Thread

Black Holes or Black Hole Stars? Astronomers Spar over 'Little Red Dots'

105 points · 61 comments · jandrewrogers

jahnu · 1789828933 · focus · HN ↗
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.”
<a href="https://en.wikipedia.org/wiki/VY_Canis_Majoris" rel="nofollow">https://en.wikipedia.org/wiki/VY_Canis_Majoris
<a href="https://nineplanets.org/vy-canis-majoris/" rel="nofollow">https://nineplanets.org/vy-canis-majoris/
1. usrnm · 1789829484 · focus · HN ↗
  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
  1. kulahan · 1789855188 · focus · HN ↗
    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.
2. dpriddle · 1789829710 · focus · HN ↗
  Yes and no. It’s fluffy on the outside, but at the core it’s likely denser than our sun.
3. Sharlin · 1789830557 · focus · HN ↗
  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.
  1. brabel · 1789844059 · focus · HN ↗
    Why doesn’t the stuff just falls in towards the center? It just floats there??
    zygentoma · 1789844563 · focus · HN ↗
    Radiation pressure! There is so much heat (= photons) radiating outwards, that it counteracts the gravitational pull.
    brabel · 1789844718 · focus · HN ↗
    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 · 1789845911 · focus · HN ↗
    These giant stars burn so very very bright. And correspondingly only live a few tens of millions years at most.
    stouset · 1789846844 · focus · HN ↗
    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 · 1789849503 · focus · HN ↗
    In most star photospheres the role of radiation pressure is negligible. They are supported by the pressure gradients. The exception is very hot stars.
    kulahan · 1789855107 · focus · HN ↗
    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.
4. FranOntanaya · 1789831189 · focus · HN ↗
  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.
  1. antognini · 1789834819 · focus · HN ↗
    The outer boundary is the point where a photon has a ~50% chance of escaping without encountering another particle.
    encrypted_bird · 1789843009 · focus · HN ↗
    Do you have a source? I'd love to read more. :)
    zamadatix · 1789852613 · focus · HN ↗
    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.
    antognini · 1789951697 · focus · HN ↗
    I'd recommend Carroll & Ostlie's Introduction to Modern Astrophysics, though it requires some basic calculus and physics.
    encrypted_bird · 1790151105 · focus · HN ↗
    Thanks!
  2. [deleted] · 1789835041 · focus · HN ↗
    [deleted]
  3. chabska · 1789863215 · focus · HN ↗
    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 · 1789928502 · focus · HN ↗
    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.
5. margalabargala · 1789836996 · focus · HN ↗
  > 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?
  1. adrianN · 1789837360 · focus · HN ↗
    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 · 1789838513 · focus · HN ↗
    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 · 1789840563 · focus · HN ↗
    There's dedicated Wikipedia articles responsive to this,
    <a href="https://en.wikipedia.org/wiki/Solar_core" rel="nofollow">https://en.wikipedia.org/wiki/Solar_core
    idiotsecant · 1789840800 · focus · HN ↗
    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 · 1789838565 · focus · HN ↗
    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 · 1789842065 · focus · HN ↗
    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 · 1789848618 · focus · HN ↗
    Stellar fusion could also be a factor in a sufficiently large compost heap.
    malfist · 1789843546 · focus · HN ↗
    Of course it's misleading, thats the point GP was trying to make. Averages belie truth in large, diverse systems with extremes
6. lrasinen · 1789837901 · focus · HN ↗
  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.
  1. Rygian · 1789849267 · focus · HN ↗
    So a locomotive floats. Interesting.
    TeMPOraL · 1789890285 · focus · HN ↗
    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.
7. peri-cl · 1789841253 · focus · HN ↗
  These objects (black hole stars) seem to be far larger than that,
  <a href="https://en.wikipedia.org/wiki/Quasi-star#Formation_and_properties" rel="nofollow">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")
8. dylan604 · 1789851841 · focus · HN ↗
  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.
  1. kulahan · 1789855321 · focus · HN ↗
    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???
  2. e_l · 1789891833 · focus · HN ↗
    Yeah, 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%)!!! <a href="https://www.esa.int/Science_Exploration/Space_Science/How_many_stars_are_there_in_the_Universe" rel="nofollow">https://www.esa.int/Science_Exploration/Space_Science/How_ma...
9. ramraj07 · 1789865027 · focus · HN ↗
  The largest supermassive blackholes have an average density less than earth's atm as well.

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