It\u2019s so far away that when you get there the Sun\u2019s light is a mere 1\/25th as bright as on Earth.<\/p>\n
And yet, if you were to make a journey of the same distance in some planetary systems, you\u2019d still be inside the star<\/i>.<\/p>\n These celestial beasts \u2013 known as hypergiant stars \u2013 are colossal. The biggest can fit 10 billion Suns inside, or 14 quadrillion Earths.<\/p>\n Such monsters are rare, but they play a crucial role in seeding the Universe with the rich array of chemistry required to sustain life.<\/p>\n Their scarcity means they\u2019ve been poorly understood in the past, but a run of recent research is giving astronomers unprecedented insights into their unique behaviour.<\/p>\n Soon we may know their secrets.<\/p>\n Hypergiants are so massive, typically dozens of times the mass of the Sun or more, that they are highly unstable.<\/span><\/p>\n They regularly cough huge quantities of their material back into space.<\/span><\/p>\n \u201cThey are throwing out the mass of Jupiter<\/a> or more in a single event,\u201d says Roberta Humphreys, an astrophysicist at the University of Minnesota<\/a>. <\/span><\/p>\n 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<\/a>.<\/span><\/p>\n Painstaking analysis concluded that it spat out <\/span>material weighing several times the mass of the Moon from its southern hemisphere.<\/p>\n That material blocked out some of Betelgeuse<\/a>\u2019s light, causing the temporary dimming.<\/p>\n It was the first time astronomers had seen such a huge ejection from the surface of a star in real time.<\/p>\n Ejected material from hypergiants forms vast and intricate clouds that can stretch out to 10,000 times the Earth\u2013Sun distance from the star\u2019s surface.<\/p>\n That\u2019s over 300 times further out than Neptune<\/a>, the farthest planet from Earth in the Solar System, sits from the Sun.<\/p>\n \u201cYou can clearly see the ejected material forming arcs, lumps, knots and jets around the star,\u201d says Humphreys.<\/p>\n Individual knots can contain 3,000 Earths\u2019 worth of material.<\/p>\n This ejected material enriches the interstellar medium with complex molecules.<\/p>\n When gas and dust from several stars mingles, it can collapse to form new solar systems.<\/p>\n Fledging planets there will already have the necessary chemical building blocks for biology.<\/p>\n 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<\/a>.<\/p>\n \u201cTwenty-five different molecules have now been identified there,\u201d says Humphreys, including water and silicon dioxide, which is the major constituent of sand.<\/p>\n However, understanding why hypergiants like VY Canis Majoris lose so much mass has been an enduring mystery.<\/p>\n A mystery deepened by the rarity of these stellar goliaths.<\/p>\n Astronomers know of just 10 hypergiants in the Milky Way<\/a>, meaning Sun-like stars outnumber them by more than a billion to one.<\/p>\n Even then our view of them is often obscured by the huge amounts of dust in our galactic disc.<\/p>\n Fortunately, astronomers have also identified a number of hypergiants in the Magellanic Clouds, two of the satellite galaxies that orbit around the Milky Way.<\/p>\n In 2022 a team led by Michalis Kourniotis from the Czech Academy of Sciences<\/a> found three of them had something in common: they\u2019re pulsating.<\/p>\n This could explain why hypergiants are often classified as variable stars<\/a>. The star\u2019s brightness changes in a repeating pattern as it throbs in and out.<\/p>\n Could these pulsations also be behind the ejected material from hypergiants?<\/p>\n Humphreys doesn\u2019t think so. \u201cPulsations cannot get the material far enough above the star\u2019s surface to the point where the dust and the molecules condense,\u201d she says.<\/p>\n It simply isn\u2019t a powerful enough mechanism to spit material out to the vast distances that ejected material has been seen from hypergiants.<\/p>\n The cloud of ejected material from VY Canis Majoris, for example, is some 300 billion kilometres wide \u2013 that\u2019s over 65 times Pluto\u2019s distance from the Sun.<\/p>\n A possible clue comes from the fact that the lost material isn\u2019t ejected symmetrically.<\/p>\n \u201cIt forms projectiles that are fired out in different directions and different angles from different regions of the star,\u201d Humphreys says.<\/p>\n That points to something going on in isolated regions at the surface, not something happening to the whole star at once.<\/p>\n Energy usually reaches the surface of a star through convection.<\/p>\n Hot material bubbles upwards, making the star\u2019s surface seeth and roil, a bit like a pan of boiling water.<\/p>\n This creates convection cells at the star\u2019s visible surface. The Sun\u2019s convection cells are typically 1,000 kilometres across, but they can take up 60% of the surface of supergiants like Betelgeuse.<\/p>\n Perhaps these huge convection cells on the surfaces of hypergiants are destabilising and blasting material into space?<\/p>\n \u201c[This idea] has the same problems as pulsations,\u201d says Humphreys.<\/p>\n Such an event would still not be powerful enough to eject material to the vast distances observed by astronomers.<\/p>\n \u201cSomething is missing \u2013 an additional mechanism is required,\u201d she says. The missing piece of the puzzle is magnetism.<\/p>\n We know from observations of our own Sun that it is highly magnetic.<\/p>\n Huge and powerful magnetic fields<\/a> twist and contort until they snap, flinging huge quantities of material into the Solar System.<\/p>\n The most violent of these solar magnetic mood swings is a coronal mass ejection<\/a> (CME), where the Sun spits out a billion tonnes of material at speeds of over a million kilometres per hour.<\/p>\n They are often associated with so-called active regions on the Sun and with other features such as sunspots<\/a>.<\/p>\n Except even they pale in comparison to similar events on hypergiants.<\/p>\n \u201cThe strength of the magnetic field is five times greater,\u201d Humphreys says. \u201cThe energy increases by a factor of a thousand.\u201d<\/p>\n 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.<\/p>\n The coronal arcs produced would be a billion times larger than those seen on the Sun.<\/p>\n How they lose mass may be getting clearer, but another big mystery remains: how hypergiant stars die.<\/p>\n It\u2019s a puzzle that dates back to 2015, when astronomers noticed the disappearance of It was there in Hubble images from 2007, but had vanished from view eight years later.<\/p>\n 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?<\/p>\n There\u2019s a hint to one possible answer in the star\u2019s name.<\/p>\n Stephen Smartt, of Queen\u2019s University Belfast<\/a>, suspects that stars heavier than 17 solar masses \u2013 in other words most hypergiants \u2013 are so massive that they directly collapse into a black hole<\/a> (hence BH) without going supernova<\/a> first.<\/p>\n Humphreys points to another option, though.<\/p>\n As hypergiants lose mass they could shrink down, heat up and evolve back into warmer stars.<\/p>\n Paradoxically, such a star wouldn\u2019t be as bright, because although it is hotter, its surface area has been reduced.<\/p>\n That could explain why N6946-BH1 faded from view without going supernova.<\/p>\n It\u2019s still there, just dimmer and so no longer visible to us from NGC 6946\u2019s distance of 25 million lightyears away.<\/p>\n Astronomers could know one way or the other soon. 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