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Black Holes or Black Hole Stars? Astronomers Spar over 'Little Red Dots'
- debo_ 14d agoSoundgarden was apparently prophetic with their hit song "Black Hole Sun."
- TimeBearingDown 14d agoMay Chris Cornell rest in peace. He should have seen this.
- jahnu 14d agoOne 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 14d 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 14d agoWhy doesn’t the stuff just falls in towards the center? It just floats there??
- zygentoma 14d agoRadiation pressure! There is so much heat (= photons) radiating outwards, that it counteracts the gravitational pull.
- brabel 14d 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 14d agoThese giant stars burn so very very bright. And correspondingly only live a few tens of millions years at most.
- stouset 14d 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 14d ago
- 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 14d agoThe outer boundary is the point where a photon has a ~50% chance of escaping without encountering another particle.
- encrypted_bird 14d agoDo you have a source? I'd love to read more. :)
- zamadatix 14d 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 14d ago
- margalabargala 14d 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 14d 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 14d 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 14d agoThere's dedicated Wikipedia articles responsive to this, https://en.wikipedia.org/wiki/Solar_core https://en.wikipedia.org/wiki/Solar_core
- idiotsecant 14d 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 14d 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 14d ago
- lrasinen 14d 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 14d 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 14d 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 14d 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.
- JumpCrisscross 14d agoIs 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 14d agoI wonder if our universe is the experiment to resolve this.
- itsalwaysgood 14d agoWe 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 13d ago[flagged]
- benji-york 14d agoAny decent human being would have called these Black Hole Suns.
- vardump 14d agoI just twisted my mouth sides upwards with my fingers to form a smile in agreement.
- pixelpoet 14d agoSadly, those won't come.
- ramraj07 13d agoBy 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.
- ck2 14d agoDr. Becky take: https://www.youtube.com/watch?v=KuTVjuXu7uE https://www.youtube.com/watch?v=KuTVjuXu7uE PBS Space Time: https://www.youtube.com/watch?v=FMdrD_jcYgE https://www.youtube.com/watch?v=FMdrD_jcYgE love those channels
- brabel 14d agoThey are great but you forgot Anton! He definitely belongs with them in my opinion, he is a wonderful person after all :) Link: https://youtu.be/gUobqtANMfE?si=NVG26d6Aoc2XNEWg https://youtu.be/gUobqtANMfE?si=NVG26d6Aoc2XNEWg
- blop 14d agoAnton is definitely wonderful! He explains everything very well with no drama and BS. High information density and yet very clear!
- Hikikomori 14d agoWas also a fan of Anton but professor Dave recently made a video about him. https://youtu.be/dst-C0IDQRU https://youtu.be/dst-C0IDQRU
- kulahan 13d agoI’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 13d agoDave does a lot debunk videos but this is just a clickbait title as it's an interview with Anton.
- andrewflnr 13d ago
- itsalwaysgood 14d agoPerhaps 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.
- joebig 14d agoWouldn't objects this ancient (following in the wake of the big bang, essentially) be redshifted anyway, on account of cosmological expansion?
- Tuna-Fish 13d agoYes, 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 13d agoAppreciated. 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 13d agoThe 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.
- NeoByte 13d ago[dead]
- deleted 12d ago[deleted]