A Truly Massive Star

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jewish_scientist
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A Truly Massive Star

Postby jewish_scientist » Thu Oct 04, 2018 7:31 pm UTC

In the comic Homestuck (which I cannot recommend enough) there is a thing called The Green Sun. It is a star with twice the mass of our universe. I was wondering what properties such a huge thing could have, if it could even exist.

Assuming this refers to the observable universe, The Green Sun is 2e55g and have a diameter of at least 3e25m. That is really all I can work out on my own.
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Re: A Truly Massive Star

Postby Pfhorrest » Thu Oct 04, 2018 7:44 pm UTC

It would be a black hole.
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Re: A Truly Massive Star

Postby jewish_scientist » Thu Oct 04, 2018 8:13 pm UTC

No, it would not. Black holes from when the density of an object gets to high, not the mass.
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Re: A Truly Massive Star

Postby Pfhorrest » Thu Oct 04, 2018 8:19 pm UTC

Stars collapse under their own weight until some force pushes back hard enough, and the more massive a star the stronger a force you need to halt that collapse, and with enough mass there is no force that will stop that collapse. Twice the mass of the universe is definitely enough mass.

Though, I mean, I guess you could have a big cloud of gas twice the mass of the universe in the process of collapsing, for a little while.
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Re: A Truly Massive Star

Postby speising » Thu Oct 04, 2018 8:27 pm UTC

or, in other words, the lifetime of a star is a function of its mass:
Image

a massive object wouldn't live very long before it burns out and collapses.

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Re: A Truly Massive Star

Postby LaserGuy » Thu Oct 04, 2018 8:37 pm UTC

jewish_scientist wrote:In the comic Homestuck (which I cannot recommend enough) there is a thing called The Green Sun. It is a star with twice the mass of our universe. I was wondering what properties such a huge thing could have, if it could even exist.

Assuming this refers to the observable universe, The Green Sun is 2e55g and have a diameter of at least 3e25m. That is really all I can work out on my own.


I think there are two possibilities. As others have stated, either it would immediately collapse into a black hole, or, if the diameter were large enough, it would just be a universe and wouldn't be properly called a star at all.

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Re: A Truly Massive Star

Postby speising » Thu Oct 04, 2018 8:41 pm UTC

if course, the thing would also have to have a diameter of billions of ly. that means that fusion would start, and burn out, in the center (or maybe in multiple places, if it's not completely isotropic) while the collapse is still going on in the outer regions.

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Re: A Truly Massive Star

Postby Xanthir » Thu Oct 04, 2018 10:34 pm UTC

Right, but that's not, by our usual meaning of the word, a "star"; it's a nebula (a gas cloud birthing stars within it).

Anything that meaningfully can be called a "star" would be a black hole at that mass, as others have said.
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Re: A Truly Massive Star

Postby gmalivuk » Thu Oct 04, 2018 10:44 pm UTC

What everyone else said, but tripled.

jewish_scientist wrote:
Assuming this refers to the observable universe, The Green Sun is 2e55g

Your own source says 3e55 for the mass, not 1e55, meaning this would need to be at least 6e55g to be twice that, and the radius would be almost 10 billion light years.
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Re: A Truly Massive Star

Postby jewish_scientist » Fri Oct 05, 2018 2:47 pm UTC

Oops! I have no idea how I read that wrong. Anyway, if we consider it a universe, the density of that universe would be a lot higher than ours. What would the effects of that be?
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Re: A Truly Massive Star

Postby Eebster the Great » Sat Oct 06, 2018 12:30 am UTC

The radius of the observable universe is actually 46.6 billion light years (4.4×1026 m), giving a volume of 3.6×1080 m3. The mass you gave (3×1055 kg) is the combined mass of everything in the observable universe, including normal matter, dark matter, and dark energy. Dividing gives the critical density (which you can also find online) of 8.5×10-26 kg m-3.

