IMPOSSIBLE STELLAR GIANT

Some 165,000 light years from Earth, the heaviest star in the known universe lies at the heart of a brilliant star cluster – a rare type of blue hypergiant with a mass of around 230 Suns

Just how big can a star get? The biggest stars in terms of diameter are red supergiants J that swell to enormous sizes as their lives come to an end. As they begin to exhaust their fuel and go through internal changes, these stars brighten, swelling in size as their surfaces become cooler and redder. But if by defining the biggest star you simply mean the most massive, the answer is very different. The most monstrous stars of all are hypergiants, with many times the mass of the Sun. The most massive of all was discovered in a neighbouring galaxy of the Milky Way in 2010 – a hypergiant star with up to 230 times the mass of the Sun, called R136a1.

Fittingly, this stellar bruiser is a resident of the largest star-forming nebula in our Local Group – the Tarantula Nebula in the Large Magellanic Cloud (LMC). Looking at first glance like a detached portion of the Milky Way in far southern skies, the LMC is one of the largest and brightest of several satellite galaxies trapped in billion-year orbits around the Milky Way. Huge tidal forces are compressing its copious reserves of gas and dust to trigger the birth of new stars at a much faster rate than in our own galaxy, giving rise to the Tarantula Nebula.

Within this region, some 650 light years across, radiation from newborn stars excites the surrounding gas to glow intensely. The Tarantula Nebula is famously so large and luminous that if it were transplanted to the location of the Orion Nebula, just 1,350 light years away, it would cover an area several times that of the full Moon and be bright enough to cast shadows at night. Inside the nebula, stars are born in waves as huge knots of gas condense in a runaway chain reaction. Radiation from the first newborn stars in any given region creates shock waves that ripple through the surrounding nebulosity and trigger its collapse into more stars. At the same time, the pressure of radiation blows away any material left unclaimed by other nascent stars. This produces distinctive caverns of star formation with newborn open star clusters at their centre.

The Tarantula Nebula has already gone through this process at least twice, producing two generations of young stars. The older group is about 25 million years old and now lies 150 light years from the centre of the nebula. It contains a couple hundred stars – about 40 of which have already exploded as supernovae. The younger R136 cluster at the very centre of the nebula, meanwhile, is thought to be just 2 million years old. This means that even its heaviest stars have not yet had time to age and die, and it’s this youth that makes R136 a rich hunting ground for astronomers in search of the heaviest stars in the universe.

WHAT IF?

If R136a1 were as close as our Sun, it would outshine it by as much as the Sun outshines the Moon

Although its powerful gravity prevents it from growing quite as large as some red supergiants, R136a1 is still a mightily impressive star. If it took the Sun’s place at the heart of our Solar System, it would stretch halfway to the orbit of Mercury and appear 35 times larger than the Sun in our skies. Its powerful gravity, meanwhile, would require the planets to orbit much faster in order to maintain their current locations – Earth would have to move along its orbit at roughly 500 kilometres (311 miles) per second, over 16 times its current average speed. More dramatically, the searing radiation from our new star would strip away our planet’s atmosphere and instantly boil Earth’s oceans off into space, making life on our planet impossible.

INSIDE THE GIANT

A high mass and compact size turn R136a1 into a superhot stellar behemoth

With a mass of up to 230 Suns, R136a1 is so huge that astronomers were shocked by its discovery – it’s almost twice as massive as any known star in the Milky Way. R136a1 is surprising because it comes close to breaking the rules of stellar physics. For over half a century we’ve understood that the heavier a star is, the brighter it shines during its main sequence lifetime. This is the stable period of its life where it generates energy by nuclear fusion of hydrogen to form helium in its core. This mass-luminosity relationship is driven by the two different processes involved in fusion. Even though R136a1 has much more hydrogen fuel than the Sun, it’s squandering it much more quickly and will reach the end of its life after around 5 million years.

Astronomers believe this puts a natural upper limit on stellar mass – a star with the mass of R136a1 should generate such fierce radiation that it would simply blow itself apart, even as it is formed. However, this giant has challenged conventional stellar theories, causing scientists to change the rules – it simply shouldn’t exist within the previous laws. Instead, it now seems that stellar behemoths up to 300 solar masses are capable of holding themselves together through their powerful gravity, which keeps

In its final days, R136a1 may become more unstable and prone to violent eruptions as it evolves into a luminous blue variable star similar to Eta Carinae, pictured here the outer layers close to the star rather than forcing them to balloon outwards.

