I have a bit of time on my hands, so let's do another 24h round of #AskAnAstrophysicist, but this time it's a thematic one.
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I heard Brian Cox talk about “escape velocity” in relation to density. What are the densest objects discovered and how dense are they?
@DaveNelson it's not a about stars, but I'll answer (I like neutron stars & black holes): the densest objects of normal matter are neutron stars - pretty much the density of an atomic nucleus but as a star or the mass of 1.4 suns squeezed into a ball of 10 km radius. Denser doesn't work - if we pile up more than 3 sun masses together at neutron star density or squeeze the neutron star further, we end up with a black hole, where things collapse to endless density in a singularity in the middle.
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@vicgrinberg I've always thought the ability of gas & plasma to sustain shockwaves means they can actually carry sound.
one of the more spectacular examples would be the "bounce" of core collapse in a supernova. but years and years after that, the shockwave is still travelling and the gas density is very thin.
does that mean these thin clouds of gas can carry sound? are the remnants dense enough to carry it? your work is with stellar winds, does it have sound? does a corona carry sound? the gas "leak" from a cataclysmic variable, what about that...
(I am not a professional astrophysicist, but I sure wanted to be one.)
@draNgNon this is not a question about stats, so very short answer: gas clouds are far too thin to carry sound in the meaning of the sound on Earth - the densest gas clouds are as dense as some of the most extreme vacuums we can create on Earth. The Universe is very, very empty.
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I have a bit of time on my hands, so let's do another 24h round of #AskAnAstrophysicist, but this time it's a thematic one.
What do you want to ask an astrophysicist about stars? (I am a professional astrophysicist, part of whose work concerns itself with high mass stars & their winds and I've also taught a variety of astro university courses)
Boosts welcome. I may not be able to reply to all in case of many questions.
@vicgrinberg Do you think it is possible to verify models in theoretical physics using publicly available data from repositories such as https://researchguides.gonzaga.edu/physics/data only ? Just order this data around some N real world physical experiments and check your theoretical physics model for each of the N experiments. My idea is developing new models in theoretical physics using AI (new geometries in general relativity for example) and check them against the public data repos.
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@Two9A to be honest, I don't know what you mean by a deeper reason here. There is no reason why things are the way they are - except that we would not exist as we are if they weren't.
For atoms, you can roughly imagine that the more (positivly charged) protons there are, the harder it is to squeeze them together. So the larger the atom, the less energy is left over to be released when it is created, until at some point you need go add energy to make them instewd of releasing.
@vicgrinberg Fair, I guess it's a question along the lines of "what if G had a different value": not something one can answer except with "in this universe it doesn't".
Thanks for taking the time!
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Since I have not seen the question already: What are those winds you mentioned? How can I imagine them? I suspect stars don't have an atmosphere like planets do.
I am so confused.
@palaress ah, thanks for the question! Stellar wind (our Sun also has a wind, called solar wind) is not something that happens in an atmosphere, but rather a thin stream of particles blown away from the star from its uppermost layers. Stellar wind is what makes Aurorae (when there is an especially strong bit of it) and that makes comets have nice tails (the tail is pretty mucbtan interaction result of the comet with the wind).
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@vicgrinberg Do you think it is possible to verify models in theoretical physics using publicly available data from repositories such as https://researchguides.gonzaga.edu/physics/data only ? Just order this data around some N real world physical experiments and check your theoretical physics model for each of the N experiments. My idea is developing new models in theoretical physics using AI (new geometries in general relativity for example) and check them against the public data repos.
@liklyhood not a question about stars and no. Physics is not something done by single with help of AI. And you need to really understand you data to compare it with models, just getting it from online repos isn't enough. I know you will not listen, but I want others to read this. No.
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@coleenwalter it's a very cool question actually! The stars move relative to the solar system and the solar system itself moves through our galaxy, so overall the position of stars changes. The timescales are very large, though, so "just" a few thousand years ago things would not look too different, possibly not even noticeable with the nakes eye. But the further in the past you go (to pre homo sapiens time), the more different it would look - same for far away future!
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I have a bit of time on my hands, so let's do another 24h round of #AskAnAstrophysicist, but this time it's a thematic one.
What do you want to ask an astrophysicist about stars? (I am a professional astrophysicist, part of whose work concerns itself with high mass stars & their winds and I've also taught a variety of astro university courses)
Boosts welcome. I may not be able to reply to all in case of many questions.
