If you’ve ever looked up at the sky, you’ve definitely seen the Sun before. You’ve probably also seen the moon too–but that depends on whether it’s day or night. It’s also quite large in the sky–while also being really far away. Like… 93 million miles (~150 million kilometers) away from Earth. Which stands to reason–that means the Sun is pretty big. Which is true, the Sun is very big. But it’s not the biggest–far from it. So what is the biggest star?
Benchmarking With the Sun
If you were ever concerned with how absolutely massive space is, we’re now going to the entire Sun as our baseline measurement. Which is already so far away from us that we can’t really comprehend it.
But from end to end, the Sun measures about 865,000 miles (1,400,000 km). One of the fastest jets we have (NASAs X-43 series) breaches Mach 9 with an airspeed around 7,000 miles per hour. Which means it would take about 5 days for this jet going over 9 times the speed of sound to fly from pole to pole of the Sun. It would take about 16 days to fly around it in a circle. Keep this in mind, we will be bringing it back.
There’s more too–the Sun is absolutely massive. No seriously, it’s about 99.8% of all the mass in the Solar System. All the planets, including the juggernauts Jupiter and Saturn, asteroids, and the space dust all around us? That all accounts for less than 1% of the Solar System’s mass. Talk about hoarding all the wealth.
Alright, you’ve probably heard of red giants before. When stars age, their cores start burning heavier elements. Most of them (main sequence stars) fuse hydrogen. But eventually they run out of hydrogen and they move on.
Some very low mass stars (red dwarfs) can keep fusing hydrogen into helium for around a trillion years until they just fizzle into white dwarfs. These stars will likely be the last, dim little candles when our universe inevitably ages into a starless void. How lonely.
Further Reading: When Will the Universe End?
But that’s neither here nor there, we’re looking for the big boys. Red giants might certainly fit that bill. Our Sun will eventually swell to the size of a gargantuan red giant and burn helium. How big will it swell? Well it will swallow both Mercury and Venus, and might get Earth as well.
This kills the Earth.
Don’t worry though, this won’t happen for another 5 billion years–so it’s not an excuse to stop making the world a better place. That hasn’t stopped politicians from trying though.
On average red giants are somewhere between 100 and 1,000 times the size of our Sun as it is. Which is a much longer flight for our Mach 9 superjet we’ve been using for reference.
That’s right, space hates you and just when you thought we got maximum size, we go bigger. Particularly large red giants are known as supergiants, which are, as the name suggests really big. But they are already eclipsed by the utter shenanigans that are hypergiants.
Hypergiant stars are so big, they’re quite literally coming apart at the seams. Gravity at their surfaces isn’t strong enough to keep everything in, so mass is constantly being ejected in the form of solar waves.
These stars are rare, and come in different colors. Like the blue hypergiant–Pistol Star, one of the most luminous in the Milky Way. It probably won’t last more than a couple more million years as its solar winds are over 10 billion times stronger than our own Sun’s (whose solar winds could knock out Earth’s atmosphere if they were in a bad mood). Eventually, it’ll explode in a massive super or hypernova. Oh, also Pistol Star radiates as much energy as our Sun does in a year in about… 20 seconds.
There are also yellow hypergiants like Rho Cassiopaeiae. Which is of a very rare class–we only know of a handful. But if you were to replace our Sun with Rho Cassiopeiae, it would swallow the inner planets like our Sun one day will.
We Need to Go Bigger
We didn’t mention Rho Cassiopaeiae being near the end of its life cycle, because it probably isn’t. The rarity of yellow hypergiants suggests that they are in a brief middling stage of their life–something like hypergiant puberty. If puberty lasted millions of years.
Enter red hypergiants.
They’re so big we can’t actually measure them properly–they’re Solar System sized monsters. Remember when we said hypergiants are literally bursting at the seams? Well red hypergiants are ejecting so much stuff into space in a cloud around them that it’s really hard to get a measure on them. It doesn’t help that things in space are really far away from us.
The current biggest boy we have on the record is Stephenson 2-18. It’s leagues bigger than the largest red supergiant we know, VY Canis Majoris–which is already 1,800 times the size of our Sun.
Stephenson’s radius is projected to be over 2,000 times our Sun’s, which would make the Sun look like a grain of dust. If you plopped it in the Solar System, it would probably eat Saturn.
Which means our Mach 9 jet would probably take almost 90 years to fly around this star. What’s more, did we mention that Stephenson is estimated to have already ejected about half its mass? So at some point it might have been twice as massive.
Speaking of other big deals in space, see if you can’t get some big things humans did in space here. Not that we’re big at all.