What is the Biggest Star in the Universe?

12 May , 2016 by Video

This article was originally published in 2008, but has been updated several times now to keep track with our advancing knowledge of the cosmos!

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, “What’s the Biggest Star in the Universe”? I had an easy answer. “The Universe is a big place,” I said, “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 once I got back home, and was able to do some research, I learned the answer and thought I’d share it with the rest of you But to answer it fully, some basic background information needs to be covered first. Ready?

Solar Radius and Mass:

When talking about the size of stars, it’s important to first 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. Speaking of which, the Sun also accounts for 99.9% of all the matter in our Solar System. In fact, you could fit one million planet Earths inside the Sun.

Using these values, astronomers have created the terms “solar radius” and “solar mass”, which they use to compare stars of greater or smaller size and mass to our own. 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 nonillion kilograms, or 2,000,000,000,000,000,000,000,000,000,000 kg.

Artist's depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons

Artist’s depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons

Another thing worth considering is the fact that our Sun is pretty small, as stars go. As a G-type main-sequence star (specifically, a G2V star), which is commonly known as a yellow dwarf, its on the smaller end of the size chart (see above). While it is certainly larger than the most common type of star – M-type, or Red Dwarfs – it is itself dwarfed (no pun!) by the likes of blue giants and other spectral classes.

Classification:

To break it all down, stars are grouped based on their essential characteristics, which can be their spectral class (i.e. color), temperature, size, and brightness. The most common method of classification is known as the Morgan–Keenan (MK) system, which classifies stars based on temperature using the letters O, B, A, F, G, K, and M, – O being the hottest and M the coolest. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. O1 to M9 are the hottest to coldest stars).

In the MK system, a luminosity class is added using Roman numerals. These are based on the width of certain absorption lines in the star’s spectrum (which vary with the density of the atmosphere), thus distinguishing giant stars from dwarfs. Luminosity classes 0 and I apply to hyper- or supergiants; classes II, III and IV apply to bright, regular giants, and subgiants, respectively; class V is for main-sequence stars; and class VI and VII apply to subdwarfs and dwarf stars.

The Hertzspirg-Russel diagram, showing the relation between star's color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com

The Hertzspirg-Russel diagram, showing the relation between star’s color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com

There is also the Hertzsprung-Russell diagram, which relates stellar classification to absolute magnitude (i.e. intrinsic brightness), luminosity, and surface temperature. The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude (expressed as Mv) to place them on a 2-dimensional chart (see above).

On average, stars in the O-range are hotter than other classes, reaching effective temperatures of up to 30,000 K. At the same time, they are also larger and more massive, reaching sizes of over 6 and a half solar radii and up to 16 solar masses. At the lower end, K and M type stars (orange and red dwarfs) tend to be cooler (ranging from 2400 to 5700 K), measuring 0.7 to 0.96 times that of our Sun, and being anywhere from 0.08 to 0.8 as massive.

Based on the full of classification of our Sun (G2V), we can therefore say that it a main-sequence star with a temperature around 5,800K. Now consider another famous star system in our galaxy – Eta Carinae, a system containing at least two stars located around 7500 light-years away in the direction of the constellation Carina. The primary of this system is estimated to be 250 times the size of our Sun, a minimum of 120 solar masses, and a million times as bright – making it one of the biggest and brightest stars ever observed.

Eta Carinae, one of the most massive stars known. Image credit: NASA

Eta Carinae, one of the most massive stars known, located in the Carina constellation. Credit: NASA

There is some controversy over this world’s size though. 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. Also, it is believed that Eta Carinae will explode in the not-too-distant future, and it will be the most spectacular supernovae humans have ever seen.

In terms of sheer mass, the top spot goes to R136a1, a star located in the Large Magellanic Cloud, some 163,000 light-years away. It is believed that this star may contain as much as 315 times the mass of the Sun, which presents a conundrum to astronomers since it was believed that the largest stars could only contain 150 solar masses. The answer to this 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, Betelgeuse serves as a good (and popular) example. Located in the shoulder of Orion, this familiar red supergiant 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. In fact, whenever we want to put our Sun’s size into perspective, we often use Betelgeuse to do it (see below)!

Yet, even after we use this hulking Red Giant to put us in our place, we are still just scratching the surface in the game of “who’s the biggest star”. Consider WOH G64, a red supergiant star located in the Large Magellanic Cloud, approximately 168,000 light years from Earth. At 1.540 solar radii in diameter, this star is currently one of the largest in the known universe.