The proposed "star," even after collapsing entirely into a black hole, would have a mass of about 6×1055 and, assuming it was approximately Schwarzschild, since the total angular momentum and electric of the universe should be negligible, it would have a radius of 8.9×1028 m, a volume of 3.0×1087 m3, and a density of 2×10-32. In other words, though this black hole would have twice the mass of our observable universe, it would have more than 8 million times the volume. Its density would therefore be only about a four-millionth of intergalactic space.

The reason is that on large scales in the real universe, dark energy dominates, and things get pulled apart rather than falling together. They certainly don't form a black hole. So at the scale of the entire observable universe, most of the energy is dark energy. But in this hypothetical scenario, we need to have a "star" with such a huge mass. For that to be possible, I have to assume we do not have a universe with accelerating expansion (dark energy), since that would make it impossible for this "star" to ever hold together, no matter how it is configured. By getting rid of dark energy, we allow the star to be the only thing such a massive object could be: a superdupermassive black hole.

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Re: A Truly Massive Star

Postby Sizik » Sat Oct 06, 2018 1:30 am UTC

I think there's a reason said "sun" resides in what is called "Paradox space".
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Re: A Truly Massive Star

Postby jewish_scientist » Sun Oct 07, 2018 8:22 am UTC

More massive black holes are actually less dense. That is a rather counter-intuitive result, which is always something I like to find.
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Re: A Truly Massive Star

Postby Quizatzhaderac » Tue Oct 16, 2018 9:31 pm UTC

I think it would have to be a question of how dark energy/ cosmic expansion would work. No expansion, and it would collapse whatever parameters you pick.

The much throny-er issue is how is this universe radiating? It's obviously not radiating into Minkowski space (aka space as relativity talks about it). I'd have to assume paradox space is some kind of probability space, rather than some kind of geometric space, with geometric spaces only corresponding to some local condition. So even if it was some kind of black hole, it would still make as much sense as anything to be radiating into paradox space.

The green sun is also indicated to have either limited heat, or limited thermal conductivity, indicated by the fact that Spades Slick and the Leprechauns were able to exit in it and not be immediately fried. Our own universe has a radiate temperature of 2.73 K, and light matter average temperature of about 800 K
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Eebster the Great
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Re: A Truly Massive Star

Postby Eebster the Great » Wed Oct 17, 2018 2:09 am UTC

800 K seems a bit cooler than I would have guessed. Where do you get that data?

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Re: A Truly Massive Star

Postby Quizatzhaderac » Thu Oct 18, 2018 4:16 pm UTC

I took the wikipedia page on the types interstellar mediumand multiplied the volume fraction of each by density by temperature then divided by the total mass of the "sample". Ignoring everything that's not gas and dust.

This is, I realize, a very rough approximation. In the first place the numbers from the Wikipedia page have huge variation. Secondly I ignored the different specific heats of types (actually just doing a correction of molecular (H2) capacity=1, atomic=2, ionized=4, changes the figure to 2400 C). I also assumed that intergalactic medium was the same as interstellar medium, for no better reason than there wasn't a nice chart describing it (and this is something like 90% of baryonic matter).

Most complexly, I've ignored everything above optical thickness (2 percent-ish of the mass, but very complex thermal properties). Anything too small to sustain fusion barely glows and barely occludes light, and as such we have pretty terrible ideas of how many there are. Asteroids are dust are generally cold/ in thermal equilibrium with radiation. Brown dwarfs are warm in the center (like Jupiter), but I couldn't begin to give you a decent estimate of how warm.

Stars are easy, hot , and rare. I'd expect a real astronomer could figure out the thermal mass of the average star, but I'm not going to try. I supposed they're hot enough to significantly raise the average even at < 1 % of mass.

Anyway my point (if I actually have one) is that the temperature of a universe is a complex subject.
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Re: A Truly Massive Star

Postby Eebster the Great » Fri Oct 19, 2018 5:03 am UTC

The intergalactic medium is not similar to the interstellar medium. According to Wikipedia, the largest mass of intergalactic plasma is the warm-hot intergalactic medium at 105–107 K. Note that this is just a majority by mass. Most of the volume of the universe consists of massive voids that are even less dense.


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