The riddle of the hypergiants
New research is uncovering the weird workings of the largest stars in the Universe, explains Colin Stuart
You feel the vibrations as the rocket lifts off from the launch pad. This cramped spaceship is your home for two years as you journey all the way out to Jupiter, the Solar System’s largest planet. It’s so far away that when you get there the Sun’s light is a mere 1/25th as bright as on Earth.
And yet, if you were to make a journey of the same distance in some planetary systems, you’d still be inside the star. These celestial beasts – known as hypergiant stars – are colossal. The biggest can fit 10 billion Suns inside, or 14 quadrillion Earths. Such monsters are rare, but they play a crucial role in seeding the Universe with the rich array of chemistry required to sustain life. Their scarcity means they’ve been poorly understood in the past, but a run of recent research is giving astronomers unprecedented insights into their unique behaviour. Soon we may know their secrets.
Bizarre behemoths
Hypergiants are so massive, typically dozens of times the mass of the Sun or more, that they are highly unstable. They regularly cough huge quantities of their material back into space. “They are throwing out the mass of Jupiter or more in a single event,” says Roberta Humphreys, an astrophysicist at the University of Minnesota.
A similar event on a smaller scale unfolded on the supergiant star Betelgeuse in 2019, when it dimmed noticeably in the night sky before brightening again. Painstaking analysis concluded that it spat out material weighing several times the mass of the Moon from its southern hemisphere. That material blocked out some of Betelgeuse’s light, causing the temporary dimming. It was the first time astronomers had seen such a huge ejection from the surface of a star in real time.
Ejected material from hypergiants forms vast and intricate clouds that can stretch out to 10,000 times the Earth–Sun distance from the star’s surface. That’s over 300 times further out than Neptune, the Solar System’s outermost planet, sits from the Sun. “You can clearly see the ejected material forming arcs, lumps, knots and jets around the star,” says Humphreys. Individual knots can contain 3,000 Earths’ worth of material.
This ejected material enriches the interstellar medium with complex molecules. When gas and dust from several stars mingles, it can collapse to form new solar systems. Fledging planets there will already have the necessary chemical building blocks for biology. In June 2022, Humphreys was part of a team that used the Atacama Large Millimeter Array (ALMA) in Chile to take a closer look at the ejected material surrounding a particularly famous hypergiant: VY Canis Majoris. “Twenty-five different molecules have now been identified there,” says Humphreys, including water and silicon dioxide, which is the major constituent of sand.
However, understanding why hypergiants like VY Canis Majoris lose so much mass has been an enduring mystery. A mystery deepened by the rarity of these stellar goliaths. Astronomers know of just 10 hypergiants in the Milky Way, meaning Sun-like stars outnumber them by more than a billion to one. Even then our view of them is often obscured by the huge amounts of dust in our galactic disc.
Stars with a heartbeat
Fortunately, astronomers have also identified a number of hypergiants in the Magellanic Clouds, two of the satellite galaxies that orbit around the Milky Way. Last year, a team led by Michalis Kourniotis from the Czech Academy of Sciences found three of them had something in common: they’re pulsating. This could explain why hypergiants are often classified as variable stars. The star’s brightness changes in a repeating pattern as it throbs in and out.
Could these pulsations also be behind the ejected material from hypergiants? Humphreys doesn’t think so. “Pulsations cannot get the material far enough above the star’s surface to the point where the dust and the molecules condense,” she says. It simply isn’t a powerful enough mechanism to spit material out to the vast distances that ejected material has been seen from hypergiants. The cloud of ejected material from VY Canis Majoris, for example, is some 300 billion kilometres wide – that’s over 65 times Pluto’s distance from the Sun.
A possible clue comes from the fact that the lost material isn’t ejected symmetrically. “It forms projectiles that are fired out in different directions and different angles from different regions of the star,” Humphreys says. That points to something going on in isolated regions at the surface, not something happening to the whole star at once.
Energy usually reaches the surface of a star through convection. Hot material bubbles upwards, making the star’s surface seethe and roil, a bit like a pan of boiling water. This creates convection cells at the star’s visible surface. The Sun’s convection cells are typically 1,000 kilometres across, but they can take up 60 per cent of the surface of supergiants like Betelgeuse.
Mega magnetism
Perhaps these huge convection cells on the surfaces of hypergiants are destabilising and blasting material into space? “[This idea] has the same problems as pulsations,” says Humphreys. Such an event would still not be powerful enough to eject material to the vast distances observed by astronomers. “Something is missing – an additional mechanism is required,” she says. The missing piece of the puzzle is magnetism.
We know from observations of our own Sun that it is highly magnetic. Huge and powerful magnetic fields twist and contort until they snap, flinging huge quantities of material into the Solar System. The most violent of these solar magnetic mood swings is a coronal mass ejection (CME), where the Sun spits out a billion tonnes of material at speeds of over a million kilometres per hour. They are often associated with so-called active regions on the Sun and with other features such as sunspots.
