INTRODUCTION
There are hundreds of sextillions of stars in the universe, and while the Milky Way contains an estimated 400 billion of those stars, that’s still a comparatively minuscule sample size. This incomprehensible expanse means that, we will never know for sure just how big the biggest star in the universe really is. The overwhelming majority of stars are simply too far away for us to ever be able to measure. That being said, we’ve discovered no shortage of giants within our own backyard, stars that dwarf our Sun, and some that could even swallow up most of our own Solar System. Increasingly accurate estimations of size and distance means that the league table for the list of the largest stars is ever-changing. But if we look for the largest star discovered today, we come to the magnificent UY Scuti. Discovered in the 1800s but reclassified as the largest star at the start of this decade, UY Scuti has some absolutely insane numbers associated with it.
HISTORY OF UY SCUTI
UY Scuti was first noted over a hundred and fifty years ago, when German astronomers at the Bonn Observatory recorded it in a survey in 1960. As you may be aware, most stars are named after the survey or catalogue they are recorded through, such as Messier objects, Kepler objects, the Henry Draper catalogue, and the list goes on. UY Scuti was no exception. It was noted during the Bonner Durchmusterung Stellar Catalogue survey, and was declared the 5055th star between 12 and 13 degrees south, as it was originally coined BD-12 5055. But in the years that followed, the brightness appears to have changed when it was surveyed again, leading to the discovery that the star was actually a variable star, more specifically a pulsating variable star. Pulsating variable stars come in many different forms, but generally they are a type of star that changes their luminosity and relative size due to the expansion and contraction of layers on the surface of the star. Variable stars are invaluable to astrophysicists. It’s through them that we learn a lot more about the interior processes of a star. Main sequence stars such as our Sun tend to be more stable, with outward pressure and inward pressure from gravity counteracted in a relatively stable equilibrium. But variable stars pulsate and vary based on the rate of radiation escaping from the core and outer shell layers of the star. As there is an international protocol for recording variable stars, BD-12 5055 was reclassified and became the 38th variable star in the constellation of Scutum. It was then renamed UY Scuti accordingly, the title we know it by today. The rate that UY Scuti pulses is considered irregular, but it seems to have a pulsation period of about 740 days roughly, a little over two calendar years on Earth. Because there is so much obscuring the star, we didn’t actually know about its enormous size until fairly recently. It wasn’t until 2012, when UY Scuti’s size was re-estimated to be about 1700 solar radii, placing it firmly above the former largest known star, Canis Majoris. While we’re almost certain that UY Scuti is the largest star we’ve ever discovered, it is far from definitive. Obstruction from light within the Milky Way is just one of the difficulties we face when taking measurements. UY Scuti’s distance from Earth is a contributing factor. Because of its surrounding area, we can’t get wholly accurate data on how far away it is, and it becomes difficult to then deduce its size. As mentioned, the former largest star was VY Canis Majoris, and was a fairly well-known star among the scientific community. While estimates of its size have also had well-defined margins of error, we have managed to refine the estimates down considerably in recent years. Initial estimates when it was first discovered put the star over 3000 times larger than the diameter of the Sun, but this has since been re-examined, and the boundary now sits between 1420 solar radii, to 2000 solar radii, which is still quite a wide margin. The 2000 solar radii estimation is a probably a bit optimistic, and so it’s likely that Canis Majoris is much nearer the 1500 solar radii mark. In comparison, UY Scuti’s radius is estimated to be approximately 1708 solar radii, give or take 192. This means that even at its lower bound estimate, it is still in excess of 1500 times the size of the Sun, and most likely the larger of the two red hypergiants. There are a few other stars that sit in the league table between Canis Majoris and UY Scuti, but the estimates of their size are considered much less reliable. When comparing stars, we always rank based on the lower bound estimate, so for example, Canis Majoris is classified as 1420 solar radii, which probably doesn’t do it justice. A star than that perhaps, is WOH-G64, a star within the Large Magellanic Cloud, a satellite galaxy at the edge of the Milky Way. Its parameters are uncertain, and sits between 1500 times the size of the Sun, and 2500 times, respectively, which would make it the largest star ever discovered. Unfortunately, it is surrounded by a thick envelope of dust and is much further away than most stars we analyse. So, until technology improves, this boundary doesn’t look set to narrow anytime soon. Something slightly closer to home is Westerlund 1-26, a star with very strong radio emissions, but again, it has very vague estimates of its size associated with it, with a difference in upper and lower bound of over 1000 solar radii. Something maybe only a fraction larger than Canis Majoris is HD-143183, a red supergiant with much more precise estimations, perhaps just under 1500 solar radii, a very similar size to the former largest known star. And then finally, there are 2 stars from RGSC-1, a young star cluster within the Milky Way, which are estimated to be also just under 1500 solar radii. But UY Scuti, of course, still takes the grand prize as the largest known star. For now, at least. But one thing to consider is, if it is the largest, what implications does that have for its chances of supporting life?
