The fastest nova star explosion ever observed has been recorded by astronomers.
They watched as a white dwarf star “stole” gas from a nearby red giant and set off an explosion bright enough to be seen with binoculars from Earth.
The nova explosion, dubbed V1674 Hercules, happened on June 12 last year 100 light-years away but lasted only a day — up to three times faster than previously observed.
A nova is a sudden explosion of bright light from a two star system. Each nova is created by a white dwarf – the very dense remaining core of a star – and a nearby companion star.
Arizona State University experts hope their observation will help answer bigger questions about the chemistry of our solar system, the death of stars and the evolution of the universe.
The fastest nova star explosion ever observed has been recorded by astronomers. This illustration shows the type of two-star system to which the research team believes V1674 Hercules belongs
WHAT IS A WHITE DWARF?
A white dwarf is the remains of a smaller star that has run out of nuclear fuel.
While large stars – more than ten times the mass of our sun – undergo a spectacularly violent climax at the end of their lives as a supernova explosion, smaller stars are spared such a dramatic fate.
When stars like the sun come to the end of their lives, they exhaust their fuel, expand like red giants, and later eject their outer layers into space.
The hot and very dense core of the former star – a white dwarf – is all that remains.
White dwarfs contain about the mass of the sun, but have about the radius of the Earth, meaning they are incredibly dense.
The gravity on the surface of a white dwarf is 350,000 times that of the gravity on Earth.
They become so dense because their electrons are knocked together, creating “degenerative matter.”
This means that a more massive white dwarf has a smaller radius than its less massive counterpart.
Material shot into space at speeds of millions of miles per hour — something visible from Earth for just over 24 hours before extinguishing.
Lead author Professor Sumner Starrfield of Arizona State University said, “It was like turning a flashlight on and off.”
Novas are different from supernovas. They occur in binary systems with a small, incredibly dense star and a much larger Sun-like companion.
Over time, the former draws matter from the latter, which falls on the white dwarf.
The white dwarf then heats this material, causing an uncontrolled reaction that releases a burst of energy and blasts the matter at high speeds, which we perceive 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 one day, and the previous fastest nova was one we studied in 1991, V838 Herculis, which faded in about two or three days.”
Nova events at this speed level are rare, making this nova a precious subject of study.
Speed wasn’t the only unusual property – the light and energy emitted also pulsed like the sound of a ringing bell.
Every 501 seconds there is a fluctuation in visible light waves and X-rays. It’s still there a year later – and will be even longer.
Mark Wagner, chief of science at the Large Binocular Telescope Observatory on Mount Graham, in southern Arizona, said: “The most unusual thing is that this oscillation was seen before the eruption.
But it was also clear when the nova was about 10 magnitudes brighter. One mystery that people struggle with is what drives this periodicity that you would see it across that range of brightness in the system.”
The US team also noticed a strange wind when they monitored the matter ejected from the nova, which they think depends on the positions of the white dwarf and its companion star.
They appear to be the flow of material in space around the system that lay in the constellation Hercules.
It is very conveniently located, in a dark east sky as the twilight fades after sunset.
Being less than 17° north of the celestial equator, it can be seen from all over the world – and photographed with an exposure time of just a few seconds.
Novae can tell us important information about our solar system and even about the universe as a whole.
It is thought that about 30 to 60 exist in the Milky Way each year, although only about 10 are discovered during that time. Most are obscured by interstellar dust.
A white dwarf collects and changes matter, then seasons the surrounding space with new material when it becomes nova.
It is an important part of the cycle of matter in space, as the materials ejected from novae will eventually form new stellar systems.
Such events also contributed to the formation of our solar system, making the Earth more than a lump of carbon.
White dwarfs are the incredibly dense remnants of stars the size of the Sun after they have exhausted their nuclear fuel, shrunk to about the size of Earth (artist’s impression)
Professor Starrfield said: ‘We are always trying to figure out how the solar system came into being, where the chemical elements in the solar system come from.
‘One of the things we’re going to learn from this nova, for example, is how much lithium was produced by this explosion.
“We’re pretty sure now that a significant amount of the lithium we have on Earth was produced by these types of explosions.”
Sometimes a white dwarf star doesn’t lose all of its accumulated matter during a nova explosion, so it gains mass with each cycle.
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 brightness level, which is why 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 at great distances in 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. That’s one of the interesting reasons we’re studying some of these systems.’
In addition, novae can tell us more about how stars in binary systems evolve towards their deaths, a process that is not well understood.
They also act as living labs where scientists can see nuclear physics in action and test theoretical concepts.
The observed nova is now too faint for other types of telescopes to see, but can still be tracked by the Large Binocular Telescope thanks to its wide aperture and state-of-the-art scanners.
Professor Starrfield and colleagues now plan to investigate the cause, the processes that led to it, the reason for the record-breaking decline, the forces behind the perceived wind and the pulsating brightness.
The sighting was published in the Research Notes of the American Astronomical Society.
HOW DO STARS FORM?
Stars are formed from dense molecular clouds — of dust and gas — in regions of interstellar space known as stellar nurseries.
A single molecular cloud, containing mostly hydrogen atoms, can be thousands of times the mass of the sun.
They undergo turbulent motions where the gas and dust move over time, disrupting the atoms and molecules, causing some regions to contain more matter than others.
When enough gas and dust converge in one area, it begins to collapse under the weight of its own gravity.
As it begins to collapse, it slowly gets hotter and expands outward, taking in more of the surrounding gas and dust.
At this point, when the region is about 900 billion miles wide, it becomes a pre-stellar core and begins to become a star.
Then in the next 50,000 years this will contract 92 billion miles to become the inner core of a star.
The excess material is ejected toward the star’s poles and a disk of gas and dust is formed around the star, forming a protostar.
This matter is then either absorbed into the star or ejected into a wider disk that will lead to the formation of planets, moons, comets and asteroids.