You might not know it to look at it, but our local star, the Sun, is really nothing special. In fact, it’s just a regular yellow dwarf star. It dominates our solar system and yet there are countless others that put ours to shame.
Our own Milky Way galaxy contains billions of stars of many different sizes and colours. From cool red dwarves to hot blue supergiants, there are thought to be about 250 billion stars within our galaxy alone. On any given clear night, you might be able to see about 2,000 of them.
What colours can you see? Which stars stand out? Which ones are hottest? Or the largest? Which ones live the longest? Today we’ll consider the blue supergiant stars of the cosmos – the kings whose reigns are short and lonely.
1. Blue Giants Are Very Hot
Earth’s average temperature is about 58 degrees Fahrenheit (or nearly 15 degrees Celsius.) However, astronomers measure temperatures in Kelvin (K) and 58F is roughly 288K. In comparison, the surface of the Sun is nearly 10,000F (5,500C) or roughly 5,800K.
That’s about 172 times hotter than the Earth, but still quite cool compared to blue supergiants. These stars are some of the hottest and can have surface temperatures in excess of 40,000K – about four times hotter than the Sun.
So why are blue stars so hot? Or, alternatively, why are hot stars blue? It really comes down to the star’s mass. As all stars produce energy through nuclear fusion, the more massive a star, more nuclear fusion is occurring and the more energy it gives out.
Most energy is emitted in the blue end of the spectrum and as light itself is energy, the more energy is produced, the more blue light is emitted. Hence, hot, blue stars.
2. They Are Very Luminous
A size comparison of a red dwarf (lower left), the Sun (centre), a blue dwarf (right) and R136a1 (background). Credit: ESO/M. Kornmesser
There is a star, some 28,000 light years away, that’s about two million times brighter than the Sun. When LBV 1806-20 was discovered, back in January 2004, it was thought to be the biggest and most luminous star known – but we now know that’s not the case.
In fact, there are a number of stars even more luminous, with the current record holder being R136a1. This blue hypergiant shines with a light that’s nearly nine million times brighter than the Sun. Not surprisingly, it’s also the most massive star, with an estimated mass of over 250 Suns and a volume large enough to contain 27,000 Suns within it.
Again, the reason blue stars are so luminous comes down to energy. The more energy is produced, the more light is emitted and energy is emitted at the blue end of the spectrum. The reason R136a1 is so luminous is that it generates more energy in four seconds than the Sun does in a year. So you have a very luminous, hot blue giant star!
3. Blue Giant Stars Are Unstable
There’s a limit to a star’s luminosity and this limit actually has a name, the Eddington Limit (or Eddington Luminosity). Named for the man who discovered it, Sir Arthur Eddington, it describes the balance between the force of radiation (energy as light) being pushed outward and the force of gravity pulling inward.
Super and hypergiant stars live close to the edge of this limit. Sometimes, when the limit is exceeded and the outward push of radiation is greater than the inward pull of gravity, the star ejects matter and suddenly brightens. Eventually, balance returns, the star slips below the Eddington Limit again, less radiation (light) is emitted and the star consequently dims.
To all intents and purposes, it’s as though the star has become a temporary supernova.
A classic example of this is the star Eta Carinae, in the southern hemisphere. This multiple star system has a combined luminosity of over five million Suns and a combined mass of about 200 Suns. Over the past 160 years, it has undergone a number of sporadic outbursts, causing it to shift back and forth from naked-eye brilliance to invisibility.
Most famously, in 1843 it became the second brightest star in the sky as a result of an event now known as the Great Eruption. The aftermath of this outburst can be seen in photographs taken with the Hubble Space Telescope. It’s currently a relatively dim magnitude 4.3 star, but it’s been consistently brightening over the past few years.
4. These Stars Don’t Live Long
“The flame that burns twice as bright burns half as long,” said Lao Tzu, the renowned philosopher of ancient China. He was, of course, talking about our mortal lives, but he could have been talking about the lives of giant blue stars.
Put simply, stars survive by fusing hydrogen into helium. The more massive the star, the quicker it burns through its supply of hydrogen and, therefore, the shorter its lifespan.
Our Sun is middle-aged; it’s been around for about four and a half billion years and probably has another five billion to go. That’s not bad. It’s certainly plenty of time for the human race to leave the Earth and find a new home.
In comparison, a red dwarf star typically has about a tenth the mass of the Sun and could theoretically survive for trillions of years. The universe itself is thought to only be about 13 billion years old, so there’s no way to test the theory yet!
(No one knows the fate of the universe or when, if at all, it will ever come to an end. Assuming the universe does come to an end, red dwarves could potentially last until the end of time!)
Now let’s consider blue supergiant stars. It’s not unusual for these stars to have masses of ten or twenty times greater than the Sun. Rigel, for example, has a mass of about 21 Suns. It’s burning through its hydrogen pretty rapidly and is losing mass ten million times faster than the Sun.
