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Comment by _justinfunk | Hacker News Reader

> Astronomers have found a star that has a magnetic field rivaling the strongest magnet humans have ever built.

This does not sound impressive. What am I missing?

As the article describes, most star's magnetic field is rather steak. For example, the sun's magnetic field is only double the strength of Earth's. We have definitely built much stronger ones.

While it is not unusual for stellar objects to be magnetic, these are usually neutron stars. It is interesting to find something like this.

Just as an interesting bit:

Inside of the Sun there are regions with significantly stronger fields. In particular the solar tachochline, a region at around 70% of the Sun's radius has fields of up to sometimes assumed 10 T. It's the region where the interior goes over to the convective zone. Due to differential rotation strong fields are produced.

I'm a bit doubtful of the 10 T number, having looked into literature about solar models ~5 years ago. More likely seems maybe 1-3 T, but maybe things are more clear now and I'm not an expert on solar magnetohydrodynamics (the strength there was just important for my work back then; but hey, if someone reads this who _is_ an expert, I'm still interested in details there haha).

I thought so, too. Upon a quick search, the most powerful magnet ever made on Earth is around 45 teslas, which according to WolframAlpha is 450,000 gauss. I'm assuming the difference is that these stars have the magnetic field strength everwhere on their surface and covering a huge region beyond the star, whereas the human made magnets, likely superconducting, only have that field strength in a very small area. That's just an intuitive guess though, as I don't know for sure.

Magnetic fields fall off with the inverse square law. A tiny magnet can easily be extremely powerful but it's very difficult for a huge one to be. Think of picking up a nail with a neodymium magnet and compare that with the earth's magnetic field affecting a compass needle.

The same rules apply to light. It's pretty easy to construct a light bulb with a filament which is hotter than the surface of the sun and which appears far brighter. But if you compare the sun and the light bulb at equal distances the sun is going to be brighter.

> Magnetic fields fall off with the inverse square law.

Inverse cube: the simplest magnets are dipoles.

> It's pretty easy to construct a light bulb with a filament which is hotter than the surface of the sun

The surface of the sun is about 10,000 F, well above the melting point of tungsten. Maybe there's some exotic ceramic that can survive those temperatures, but I'm not aware of any.

Seems like ceramics have hit ~7,000F: https://en.wikipedia.org/wiki/Ultra-high_temperature_ceramic

But yeah. No filaments at that temperature, though 10,000F is rather easy to hit if you're not trying to keep whatever you're heating.

Though this does make me wonder how hot a star can get. Stable states are likely not all that hot, but what about supernova?

My favourite type of supernova is the pair-instability collapse, where it's hot enough that, thanks to the Maxwell-Boltzman distribution and Stefan–Boltzmann law, a nontrivial fraction of the photons spontaneously turn into positron-electron pairs, causing a runaway loss of pressure, leading to collapse, leading to more heat, leading to more sufficiently energetic photons… leading very quickly to multiple tens of solar masses of hydrogen turning directly into nickel-56 in a matter of seconds.

It sounds very interesting, like a scenario from https://what-if.xkcd.com/ but in real life. Do you know a good (not specialized) article/blog post/Wikipedia article about it? Something at the ELI25 level.

https://en.m.wikipedia.org/wiki/Pair-instability_supernova

Check out Wolf-Rayet stars ([1]); effective surface temperature from 10,000 to 210,000 K. Tungsten melts at 3956 K, hafnium carbonitride ([2]) at 4370 K.

[1] https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet_star

[2] https://en.wikipedia.org/wiki/Hafnium_carbonitride

> Stable states are likely not all that hot, but what about supernova?

As context first, at some point temperature just sort of, stops being useful as a comparison. Not because it can't be computed, but because there really isn't any point of comparison for it. The coldest temperature in Antarctica was -89.2 Celsius [0]. The hottest day in Death Valley was 56.7 Celsius [1]. Tungsten melts at 3400 Celsius [2]. And papers about supernova will casually throw around phrases like "for temperatures exceeding a few 100 keVs" [3], to state that pretty much everything that happens in a supernova will be hotter than that. That's an energy measurement equivalent to about 1 trillion kelvin.

The second piece of context, for anything that deals with energy output, supernova are basically the "I win." answer. XKCD's "What If?" series has a good comparison for this, that for pretty much any comparison you can conceive, if the question is about energy output, the supernova wins.

> Which of the following would be brighter, in terms of the amount of energy delivered to your retina: (1) A supernova, seen from as far away as the Sun is from the Earth, or (2) The detonation of a hydrogen bomb pressed against your eyeball? ... [The supernova] is ... by nine orders of magnitude. [4]

So when temperatures are reported as 100 billion kelvin for the neutron star remnant [5], those are the cold (by comparison only) embers of a dying fire. The supernova itself has a "typical core temperature of 1 MeV" [3], which translates to about 10 trillion kelvin.

[0] https://en.wikipedia.org/wiki/Lowest_temperature_recorded_on...

[1] https://en.wikipedia.org/wiki/Highest_temperature_recorded_o...

[2] https://en.wikipedia.org/wiki/Tungsten

[3] https://arxiv.org/abs/astro-ph/0612072

[4] Xkcd, What If, #73: https://what-if.xkcd.com/73/

[5] https://en.wikipedia.org/wiki/Supernova#Detailed_process

Positive attraction.

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