But there’s also RW Cephei, an orange hypergiant star in the constellation Cepheus, located 3,500 light years from Earth and measuring 1,535 solar radii in diameter. Westerlund 1-26 is also pretty huge, a red supergiant (or hypergiant) located within the Westerlund 1 super star cluster 11,500 light-years away that measures 1,530 solar radii in diameter. Meanwhile, V354 Cephei and VX Sagittarii are tied when it comes to size, with both measuring an estimated 1,520 solar radii in diameter.

The Largest Star: UY Scuti

As it stands, the title of the largest star in the Universe (that we know of) comes down to two contenders. For example, UY Scuti is currently at the top of the list. Located 9.500 light years away in the constellation Scutum, this bright red supergiant and pulsating variable star has an estimated average median radius of 1,708 solar radii – or 2.4 billion km (1.5 billion mi; 15.9 AU), thus giving it a volume 5 billion times that of the Sun.

However, this average estimate includes a margin of error of ± 192 solar radii, which means that it could be as large as 1900 solar radii or as small as 1516. This lower estimate places it beneath stars like as V354 Cephei and VX Sagittarii. Meanwhile, the second star on the list of the largest possible stars is NML Cygni, a semiregular variable red hypergiant located in the Cygnus constellation some 5,300 light-years from Earth.

A zoomed-in picture of the red giant star UY Scuti. Credit: Rutherford Observatory/Haktarfone

A zoomed-in picture of the red giant star UY Scuti. Credit: Rutherford Observatory/Haktarfone

Due to the location of this star within a circumstellar nebula, it is heavily obscured by dust extinction. As a result, astronomers estimate that its size could be anywhere from 1,642 to 2,775 solar radii, which means it could either be the largest star in the known Universe (with a margin of 1000 solar radii) or indeed the second largest, ranking not far behind UY Scuti.

And up until a few years ago, the title of biggest star went to VY Canis Majoris; a red hypergiant star in the Canis Major constellation, located about 5,000 light-years from Earth. Back in 2006, professor Roberta Humphrey of the University of Minnesota calculated its upper size and estimated that it could be more than 1,540 times the size of the Sun. Its average estimated mass, however, is 1420, placing it in the no. 8 spot behind V354 Cephei and VX Sagittarii.

These are 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 even if we cannot find these stars, it is possible to theorize about their likely size and mass. So just how big can stars get? Once again, Professor Roberta Humphreys of the University of Minnesota provided the answer.

VY Canis Majoris. The biggest known star.

Size comparison between the Sun and VY Canis Majoris, which once held the title of the largest known star in the Universe. Credit: Wikipedia Commons/Oona Räisänen

As she explained when contacted, the largest stars in the Universe are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and therefore only 250 solar radii big. The largest stars, in contrast, will be cool supergiants. Case in point, VY Canis Majoris is only 3,500 Kelvin, and a really big star would be even cooler.

At 3,000 Kelvin, Humphreys estimates that cool supergiant would be as big as 2,600 times the size of the Sun. This is below the upper estimates for NML Cygni, but above the average estimates for both it and UY Scutii. Hence, this is the upper limit of a star (at least theoretically and based on all the information we have to date).

But as we continue to peer into the Universe with all of our instruments, and explore it up close through robotic spacecraft and crewed missions, we are sure to find new and exciting things that will confound us further!

And be sure to check out this great animation that shows the size of various objects in space, starting with our Solar System’s tiny planets and finally getting to UY Scuti. Enjoy!

We have written many articles about stars for Universe Today. Here’s The Sun, What’s the Brightest Star in the Sky Past and Future?, and What Is The Smallest Star?

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.

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Sort by:   newest | oldest | most voted
Polaris93
Member
April 6, 2008 10:58 PM

Erm, the animation wasn’t there. Could somebody please redo this page?

tsuchan
Member
tsuchan
May 12, 2016 8:57 PM

@Polaris93: Are you sure you haven’t got some Google-blocker or something like that, preventing the animation (video?) you’re referring to from loading?

Haplo
Guest
Haplo
April 6, 2008 11:57 PM

I leave you the same animation but with music, Also Sprach Zaratustra (sp?), and it adds a lot more to the effect.