KNOW ABOUT UY SCUTI
UY Scuti is a red hypergiant pulsating variable star, located roughly 5,100 lightyears from the Earth, in the Scutum constellation. The star sits in a similar area of the sky to Sagittarius A*, the engine black hole at the centre of the Milky Way, in a fairly central galactic location. At just over 7 astronomical units in diameter, this gives the star a radius of a massive 1,700 times that of our Sun. However, despite this massive disparity in size, we predict its mass to be between only 7 to 10 times that of our Sun, and is nowhere near the record for the most massive star ever discovered. But then again, we don’t have a lot in the neighbouring regions to compare it to, so it is quite difficult to estimate this star’s mass. Speaking of neighbouring regions, UY Scuti is believed to be extremely luminous, as you might expect given the size – 400 thousand times brighter than the Sun. However, it is not visible to the naked eye, and does not appear that bright in the night sky due to the star’s position within the Cygnus Rift, an area within the Milky Way that is obstructed by its dark bands of dust and gas. Furthermore, UY Scuti is also close to the Zone of Avoidance, an area of the night sky we can’t really observe because the Milky Way’s bright galactic plane obstructs it from our perspective. Given this luminosity, you would expect UY Scuti to be one of the hottest stars out there, but that isn’t the case, either. In fact, quite the opposite. Its surface temperature is estimated to be just under 3,000 Kelvin. In comparison, the Sun’s surface temperature is almost double this, at 5,778 Kelvin. Just as it doesn’t correlate to mass, size does not always correlate to temperature. This is because stars that are this enormous in size are almost always no longer in their main sequence, the main part of the star’s life when the hydrogen fuel the star was created with is being fused in the core. When this hydrogen in the core runs out, heavier elements such as helium are fused. But hydrogen continues being fused on the outer layers of the core. When this occurs, the outward pressure of the radiation generated is much greater than the immense pressure of the star’s own gravity, which causes the star to expand and balloon to hundreds of times larger than it was during the main sequence. This is how UY Scuti and the vast majority of other red giants and red super-giants came to be so large. But it also means that it is in the dying stage of its life. Given that the star is out of its main sequence, you’d expect it to be at least a few billion years old. But this post-production phase doesn’t actually provide us with much insight as to how old the star is. It could be billions of years old, but because it’s so large and ejecting a lot of its mass, this instability could be indicative that the star is merely a few million years old, and is just an inefficient star with a short lifespan. While this instability doesn’t tell us how long it’s been around for, what it does tell us is that it hasn’t got long left to live, on a cosmological timescale anyway. As we mentioned earlier, this star is the largest known star in the galaxy. Its radius is approximately 1700 times larger than the Sun’s. So how do we put this in some kind of perspective? Well, for starters, this impressive volume means that you could fit our Sun into the star 5.1 billion times. Photons traveling at the speed of light would take about 14 and a half seconds to circle the circumference of our Sun, but for UY Scuti, it would take light the best part of seven hours. That is seven hours traveling at the fastest speed in the universe. Were you to fly something around it at a more human speed, say if you flew a standard Boeing 777 aeroplane around the surface at its normal operating speed, it would take well over 1200 years to circle it just once. You may have already heard this stat, but were you to place UY Scuti at the centre of our Solar System, its seven astronomical unit excess diameter would engulf everything as far out as the orbit of Saturn. That’s 5 planets, 82 moons, and the Asteroid Belt, all consumed by this one star. While we can’t estimate UY Scuti’s mass very accurately, we know it appears to be ejecting a lot of its mass following the advent of the dying stage of its life. It is losing mass at a rate of about 5.8 times 10 to the -5 solar masses per year, so that’s 0.00058 of a solar mass. This seems insignificant, but in tons, our solar mass is 2 with 27 zeroes after it. So, while this amount is not catastrophic when spread out over the entire surface, it is still ejecting millions of trillions of tons of solar radiation every year, and much more than the average star. This gives it a considerable extrasolar surrounding of dust and gas, and is yet another indication that this beast is going to go supernova in the not-too-distant future. For a star so far away and obscured by so many variable factors, we actually know quite a lot about UY Scuti. Our fascination with the enormous has led us to tirelessly adapt and refine our techniques of star observation and cataloguing, but it hasn’t always been this way. So, the question is, how did the star’s profile begin?
