WITH hundreds of billions of stars in our galaxy alone, you would expect a few oddballs – and you would be right. Stars do follow a more or less set life path, determined by their mass. But we are increasingly finding that the details of those lives can diverge more than we ever imagined. In some cases, we are discovering stellar characteristics and habits so outlandish that they challenge our understanding of physics.
From a cannibalistic star to one that makes impossible elements and another that refuses to die, here is our introduction to some of the strangest stars in the universe.
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The one that got away
Our galaxy is leaking stars. That is the only conclusion astronomers have been able to draw from the discovery of a few dozen stars travelling so fast that not even the gravity of the Milky Way can hold on to them. The record holder is S5-HVS1, which clocks in at 1700 kilometres per second – so fast that it has already broken out into the lonely reaches of intergalactic space. But how has an enormous ball of gas accelerated to such a speed?
When Warren Brown at the Harvard-Smithsonian Center for Astrophysics identified the first hypervelocity star in 2005, it appeared to have come from the centre of the galaxy. That pointed the finger at the supermassive black hole there. Brown’s calculations showed that if a pair of stars passed close enough, the black hole would snatch hold of one of them and shoot the other off into space.
The plot thickened last year, when Monica Valluri at the University of Michigan led a team to investigate LAMOST-HVS1, the closest hypervelocity star to the sun. This one seems to have been ejected from a star cluster, which is odd, because there are no black holes in star clusters – or so we thought. But it would make sense if these clusters contained a suspected class of black holes, known as intermediate mass black holes, that have so far evaded definitive detection. “It was very surprising,” says Valluri. “If we find more hypervelocity stars coming from star clusters, then we really need to investigate what’s sitting at the centre of those clusters,” she says.
Perhaps the most intriguing thing about hypervelocity stars, however, is what they could reveal about dark matter. This is the hypothetical substance thought to hold galaxies together. By studying the trajectories of the hypervelocity stars ejected from the galactic centre, astronomers hope to deduce the shape of the dark matter cloud surrounding the Milky Way. That could give us a better idea about how dark matter interacts with itself, providing a vital new clue to its identity.
The star that makes “impossible” elements
What it lacks in vowels, Przybylski’s star (pronounced shi-BILL-skee) makes up for in intrigue, being the only star suspected to point to a radical extension of atomic physics.
The mystery began in 1961, when astronomer Antoni Przybylski discovered something peculiar about the spectrum of light from star HD 101065. Spectra are nature’s barcodes. They feature dark lines superimposed on a spectrum of colours, with the positions of the lines relative to the colours telling us about the star’s chemical constituents. Ordinarily, the patterns are easy enough to decipher, but in the case of Przybylski’s star, the sheer number of lines makes it difficult to identify anything with certainty.
In 2008, a team of astronomers claimed some of the spectral lines from Przybylski’s star from a group of heavy radioactive elements known as the actinides. That was startling because, although actinides can be produced in particle accelerators, the heavier members of the group weren’t thought to appear in nature. Not only are they extremely difficult to produce, they decay quickly too. Einsteinium, for instance, has a half-life of just under 472 days. So any such elements should be long gone from stars, unless there is something to replenish them.
As a result, some astronomers suspect the weird spectra from Przybylski’s star could be the first evidence for an “island of stability” – a fabled place beyond the existing periodic table where superheavy elements long thought impossible in nature are stable enough to persist, perhaps for tens of millions of years.
The only place where these superheavy elements might be created is within supernovae, the explosive death throes of certain stars. With that in mind, Vladimir Dzuba at the University of New South Wales in Australia and his colleagues suggested in 2017 that from a nearby supernova could have triggered the formation of Przybylski’s star and loaded it with superheavy elements. The idea is that those elements are decaying slowly, ensuring the star’s atmosphere contains an abundance of decay products such as einsteinium. “It’s a beautifully elegant solution,” says Jason Wright at Pennsylvania State University, before pointing out a major caveat: the original identification of einsteinium may not be secure.
The reason for suspicion is that at 6600 kelvin, the surface temperature of Przybylski’s star is hotter than most other stars, and its atmosphere is likely to be hotter still. In these conditions, atoms have their outer electrons stripped away, turning them into ions, and this changes the pattern of spectral lines they emit. No lab has compiled a complete catalogue of the spectral lines emitted by such ions, so what we think is einsteinium could well be a familiar element that has been misidentified.
