Look up at the sky on a dark night, and you’ll see hundreds of stars. But only a few will really stand out – have you ever wondered why?
For some, it’s simply because they’re quite close to Earth. For instance, Sirius is just 8.6 light years away – so, even though it’s a fairly average star (though still 25 times more luminous than our Sun) it appears as the brightest star in our sky.
But other stars appear bright because they really are. The second brightest star in the sky, Canopus, is one such star – 310 light years from Earth and some 15,000 times more luminous than the Sun.
Stars in this class are usually known as supergiants – they have the mass of ten or more Suns, and evolve in a very different way from lower-mass ‘Sun-like’ stars, living fast, squandering their nuclear fuel and dying young in spectacular supernova explosions. The most massive stars of all, containing many tens or even hundreds of solar masses of material, are hypergiants, the most extreme stars known.
“In astronomy I think there’s a natural tendency to be attracted to extremes,” explains Professor Paul Crowther from Sheffield University. “Whether that’s the most extreme by physical size, which are generally the cool red supergiants, or the most extreme by mass, which are the hottest and brightest blue hypergiants.”
And Crowther should know -he’s devoted much of his career to studying these stellar monsters, and in 2010 discovered the most extreme hypergiant so far, a stellar beacon 165,000 light years from Earth in the independent Large Magellanic Cloud galaxy an incredible 9 million times more luminous and 265 times more massive than the Sun.
Supergiants and hypergiants were first discovered through the theoretical tools of astronomy – in particular the Hertzsprung-Russell (H-R) diagram which allows astronomers to visualize the properties of stars en masse. However, the word ‘giant’ can be somewhat confusing, because in this case it combines concepts of mass and size. The largest stars by diameter can all be loosely defined as ‘red giants’ -an evolutionary phase that most stars pass through near the end of their lives, during which they swell tol huge diameters (often larger than Earth’s orbit around the Sun) and become far more luminous as they pump out more energy, but conversely turn red thanks to the coolness of their vast outer surfaces. The more massive a star is, the bigger it will grow as a red giant, and red supergiants with tens of solar masses (such as VY Canis Majoris, with a diameter larger than Jupiter’s orbit around the Sun) are indeed the largest stars of all. However, really monstrous heavyweight stars never actually reach this stage, so while the larger a red giant is, the more massive it will be, the most massive stars of all aren’t actually the largest.
The most massive stars are born at the heart of collapsing star-forming nebulas, where gas and dust are most readily available. Unlike the more sedate, Sun-like stars, which form around the edges and coalesce over many millions of years, these stellar heavyweights grow to their enormous proportions in just a hundred thousand years. The overall amount of raw material in the nebula (reflected in the size of the star cluster that emerges from it) also has a role to play.
“There seems to be a broad relationship between the total mass of a cluster, and the most massive star within it – so for instance the Orion Nebula has a mass of 1,000 Suns, and its most massive stars are about 30 times that of the Sun, while the NGC 3603 cluster has about 10,000 solar masses of material, and its most massive stars weigh around 100 solar masses. We don’t know quite why this ‘mass function’ is the way it is in young star clusters, but it seems to be a universal rule,” says Crowther
Competition between the massive central stars seems to act as a throttle to the formation process, ensuring that really massive stars are increasingly rare. “The next obvious question is whether if you had an even more massive cluster, would the mass of its biggest star keep going higher?” says Crowther. “And the answer seems to be no – we suspect there’s a limit and it’s linked to the star formation process. A star forms in a collapsing nebula full of competing stellar ‘seeds’, and it has a limited time to grab as much material as it can, or else its neighbours will. It’s a bit like throwing a handful of sweets into a crowd of children – the ones nearest the centre will grab most of them really quickly, while those at the edges hardly get any. It’s a competitive environment, and that probably puts an upper limit on how massive a star can get.”
Another major difference between normal and monster stars lies in the nuclear reactions that keep them shining. In low-mass stars, these reactions are dominated by the ‘proton-proton (p-p) chain’, a process in which individual hydrogen nuclei fuse together one reaction at a time, to eventually produce nuclei of helium, the next heaviest element. The p-p chain releases small amounts of energy at every step, but proceeds relatively slowly allowing Sun-like stars to keep shining for billions of years.
In more massive stars, however, another process called the CNO cycle becomes important. This fusion chain also converts hydrogen nuclei into helium, but it uses carbon nuclei as a sort of ‘catalyst’, allowing the reactions to happen at a much faster rate. The CNO cycle becomes increasingly dominant at higher temperatures and densities, and causes heavyweight stars to shine many thousands of times more brightly than their less massive neighbours. But the price for this brilliance is a drastically shortened life span – even though their cores contain much more nuclear fuel than those of Sun-like stars, massive stars exhaust themselves in just a few million years and begin to swell into supergiants or hypergiants.
This short life span means that supergiants are almost always found at the heart of newborn star clusters – these clusters disintegrate over millions of years, eventually scattering their longer-lived stars over a broad region of space, but supergiants simply don’t live long enough to make it out of their stellar nurseries.