This makes radical changes to the other properties of such hypergiants. The amount of energy blasting its way out through a relatively small surface area heats them to searing temperatures, and this creates a powerful stellar wind as hot gas from the surface blows away into space. This effect enables stars like R136a1 to shed an entire solar mass of material every few hundred thousand years. This exposes more of its hot internal layers at the surface, which only serves to strengthen the wind further.

The result is an extreme Wolf-Rayet star whose exposed surface exceeds 50,000 degrees Celsius (90,000 degrees Fahrenheit). Although astronomers have not yet obtained information about R136a1’s composition from a spectrum of its light, evolutionary models show that as R136a1 approaches the end of its life, perhaps 2 million years from now, it will develop a complex layered structure thanks to internal changes, which enable it to keep shining.

Ultimately, it will die in a huge supernova, but the precise details of this event are still uncertain. It may, like less massive stars, keep shining to the bitter end, becoming unstable as it burns heavier and heavier elements. Eventually, a doomed attempt to generate energy from the fusion of iron would tip it into a Type II supernova. This cataclysm would be one of the most violent ever seen and would leave behind a massive black hole. Alternatively, R136a1’s intense energy could trigger a rare event known as a pair-instability supernova, in which the creation of antimatter in the star’s core triggers a sudden drop in pressure and a premature core collapse. This triggers runaway nuclear reactions that blow the star apart, scattering its material across surrounding space.

OBSERVING R136A1

Despite its brilliance, this massive star is challenging to observe thanks to its crowded surroundings

Seen across 165,000 light years of space, R136a1’s brilliant light dwindles to an apparent magnitude well below naked-eye visibility at +12.3. Nevertheless, it remains within reach of all but the smallest telescopes. The real problem lies in its proximity to other stars. In the early 1980s, astronomers speculated that the central object might be a single bright star with a mass of 1,500 Suns, but many doubted that such a monster could exist. They were proven right in 1985 when R136 was confirmed to be a densely packed cluster.

As seen from Earth, the stars R136a1, R136a2 and R136a3 are separated by approximately 0.1 and 0.5 arcseconds. While this might be a tiny angle, according to optical theory this is the kind of detail that should be resolvable through a telescope with a diameter of around a metre (3.3 feet) or more. The problem lies in Earth’s atmosphere, which complicates the theoretical behaviour of light in unpredictable ways. Moving masses of air act like tiny lenses, bending and warping the path of light rays while blurring and shifting the images seen from the surface. This is the phenomenon that causes stars to flicker and twinkle in the sky, while on long-exposure photographs it causes the images of stars to smear out from points into fuzzy-edged discs. Even at mountaintop altitudes above most of the atmosphere, it reduces the resolving power of even the largest telescope to about 0.5 arcseconds, blurring images of R136 and making its individual stars indistinguishable.

In the past two decades, astronomers have finally started to overcome this problem using the ingenious technology known as adaptive optics. The principle is simple – adjusting the configuration of the telescope itself to correct for changing atmospheric turbulence – but putting it into practice requires huge amounts of computing power. It was the adaptive optics system on the European Southern Observatory’s Very Large Telescope (VLT) that enabled a team of astronomers led by Paul Crowther of the University of Sheffield to produce the first resolved image of the cluster’s heart. Based on the brightness of its individual stars and a comparison with a relatively nearby monster star with better known properties, the team was able to calculate R136a1’s physical properties. They revealed the secrets of what remains, for the moment, at least, the heaviest star in the universe.

WEIGHING A GIANT

Confirmation of R136a1’s enormous mass came from data collected using the Multi-conjugate Adaptive optics Demonstrator on the VLT. This system monitors the appearance of a guide star very close to the target object in the sky, analysing the atmospheric changes that distort its image from moment to moment.

This information is then used to drive tiny motors that deform two small mirrors along the telescope’s overall light path, correcting the original distortion to produce sharper images. Nevertheless, R136a1 and its neighbour R136a2 are only just distinguishable from each other in this near-infrared image.