Cheers.

sofista
Member
April 6, 2008 5:53 PM

Mi hija de seis años es una máquina de hacer preguntas. Hace un par de días regresábamos a casa en el auto y ella me torturaba con preguntas sobre la naturaleza del universo. Una de sus preguntas más ocurrentes fue ¿Cuál es la estrella más grande del universo? […] Fuente: Fraser Cain para Universe Today.

KK
Guest
KK
April 6, 2008 11:10 PM

Gotta love the curiosity and the ability of asking simple but almost impossible to answer questions of 6-year olds…
Thank your daughter from me!
Indirectly she increased my feeling of awe for the universe.

marcellus
Guest
marcellus
April 7, 2008 6:19 AM

Mike Portugal.

You watch a cool video like that and the most important thing to pop into your head is to complain about Mr. Bush’s ego?

Get a life.

dknie@netnitco.net
Member
[email protected]
April 7, 2008 4:44 AM

Welcome back, Fraser. I was worried something might have happened to you. Ian is doing a good job. However, please don’t scare us like that again.

alphonso richardson
Guest
alphonso richardson
April 7, 2008 5:25 AM

When I was that age, I was told to shut up when I asked questions like that @ school.
Full marks on attempting to answer a very curious child’s question.
Cool animation

Mark Van Loon
Guest
Mark Van Loon
April 7, 2008 5:53 AM

The followup question is what is the star with the most mass?

Mike - Portugal
Guest
Mike - Portugal
April 7, 2008 6:06 AM

Damn we are so small… yet our Human ego wouldn’t fit on any of those stars… In fact it would take a lot more than that just to accomodate Mr. Bush’s ego wouldn’t it ?? lol

kootstar
Member
kootstar
April 7, 2008 2:24 PM

My poor dsl hookup is fussy, but by going to that other (Thanks Haplo!!) on Youtube I got one reaction, W O W ! ! And that was repeated all 5 times I watched and listened to it.

dre
Guest
dre
April 7, 2008 7:13 PM

Crazy! My almost-5-year-old son asked me the same question in the car today! I told him I really didn’t know, but I guessed maybe a star might exist that was 1 million solar masses (he pretended to understand), but if it did, it wouldn’t last long. That leads to the next question, mentioned by Mr. van Loon in a comment above: what’s the greatest know mass of a star, and is there a theoretical upper limit to a star’s mass? My son is waiting…

alokmohan
Guest
alokmohan
April 7, 2008 10:10 PM

2600 solar raddi is biggest star in universe.Right?

alokmohan
Guest
alokmohan
April 7, 2008 10:13 PM

Your little daughter is asking difficult questions at the age of 6.Be ware.

Dako
Guest
April 8, 2008 12:43 AM

Hmmm. It seams there is a larger star named A1. This new Star has an apparent mass of 114 times the Sun which would be 3.2 times larger then VV Cephei. With a possible Diameter of 5140-6080 times the Sun. So yes it would be even LARGER then VV Cephei and VY Canis Majoris.

http://www.cosmosmagazine.com/node/1383

Ella Kosta
Guest
Ella Kosta
April 8, 2008 2:13 AM

dre, there isn’t a star over 200 solar masses.

Mike - Portugal
Guest
Mike - Portugal
April 8, 2008 5:09 AM
Hey Mr. Marcellus, I DO have a life, for the time being at least, and YES that IS what comes to mind because this is where we all live, breath and everything else so i am much more interested in what is going on here on THIS planet and the politics which govern us all. So if you do not think that we should all take a “closer” look to what is going on here you sir are a child without a hint of a brain. So YOU get a life and by all means start contributing with something other than love for Bush cause we ( outside the US ) have already seen and felt what it… Read more »
davey
Member
davey
April 8, 2008 8:10 AM

Is it me, or do these two ginormous examples seem uncomfortably close (reletively) to us?

It’s me, right??

Harin
Guest
Harin
April 9, 2008 5:21 AM

There’s a star called LBV 1806-20 that’s ridiculously massive (150 solar masses), unbelievably bright (40 MILLION times solar) and sits right in our backyard – the Milky Way! This monster is so huge that if placed side-by-side with Eta Carinae, the latter would not be detectable. I for one don’t want this monster going hypernova any time soon…

StarFox
Guest
StarFox
May 13, 2016 5:08 PM

Harin, the star LBV 1806-20 is 2 million times as bright as the sun, not 40 mil. It is also 130 solar masses, not 150 and it’s diameter is 150 solar. Please check things out before spreading garbage. https://en.wikipedia.org/wiki/LBV_1806-20

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