Just how big is UY Scuti?

I was wondering just how big stars like UY Scuti really is, as it is the largest one we have ever found. There aren’t consistent sources about it’s size, but have it placed where the sun is, it’s photosphere would extend past Jupiter, but what exactly does that mean? How far would it’s surface extend to?

UY Scuti (the bright star in the middle of the image above) is possibly one of the largest stars we’ve observed to date - I say possibly because the exact radius is still somewhat uncertain, so if it’s at the smaller edge of it’s potential range, there are other stars which could be bigger.

UY Scuti is, in any case, classified as a ‘hypergiant’ star - which is the classification which comes after 'supergiant’ and regular old 'giant’. Its 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 is only 30 times the radius of the sun). Mass and physical size don’t always correlate for stars nearing the end of their lives.

UY Scuti is only around 30 times more massive than the sun, but 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 be physically changing over time. The margin of error on this measurement is about 192 solar radii, which is a lot of possible error (although fractionally it’s pretty small), and if we consider that the star was measured not at its brightest, the size of the star will probably reach past the high end of this uncertainty when it brightens.

The size of the star from the centre to the edge (1700 solar radii), is equivalent to 750 million miles, or nearly 8 astronomical units, where 1 astronomical unit is the distance between the earth and the Sun. As you mention, this is large enough that it would extend past Jupiter. The complication with stars is that they become diffuse at their edges. Most stars don’t have a rigid surface where the gas ends and vacuum begins, as a harsh divider and easy marker of the end of the star. So in order to usefully define the ‘edge’ of the star, we use the location of the photosphere. The photosphere is where the star becomes transparent to light, and photons 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. Any further below this level, and the photons are caught in a series of bounces, and can’t 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, which extend beyond the photosphere.

For UY Scuti, whose photosphere extends beyond the orbit of Jupiter, this means that the light produced in the centre of that 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 had been lost from the star over the years - the 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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