@vicgrinberg So on 12 May 2026 at around 2000 hours to 2100 hours (GMT+5), I observed a star/celestial object in the sky which was way too on low down the horizon line and I think it was somewhere in the West (cos it was in the opposite direction where the sun rise). The geographical co-ordinates of my city are 24.860966° N and the longitude is 66.990501° E.
From where can I learn more about what it is/was ?! I don't know if this might sound like a stupid noob question but it's been bugging me
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@vicgrinberg @johnnythan This XKCD is more accurate than one might think. I've been guilty of it too.
@internic @johnnythan ah yes, xkcd forever
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@vicgrinberg @bkahn Interesting! So how do we know that these black holes were caused by single supermassive stars rather than having grown over time?
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@vicgrinberg Thanks Dr. Grinberg. Enjoy your vacation.
@GranVegas thank you
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@glc xkcd always wins
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@vicgrinberg How bad has Starlink become to hinder professional examination of the stars from earth or does it all depend on the space telescopes now?
@Bugspriet @vicgrinberg I think that question can best be anwered by @sundogplanets
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I have a bit of time on my hands, so let's do another 24h round of #AskAnAstrophysicist, but this time it's a thematic one.
What do you want to ask an astrophysicist about stars? (I am a professional astrophysicist, part of whose work concerns itself with high mass stars & their winds and I've also taught a variety of astro university courses)
Boosts welcome. I may not be able to reply to all in case of many questions.
Oo, oo! I've got one!
If 'dark matter', whatever it is, interacts with other stuff by gravity, why do we seem not to have 'dark matter black holes'? And obvs, without any coherent theory of what 'dark matter' actually is, how would one know, or test the theory of how they behave or form?
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@vicgrinberg Maybe more of a physics question than a star question; if so, I apologize: what is your take on the new possibility that black holes may not contain singularities after all?
https://www.sciencenewstoday.org/no-more-singularity-physicists-propose-new-black-hole-paradigms
@secbox it's indeed not a star question. I discuss singularities as ideas eg in my book https://www.kosmos.de/de/schwarze-locher_1180792_9783440180792 - it would be too long for a post here.
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@vicgrinberg How much impact does dark matter have on stars? (Does the extra gravity influence internal processes, the number of CMEs, etc.) How much does it vary based upon the star's size? Is there the same relative regular matter to dark matter ratio regardless of size?
@clonezone none at all likely. Dark matter is distributed on much, much longer scales. It does play a major role in the formation of galaxies and thus, in a way, on the stars that will be later formed in them (no galaxies, no or very little stars), but to today's very detailed knowledge, it plays zero role in birth, life or death of individual stars.
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@vicgrinberg
This could be a question specific to stars (or even specific to massive stars) or it could be a question for all of astrophysics, I guess. Are we collecting data about the universe and its. contents faster than we (meaning you astrophysicists primarily) can analyze it? I know there are citizen science projects that help to classify galaxies and whatnot (Galaxy Zoo? I don't remember) but I imagine the flood of data far outstrips the ability of humans to parse it all. True? Are we getting ever further behind?@bkahn there is a lot of data, bit we are also pretty good at analysing it. The problem is more that there is a lot of additional stuff that can be learned from the same data (data collected to answer question A, can also be used for answer question B and, together with different data, possibly also C), but we don't have enough funding for professional astrophysicists to do the work.
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I have a bit of time on my hands, so let's do another 24h round of #AskAnAstrophysicist, but this time it's a thematic one.
What do you want to ask an astrophysicist about stars? (I am a professional astrophysicist, part of whose work concerns itself with high mass stars & their winds and I've also taught a variety of astro university courses)
Boosts welcome. I may not be able to reply to all in case of many questions.
@vicgrinberg which elements are made in which kind of stars and when during the Star lifetime?
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I have a bit of time on my hands, so let's do another 24h round of #AskAnAstrophysicist, but this time it's a thematic one.
What do you want to ask an astrophysicist about stars? (I am a professional astrophysicist, part of whose work concerns itself with high mass stars & their winds and I've also taught a variety of astro university courses)
Boosts welcome. I may not be able to reply to all in case of many questions.
@vicgrinberg
Any kind of exotic/theoretical star that science says could potentially exist but haven't been observed yet? -
@vicgrinberg which elements are made in which kind of stars and when during the Star lifetime?
@confusedMiddleAgedDad funnily, one of my very first scicomm pists on mastodon was on this https://mastodon.social/@vicgrinberg/109461953352960242 - for more info see the link in the image - here a nice article by the author herself
https://www.americanscientist.org/article/a-chemical-history-of-the-universe
