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Compared to VY Canis Majoris, our Sun is more like a speck of space dust. Is there any planet or star which is known to us that is bigger than the VY Canis Majoris?

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    $\begingroup$ Note: There is no way for a planet to be that big. At that size, the object either collapses into a black hole, or resists collapse due to pressure from internal heat generated via the normal fusion in stars. It has to be a star, actively producing heat, in order to avoid collapse. As for stars bigger than that, it's possible (barely), but I'm not aware of any examples. At that size, even stellar objects cannot live very long, and therefore must be pretty rare. $\endgroup$ – Florin Andrei Jan 20 '15 at 19:23
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    $\begingroup$ See here and here. None are known exactly, and there's some debate on the subject. $\endgroup$ – HDE 226868 Jan 20 '15 at 19:43
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Part 1: Assuming "larger" means greater in diameter.

Stars
Estimates on the size of stars are just that, estimates, and estimates based on rather fuzzy observations. VY Canis Majoris has been bumped down to size. The current thinking is that there are seven known stars larger than VY Canis Majoris, the largest of which is UY Scuti.

Current models indicate that the first generation of stars were much, much larger than anything we see now. It will be quite some time before we can resolve a first generation star. So far, they are just theoretical objects.

Planets
Jupiter-mass planets are about as large as a planet can get. There are some exoplanets that are larger than Jupiter, but that's because they orbit much closer to their parent star than does Jupiter. This makes them puff up a bit. The reason Jupiter-mass planets are deemed to be the largest possible is that planets of this mass are presently assumed to have a core of degenerate hydrogen. A funny thing happens to degenerate masses when mass is added to them: They shrink in diameter. (The shrinkage becomes catastrophic as the mass approaches the Chandrasekhar limit.)

This means that assuming all other things are equal (temperature, composition), a planet more massive than Jupiter will be small in diameter than Jupiter is. Even if all other things aren't equal, a Jupiter-diameter planet is (give or take) about as large as planet can get.

Part 2: Assuming "larger" means greater in mass.

Stars
In terms of mass, VY Canis Majoris doesn't even make the top ten, not even close! The most massive known star is R136a1. Again, these are estimates, but mass is a bit easier to pin down than is radius (or diameter).

As is the case with physical extent, the first generation stars are presently modeled as being much, much more massive than anything we see now.

Planets
There's not much difference between the largest planet and the smallest brown dwarf. I would argue there's very little difference. It's a spectrum with no distinguishing characteristic that lets one say "this is a planet" and "that is a brown dwarf". Ignoring the distinction between super-Jupiters and brown dwarfs the largest is about 80 Jupiter masses. V1581 Cygni C is 79 Jupiter masses. (More massive than that and they start burning hydrogen, thus making them a small red dwarf.)

The current factor that is used to distinguish between brown dwarfs and super Jupiters is mass. Anything larger than 13 Jupiter masses is a brown dwarf, anything smaller, a planet. That boundary is very arbitrary.

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  • $\begingroup$ I thought the boundary between Jupiter-likes and brown dwarfs was deuterium fusion. That doesn't seem particularly arbitrary to me. Wikipedia says this definition is under debate, though I don't know much about that personally. I suppose an unsettled definition can be called arbitrary. $\endgroup$ – zibadawa timmy Mar 25 '15 at 4:40
  • $\begingroup$ @zibadawatimmy it's arbitrary because it's hard to detect. The heat of formation is greater than the heat from limited fusion. 23 PPM D to H ratio. en.wikipedia.org/wiki/Deuterium#Abundance isn't much and at the cutoff, about 13 Jupiter mass, the fusion process proceeds slowly. $\endgroup$ – userLTK Nov 22 '15 at 19:35
  • $\begingroup$ It's funny than in the top 6 heaviest stars, 4 are from the same region 2 parsecs across! At least according to this wikipedia article. $\endgroup$ – user18466 Sep 29 '17 at 12:03
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Looking at the primary literature (and the source of the wikipedia information), it appears that Arroyo-Torres et al. (2013) have estimated a radius of $1708\pm 192R_{\odot}$, using an interferometric angular diameter and a highly uncertain distance (where I'm not sure the distance uncertainty has been adequately included). Using similar methods Wittkowski et al. (2012) estimated a radius of $1420\pm 120 R_{\odot}$ for VY CMa.

Taking these numbers at face value then UY Scuti is probably larger than VY CMa. However, I wouldn't take these numbers at face value. I don't believe the distances to these stars are known to better than 10% as claimed in these papers (they soon will be with Gaia trigonometric parallaxes). You also have to bear in mind that these are pulsational variables (hence the variable star designations), so their radii are variable.

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protected by HDE 226868 Jan 9 '16 at 16:34

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