The one that refuses to die
A giant star blows itself to smithereens somewhere in the universe every single day. These supernovae are the final act of a massive star’s life, and they occur only when a star runs out of fuel for its nuclear reactions. By definition, it can only happen once to any given star. Imagine the shock, then, when Iair Arcavi, then at Las Cumbres Observatory in California, found a star that seemed to explode over and over again.
Supernova iPTF14hls was discovered in 2014. Over the course of almost two years, Arcavi and his colleagues watched iPTF14hls flare up no less than five times. Archive images from the Palomar Observatory in California show that the star could well have been exploding as early as 1954, but that it wasn’t in 1993, when another image of its host galaxy was taken.
Perhaps the strangest thing was that analysis of the light coming from the star always suggested it was a completely normal, freshly exploded supernova, even hundreds of days into the explosion. Then, just when the researchers were getting used to its persistence, . “It’s the weirdest supernova we’ve ever seen,” says Arcavi, now at Tel Aviv University in Israel.
There is currently no conclusive explanation. The best stab so far is that the star is so enormous that its interior is hot enough to form antimatter, triggering an explosion. Arcavi’s team suggests that this process has happened several times in the past, leading not to the complete destruction of the star, but to a series of eruptions in which shells of gas were ejected into space. The most recent explosion then blasted out a final shock wave of material that caught up and collided with the other ejected shells one after another, creating the fresh-looking flares observed from Earth.
The trouble is, the first shell released should carry away all the hydrogen gas. Yet in the case of iPTF14hls, every time the light peak rose, the team saw the chemical signature of this element. To figure it out, Arcavi wants to find similar supernovae. But perhaps he should just wait, because the 2014 explosion may not be the last. “Maybe it’s still there, and it will surprise us again in 10, 50 or 60 years’ time,” he says.
The oldest star in the cosmos?
There is a star that seems to be older than the universe, unless the universe is older than we thought. HD 140283, around 200 light years from Earth, is made almost entirely of hydrogen and helium. This suggests it is one of the very earliest stars to form because stars reveal their approximate ages by the amount of “metals” – the astronomical term for any element heavier than helium – they contain.
“Stars can only go supernova once – but one seems to explode again and again”
140283’s age come in at around 14.46 billion years, whereas the universe is thought to be around 13.8 billion years old. And there must have been at least one previous generation of stars to have produced the metals in this star. The upshot is that either the cosmology used to calculate the universe’s age or the stellar astrophysics used to calculate the star’s age is wrong.
Howard Bond at the Space Telescope Science Institute in Maryland, who did the estimations, thinks they could be out by 800 million years either way. That could bring the star’s age down to fit within the age of the universe, but it would leave precious little time for the first generation of stars to live and die. All of which goes to show how little we know about the early days of the cosmos.
The one so big it would engulf Jupiter
When it comes to deciding on the biggest star in the galaxy, there are some gargantuan contenders. In 2013, astronomers measured three red supergiant stars. They found that all three were 1000 times larger than the sun, but the winner was UY Scuti.
This one is actually a hypergiant, because estimates suggest it is 1708 times larger than the sun. If it were to replace our star, it would reach most of the way to Saturn’s orbit. And talk about bright: UY Scuti is 300,000 times more luminous than the sun.
Its status as the biggest of the big is disputed, however. Another group of astronomers thinks the star is much closer to Earth than is generally thought. If so, UY Scuti must be smaller, perhaps just 825 times the sun’s size.
Even at its largest estimate, UY Scuti could still be dwarfed by the star WOH G64, which resides in a nearby galaxy, the Large Magellanic Cloud. Estimates place it somewhere , although its greater distance from us makes it even harder to measure accurately.
The star that eats its twin
Algol is one of the only variable stars in the night sky noticeable to the naked eye. Every 2.867 days, it fades for 10 hours, then returns to normal – a wink that led the ancient Greeks to draw the constellation of Perseus around it and label Algol the eye of the snake-haired Medusa.
We now understand that Algol is part of a system in which two stars regularly eclipse each other, but the 2.867-day orbit still presents a puzzle. To travel that fast around each other, the stars should be so close that one pulls gas from the other. This would slowly push the pair away from one another, lengthening the orbit, but astronomers haven’t seen any sign of this.
It turns out the ancients may provide the answer. Sebastian Porceddu at the University of Helsinki in Finland believes that the ancient Egyptians based some of their calendar’s unlucky days on Algol’s eclipses. – an amount that could easily be explained by one star pulling gas from the other.
