One thing worth noting is our popular notion of what a star seems to be is quite different from what astronomers have.
One example, Canis Majoris has a radius of about 1420 solar radii. But its mass is roughly
17±8 times the mass of the Sun and therefore an average density of 5.33 to 8.38 mg/m3. “It is over 100.000 times less dense than Earth’s atmosphere at sea level.”
The surface radius of a star is defined based on optical thickness. If you were there, the star would in fact look like a fairly well-defined opaque spheroid with the reported radius. The density of the photosphere plasma is near zero, but there’s a lot of it.
brabel 4 hours ago [-]
Why doesn’t the stuff just falls in towards the center? It just floats there??
zygentoma 4 hours ago [-]
Radiation pressure! There is so much heat (= photons) radiating outwards, that it counteracts the gravitational pull.
stouset 4 hours ago [-]
What’s particularly interesting to me is that for stars the size of our sun, regular old gas pressure dominates. The sun’s atmosphere is held up essentially just from the temperature (and therefore high kinetic energy) of the plasma.
Only once you get to 10+ solar masses does radiation (light) pressure begin to become significant, and at 50+ solar masses is when it dominates and the atmosphere is held up by the momentum of light.
sega_sai 3 hours ago [-]
In most star photospheres the role of radiation pressure is negligible. They are supported by the pressure gradients. The exception is very hot stars.
brabel 4 hours ago [-]
Wow I knew that stars have that radiation pressure but had no idea it caused mass to get so crazy far away from the ignited area of the star!
Sharlin 4 hours ago [-]
These giant stars burn so very very bright. And correspondingly only live a few tens of millions years at most.
kulahan 1 hours ago [-]
To add on to the other response, think of it like the star “leaking” into space. It’ll be defined as a pretty radically different size at some point in the future, even as it’s still undergoing fusion.
FranOntanaya 8 hours ago [-]
I always wonder when do they stop considering the outer layers part of the star and not relatively dense solar wind soup. Nobody says Earth has 7000km radius, even tho there's traces of exosphere past 500km.
antognini 7 hours ago [-]
The outer boundary is the point where a photon has a ~50% chance of escaping without encountering another particle.
encrypted_bird 5 hours ago [-]
Do you have a source? I'd love to read more. :)
zamadatix 2 hours ago [-]
Amateur stargazer's understanding:
Depending what kind of reading material you're looking for (e.g. high level details or mathematically/jargon dense papers) look for things discussing "Rosseland optical depth" and "grey atmosphere approximation".
At a high level, the common convention is to define the radius by finding where the optical depth is 2/3 when using Rosselands clever way of calculating a weighted mean of the opacity from that layer in the star to space across different wavelengths of light. 2/3 being a clever derivation from Eddington where, in an idealized model of a star, that's when the actual temperature of the star should equal its blackbody equivalent temperature.
Pedantically, this distance to the point of equality is an ever so slightly different value than the "distance from the center where there is a 50% chance a photon traveling directly outward will escape the star without another interaction" rule of thumb (in the same idealized grey model). Practically, that the difference is so small is why it's a fantastic rule of thumb explanation.
7 hours ago [-]
usrnm 8 hours ago [-]
My layman definition of what a star is is "it's somewhere in the sky and it gives off light". The word "density" wouldn't even come to my mind
kulahan 1 hours ago [-]
Thanks to blackbody radiation, every human skydiver briefly becomes a star by this definition! ;)
It needs a core generating energy through fusion, which has density requirements, for what it’s worth.
margalabargala 6 hours ago [-]
> therefore an average density of 5.33 to 8.38 mg/m3
That's nuts. How on earth does fusion happen at that density? Is there a denser core that actually fuses, and the outer fluffy bits just glow from the inner heat?
adrianN 6 hours ago [-]
Averages are quite misleading. The core is obviously a lot denser. Our sun has an average energy output per cubic meter that is comparable to a compost heap.
throwaway89864 6 hours ago [-]
Astra says this claim is misleading. Sun's average energy output per cubic meter is three orders of magnitude smaller than that of a compost heap. The fusion core is comparable though.
Retric 5 hours ago [-]
The energy density of specific compost heaps varies by several orders of magnitude it’s not a single number. Volume, moisture content, internal temperature, external temperature, materials being composted, etc all play a significant role.
mordechai9000 3 hours ago [-]
Stellar fusion could also be a factor in a sufficiently large compost heap.
malfist 5 hours ago [-]
Of course it's misleading, thats the point GP was trying to make. Averages belie truth in large, diverse systems with extremes
margalabargala 6 hours ago [-]
Sure, but now I'm left wondering what the size of the body doing fusion is. If most of the star is glorified glowing atmosphere, I want to know the mass and radius of the fusing bits.
peri-cl 5 hours ago [-]
There's dedicated Wikipedia articles responsive to this,
Depends on the class of the star. Our sun? About 1/4 radius and in is fusion reaction. But it gets weird in other stars. In red giants the fusion zone is a very small shell around a dead core. Maybe something like earths orbit in radius, but very very thin.
dpriddle 8 hours ago [-]
Yes and no. It’s fluffy on the outside, but at the core it’s likely denser than our sun.
dylan604 2 hours ago [-]
I love astronomy simply for things like "17±8 times the mass". That's ~50% acceptable variance. I wish I could apply that logic to my creditors.
kulahan 1 hours ago [-]
I worked for a financial org with over a trillion in assets, and they had a 100% acceptable variance on project length predictions. Made for a laid-back atmosphere at work but boy. How they ever swung that is beyond me.