Despite only being about eight million years old, it’s already burned through its hydrogen and could be entering the final stages of its life. In fact, at a distance of about 800 light years, it could have already become a supernova and we simply don’t know it yet. (Realistically though, it’s probably still got millions of years to go.)
5. Blue Supergiants Don’t Have Planets
As of this date (April 2019), there are over 2,800 stars with confirmed planets. None of them are blue giant stars. Part of the reason is that it takes billions of years for planets to form and, unfortunately, blue giants simply don’t live that long.
Another reason is that blue giants tend to have very strong solar winds that make it difficult for planets to form in the first place. Having said that, there have been two giant stars discovered with dusty disks around them.
Both were discovered by the Spitzer space telescope in 2006 and are located in our nearest galactic neighbour, the Large Magellanic Cloud. One, HD 37974 (R126) is over 70 times more massive than the Sun and is over a million times more luminous.
Both stars have dusty discs that extend 60 times further than the orbit of Pluto and could contain ten times as much mass as our own Kuiper belt. Nobody knows if these discs represent the formation of a solar system or the destruction of one.
6. Some Blue Giants Are Quite Rare
Stars are classified by their characteristics and typically fall into one of seven groups, with each group being assigned a letter of the alphabet. The seven groups (or types) are O, B, A, F, G, K, and M. The hottest and most luminous stars are known as Type O stars, while the coolest and least luminous are Type M.
Type O stars are the rarest and you may have already guessed the reason why. They’re typically very hot, very luminous and very massive, so they appear blue and tend to burn through their fuel very quickly and then explode.
If we could watch a time-lapse video of the night sky over the past billion years, the chances are we’d see a lot of blue stars quickly appear and then suddenly vanish, like fireworks on New Years’ Eve.
They’re rare because they simply don’t live as long as the other stars and so they’re not as numerous. If you want to spot a couple of easy examples, turn towards Orion. Of the three stars that form his belt, two are type O blue giant stars: Mintaka, the westernmost, and Alnitak, the easternmost.
Both were probably born from the same star cloud, the Orion Molecular Cloud Complex. Alnitak is actually the brightest example of a type O star in the entire night sky. It’s a multiple star system, some 1,250 light years away, with the largest member being a blue supergiant some 33 times the mass of the Sun and with a luminosity of over 200,000 Suns.
7. Other Blue Giants Are More Common
The other type of blue star, type B, is actually fairly common. As type B stars tend to be less massive, they take longer to burn through their fuel and, hence, they have longer lifespans and there’s more of them around.
Type B blue supergiants are relatively common in the night sky because of their luminosity; despite the distance, they’re still easily seen because they’re bright.
If you want to see some examples, you only have to look again at Orion. Rigel, the star that marks the hunter’s knee, is the seventh brightest star in the sky and the brightest example of a Type B blue supergiant.
At a distance of over 850 light years, it must be very luminous to be so easily seen. However, estimates of its luminosity wildly vary. The star could be anywhere between about 60,000 and 360,000 times more luminous than the Sun.
Another example can, again, be found in Orion. Alnilam is the middle and brightest star in Orion’s belt, with neighbouring Mintaka and Alnitak on either side. Alnilam is, arguably, more impressive than any of the other blue supergiants to be found in the region.
Not only is it further away (about 2,000 light years) but with some estimates placing its luminosity at over 500,000 Suns, it’s the most luminous of the seven stars that form the brightest part of the constellation.
8. Where Can Blue Super Giants be Found?
Besides Orion, a winter constellation, there’s an easy way to find blue giants at any time of year: open star clusters.
Open star clusters (such as the Pleiades) are typically formed of young, hot, blue-white stars that have yet to drift apart from one another. The Pleiades is the most famous example, but there are, of course, many others to be found across the night sky.
Northern hemisphere observers can admire the giant blue stars of the Pleiades, but those in the southern hemisphere have their own prize.
NGC 4755, the famed Jewel Box cluster in the constellation of Crux, is a stunning open cluster and contains a number of blue giant stars. Messier 47, in Puppis, another winter sight, has a multitude of giant stars while Messier 18, in the summer constellation of Sagittarius, has nearly thirty type B stars. In the autumn, NGC 663, in Cassiopeia, has over twenty.
9. Remember the Blue Giant Kings
Spare a thought for the blue giant stars. These stellar kings are as luminous as they are rare. Their reign can be cut short in the blink of a cosmic eye, leaving no planetary legacy to remember them by.
As you look upon them, be grateful that you have the opportunity to see them at all. These brilliant beacons of light may be here today but will be gone for our ancestors of tomorrow. How many have come and gone since the dinosaurs walked the Earth? How many others will live and die before the Earth itself is gone? Ultimately, they all may come, but few may be remembered.
