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.
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.
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.
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.
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.
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.
> 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?
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.
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.
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.
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.
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.
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.
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.
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???
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.
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.
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.
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?
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
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