This write-up was originally published in 2008, yet has been updated several times currently to store track v our advancing expertise of the cosmos!

My six-year old daughter is a question-asking machine. We were driving house from school a couple of job ago, and she was grilling me about the nature the the Universe. One of her zingers was, “What’s the biggest Star in the Universe”? i had simple answer. “The universe is a huge place,” i said, “and there’s no means we deserve to possibly understand what the biggest star is”. However that’s not a real answer.

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So she refined the question. “What’s the biggest star the we understand of?” that course, ns was grounding in the car, and also without access to the Internet. But once i got ago home, and also was may be to execute some research, ns learned the answer and thought I’d share it with the rest of you but to answer it fully, some simple background info needs to be spanned first. Ready?

## Solar Radius and also Mass:

When talking about the size of stars, it’s essential to first take a look in ~ our very own Sun because that a sense of scale. Our acquainted star is a mighty 1.4 million km throughout (870,000 miles). That’s such a huge number the it’s difficult to obtain a sense of scale. Speaking of which, the Sun also accounts for 99.9% of every the issue in our Solar System. In fact, you could fit one million planet Earths within the Sun.

Using this values, astronomers have produced the terms “solar radius” and also “solar mass”, i beg your pardon they usage to to compare stars of higher or smaller size and also mass to our own. A solar radius is 690,000 kilometres (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 relenten of the Morgan-Keenan spectral diagram, mirroring the difference between main sequence stars. Credit: Wikipedia Commons

Another thing worth considering is the reality that our sunlight is nice small, as stars go. Together a G-type main-sequence star (specifically, a G2V star), i beg your pardon is frequently known together a yellow dwarf, its ~ above the smaller end of the size chart (see above). While it is definitely larger 보다 the many common form of star – M-type, or Red Dwarfs – it is itself dwarfed (no pun!) by the likes that blue giants and also other spectral classes.

## Classification:

To break it all down, stars room grouped based upon their necessary characteristics, which have the right to be their spectral course (i.e. Color), temperature, size, and also brightness. The many common an approach of group is well-known as the Morgan–Keenan (MK) system, i m sorry classifies stars based on temperature utilizing the letters O, B, A, F, G, K, and also M, – O gift the hottest and M the coolest. Every letter class is climate subdivided using a numeric digit v 0 gift hottest and 9 gift coolest (e.g. O1 to M9 room the hottest come coldest stars).

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

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

There is likewise the Hertzsprung-Russell diagram, i m sorry relates stellar classification to absolute magnitude (i.e. Intrinsic brightness), luminosity, and surface temperature. The same classification for spectral species are used, varying from blue and also white in ~ one end to red at the other, i beg your pardon is then merged with the stars absolute Visual magnitude (expressed together Mv) to place them ~ above a 2-dimensional chart (see above).

On average, stars in the O-range space hotter than various other classes, reaching efficient temperatures of approximately 30,000 K. At the very same time, lock are likewise larger and more massive, reaching sizes of end 6 and a fifty percent solar radii and also up come 16 solar masses. At the reduced end, K and M form stars (orange and also red dwarfs) tend to be cooler (ranging indigenous 2400 come 5700 K), measure 0.7 to 0.96 times that of our Sun, and being anywhere from 0.08 to 0.8 as massive.

Based top top the full of category of our sunlight (G2V), we can therefore say the it a main-sequence star with a temperature roughly 5,800K. Currently consider one more famous star system in ours galaxy – Eta Carinae, a mechanism containing at the very least two stars located about 7500 light-years far in the direction the the constellation Carina. The major of this mechanism is approximated to be 250 time the size of our Sun, a minimum the 120 solar masses, and a million times as bright – do it one of the biggest and also brightest stars ever observed.

