White dwarfs – the stellar remains of long-dead stars – ‘appear more youthful than they actually are’, a new study claims.
Thanks to data from the Hubble Space Telescope, astronomers have discovered the first evidence that white dwarfs can slow down their rate of ageing by burning hydrogen on their surface.
The experts compared cooling white dwarfs in two massive collections of stars – the globular clusters M3 and M13.
Around 70 per cent of all white dwarfs in M13 have an outer envelope of hydrogen, allowing them to burn for longer and hence cool more slowly, they found.
White dwarfs are the incredibly dense remains of sun-sized stars after they exhaust their nuclear fuel, shrunk down to roughly the size of Earth.
Roughly 98 per cent of all the stars in the universe will ultimately end up as white dwarfs, including our own Sun.
The experts compared cooling white dwarfs in two massive collections of stars – the globular clusters M3 and M13. Globular clusters are dense balls of about one million ancient stars, all bound by gravity. This image shows a wide-field view of M3. ‘Essentially zero’ of all white dwarfs in M13 are slow-burning, the researchers reveal
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 – those exceeding ten times the mass of our sun – suffer a spectacularly violent climax as a supernova explosion at the ends of their lives, smaller stars are spared such dramatic fates.
When stars like the sun come to the ends of their lives they exhaust their fuel, expand as red giants and later expel 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 approximately the mass of the sun but have roughly the radius of Earth, meaning they are incredibly dense.
The gravity on the surface of a white dwarf is 350,000 times that of gravity on Earth.
They become so dense because their electrons are smashed together, creating what’s caused ‘degenerative matter’.
This means that a more massive white dwarf has a smaller radius than its less massive counterpart.
According to the European Space Agency, the study challenges the prevalent view of white dwarfs as inert, slowly cooling stars.
‘We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity,’ said study author Jianxing Chen at the University of Bologna and the Italian National Institute for Astrophysics.
‘This was quite a surprise, as it is at odds with what is commonly believed.’
White dwarfs are common objects in the cosmos. They’re the slowly cooling stars that have cast off their outer layers during the last stages of their lives.
Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages.
Researchers looked at clusters M3 and M13, as they share many physical properties such as age and ‘metallicity’ – elements other than hydrogen and helium.
But the populations of stars that will eventually give rise to white dwarfs are different in the two clusters.
In particular, the overall colour of stars at an evolutionary stage known as the Horizontal Branch are bluer in M13, indicating a population of hotter stars.
This makes M3 and M13 together a ‘perfect natural laboratory’ in which to test how different populations of white dwarfs cool.
‘The superb quality of our Hubble observations provided us with a full view of the stellar populations of the two globular clusters,’ said Chen. ‘This allowed us to really contrast how stars evolve in M3 and M13.’
Using Hubble’s Wide Field Camera 3, the team observed M3 and M13 at near-ultraviolet wavelengths, allowing them to compare more than 700 white dwarfs in the two clusters.
To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity but the populations of stars which will eventually give rise to white dwarfs are different. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool
This image shows a wide-field view of M13, a globular cluster. Around 70 per cent of all white dwarfs in M13 are slow-burning, according to the experts
They found that M3 contains standard white dwarfs that are simply cooling stellar cores.
WHAT IS METALLICITY?
Astronomers use the word ‘metallicity’ to describe the proportion of a star which is composed of elements other than hydrogen and helium.
The vast majority of matter in the universe is either hydrogen or helium.
To take the Sun as an example, 74.9 per cent of its mass is hydrogen, 23.8 per cent is helium and the remaining 1.3 per cent is a mixture of all the other elements, which astronomers refer to as ‘metals’.
M13, on the other hand, contains two populations of white dwarfs – standard white dwarfs and those that have managed to hold on to an outer envelope of hydrogen, allowing them to burn for longer.
Comparing their results with computer simulations of stellar evolution in M13, the team found roughly 70 per cent of the white dwarfs in M13 are burning hydrogen on their surfaces, slowing down the rate at which they are cooling.
M3, meanwhile, has ‘essentially zero’ slow burning white dwarfs, according to study author Professor Francesco Ferraro, also at the University of Bologna and the Italian National Institute for Astrophysics.
‘At the moment, no stellar systems with 100 per cent slow white dwarfs is known,’ he told MailOnline.
This discovery could have consequences for how astronomers measure the ages of stars in the Milky Way.
The evolution of white dwarfs has previously been modelled as a predictable cooling process, according to the team.
This relatively straightforward relationship between age and temperature has led astronomers to use the white dwarf cooling rate as a natural clock to determine the ages of star clusters, particularly globular and open clusters.
NASA image shows the Hubble Space Telescope floating against the background of space
However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as 1 billion years – unless other methods of ageing stellar systems are used.
‘Our work suggests to use caution in adopting white dwarf cooling sequence as a clock,’ said Professor Ferraro.
‘Of course this can affect the age of any stellar system whose age is based exclusively on the white dwarf cooling sequence.
‘Fortunately, white dwarfs are not the most used clocks to measure the age of stellar systems.’
Our Sun will become a red giant in about five billion years before it eventually shrinks down into a compact white dwarf.
When this happens, Professor Ferraro said our Sun will be a normal white dwarf – not a slow-burning one.
Slow-burning white dwarfs are essentially generated by low-mass, low-metallicity progenitor stars, he said, so there’s ‘no possibility of it to get a slow ageing’.
The research team are now investigating other clusters similar to M13 ‘to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly’, according to Professor Ferraro.
‘Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old,’ he said.
The study has been published in Nature Astronomy.
SORRY EARTHLINGS: OUR SUN WILL BECOME A RED GIANT IN ABOUT 5 BILLION YEARS BEFORE SHRINKING DOWN TO A COMPACT WHITE DWARF
The Sun is only 4.6 billion years through its roughly 10-billion-year lifetime.
When hydrogen fuel at the centre of a star is exhausted, nuclear reactions will start move outwards into its atmosphere and burn the hydrogen that’s in a shell surrounding the core.
As a result, the outside of the star starts to expand and cool, turning much redder.
Over time, the star will change into a red giant and grow to more than 400 times its original size.
As they expand, red giants engulf some of their close-orbiting planets. In the Sun’s case, this will mean the fiery end of all the inner planets of our Solar System, which might also include the Earth.
But don’t worry – this won’t happen for another 5,000,000,000 years.
Once swelled into a red giant, engulfing the inner planets and searing the Earth’s surface, it will then throw off its outer layers, and the exposed core of the Sun will be left as a slowly cooling white dwarf.
This stellar ember will be incredibly dense, packing a large fraction of the mass of the Sun into a roughly Earth-sized sphere.
Source: ESA/National Schools’ Observatory