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What is the Biggest Star in the Universe?

This article was originally published in 2008, but I’ve updated it and added this cool video.

My six-year old daughter is a question asking machine. We were driving home from school a couple of days ago, and she was grilling me about the nature of the Universe. One of her zingers was this, “What’s the Biggest Star in the Universe”? I had an easy answer, the Universe is a big place, and there’s no way we can possibly know what the biggest star is. But that’s not a real answer. So she refined the question. What’s the biggest star that we know of?

Of course, I was stuck in the car, and without access to the Internet. But now I’m back at home, doing some research, and I thought I’d share the answer with the rest of you too.

Before we jump straight to the answer, let’s take a look at our own Sun for a sense of scale. Our familiar star is a mighty 1.4 million km across (870,000 miles). That’s such a huge number that it’s hard to get a sense of scale. The Sun accounts for 99.9% of all the matter in our Solar System. In fact, you could fit one million planet Earths inside the Sun.

Astronomers use the terms “solar radius” and “solar mass” to compare large and smaller stars, so we’ll do the same. A solar radius is 690,000 km (432,000 miles) and 1 solar mass is 2 x 1030 kilograms (4.3 x 1030 pounds). That’s 2,000,000,000,000,000,000,000,000,000,000 kg.

One huge, famous star in our galaxy is the monster Eta Carinae, located approximately 7,500 light years away, and weighing in at 120 solar masses. It’s a million times as bright as the Sun. Most stars blow with a solar wind, losing mass over time. But Eta Carinae is so large that it casts off 500 times the mass of the Earth every year. With so much mass lost, it’s very difficult for astronomers to accurately measure where the star ends, and its stellar wind begins.

So the best answer astronomers have right now is that Eta Carinae’s radius is 250 times the size of the Sun. And as star size estimates go, that’s pretty accurate.

And one interesting side note: Eta Carinae should explode pretty soon as one of the most spectacular supernovae humans have ever seen.

But the most massive star in the Universe is thought to be R136a1, located in the Large Magellanic Cloud. There’s some controversy, but it might contain as much as 265 times the mass of the Sun. And this is a puzzle for astronomers, since the largest theoretical stars were thought to be about 150 solar masses, formed in the early Universe, when stars were made of hydrogen and helium left over from the Big Bang. The answer to this contradiction is that R136a1 was probably formed when several massive stars merged together. Needless to say, R136a1 is set to detonate as a hypernova, any day now.

In terms of large stars, let’s look at Betelgeuse, the familiar star located in the shoulder of Orion. This red supergiant star has a radius of 950-1200 times the size of the Sun, and would engulf the orbit of Jupiter if placed in our Solar System.

But that’s nothing. The largest known star is VY Canis Majoris; a red hypergiant star in the Canis Major constellation, located about 5,000 light-years from Earth. University of Minnesota professor Roberta Humphreys recently calculated its upper size at more than 1,540 times the size of the Sun. Placed in our Solar System, its surface would extend out past the orbit of Saturn.

That’s the biggest star that we know of, but the Milky way probably has dozens of stars that are even larger, obscured by gas and dust so we can’t see them.

But let’s see if we can work out the original question, what’s the biggest star in the Universe? Obviously, it’s impossible for us to actually find it – the Universe is a big place, and there’s no way we can peer into every corner.
Pistol Star, another star thought to be one of the largest. Image credit: Hubble
But according to theorists, how big can stars get?

I contacted Roberta Humphreys from the University of Minnesota, the researcher who calculated the size of VY Canis Majoris, and posed this question to her. She noted that the largest stars are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and so only a mere 250 solar radii.

The largest stars will be the cool supergiants. For example, VY Canis Majoris is only 3,500 Kelvin. A really big star would be even cooler. At 3,000 Kelvin, a cool supergiant would be 2,600 times the size of the Sun.

That, she believes, is the largest possible star.

Finally, here’s a great animation that shows the size of various objects in space, starting with our tiny planet and finally getting to VV Cephei A. I guess they didn’t have the new info on VY Canis Majoris to include it in the animation.