Eta Carinae, among the most huge stars known, located in the Carina constellation. Credit: NASA

There is some controversy over this world’s dimension though. Many stars blow through a solar wind, losing mass end time. Yet Eta Carinae is so large that the casts off 500 times the mass of the earth every year. V so much mass lost, it’s very difficult for astronomers come accurately measure whereby the star ends, and also its stellar wind begins. Also, the is thought that Eta Carinae will certainly explode in the not-too-distant future, and also it will be the many spectacular supernovae people have ever before seen.

In terms of sheer mass, the top spot goes come R136a1, a star situated in the huge Magellanic Cloud, some 163,000 light-years away. The is believed that this star may contain as much as 315 time the fixed of the Sun, which presents a conundrum come astronomers since it was believed that the biggest stars might only contain 150 solar masses. The answer come this is that R136a1 was probably created when several huge stars an unified together. Needless come say, R136a1 is collection to detonate as a hypernova, any day now.

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

Yet, also after we usage this hulking Red giant to put us in our place, we are still just scratching the surface ar in the game of “who’s the best star”. Consider WOH G64, a red supergiant star situated in the big Magellanic Cloud, roughly 168,000 irradiate years from Earth. At 1.540 solar radii in diameter, this star is currently one the the largest in the well-known universe.

But there’s also RW Cephei, one orange hypergiant star in the constellation Cepheus, situated 3,500 light years indigenous Earth and also measuring 1,535 solar radii in diameter. Westerlund 1-26 is additionally pretty huge, a red supergiant (or hypergiant) located within the Westerlund 1 at sight star swarm 11,500 light-years far that actions 1,530 solar radii in diameter. Meanwhile, V354 Cephei and also VX Sagittarii space 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 location of the biggest star in the universe (that we recognize of) comes down to two contenders. Because that example, UY Scuti is currently at the optimal of the list. Situated 9.500 light years far in the constellation Scutum, this shining red supergiant and also pulsating change star has actually an approximated average typical radius of 1,708 solar radii – or 2.4 billion kilometres (1.5 exchange rate mi; 15.9 AU), thus offering it a volume 5 billion times the of the Sun.

However, this average estimate consists of a margin of error that ± 192 solar radii, which method that it could be as big as 1900 solar radii or as little as 1516. This reduced estimate places it in ~ stars favor as V354 Cephei and VX Sagittarii. Meanwhile, the 2nd star on the perform of the largest feasible stars is NML Cygni, a semiregular variable red hypergiant situated in the Cygnus constellation part 5,300 light-years from Earth.

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

Due come the place of this star within a circumstellar nebula, the is heavily obscured through dust extinction. As a result, astronomers estimate that its size can be everywhere from 1,642 to 2,775 solar radii, which way it could either it is in the biggest star in the recognized Universe (with a margin the 1000 solar radii) or indeed the 2nd largest, ranking not far behind UY Scuti.

And up till a few years ago, the title of best star saw VY Canis Majoris; a red hypergiant star in the Canis significant constellation, located about 5,000 light-years from Earth. Earlier in 2006, professor Roberta Humphrey of the university of Minnesota calculate its top size and estimated that it could be more than 1,540 time the dimension of the Sun. Its average approximated mass, however, is 1420, placing the in the no. 8 clues behind V354 Cephei and VX Sagittarii.

These space the greatest star the we understand of, however the Milky method probably has actually dozens that stars that are also larger, hidden by gas and also dust so we can’t watch them. Yet even if us cannot uncover these stars, it is feasible to theorize about their likely size and also mass. So just how huge can stars get? when again, Professor Roberta Humphreys the the college of Minnesota noted the answer.

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

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

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

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But together we continue to peer right into the universe with all of our instruments, and also explore it up close through robotic spacecraft and crewed missions, we space sure to find new and exciting things that will certainly confound united state further!

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

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

Want come learn an ext about the birth and also death of stars? we did a two component podcast in ~ Astronomy Cast. Here’s part 1, wherein Stars Come From, and here’s component 2, just how Stars Die.