How Big Is The Biggest Star We Have Ever Found?

How Big Is The Biggest Star We Have Ever Found? 1

The universe is such a big place that it is easy to get baffled by the measurements that astronomers make. The size of UY Scuti, possibly one of the largest stars we have observed to date, is certainly baffling.

Above: UY Scuti (Rutherford Observatory/CC).

Not surprisingly, UY Scuti is classified as a “hypergiant” star – which is the classification which comes after “supergiant” and regular “giant.” Its size may make it the biggest, but it is not the most massive star (where the “mass” in massive stands for the amount of matter in the star).

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UY Scuti’s mass is probably slightly more than 30 times the mass of our Sun, which places it nowhere near the top of the most massive stars list. That honor is held by a star with the charming name of R136a1, which clocks in at 265 times as massive as the Sun, but only 30 times the radius of the sun. Mass and physical size don’t always correlate for stars, particularly the case for giant stars.


UY Scuti size comparison to the Sun. (Philip Park/CC).

So while UY Scuti is only around 30 times more massive than the sun, it has a radius somewhere in the region of 1,700 times larger than the radius of the sun. This star is one of a class of stars that varies in brightness because it varies in size, so this number is also likely to change over time. The margin of error on this measurement is about 192 solar radii. This uncertainty is why I used “possibly one of the largest stars” in my description of UY Scuti. If it is smaller by 192 solar radii, there are a few other candidates that would beat UY Scuti.

The approximate size of the star is about 750m miles, or nearly eight astronomical units, where one astronomical unit is the distance between the earth and the sun. This is large enough that it would extend past Jupiter.

The complication with stars is that they have diffuse edges. Most stars don’t have a rigid surface where the gas ends and vacuum begins, which would have served as a harsh dividing line and easy marker of the end of the star.

So in order to usefully define the “edge” of a star, we use the location of the photosphere. The photosphere is where the star becomes transparent to light, and where photons – that is, light particles – can escape. As far as an astrophysicist is concerned, this is the surface of the star, as this is the point at which photons can leave the star. Going any further towards the center of the star would mean photons would get caught in a series of bounces, and unable to stream freely.

To be clear, the photosphere is not where the gas of the star ends – stars also have atmospheres, which are transparent to light, and which extend beyond the photosphere. But the atmosphere is not considered to be part of the star when it comes to defining its radius.

For UY Scuti, whose photosphere extends beyond the orbit of Jupiter, this means that the light produced in the centre of the star would not be able to stream freely from the star until it had made it beyond Jupiter. Beyond that, there would be a diffuse, hot atmosphere which would extend even further out into our solar system, and a large amount of gas and dust which was lost from the star over the years. This nebula of gas lost from the star extends out to 400 astronomical units, which is 10 times further out than Pluto.


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  1. Its Moons would still be visible! A brown dwarf star would exist only in the Fusion based mainstream stellar model in an electric universe this would just be a gassy planetary body!

  2. Number One…. A darkstar Emits no light because it is a gassy star not lit yet.. It core temperature of a darkstar would be around -455 degrees and that is almost the same temperature of deep space as absolute zero is -458.. You need an infrared telescope based in space to see it coming.. But this Star would emit lots of Gamma Radiation and it would have a very strong magnetic influence on our star..

  3. If it were that close it would be seen in the night sky by now! Oh I forgot you can only see it with a camera pointed at the sun! Next year we’ll be seeing this story all over again ! So on september 13th I will stick my head between my legs and kiss my hairy butt goodbye and again on 14th and again on 15th Then on 16th I’ll kiss my wife! 😛

  4. September 28th is the day they say this will occur.. And they said the Gates of Hell and the Gates of Heaven opened July 27th… My Nickname is Rayzor, born July 27th, My Wife’s Nickname is DarkStar, born September 28th.. These dates have been played on for two years now, Why is always my question…

  5. As the DarkStar arrives it will clear a path through what they call the asteroid field as this asteroid field was a planet at one time that was hit by Nibiru and it smashed into it and the explosion of this planet scraped the outer surface off of Mars making the comets that circle our sun as this comet debris is the scrapped of surface of Mars…

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