Except even they pale in comparison to similar events on hypergiants. “The strength of the magnetic field is five times greater,” Humphreys says. “The energy increases by a factor of a thousand.” She argues that, combined with convection, magnetic fields and coronal mass ejections could be the driving force behind the coughing fits of VY Canis Majoris and other hypergiants. The coronal arcs produced would be a billion times larger than those seen on the Sun.
A whimper not a bang
How they lose mass may be getting clearer, but another big mystery remains: how hypergiants die.
It’s a puzzle that dates back to 2015, when astronomers noticed the disappearance of a hypergiant star called N6946-BH1 in the spiral galaxy NGC 6946. It was there in Hubble images from 2007, but had vanished from view eight years later. Usually such a massive star would detonate as a cataclysmic supernova when it dies, which would be hard to miss. How could it have simply faded away without so much as a whimper?
There’s a hint to one possible answer in the star’s name. Stephen Smartt, of Queen’s University Belfast, suspects that stars heavier than 17 solar masses – in other words most hypergiants – are so massive that they directly collapse into a black hole (hence BH) without going supernova first.
Humphreys points to another option, though. As hypergiants lose mass they could shrink down, heat up and evolve back into warmer stars. Paradoxically, such a star wouldn’t be as bright, because although it is hotter, its surface area has been reduced. That could explain why N6946-BH1 faded from view without going supernova. It’s still there, just dimmer and so no longer visible to us from NGC 6946’s distance of 25 million lightyears away.
Astronomers could know one way or the other soon. There’s another hypergiant star that might just be gearing up for a similar feat. Except this one is in our own Galaxy, giving astronomers a much better chance to see what’s going on. It’s called Rho Cassiopeiae (or Rho Cas for short).
In June 2022, Grigoris Maravelias, from the National Observatory of Athens, published details of its ejected material. Astronomers have seen Rho Cas experience four major outbursts in the last century, most recently in 2013. Maravelias analysed these episodes and concluded that the outbursts are getting shorter and more frequent. “This activity indicates that Rho Cas may be preparing to pass to the next evolutionary phase,” he writes in the paper outlining his findings.
Again, Humphreys is cautious about getting carried away. She points out that there aren’t substantial amounts of dust around Rho Cas. “The lack of dust and ejecta could mean the star still has a long way to go,” she says.
What will ultimately happen to Rho Cas remains unclear, but it could hold the key to getting under the hood of hypergiants. Astronomers will continue to watch with keen interest, as they look to add the final pieces to the enduring puzzle of understanding the Universe’s biggest stars.
How big is a hypergiant?
The staggering size of these largest of stars is hard to comprehend
How VY Canis Majoris would look if it were our Sun (above right, top), swallowing up the inner planets and extending beyond Jupiter. The mammoth star dwarfs Earth’s orbit (above right, bottom) and could accommodate almost three billion of our Suns
You could fit at least 1,420 Suns across the face of VY Canis Majoris, making its total diameter close to two billion kilometres. Some estimates put it at over 2,000 Suns across, or almost three billion kilometres wide. Earth orbits a mere 150 million kilometres from the Sun, meaning VY Canis Majoris is at least 13 times wider than the Earth–Sun distance.
It would take almost three billion Suns to fill up the star. Even Mercury, the smallest planet, can only fit inside the Sun 21 million times. A supergiant star like Betelgeuse would fit inside this hypergiant eight times over.
Despite travelling at around 300,000 kilometres per second, it would take a beam of light six hours to travel around the circumference of VY Canis Majoris. That’s about the same amount of time it took for photos of Kuiper Belt object Arrokoth to travel back to Earth from NASA’s New Horizons probe.
Four hypergiant stars to find
Focus your scope on these bright, giant oddities of the Universe
Rho Cassiopeiae
Magnitude: +4.1 to +6.2
This yellow hypergiant star is located in the prominent W-shaped constellation of Cassiopeia. It’s a semi-variable star, meaning its brightness changes over time. Usually it is bright enough to be seen with the unaided eye, but in the past it has temporarily dimmed enough to make binoculars necessary.
VY Canis Majoris
Magnitude: +6.5 to +9.6
VY Canis Majoris is one the biggest stars in the Universe. This pulsating red hypergiant is located in the constellation of Canis Major, or the Great Dog, diagonally down to the left of Orion’s Belt. It’s visible from the UK in the winter and very close to the horizon below Sirius.
P Cygni
Magnitude: +4.8
Despite the fact that it is over 5,000 lightyears away, this luminous blue hypergiant is so bright that it’s visible to the unaided eye in the constellation of Cygnus, the Swan. You’ll find it shining with a magnitude of +4.8 close to where the tail and wings of the Swan meet.
V 4030 Sagittarii & V4029 Sagittarii
Magnitude: +8.3
This pair of blue hypergiants is located to the southeast of the Omega Nebula (M17) in the constellation of Sagittarius. The lid of the Teapot asterism points towards them. Sharing a similar brightness, they can be easily seen through a modest amateur telescope low in the south during the summer months.
Colin Stuart (@skyponderer) is an astronomy author and speaker. Get a free e-book at colinstuart.net/ebook