This article was initially published in 2008, however has been updated several times currently to keep track v our advancing knowledge of the cosmos!

My six-year old daughter is a question-asking machine. Us were driving house from institution a couple of work ago, and she to be grilling me about the nature of the Universe. One of her zingers was, “What’s the greatest Star in the Universe”? ns had an easy answer. “The universe is a huge place,” ns said, “and there’s no way we can possibly understand what the biggest star is”. Yet that’s not a real answer.

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So she sleek the question. “What’s the biggest star that we recognize of?” the course, i was grounding in the car, and without accessibility to the Internet. However once ns got earlier home, and also was may be to perform some research, i learned the answer and also thought I’d re-superstructure it with the rest of you yet to answer the fully, some basic background info needs to be extended first. Ready?


Solar Radius and also Mass:

When talking about the dimension of stars, it’s vital to first take a look at our very own Sun for a feeling of scale. Our familiar star is a mighty 1.4 million km across (870,000 miles). That’s together a large number the it’s tough to gain a feeling of scale. Speak of which, the Sun also accounts for 99.9% of every the matter in ours Solar System. In fact, you might fit one million earth Earths within the Sun.

Using these values, astronomers have produced the state “solar radius” and “solar mass”, which they usage to to compare stars of greater or smaller sized size and also mass to our own. A solar radius is 690,000 kilometres (432,000 miles) and also 1 solar massive 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.

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Artist’s relenten of the Morgan-Keenan spectral diagram, reflecting the difference in between main succession stars. Credit: Wikipedia Commons

Another thing worth considering is the truth that our sun is nice small, as stars go. As a G-type main-sequence star (specifically, a G2V star), which is typically known as a yellow dwarf, its ~ above the smaller finish of the dimension chart (see above). While that is definitely larger than the most common type of star – M-type, or Red Dwarfs – that is itself dwarfed (no pun!) through the likes the blue giants and also other spectral classes.


Classification:

To break it all down, stars space grouped based on their essential characteristics, which deserve to be their spectral course (i.e. Color), temperature, size, and brightness. The most common technique of group is well-known as the Morgan–Keenan (MK) system, which classifies stars based upon temperature utilizing the letter O, B, A, F, G, K, and also M, – O gift the hottest and also M the coolest. Each letter course is climate subdivided using a numeric digit through 0 being hottest and also 9 gift coolest (e.g. O1 come M9 room the hottest to coldest stars).

In the MK system, a luminosity course is included using roman numerals. This are based upon the broad of particular absorption present in the star’s spectrum (which vary through the density of the atmosphere), thus separating giant stars indigenous dwarfs. Luminosity great 0 and I use to hyper- or supergiants; class II, III and IV apply to bright, consistent giants, and also subgiants, respectively; course V is because that main-sequence stars; and also class VI and VII apply to subdwarfs and also dwarf stars.

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

There is also the Hertzsprung-Russell diagram, which relates stellar group to absolute magnitude (i.e. Intrinsic brightness), luminosity, and also surface temperature. The same classification for spectral species are used, ranging from blue and also white at one finish to red in ~ the other, i beg your pardon is then an unified with the stars pure Visual size (expressed together Mv) to ar them top top a 2-dimensional chart (see above).


On average, stars in the O-range space hotter than other classes, reaching efficient temperatures of as much as 30,000 K. In ~ the exact same time, castle are additionally larger and an ext massive, getting to sizes of end 6 and also a half solar radii and also up to 16 solar masses. At the lower end, K and also M kind stars (orange and red dwarfs) tend to be cooler (ranging indigenous 2400 come 5700 K), measure up 0.7 to 0.96 times that of ours Sun, and being almost everywhere from 0.08 to 0.8 as massive.

Based ~ above the complete of category of our sun (G2V), us can thus say that it a main-sequence star with a temperature around 5,800K. Now consider one more famous star mechanism in our galaxy – Eta Carinae, a mechanism containing at the very least two stars located approximately 7500 light-years far in the direction that the constellation Carina. The major of this mechanism is estimated to it is in 250 times the size of our Sun, a minimum that 120 solar masses, and a million times as bright – make it one of the biggest and also brightest stars ever before observed.

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

There is some dispute over this world’s size though. Most stars blow through a solar wind, losing mass end time. However Eta Carinae is so big that it casts turn off 500 times the fixed of the planet every year. V so much mass lost, it’s very challenging for astronomers come accurately measure where the star ends, and its mainly wind begins. Also, the is believed that Eta Carinae will certainly explode in the not-too-distant future, and also it will be the many spectacular supernovae humans have ever seen.


In regards to sheer mass, the optimal spot goes come R136a1, a star situated in the large Magellanic Cloud, part 163,000 light-years away. The is thought that this star might contain as much as 315 time the fixed of the Sun, which presents a conundrum come astronomers since it was thought that the largest stars could only save on computer 150 solar masses. The answer come this is the R136a1 was probably formed when several massive stars an unified together. Needless to say, R136a1 is collection to detonate together a hypernova, any kind of day now.