We have written many articles about stars for Universe Today. Here’s a list of the 10 brightest stars.

Want to learn more about the birth and death of stars? We did a two part podcast at Astronomy Cast. Here’s part 1, Where Stars Come From, and here’s part 2, How Stars Die.

Sources:
NASA: Eta Carinae
NASA: What is the biggest star we know?

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Comments on this entry are closed.

  • Hayley October 6, 2008, 1:24 PM

    ohmygoodness, your six year old daughter sounds so adorable. :) it’s amazing how impressionable and interested she is. thank you for sharing, this was very helpful, and very interesting too. it’s amazing how although we look at the sun and think wow, that is HUGE, we don’t realise how tiny it actually is.

    wow, what a big universe. :)

  • Chris Thompson October 9, 2008, 11:46 AM

    Just a passing note – not a criticism or anything – we all do this sort of thing … but I had to smile when I read: “Light takes more than 8 hours to cross its circumference.”

  • Chris Thompson October 9, 2008, 11:59 AM

    Oh yes, and also … on the topic of children. We (local astro club) took our telescopes out to a (primary) school yesterday and had the lovely report from the headmistress that several of the children had dragged their parents outside, away from their “boring television shows”, to look at the cool things in the sky. The future may be uncertain, but there are still some bright young minds around to inherit it.

  • J.E October 29, 2008, 11:00 AM

    Kamal: If a planet was that big, the gravitational field would be so strong that it would make life impossible on the planet. That’s if there really is life outside this planet anyway, which I tend to doubt. By the way, I believe God created the heavens and the earth, if not, how did the earth get to just the right place to support life. If our orbit was off by the slightest margin, or the sun was a little bit farther away from us, life could not exist.

  • Interested November 6, 2008, 3:03 AM

    Well all this information on VY Canis Majoris and my father believes its till only a theorie and that scientists are guessing it exists without proof is that true or is there some proof which can convice him otherwise?

  • Angel Elf November 12, 2008, 5:44 AM

    I don’t understand why VY Canis Majoris isn’t marked on the map of the constellation Canis Major. Or am I missing something?

  • RobRob November 18, 2008, 10:37 AM

    i have many questions and so on but i have one big but simple question!?!? have we ever recorded the sun moving in circles as we rotate around it?? picture a dizzy symple circle till it comes to its comlete center and spins on spot….???? i havent search that part of the internet yet just checking my theories……and i liked the way you went to explain the biggest star…..but i’m a fanatic about everything that can be questioned haha like your daughter

  • RobRob November 18, 2008, 10:40 AM

    hah i’m tired i re read it and still missed that symbole haha

  • AKASHOW December 2, 2008, 1:12 AM

    have we ever thought of the universe as one GIANT ATOM???? cuz the atom is so complexed like the mind……… so….. i dont know but i do know that i dont know was the first thing i knew i believe before i even knew what i was doing :)

  • jesus December 3, 2008, 7:34 PM

    My stars bigger than yours! :) :p

  • jesus December 3, 2008, 7:34 PM

    lolololollol eat a penis!
    Internet lol!

  • João Martins December 4, 2008, 12:56 PM

    Hi.
    I’m just asking if VY Canis Majoris is only 2’100 times the size of the Sun, because I heard that Beteigeuze is 3’600 times the size of the Sun. I wish you could answer me this question because I love studying the Universe and this doubt is making me think.
    Thank you for your patience,
    João Martins, 11 years old, Portugal.

  • AGuy December 11, 2008, 2:29 PM

    Wow, those stars are HUGE! And only 2100 times the size of the sun? The theorectical limit is actually 2600, so imagine that. :)

  • Meme virus January 7, 2009, 11:05 PM

    Yes.

  • william January 9, 2009, 2:13 AM

    Christian humor: I’m trying to to an interview for the biography/ autobiography for the book
    “The bible, the inspired word of God” I tried to google him after I got his real name and aliases. I tracked down and found lots of satellite offices all over the world.