IMPLICATION FOR LIFE
Like most stars, UY Scuti may have a planetary system orbiting it. But in this case, we will likely never be able to observe it due to its low apparent visual magnitude and its sheer size. We detect planetary systems orbiting stars by detecting small dips in brightness, which can be indicative of a planet passing in front of the star and blocking a small fraction of the light. The problem with UY Scuti is that it is so enormous that even if a planet did pass in front of it, it would be far too small and insignificant to be detectable. These small variations in magnitude would be completely distorted by the pulsing and fluctuations in brightness of the star, too. Furthermore, matter is constantly being ejected out of the star at a somewhat substantial rate. Given its size and the massive amounts of stellar radiation this would equate to, the possibility of UY Scuti having its own planetary system is statistically unlikely. But if it were to sustain a human-like civilisation there, how far away would the planet need to orbit to be in its habitation zone? Well, we sit at one astronomical unit away from the Sun, approximately 150 million kilometres. For UY Scuti, the habitation zone is more likely to be about 1.5 trillion kilometres away, about 1 thousand astronomical units, meaning light and heat reaching the unit would take around a month to travel the distance. This also means that in order to complete a full calendar year on the planet, you would need about 10 thousand years to orbit the star just once. This means that the planet’s seasons, as we know them at least, would last 2500 years each. Entire civilisations would likely exist never knowing any other season than the one they were born out of. We’re talking such vast amounts of time that any intelligent civilisations probably wouldn’t even see them as seasons, but rather natural climate change. If the star ever supported life at any point, no matter what form it took, the odds suggest that this would have been at some point during the main sequence. Now, this star is far too volatile and ejects far too much radiation to give life a realistic chance of evolving. And there’s also something else. UY Scuti is only expected to burn for another 1 million years. Its ejection of mass is a sign that it’s reaching the end of its life. Stars outside of the main sequence burn for a fraction of the time they would have during the main stage, because they are ejecting their outer layers and becoming less stable. All of the hydrogen fuel in the core UY Scuti was formed with has been used up, and the star is now fusing heavier elements, turning them into oxygen and carbon. However, this process will continue, and eventually the helium will all be fused up, and the process will reach a physically unsustainable point. It is thought that UY Scuti could evolve into another stage before it goes supernova, either into a yellow hypergiant, or a much more luminous blue hypergiant, or to what is known as a Wolf-Rayet star. Whatever it evolves into, all 3 eventualities will create strong stellar winds that will expose the core, and will mark the beginning of the end. Based on its location within the Milky Way Galaxy, we know UY Scuti to be a metal-rich star, and so eventually the core will begin to fuse iron. When it does this, no heavier elements can be produced, and the outward radiation which once caused it to expand to this incredible size will no longer be strong enough to counterbalance its own gravity, causing the star’s outer layers to collapse in an extreme supernova. Except, because of its size, this won’t just be a supernova, it will be a more appropriately named hypernova. It will explode with the power of over a hundred supernova explosions, emitting high amounts of gamma radiation, which will decimate any planetary system around it. Thankfully, Earth will be much too far away to be affected, and this still isn’t likely to happen for at least another million years. But then again, 1 million years is a pretty short time span on a cosmological timescale. Following this tremendous explosion, if UY Scuti had evolved into a Wolf-Rayet star beforehand, then the gases dispersed by the supernova could act as incubators for new stars following its death, like a phoenix rising from the ashes. The amount of mass lost in the hypernova will determine what becomes of the leftover dead core. If what remains intact exceeds four times the mass of the Sun, then it will likely collapse in on itself to form a black hole. But if not, it will form a neutron star. UY Scuti is not particularly massive, as we mentioned earlier, so if it loses the majority share of its mass, it will probably not be massive enough to become a terrifying black hole you would typically expect from such a huge star.
CONCLUSION
UY Scuti is a giant of creation, but only within the abridged sample of the Milky Way Galaxy that we have to observe. With 2 trillion galaxies out there, and trillions more beyond the boundary of the observable universe, it seems a bit presumptuous to assume that UY Scuti is the biggest star in the universe, as some refer to it as. But here’s an exciting thought – say humanity does manage to colonise the Milky Way Galaxy, if we can survive approximately 3 and a half billion years into the future, then we will probably be around to witness the collision of the Milky Way and Andromeda. When this happens, our nearest neighbour is due to bring almost 1 trillion new stars with it to bolster our galactic family. And with this new compound galaxy containing nearly 3 times the amounts of stars we have currently, imagine what other record-breaking stars we are likely to find. But that’s a long way off in the future, and there are plenty of scientific barriers to cross before them. In the meantime, thank you very much for watching, and don’t forget to keep reaching for the stars.