In terms of large stars, Betelgeuse serves together a good (and popular) example. Located in the shoulder of Orion, this familiar red supergiant has actually a radius the 950-1200 time the size of the Sun, and would engulf the orbit that Jupiter if placed in our Solar System. In fact, anytime we want to placed our Sun’s size right into perspective, we often use Betelgeuse to do it (see below)!

Yet, also after we usage this hulking Red giant to put us in ours place, we space still just scratching the surface in the game of “who’s the biggest star”. Think about WOH G64, a red supergiant star situated in the big Magellanic Cloud, around 168,000 light years from Earth. At 1.540 solar radii in diameter, this star is currently one the the largest in the recognized universe.


But there’s likewise RW Cephei, one orange hypergiant star in the constellation Cepheus, situated 3,500 light years from Earth and measuring 1,535 solar radii in diameter. Westerlund 1-26 is additionally pretty huge, a red supergiant (or hypergiant) situated within the Westerlund 1 at sight star swarm 11,500 light-years away that procedures 1,530 solar radii in diameter. Meanwhile, V354 Cephei and VX Sagittarii are tied once it comes to size, with both measure an estimated 1,520 solar radii in diameter.

The biggest Star: UY Scuti

As that stands, the title of the largest star in the cosmos (that we understand of) comes under to 2 contenders. Because that example, UY Scuti is currently at the peak of the list. Located 9.500 light years far in the constellation Scutum, this bright red supergiant and pulsating change star has an estimated average average radius the 1,708 solar radii – or 2.4 billion kilometres (1.5 billion mi; 15.9 AU), thus offering it a volume 5 billion times that of the Sun.

However, this median estimate includes a margin of error that ± 192 solar radii, which way that it can be as large as 1900 solar radii or as tiny as 1516. This lower estimate places it in ~ stars like as V354 Cephei and also VX Sagittarii. Meanwhile, the 2nd star on the perform of the largest feasible stars is NML Cygni, a semiregular change red hypergiant situated in the Cygnus constellation part 5,300 light-years native Earth.


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

Due come the place of this star within a circumstellar nebula, that is heavily obscured by dust extinction. Together a result, astronomers estimate that that size could be everywhere from 1,642 to 2,775 solar radii, which method it might either be the largest star in the recognized Universe (with a margin the 1000 solar radii) or certainly the second largest, ranking not far behind UY Scuti.

And up till a couple of years ago, the title of best star went to VY Canis Majoris; a red hypergiant star in the Canis major constellation, located about 5,000 light-years from Earth. Ago in 2006, professor Roberta Humphrey of the college of Minnesota calculation its top size and also estimated the it could be an ext than 1,540 times the dimension of the Sun. The average approximated mass, however, is 1420, placing that in the no. 8 clues behind V354 Cephei and VX Sagittarii.

These are the greatest star that we understand of, yet the Milky means probably has actually dozens the stars the are also larger, hidden by gas and dust so us can’t see them. Yet even if us cannot uncover these stars, the is feasible to theorize around their most likely size and mass. So just how huge can stars get? once again, Professor Roberta Humphreys of the college of Minnesota provided the answer.


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Size comparison in between the Sun and VY Canis Majoris, i beg your pardon once hosted the title of the largest recognized star in the Universe. Credit: Wikipedia Commons/Oona Räisänen

As she defined when contacted, the largest stars in the Universe room the coolest. So also though Eta Carinae is the many luminous star we understand of, it’s extremely hot – 25,000 Kelvin – and also therefore only 250 solar radii big. The biggest stars, in contrast, will be cool supergiants. Instance in point, VY Canis Majoris is just 3,500 Kelvin, and also a really huge star would certainly be also cooler.

At 3,000 Kelvin, Humphreys estimates that cool supergiant would be as huge as 2,600 time the size of the Sun. This is below the upper approximates for NML Cygni, but over the average estimates for both it and UY Scutii. Hence, this is the upper limit of a star (at the very least theoretically and also based on every the info we have to date).

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But as we continue to peer right into the world with all of our instruments, and explore it up close with robotic spacecraft and also crewed missions, we room sure to find brand-new and exciting things that will certainly confound us further!


And be sure to inspect out this an excellent animation that mirrors the size of assorted objects in space, starting with our Solar System’s small planets and also finally getting to UY Scuti. Enjoy!

We have actually written many articles around stars for world Today. Here’s The Sun, What’s the Brightest Star in the skies Past and Future?, and also What Is The the smallest Star?

Want come learn more about the birth and also death that stars? us did a two part podcast at Astronomy Cast. Here’s part 1, whereby Stars Come From, and also here’s part 2, just how Stars Die.