The fastest nova star explosion ever seen has been recorded by astronomers.
They saw a white dwarf star “steal” gas from a nearby red giant and trigger an explosion bright enough to be seen from Earth with binoculars.
Named V1674 Hercules, the nova explosion occurred 100 light-years away on June 12 last year, but lasted just a day – up to three times faster than any previously seen .
A nova is a sudden burst of bright light from a two-star system. Each nova is created by a white dwarf – the very dense residual core of a 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.
The fastest nova star explosion ever seen has been recorded by astronomers. This illustration shows the type of two-star system the research team believes V1674 Hercules belongs to
WHAT IS A WHITE DWARF?
A white dwarf is the remnant of a small star that has run out of nuclear fuel.
While large stars – those that exceed ten times the mass of our sun – experience a spectacularly violent climax in the form of a supernova explosion at the end of their lives, small stars are spared such a dramatic fate.
When stars like the sun come to the end of their lives, they exhaust their fuel, grow into red giants, and later expel their outer layers into space.
The hot, very dense core of the old star – a white dwarf – is all that remains.
White dwarfs contain approximately the mass of the sun but have about the radius of the Earth, which means they are incredibly dense.
The surface gravity of a white dwarf is 350,000 times that of Earth’s gravity.
They become so dense because their electrons are crushed together, creating what caused “degenerative matter”.
This means that a more massive white dwarf has a smaller radius than its less massive counterpart.
Material was hurled into space at speeds of millions of miles per hour – which was visible from Earth for just over 24 hours before fading away.
Lead author Professor Sumner Starrfield of Arizona State University said: “It was like someone was turning a flashlight on and off.”
Novas differ from supernovas. They occur in binary systems where there is an incredibly dense small star and a much larger solar companion.
Over time, the first draws matter from the second, which falls on the white dwarf.
The white dwarf then heats this material, causing an out-of-control reaction that releases a burst of energy and shoots the material out at high speed, which we observe as visible light.
The bright nova usually fades in a few weeks or more, but V1674 Hercules was over in a day.
Professor Starrfield said: “It was only about a day, and the previous fastest nova was the one we studied in 1991, V838 Herculis, which decayed in about two or three days.”
Nova events at this level of speed are rare, making this nova a valuable subject of study.
His speed wasn’t his only unusual trait – the light and energy sent out also pulsed like the sound of a reverberating bell.
Every 501 seconds, there’s a detectable oscillation in visible light waves and X-rays. It’s still there a year later – and expected to continue for even longer.
Mark Wagner, chief scientist at the Large Binocular Telescope Observatory on Mount Graham in southern Arizona, said: “The most unusual thing is that this oscillation was observed before the explosion.
“But it was also evident when the nova was about 10 magnitudes brighter. One mystery people are trying to wrestle with is what drives this periodicity at which you would see it over this range of brightness in the system.
The US team also noticed a strange wind as they monitored material ejected by the nova, which they believe may depend on the positions of the white dwarf and its companion star.
They seem to shape the flow of matter in the space surrounding the system which is in the constellation of Hercules.
It is very well placed, being in dark skies to the east as twilight fades after sunset.
As this places it less than 17° north of the celestial equator, it could be seen from anywhere in the world – and photographed with just a few seconds of exposure.
Novae can give us important information about our solar system and even the universe as a whole.
About 30 to 60 are believed to occur each year in the Milky Way, although only about ten are discovered during this time. Most are obscured by interstellar dust.
A white dwarf collects and modifies matter, then seasons the surrounding space with new material as it goes nova.
It is an important part of the cycle of matter in space because materials ejected by novae will eventually form new star systems.
Such events also contributed to the formation of our solar system, ensuring that Earth is more than a lump of carbon.
White dwarfs are the incredibly dense remnants of sun-sized stars after exhausting their nuclear fuel, shrunk to about the size of Earth (artist’s impression)
Professor Starrfield said: ‘We are still trying to understand how the solar system formed, where the chemical elements of the solar system came from.
“One of the things we are going to learn from this nova is, for example, the amount of lithium that was produced by this explosion.
“We are pretty sure now that a significant fraction of the lithium we have on Earth was produced by these types of explosions.”
Sometimes a white dwarf star does not lose all of its collected matter in a nova explosion, so with each cycle it gains mass.
This would eventually make it unstable and the white dwarf could generate a type 1a supernova, which is one of the brightest events in the universe.
Each type 1a supernova reaches the same level of brightness, so they are called standard candles.
Co-author Professor Charles Woodward of the University of Minnesota said: ‘Standard candles are so bright that we can see them from great distances across the universe.
“By observing 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 of the interesting reasons why we study some of these systems.
Additionally, novae can tell us more about how stars in binary systems evolve until they die, a process that is not well understood.
They also act as living laboratories where scientists can see nuclear physics in action and test theoretical concepts.
The observed nova is now too faint to be seen by other types of telescopes, but it can still be monitored by the Large Binocular Telescope thanks to its large aperture and advanced scanners.
Professor Starrfield and his colleagues now plan to investigate the cause, the processes that led to it, the reason for its record decline, the forces behind the observed wind and pulsed luminosity.
The observation was published in the American Astronomical Society Research Notes.
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, which contains mostly hydrogen atoms, can be thousands of times the mass of the sun.
They experience turbulent motion with the gas and dust moving over time, disrupting the atoms and molecules, causing some regions to have more matter than other parts.
If enough gas and dust collect in an area, it begins to collapse under the weight of its own gravity.
As it begins to collapse, 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 kilometers, it becomes a pre-stellar core and the starting process to become a star.
Then, over the next 50,000 years, it will contract 92 billion miles to become the inner core of a star.
Excess material is ejected toward the star’s poles and a disc of gas and dust forms around the star, forming a protostar.
This material is then either incorporated into the star or expelled into a larger disk which will lead to the formation of planets, moons, comets and asteroids.
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