3 ms·

So let me see if I'm understanding this, knowing nothing about cosmology other than what I've read on Wikipedia: Black holes pull in matter, but as the matter

So let me see if I'm understanding this, knowing nothing about cosmology other than what I've read on Wikipedia: Black holes pull in matter, but as the matter falls into the black hole, before it reaches the event horizon, it starts to collide into other matter, which can cause a lot of heating, which can cause gas jets to erupt outward. In the case of a supermassive black hole (such as the one theorized to be at the center of our galaxy), the gas jets could be so massive and hot that the gas could pull itself together (via gravity) and become one or more stars.

ncmncm 5y ago
To my knowledge nobody has explained how heating in an accretion disc turns into a collimated polar jet thousands of light years long. "It must be magnets" does not qualify as such an explanation.
- antognini 5y ago
  It's pretty well accepted that the jets are powered by the Blandford-Znajek mechanism, and MHD simulations of accretion disks do show that the BZ mechanism can in fact generate jets. There are, of course, open questions about the details, in particular where the magnetic fields come from.
  - ncmncm 5y ago
    I.e., not just heating.
nivekastoreth 5y ago
IANAS but: You were there up until the end. The gas jets aren’t forming stars near the black hole nor are the stars formed using material from around it. Instead, they are sending out the equivalent of pilot lights that kick off star formation in another part of the galaxy where clouds of star forming gas were already collecting
- AnimalMuppet 5y ago
  I believe the jets are compressing the gas clouds that were already there. That definitely could trigger star formation.
  - nivekastoreth 5y ago
    Agreed. To go a little further: the main point of the paper is that there is a subtlety with how black hole outflow can effect the neighborhood: 1. the outflow is so energetic that it blows the gas clouds away and shuts down star formation 2. the outflow is energetic enough to compress/raise the temperature of the gas clouds, leading to star formation This second is the newly observed mode, but there is also yet more subtlety there since this burst of star formation may result a reduction in future star formations, but that is quickly approaching the limit of my layman/enthusiast understanding.
    - ncmncm 5y ago
      It matters enormously how ionized the gas cloud is. If it is neutral, the ion jet blasts right through, hardly noticing it: it is all, after all, "vacuum". Fully ionized, every particle in the jet interacts with every particle in the cloud. Most such clouds are partially ionized, often to different degrees in different parts. Even a 0.01% ionized cloud behaves differently to a gas. The proper name for such a cloud is "plasma". Many astronomers are allergic to this word, probably because the maths for plasma fluid dynamics is all just way too hard, and they steer clear, preferring literally any alternative (heating! gravitation! shocks!) over engaging. Ordinary fluid dynamics is tricky enough to be often intractable. Plasma fluid dynamics is freaky because usually the positive ions are at least 1836 times as massive as the negative ions. Dust can be ionized, too, either direction. Then the ratio might be 6, 7, 10 orders of magnitude. Vacuum is usually treated, in astrophysics, as infinitely conductive, and the charge carriers as massless, making static electric fields impossible, and freezing magnetic fields in place. But moving ion clouds (including jets) carry magnetic fields, and thus generate varying local electric fields. And, of course, the carriers are not really massless at all.

ncmncm 5y ago

To my knowledge nobody has explained how heating in an accretion disc turns into a collimated polar jet thousands of light years long. "It must be magnets" does not qualify as such an explanation.

antognini 5y ago
It's pretty well accepted that the jets are powered by the Blandford-Znajek mechanism, and MHD simulations of accretion disks do show that the BZ mechanism can in fact generate jets. There are, of course, open questions about the details, in particular where the magnetic fields come from.
- ncmncm 5y ago
  I.e., not just heating.

nivekastoreth 5y ago

IANAS but: You were there up until the end. The gas jets aren’t forming stars near the black hole nor are the stars formed using material from around it. Instead, they are sending out the equivalent of pilot lights that kick off star formation in another part of the galaxy where clouds of star forming gas were already collecting

AnimalMuppet 5y ago
I believe the jets are compressing the gas clouds that were already there. That definitely could trigger star formation.
- nivekastoreth 5y ago
  Agreed. To go a little further: the main point of the paper is that there is a subtlety with how black hole outflow can effect the neighborhood: 1. the outflow is so energetic that it blows the gas clouds away and shuts down star formation 2. the outflow is energetic enough to compress/raise the temperature of the gas clouds, leading to star formation This second is the newly observed mode, but there is also yet more subtlety there since this burst of star formation may result a reduction in future star formations, but that is quickly approaching the limit of my layman/enthusiast understanding.
  - ncmncm 5y ago
    It matters enormously how ionized the gas cloud is. If it is neutral, the ion jet blasts right through, hardly noticing it: it is all, after all, "vacuum". Fully ionized, every particle in the jet interacts with every particle in the cloud. Most such clouds are partially ionized, often to different degrees in different parts. Even a 0.01% ionized cloud behaves differently to a gas. The proper name for such a cloud is "plasma". Many astronomers are allergic to this word, probably because the maths for plasma fluid dynamics is all just way too hard, and they steer clear, preferring literally any alternative (heating! gravitation! shocks!) over engaging. Ordinary fluid dynamics is tricky enough to be often intractable. Plasma fluid dynamics is freaky because usually the positive ions are at least 1836 times as massive as the negative ions. Dust can be ionized, too, either direction. Then the ratio might be 6, 7, 10 orders of magnitude. Vacuum is usually treated, in astrophysics, as infinitely conductive, and the charge carriers as massless, making static electric fields impossible, and freezing magnetic fields in place. But moving ion clouds (including jets) carry magnetic fields, and thus generate varying local electric fields. And, of course, the carriers are not really massless at all.