfs have roughly the mass of the Sun but roughly the radius of Earth, which means they are incredibly dense.
The gravity on the surface of a white dwarf is 350,000 times the gravity on Earth.
It becomes very dense because its electrons collide together, resulting in the ‘degenerative substance’.
This means that the radius of the most massive white dwarf is smaller than its less massive counterpart.
The material was launched into space at millions of miles per hour – which was visible from Earth for just over 24 hours before fading away.
“It was like turning on a flashlight and turning it off,” said lead author Professor Sumner Starfield, from Arizona State University.
Novas are different from supernovae. They occur in binary systems where there is an incredibly dense small star and a larger companion, similar to the Sun.
Over time, the former derives matter from the latter, which is located on the white dwarf.
The white dwarf then heats up this material, causing an uncontrolled reaction that releases a burst of energy and fires the material away at high speeds, which we observe as visible light.
The bright nova usually fades in two weeks or more, but V1674 Hercules is gone in a day.
Professor Starrfield said: “It was about a day away, and the previous fastest nova was one we studied back in 1991, the V838 Herculis, which went down in about two or three days.”
Nova events at this level of speed are rare, which makes this nova a valuable subject of study.
Its speed wasn’t the only unusual feature – light and energy also sent out impulses like the reverberating sound of a bell.
Every 501 seconds, there is a detectable oscillation in the visible and X-ray light waves. There’s still a year left – and it’s set to last longer.
‘The most unusual thing is that this wobble was seen before the explosion,’ said Mark Wagner, chief science officer at the Big-Eyed Telescope Observatory at Mount Graham in southern Arizona.
But it was also evident when the nova was 10 degrees brighter. The mystery people are trying to contend with is what drives this periodicity that you might see above the bright band of the system.
The American team also noticed strange winds while observing the material emitted by the nova, which they believe may be dependent on the locations of the white dwarf and its companion star.
They seem to constitute the flow of material into the space surrounding the system that lies in the constellation Hercules.
It is a very convenient place, set in a dark sky in the east as the twilight fades after sunset.
Since this puts it less than 17 degrees north of the celestial equator, it can be seen from all over the world – and can be photographed with an exposure of just a few seconds.
Novae can tell us important information about our solar system and even the universe as a whole.
About 30 to 60 are thought to occur each year in the Milky Way, although only about 10 were discovered during that time. Most of them are obscured by interstellar dust.
A white dwarf collects and alters matter, then saturates the surrounding space with new matter when it transforms into a supernova.
It is an important part of the cycle of matter in space as the material ejected by novas will eventually form new star systems.
Events like these helped shape our solar system as well, ensuring that Earth isn’t just a lump of carbon.
White dwarfs are the incredibly dense remnants of sun-sized stars that have exhausted their nuclear fuel, shrinking to roughly the size of Earth (artist’s impression)
Professor Starfield said: “We are always trying to figure out how the solar system was formed, and where the chemical elements in the solar system came from.
One of the things we’ll learn from this supernova is, for example, the amount of lithium that this explosion produced.
“We’re fairly certain now that a significant portion of the lithium on Earth resulted from these types of eruptions.”
Sometimes a white dwarf star does not lose all the material collected during a nova explosion, so it gains mass with each cycle.
This will eventually make it unstable, and the white dwarf can generate a Type 1a supernova, one of the brightest events in the universe.
Each Type 1a supernova reaches the same level of brightness, so they are known as standard candles.
Co-author Professor Charles Woodward from the University of Minnesota said: “Standard candles are so bright that we can see them at great distances across the universe.
By looking at how the brightness of light changes, we can ask questions about how the universe is accelerating or about the overall three-dimensional structure of the universe. This is one interesting reason for our study of some of these systems.
In addition, novae can tell us more about how stars in binary systems evolve to their death, a process not well understood.
They also serve as living laboratories where scientists can see nuclear physics in action and test theoretical concepts.
The observable nova is now too faint to be seen by other types of telescopes, but it can still be observed with the big-eyed telescope thanks to its wide aperture and modern scanners.
Professor Starfield and colleagues now plan to investigate the cause, the processes that led to it, the reason for the record decline, and the forces behind the observed winds and pulsating brightness.
The note was posted on Research notes of the American Astronomical Society.
How are stars formed?
Stars form from dense molecular clouds – of dust and gas – in regions of interstellar space known as stellar nurseries.
A single molecular cloud, containing mainly hydrogen atoms, can be thousands of times the mass of the Sun.
They undergo turbulent motion as gas and dust move over time, disturbing atoms and molecules causing some areas to contain more matter than others.
If enough gas and dust gather in one area, it will begin to collapse under the weight of its own gravity.
As it begins to break down, it slowly gets hotter and expands outward, absorbing more of the surrounding gas and dust.
At this point, when the region is about 900 billion miles wide, it becomes the pre-stellar nucleus and the beginning process for becoming a star.
Then, over the next 50,000 years, that width will shrink by 92 billion miles to become the star’s inner core.
Excess material is ejected toward the star’s poles and a disk of gas and dust forms around the star, forming a protostar.
This material is then incorporated into the star or expelled into a wider disk that leads to the formation of planets, moons, comets and asteroids.
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