“These stars are incredibly rare -they only form in a few places and have very short lifetimes, so even if you find a star cluster that’s just 5 million years old, its most massive stars will already have died,” says Crowther. “There’s only a handful of really young, massive clusters close enough to Earth for us to look for these guys and they’re losing mass at a terrific rate, so the mass we measure depends on just how old the stars happen to be. The places where you usually find these really massive clusters tend to have enhanced star formation rates, usually due to galactic collisions or interactions.”
So what do supergiants and hypergiants look like? The truth is that they’re surprisingly varied – while the H-R diagram might suggest that they’d all have extremely hot surfaces and appear blue in colour, in reality they range across the spectrum of colours. Supergiants show the most variety, and it seems that their colours simply reflect the precise balance between the inward pull of gravity and the outward pressure generated by its radiation at a particular phase in their lives. This balancing act, known as ‘hydrostatic equilibrium’ governs a star’s overall diameter and therefore its surface area: even highly luminous stars can display Sun-like yellow, or even cooler red surfaces if they are large enough for the heating effect of their escaping radiation to be thinly spread.
Most stars retain more or less the same mass throughout their lives, and therefore maintain the same gravity, so their equilibrium is mostly affected by changes to their luminosity as the nuclear reactions in their cores change and evolve – from this, we can work out that blue supergiants are still close to the ‘main sequence’ of stellar evolution, while yellow ones have begun to swell in size as they reach the end of their lives. Red supergiants are even further along their life cycle, and are the largest stars of all.
But for really massive hypergiant stars, there’s a different story. These stars never make it across to the red side of the H-R diagram – instead their brilliant radiation generates such huge pressure that it blows their outer layers away into space, exposing the interior and ensuring that such stars remain hot, maintaining blue or white-hot surfaces throughout their lives. This strong outflow of hydrogen-rich material gives itself away in a hypergiant’s spectrum and is one of the key means of distinguishing them from really bright supergiants.
The borderland between supergiants and hypergiants is filled with a strange variety of unusual stars, and no two astronomers really agree on the precise dividing lines between them. For example, luminous blue variables are extremely bright stars that show long, slow changes in brightness with occasional outbursts, and include both supergiant and hypergiant stars.
Most of the rare so-called ‘yellow hypergiants’, despite their name, actually seem to be red supergiants that are shedding their outer layers and heating up. And, as we’ve seen, astronomers also differ about whether red hypergiants even exist! Depending on their features displayed in their light, other categories of supergiant or hypergiant bear exotic names such as Wolf-Rayet stars and Ofpe stars.
However, until recently, the only certain means of weighing really massive stars, and identifying supergiants and hypergiants, was to pick them out in binary systems. Here, the orbital motions of the two stars can be used to calculate their masses. Fortunately, a recent breakthrough in modelling the behaviour of really high-mass stars promises to remove some of these limitations.
Supergiant and hypergiant stars live fast and die young, but what fate awaits them at the end of their lives? Once a star has exhausted the hydrogen fuel in its core, it has reached the end of its main sequence lifetime and can only continue to shine by burning hydrogen from the shell surrounding the core, and heavier elements in the core itself. These processes cause the dying star to brighten and swell, shifting it towards ‘red supergiant’ territory, while its core develops a complex layered structure of increasingly heavy elements. Each new phase of fusion produces less energy than the previous one, and is exhausted more quickly, but the radiation that continues to pour from the core still helps to support it against its own enormous gravity.
That all changes when the star attempts to fuse iron – the first element whose fusion absorbs energy. Abruptly, the star’s power supply falters and dies, and the huge weight of its outer layers comes crashing down. In what is known as a ‘core-collapse supernova’, the iron-rich core is compressed to a tiny size, while a tremendous shockwave rebounds through the remainder of the star, heating and compressing it until the whole star ignites in a blaze of nuclear fusion that may last for months and outshine a billion stars. As the supernova fades and the debris clears, the compressed remains of the core may be revealed as a super-dense neutron star, or even a black hole.
But, for the most massive stars of all, there may be a third option. “Theorists tell us that if a star dies with roughly 200 solar masses of material remaining, it could just blow up – it wouldn’t be the usual core-collapse event, but a ‘pair-instability supernova’, which would blow itself to bits before it could form a super-dense core. These things would be amazingly bright and there have been a few observations of events that might be this kind of ‘superluminous supernova’.”
So, while they may be rare, these monster stars are certainly making their presence felt – and interest is only likely to increase in the next few years. Astronomers believe that supergiants and hypergiants would have been far more widespread in the early universe, when the lack of heavy elements would have given them a more compact structure with a hotter surface. Thanks to the expansion of the universe, the ultraviolet radiation that poured from the surface of these superhot stars should now be stretched or ‘Doppler-shifted’ to infrared wavelengths. Here it should be visible to NASA’s James Webb Space Telescope when it launches in 2018 to give us our first view of the earliest stellar generations.