How big is the largest known star? Compared to planets, stars will always be the overall group winners in terms of superior size. When you look at the night sky on a clear night and away from city lights, you will see that there are stars of varying sizes and brightnesses, some of which are sure to outsize even our own Sun. What your observations will not tell you is just how much bigger than the Sun some recently-discovered stars are. However before we can truly appreciate the size of other stars in the Universe, we need to have an understanding of the staggering scale at which our own nearest star stands within our Solar System.
The Sun, despite its actual size, can appear small in our sky but is a massive 150 million km (93 000 000 miles) away.
Stars have been forming for billions of years. Countless have died, many exist as we speak, and no doubt millions are yet to be born in the future. One of the key factors that a star’s size will depend on is what stage of its life it is at. Converting hydrogen into helium and releasing energy as a by-product is what stars do for the majority of their lives. This phase is called a star’s ’main sequence’, from its childhood right up until middle-age. But the final stages of a star’s life are different; most stars will radically change in size and appearance, suddenly swelling and getting larger in old age. An average-sized star becomes a bloated red giant. Larger still stars in their final life phases expand to become red supergiant stars.
So the biggest stars currently in the Universe are those who were born big but have also matured and reached old age. However beyond a supergiant star there is bigger still. Compared to the numerous other stars in the night sky these special stars, known as hypergiants, are few in number and very far away. This is why what we know about them and what they will do in the future remains limited as astronomers get few opportunities to study them.
So what do we know about hypergiants? Just like all the other stars in the Universe, these huge stars can vary in size and colour. Of the very few discovered hypergiants, they fall into three main colour categories: blue, yellow, and red. A couple of characteristics can be recognised in all of them however. Firstly they are all very hot (at least within their cores), and are very luminous. Astrophysicists agree that the bigger the star’s mass, the greater the pressure and temperature within the star. From this it follows that the most massive stars are the most powerful. With the greatest pressures in their cores they will produce the most helium at the fastest rate, giving off the most heat and light hence these gigantic stars live much shorter lives than smaller stars, like our Sun. Hypergiants live life fast and furious, whereas dimmer dwarf stars like the Sun have a much steadier and sedate existence. While the largest of the stars will only live millions of years, our Sun will most likely continue to live for billions of years yet.
As these stars are burning up vast quantities of hydrogen fuel at such a great rate, they are unstable and violent, and spew out much of their contents into space in great stellar winds. The quantity of starry material fired into space in a hypergiant’s solar wind can actually be so great that it can obscure its true shape when observed from hundreds of light years away. Hypergiants also tend to have extended atmospheres reaching far into space, this and the fact that some of the more unstable appear to pulsate and alter in brightness makes measuring these space monsters to a high degree of accuracy rather difficult.
The ‘most massive’ star actually means something quite different from the term ‘largest star’. If stars could be placed on some stellar weighing pan, the most massive star will be the heaviest measured, while the largest will be the widest measured in kilometres or miles.
Firstly, what is the most massive star? The answer is R136a1, which can be seen as the biggest and brightest star at the centre of a large cluster of stars in the Tarantula Nebula, in the constellation of Dorado. This cosmic kingpin weighs in at an impressive 265 solar masses. In other words it has 265 times the amount of ‘star stuff’ inside it than our Sun. When discovered in 2010 this blue hypergiant star broke the (then) accepted laws of science. Its calculated mass was twice what astronomers thought was possible for a star to be! This star also holds another significant record. As the heaviest star, that added weight means its contents must be under even greater pressure at its core. That excess energy overflow from the nuclear fusion reaction within R136a1 means that more energy pours out from it into outer space in the form of heat and light than from any other known star. In other words, R136a1 is the most luminous star known.
Secondly, what is the largest known star in outer space, in terms of volume?
Before we answer this question we also need to remove one other possible obstacle from our comprehension. Are we supposing that the largest star in space will be one that ‘looks’ like the biggest in the sky? The answer of course is no. We already know that there are hugely varying distances between all of the stars we can observe from Earth and so stars in a closer proximity to our Solar System have the unfair advantage of ‘looking big’ compared to those that are millions of light years away. If however for a moment we imagined that the stars were all the same distance from our planet on Earth’s celestial sphere, our search for the largest star would end with the star R Dorado. This red giant star, found in the southern hemisphere constellation of Dorado, the ‘Dolphinfish’, appears to have the biggest stellar disk in the sky (after only the Sun). So while the truly greatest star in space still quietly lurks out there somewhere, R Dorado’s admittedly substantial presence of 370 solar radii and its prominent stellar pedestal of just 178 light years from Earth enable it to successfully masquerade as the largest star to the ground-based observer.
So finally, what is the star with the greatest volume, the greatest equatorial circumference, the one that currently holds the title of largest known star in the Universe? The answer is NML Cygni, also known as V*V1489Cyg, a variable red hypergiant which can be found in the constellation Cygnus the swan. Discovered in 1965 by Neugebauer, Martz and Leighton (hence its name), this super-colossus, 25-50 times as heavy as our Sun, squeezes in to the red corner of the cosmos about 5300 light years away with its staggering girth of 1639-1650 solar radii! That is around 1.2 billion km (720 million miles), were this star to replace the Sun at our Solar System’s centre, it would engulf Mercury, Venus, Earth, Mars, and even Jupiter, with its scorching surface looming ominously at Saturn! Furthermore, just for the record, this cosmic super-sphere’s diameter also outmatches that of its closest stellar rival in space, WOH G64, by at least 70 million km.
Although a red hypergiant can have a surface temperature as low as 3 500 K, much cooler than its blue and yellow siblings, crimson-coloured NML Cygni is one of the most luminous red stars of its class. If average-sized stars and supergiants give any indication: after the stellar fuel is expended and total inner collapse – we might be talking about a ‘hypernova’, a cataclysmic explosion of such deadly power that gamma rays would damage its own galaxy and irradiate all cellular life for light years around it. Just like so many other aspects of the life of hypergiants, their inevitable deaths remain shrouded in mystery. While some astronomers think NML Cygni’s demise will be within the next hundred thousand years, with one of the highest mass-loss-rates-per-year known, a symptom of stellar instability, others think it could be sooner.Two facts are clear however, as the story of the Universe generally rewards us with a supernova once every 50 years… and as the largest star currently has a very temperamental personality, we may not be disappointed!
NB this popular article was substantially revised on 22 July 2013 to reflect the latest discoveries.)
(Article by Nick Parke, Education Support Officer)