How do magnetic fields mess with astronomers' observations?

Question

Does anyone else watch 'Dr. Becky' on YouTube?

Her March 'Night Sky News' video, subtitled 'The biggest black hole burp, a wobbly Milky Way & Betelgeuse is brighter' says that...

Obviously lots of theories that have been thrown around this by various different people in the academic community, the amateur astronomy community, and also just people commenting on line as well.

But a paper that caught my eye this month was by (Emily M.) Levesque and (Philip) Massey (2020)¹ who pose the explanation... dust... which if you know anything about astrophysics in general, you know magnetic fields and dust are just the bane of astronomers' lives.

¹Betelgeuse Just Isn't That Cool: Effective Temperature Alone Cannot Explain the Recent Dimming of Betelgeuse

Why exactly are magnetic fields "just the bane of astronomers' lives"? How do magnetic fields mess with astronomers' observations?

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Cued at 09:16 for the quote, but the topic of Betelgeuse (and Dr. Becky's "I told you so" begins after 07:32

Answer 1

tl;dr, she might be talking about sunspots (or in this case, Betelgeuse-spots)

There are two things that come to mind, although one of them definitely seems more likely than the other as far as the context.

The first is Zeeman broadening. This is a broadening mechanism for spectral lines brought on by magnetic fields, and the broadening of spectral lines can make resolving certain spectral lines rather difficult, especially if they are close together. I personally haven’t ever done much hands-on work with spectra, but I could see how this could be frustrating. That being said, every time I’ve heard of Zeeman broadening, it’s been as a footnote in a discussion of more dominant broadening mechanisms.

The second is magnetohydrodynamics. This might be the more relevant, seeing as we’re talking about Betelgeuse, and recent speculation has brought up the fact that the temperature changes observed in it might be from sunspots.

Sunspots are the results of magnetic field lines coming out of the star, and this along with other stellar behaviors resulting from magnetohydrodynamics are notoriously difficult to model, and to do so one must often take simplifying assumptions that compromise models in certain limits.

All of that being said, it’s difficult to know exactly what she was talking about, but these situations were the first that came to my mind. I could’ve expounded upon some of these terms a bit more, so if this was a bit too esoteric, let me know which terms were confusing and I’ll put forth a bit more explanation.

Answer 2

On cosmological scales (i. e., past the Milky Way and Local Group) visible-light photons are not just blocked outright, but reradiated by dust. The source information has, in some dust-rich areas of the sky, been scrambled. Visible-band observing is the most common and cheapest way, so this is a major setback. Astronomers must turn to infrared, sub-mm, radio, and to an extent X-ray instruments, at higher cost.

Magnetic fields are a second-order problem. Visible-band photons are not much of an issue, though dust grains can be aligned by magnetic fields causing effects like polarization. We would like to try studying the cosmic rays which would be information carriers. But galactic fields bend the paths of these ions, and they are once again scrambled, versus a linear path back to their source.

There are also local sources, like new stars that haven’t cleared their areas of gas and dust, or old stars that are often actively shedding (or just shed). Many large, old stars are also mega-magnetic sources. It’s hard to measure these objects, despite the fact that they’re different and interesting, relative to the unobscured and ‘boring’ stars that are clear and directly observable and well-sampled.

Answer 3

On the theory side magnetic fields are one of those things we know are important to the structure and evolution of a star, but find hard to include in our models. Internal to the star, they can transport angular momentum from the core to the envelope changing its spin rate and spin profile. But depending on your model this, they can either be not very efficient at this or very efficient at this. So trying to predict how a star evolves with magnetic fields (and rotation in general) is tricky, and quite often is lumped into the "future work" (that never actually gets done) part of research.