    Everyone was very helpful, I went into a few and asked if he was here, I was told yes absolutely ! I did as they asked and I took a seat. I waited , but he never showed up, I sat there and heard many people talk about his book but again I never caught him showing up, boy even Billy Graham is easier to get in touch with!

    Some offered to take a message and promised it would get to him. I was told he checks in from time to time.

    I thought I would read his book and see if there were any clues as to where his house was.

    This is what I found out,

    Lets see there are 365 days in one earth year, and if 1000 years to us is a day to God. Then that means: The earth in 365 days travels 584 million miles . so 584 X 1000 years=
    584 billion miles is Gods circumference he must cover in one year if his orbit was as fast as ours, his diameter of his obit would be from one side to the other is about 185. billion miles, and the radius, if the sun was the center would be about 92 billion miles from the sun.

    That would make one of his days equal to 1000 of our years. Sounds right? Maybe I’m just guessing. That would make Sedna Gods Home area, but its orbit is 10,000 years for one of ours. So I must have forgotten something..hum? that means 365,000 years is needed for 365 of our days. so that one day is a thousand, right? so we will divide 365,000 years by 10,000 and we get 36.5 X 92 billion miles and we get 3.358 trillion miles as his radius and 6.716 trillion miles would be his diameter to travel from one side of his orbit to the other. He would have to travel about 21 trillion (circumference) miles to do one orbit around a sun. so It would still take us a half a light year to get to God’s place, right? Nope he could be anywhere.

    What if his orbit goes around, lets say Alpha Centauri? or any other star/sun in the universe..I guess he was just trying to make a comparison..Oh well, at least I know his aproximate orbit he needs to have around a sun to to equal 1000 of our years to 1 of his days.. Now I just have to narrow it down to the billions of solar systems in the universe.. Hey I have an idea to narrow it down! I need to find a sun bright enough to warm a planet that has a 365,000 year orbit. We were made in his image, so I guess we can assume he is warm blooded like us.
    back to the drawing board..He could be anywhere..

    I looked for him once again and tried some star maps. Still no address found? However, I did find some of his neighbors, I thought.
    Background of new info:
    Sedna’s:, but it is calculated at between 10.5 and 12.0 thousand years to orbit the sun.
    Eris ( new dwarf planet) takes 557 years to orbit the Sun,

    I tried again to Google Gods location with information of places between Sedna and Eris, but they have not found his house yet.

    I will keep trying! I’m sure he will send me a reply, I guess he wants to remain mysterious. Maybe he has been looking over my shoulder all the time. I think that address clue he gave was a red herring. I was told to try Red Antares it is only 600 light-years away..Now that is way out of the way! I sure don’t have that much gas money…I’m sure he is not there.

    I found out the government is looking for him to, they sent a rocket named New Horizons to go and find his address out near some Kuiper belt.
    ( I wonder if his tax exemption has to be verified?) oh well,

    I’m sure if I catch up with him and get the interview, I will get the Pulitzer! I will keep you updated.

    He seems to be going around and doing good things for people in secret. Maybe I will check with some of them and see if they know where I could find him

  • william January 9, 2009, 2:47 AM

    Here is my question to the above information,

    Could the whole universe be the mass like the sun to us as it is to his domain? Meaning , could his domain could be like Mercury to the universe? Could his domain travel around the universe at a higher rate of speed? Making his rotation last 365,000 years? If you added all the mass in the universe what would be the gravitational pull to it, and at what distance to the universe would his domain have to be?

    Doe that question make any sense?

    I guess that is an enigma.

  • william January 9, 2009, 3:29 AM

    The scale is the enigma,

    If the universe was 88 million light year round?
    and the diameter was 28 million light years wide
    and the radius was 14 million light years to the center.

    how fast would he have to travel to orbit the universe, if 1 day to him is 1000 years to us

  • Said January 13, 2009, 7:16 AM

    Omg how is our poor planet big , a last star is too big and i hope it will not be our sun :)

  • Johnson Lin January 15, 2009, 7:25 PM

    interesting facts :)

  • jgvv February 2, 2009, 1:56 AM

    nice web

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