3 ms·
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
by jahnu 14d 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.”
https://en.wikipedia.org/wiki/VY_Canis_Majoris https://en.wikipedia.org/wiki/VY_Canis_Majoris
https://nineplanets.org/vy-canis-majoris/ https://nineplanets.org/vy-canis-majoris/
- usrnm 14d agoMy 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 13d agoThanks 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.
- dpriddle 14d agoYes and no. It’s fluffy on the outside, but at the core it’s likely denser than our sun.
- Sharlin 14d agoThe 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 13d agoWhy doesn’t the stuff just falls in towards the center? It just floats there??
- zygentoma 13d agoRadiation pressure! There is so much heat (= photons) radiating outwards, that it counteracts the gravitational pull.
- brabel 13d agoWow 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 13d agoThese giant stars burn so very very bright. And correspondingly only live a few tens of millions years at most.
- stouset 13d agoWhat’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 13d agoIn most star photospheres the role of radiation pressure is negligible. They are supported by the pressure gradients. The exception is very hot stars.
- kulahan 13d agoTo 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 14d agoI 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 13d agoThe outer boundary is the point where a photon has a ~50% chance of escaping without encountering another particle.
- encrypted_bird 13d agoDo you have a source? I'd love to read more. :)
- zamadatix 13d agoAmateur 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.
- antognini 12d agoI'd recommend Carroll & Ostlie's Introduction to Modern Astrophysics, though it requires some basic calculus and physics.
- deleted 13d ago
- margalabargala 13d 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 13d agoAverages 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 13d agoSure, 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 13d agoThere's dedicated Wikipedia articles responsive to this, https://en.wikipedia.org/wiki/Solar_core https://en.wikipedia.org/wiki/Solar_core
- idiotsecant 13d agoDepends 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 13d agoAstra 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 13d ago
- lrasinen 13d agoAverage 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.
- peri-cl 13d agoThese objects (black hole stars) seem to be far larger than that, https://en.wikipedia.org/wiki/Quasi-star#Formation_and_properties 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")
- dylan604 13d agoI 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 13d agoI 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???
- e_l 13d agoYeah, Physics (and particularly astronomy/cosmology) often have huge variances, some of which are even greater. 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_many_stars_are_there_in_the_Universe https://www.esa.int/Science_Exploration/Space_Science/How_ma...
- ramraj07 13d agoThe largest supermassive blackholes have an average density less than earth's atm as well.