Hypergiant Question

[Umlüx]

so, there are many very.. VERY big stars out there. and i was asking myself how would the surface of such a star look like? would they have a visible/defineable "surface" comparable to the photosphere of our sun? or are they more fluffy like clouds? or simply unshapely gas clumps because of the immense forces?

[WestAir]

I imagine the gravity of these super-giants and hyper-giants would pull all of the hydrogen plasma in as packed and tightly as possible. There would most certainly be a definitive "surface". That said, I also remember reading somewhere that the light pressure from these hyper-giants is so strong that its force can actually overcome gravity and force the stars outer layers into space. Apparently this means there is a maximum size a star can be before it sheds off mass. I like to believe even hyper-giants at this limit have a definitive surface where ionized gas becomes tightly packed plasma.

As always, I'm only assuming and have no evidence to back what I've said here.

[magnemoe]

As stars live shorter who larger they are I guess it would live a million year or less and blow up before fully formed.

Yes it would probably blow away the remaining gas but would never be really stable you will still have stuff incoming while it would not be very stable and probably throw stuff out in large quantities.

[Armchair Rocket Scientist]

I imagine the gravity of these super-giants and hyper-giants would pull all of the hydrogen plasma in as packed and tightly as possible. There would most certainly be a definitive "surface". That said, I also remember reading somewhere that the light pressure from these hyper-giants is so strong that its force can actually overcome gravity and force the stars outer layers into space. Apparently this means there is a maximum size a star can be before it sheds off mass. I like to believe even hyper-giants at this limit have a definitive surface where ionized gas becomes tightly packed plasma.

As always, I'm only assuming and have no evidence to back what I've said here.

Unfortunately, the first bit is almost completely wrong. Even the hotter and more compact blue supergiants and hypergiants are far less dense than air at sea level on Earth.

In general, stars, including our sun, don't have a defined "surface" any more than gas giants do. According to wikipedia:

The photosphere of an astronomical object is the depth of a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, ÆῶÂ, ÆÉÄÃŒÂ/phos, photos meaning "light" and ÃÆαῖÃÂα/sphaira meaning "sphere", in reference to the fact that it is a spherical surface that is perceived to emit light. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately 2/3.[1] In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.

Basically, the plasma just gets denser and denser until we define it arbitrarily as opaque. It's not really a surface.

[Umlüx]

true. but if you look at a picture of our sun, it looks like a solid sphere. would a giant star look the same?

[Dodgey]

As a virtue of its immense size yes.

[Alchemist]

Blue supergiants would probably look rather similar (that is plasma density increasing from almost fully transparrent to about fully opaque rather quick, forming this kind of visible "surface"). Red supergiants, on the other hand, can have significant layer of semi-transparent atmosphere. And as the red giant grows it becomes more and more transparent until you can see the white dwarf in the middle of this planetary nebula

[Tommygun]

I think I see what Umlüx is saying. In this picture it shows the scale of different hypergiants to our solar system (the blue half circle is the orbit of Neptune) .

Would the largest red hypergiant look more like a glowing gas cloud than say our Sun, at least if viewed with a lens filter?

Edit: I guess this mostly answers it.

Blue supergiants would probably look rather similar (that is plasma density increasing from almost fully transparrent to about fully opaque rather quick, forming this kind of visible "surface"). Red supergiants, on the other hand, can have significant layer of semi-transparent atmosphere. And as the red giant grows it becomes more and more transparent until you can see the white dwarf in the middle of this planetary nebula

Edited August 27, 2014 by Tommygun

[peadar1987]

I think I see what Umlüx is saying. In this picture it shows the scale of different hypergiants to our solar system (the blue half circle is the orbit of Neptune) .

Would the largest red hypergiant look more like a glowing gas cloud than say our Sun, at least if viewed with a lens filter?

Edit: I guess this mostly answers it.

Wow! That red line is the orbit of Saturn. And even the second largest star there comes out past it.

I feel kind of tiny right now!

[lajoswinkler]

It's difficult to say. They are so huge that any visible changes in real time, at a distance where they look large as our Sun, would be nonexistent. Even the speed of light is too slow. They are absolutely huge.