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jahnu 1 days ago [-]
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 1 days ago [-]
Why doesn’t the stuff just falls in towards the center? It just floats there??
zygentoma 1 days ago [-]
Radiation pressure! There is so much heat (= photons) radiating outwards, that it counteracts the gravitational pull.
stouset 1 days 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 1 days 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 1 days 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 1 days ago [-]
These giant stars burn so very very bright. And correspondingly only live a few tens of millions years at most.
kulahan 23 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.
usrnm 1 days 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 23 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.
FranOntanaya 1 days 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 1 days ago [-]
The outer boundary is the point where a photon has a ~50% chance of escaping without encountering another particle.
encrypted_bird 1 days ago [-]
Do you have a source? I'd love to read more. :)
zamadatix 24 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.
chabska 21 hours ago [-]
The radius is based on visual observation. We have good theories and models about what's happening inside the star. We are quite sure that it's mostly empty space inside the radius. But that's not a good enough reason to say that the star's radius is anything other than what we see through our telescope.
zamadatix 2 hours ago [-]
Importantly, this works for stars relatively close to us (the farthest I can find is 16,000 light years for one hyper giant) but the majority of the stars catalogued in even our own galaxy have only been assigned a radius via taking the temperature (via color) and luminosity.
1 days ago [-]
margalabargala 1 days 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 1 days 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.
margalabargala 1 days 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 1 days 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.
throwaway89864 1 days 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 1 days 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 1 days ago [-]
Stellar fusion could also be a factor in a sufficiently large compost heap.
malfist 1 days ago [-]
Of course it's misleading, thats the point GP was trying to make. Averages belie truth in large, diverse systems with extremes
lrasinen 1 days 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 1 days ago [-]
So a locomotive floats. Interesting.
TeMPOraL 13 hours ago [-]
Locomotive itself? Probably not. If you turned its abstract bounding box into real box - thin, negligible mass, enough strength not to break up - and evacuated the air inside, then yes, the box with locomotive in it should float.
dpriddle 1 days ago [-]
Yes and no. It’s fluffy on the outside, but at the core it’s likely denser than our sun.
ramraj07 20 hours ago [-]
The largest supermassive blackholes have an average density less than earth's atm as well.
dylan604 24 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.
e_l 13 hours ago [-]
Yeah, Physics (and particularly astronomy/cosmology) often have huge variances, some of which are even greater.
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???
peri-cl 1 days 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 1 days ago [-]
I just twisted my mouth sides upwards with my fingers to form a smile in agreement.
ramraj07 20 hours ago [-]
By all indications Chris Cornell likely tried to paint some poignantly depressing mindset with that lyric, so its doubly ironic the universe is choke full of it. Or at least was.
pixelpoet 1 days ago [-]
Sadly, those won't come.
JumpCrisscross 1 days 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 1 days ago [-]
I wonder if our universe is the experiment to resolve this.
itsalwaysgood 1 days 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.
wraith_ilands 17 hours ago [-]
[flagged]
joebig 22 hours ago [-]
Wouldn't objects this ancient (following in the wake of the big bang, essentially) be redshifted anyway, on account of cosmological expansion?
Tuna-Fish 17 hours ago [-]
Yes, and we are already accounting for it. The objects as seen by us are actually very deep in infrared, when you correct for the redshift to match the key absorbtion lines of hydrogen, they still remain very red.
joebig 11 hours ago [-]
Appreciated. May I ask regarding the determination of the magnitude of correction to apply? Is it a case-to-case basis? In other words, is it very sensitive to the distance assumped for the LRDs?
Tuna-Fish 10 hours ago [-]
The light curve is not smooth, there is a clear spectrum caused by absorption lines. You find the pattern in the lines that you know to be hydrogen, and measure how much redder it is than it would be without redshift.
itsalwaysgood 1 days 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.
I’m not really down to watch a 90 minute video on the dramas of astrophysics, so I’m not sure what it says, but interestingly Sabine Hossenfelder took an extremely minor swipe at him not too long ago. Guess the dude is at least legit-enough for that? Haha
Hikikomori 11 hours ago [-]
Dave does a lot debunk videos but this is just a clickbait title as it's an interview with Anton.
andrewflnr 7 hours ago [-]
Clickbait which you made worse? Come on man, don't do that.
Hikikomori 6 hours ago [-]
Continuing the joke on Antons titles, lighten up.
mxmilkiib 5 hours ago [-]
you made it dark
kulahan 1 hours ago [-]
Roger, that makes sense. I… might actually watch this now LOL
debo_ 1 days ago [-]
Soundgarden was apparently prophetic with their hit song "Black Hole Sun."
TimeBearingDown 1 days ago [-]
May Chris Cornell rest in peace. He should have seen this.
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.
It needs a core generating energy through fusion, which has density requirements, for what it’s worth.
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.
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
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.
An example being the number of stars in the (observable) universe which ranges from 10^22 to 10^24 stars...a variance of 100x (or ~10,000%)!!! https://www.esa.int/Science_Exploration/Space_Science/How_ma...
“We predict a year, so it’ll take anywhere between 8 seconds and 24 months!” AND IT WORKED???
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")
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.
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.
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