“We predict a year, so it’ll take anywhere between 8 seconds and 24 months!” AND IT WORKED???
lrasinen 6 hours ago [-]
Average density for non-uniform objects is pretty useless, since the cubic volume scaling makes a mess of things.
As an example, I pulled the stats for an electric locomotive. 19x3x4.4 meters, mass 90 metric tonnes. That's an average density of about 1/3rd of water.
Rygian 3 hours ago [-]
So a locomotive floats. Interesting.
peri-cl 5 hours ago [-]
These objects (black hole stars) seem to be far larger than that,
Any decent human being would have called these Black Hole Suns.
vardump 6 hours ago [-]
I just twisted my mouth sides upwards with my fingers to form a smile in agreement.
pixelpoet 4 hours ago [-]
Sadly, those won't come.
joebig 45 minutes ago [-]
Wouldn't objects this ancient (following in the wake of the big bang, essentially) be redshifted anyway, on account of cosmological expansion?
itsalwaysgood 3 hours ago [-]
Perhaps they are somehow related to the elusive white hole: exit points of black holes.
Fuel for a Big Bang, if you can imagine. A universe within itself.
The article says they're sucking up gas though so I don't know. The arrow of time is weird. It would feel elegant to me, but I'm way out of my league.
The imagined scenario only works if you picture all black holes 'twisting time' towards its beginning.
Sort of like recursion, or a tesseract, where there's one beginning and many endings.
JumpCrisscross 7 hours ago [-]
Is there an experiment that could resolve this? Literally resolve these objects better, or otherwise distinguish between these hypotheses?
Is there anything behind these objects they could lens?
vardump 3 hours ago [-]
I wonder if our universe is the experiment to resolve this.
itsalwaysgood 3 hours ago [-]
We don't have a good fuel source or reason for expansion. We also don't know where all the theorized white
holes could be.
To me it's an interesting coincidence, thought description of these objects baffles me.
Unless the universe is recursively within itself: the black hole stars have black holes within them that also exit at the big bang. But then so do all other smaller big bangs we see: exact same exit point and coincidentally, the same moment in time.
Oh, and there is a beautiful symmetry here: only one white hole exists.
One beginning, many endings. It's armchair philosophy but fun to imagine.
One example, Canis Majoris has a radius of about 1420 solar radii. But its mass is roughly 17±8 times the mass of the Sun and therefore an average density of 5.33 to 8.38 mg/m3. “It is over 100.000 times less dense than Earth’s atmosphere at sea level.”
https://en.wikipedia.org/wiki/VY_Canis_Majoris
https://nineplanets.org/vy-canis-majoris/
Only once you get to 10+ solar masses does radiation (light) pressure begin to become significant, and at 50+ solar masses is when it dominates and the atmosphere is held up by the momentum of light.
Depending what kind of reading material you're looking for (e.g. high level details or mathematically/jargon dense papers) look for things discussing "Rosseland optical depth" and "grey atmosphere approximation".
At a high level, the common convention is to define the radius by finding where the optical depth is 2/3 when using Rosselands clever way of calculating a weighted mean of the opacity from that layer in the star to space across different wavelengths of light. 2/3 being a clever derivation from Eddington where, in an idealized model of a star, that's when the actual temperature of the star should equal its blackbody equivalent temperature.
Pedantically, this distance to the point of equality is an ever so slightly different value than the "distance from the center where there is a 50% chance a photon traveling directly outward will escape the star without another interaction" rule of thumb (in the same idealized grey model). Practically, that the difference is so small is why it's a fantastic rule of thumb explanation.
It needs a core generating energy through fusion, which has density requirements, for what it’s worth.
That's nuts. How on earth does fusion happen at that density? Is there a denser core that actually fuses, and the outer fluffy bits just glow from the inner heat?
https://en.wikipedia.org/wiki/Solar_core
“We predict a year, so it’ll take anywhere between 8 seconds and 24 months!” AND IT WORKED???
As an example, I pulled the stats for an electric locomotive. 19x3x4.4 meters, mass 90 metric tonnes. That's an average density of about 1/3rd of water.
https://en.wikipedia.org/wiki/Quasi-star#Formation_and_prope... (Caption: "Size comparison of a hypothetical quasi-star to some of the largest known stars")
Fuel for a Big Bang, if you can imagine. A universe within itself.
The article says they're sucking up gas though so I don't know. The arrow of time is weird. It would feel elegant to me, but I'm way out of my league.
The imagined scenario only works if you picture all black holes 'twisting time' towards its beginning.
Sort of like recursion, or a tesseract, where there's one beginning and many endings.
Is there anything behind these objects they could lens?
To me it's an interesting coincidence, thought description of these objects baffles me.
Unless the universe is recursively within itself: the black hole stars have black holes within them that also exit at the big bang. But then so do all other smaller big bangs we see: exact same exit point and coincidentally, the same moment in time.
Oh, and there is a beautiful symmetry here: only one white hole exists.
One beginning, many endings. It's armchair philosophy but fun to imagine.
PBS Space Time: https://www.youtube.com/watch?v=FMdrD_jcYgE
love those channels
Link: https://youtu.be/gUobqtANMfE?si=NVG26d6Aoc2XNEWg
https://youtu.be/dst-C0IDQRU