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We’ve created simulations to recreate the difference in time it takes for the Sun’s equator and poles to complete rotations, and the way we’ve solved is a bit surprising. And it looks like the Milky Way may not be great at mixing metals, which gives us insight into the goings on in other galaxies.

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Sources: (paywall)
De Cia, A., Jenkins, E.B., Fox, A.J. et al. Large metallicity variations in the Galactic interstellar medium. Nature 597, 206–208 (2021).

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If you’d like to support a great cause, check out [♪ INTRO]. On Earth, the length of a day is the same no matter where you live.

Because the planet’s surface is solid, every piece of ground takes a little less than 24 hours to rotate fully, so that the stars in the sky appear to move around it in a predictable path. But for the Sun, which is a big ball of plasma, a day at the solar equator is about 24 Earth days, while a day at the poles is 35. And until now, astronomers have had some trouble creating detailed computer models that replicated that reality.

A new simulation, described this week in Nature Astronomy, appears to have done just that. Deep inside the Sun, energy moves in the form of radiation, photons of light bouncing around and slowly making their way upward. But starting at around 200,000 kilometers beneath the Sun’s surface, or the closest analog to a surface that it has, temperatures start to drop to around 2 million degrees C.

That is still quite hot, but it’s low enough that the Sun's plasma becomes less see-through. And since radiation is carrying energy away from the center of the Sun via light, that means that energy can't get through it as easily. So instead, globs of hot plasma take over by physically floating upward.

The globs expand and cool to around 5,427 degrees C until they reach the “surface,” and then they sink back down. That process is called convection, and that’s what astronomers believe is the source of the difference between the Sun’s rotation rates. But when you plug numbers from actual observations of the Sun into previous models based on the principles of physics, they create a Sun where the poles rotate faster than the equator.

That is the opposite of what’s actually going on. Basically, the material undergoing convection was moving too fast in the models. The only way to replicate reality was to manually go into the simulation and tweak the results.

So one team of astronomers tried its hand at creating a new simulation, focusing on the influence that the Sun’s magnetic field could have on the convection. They looked at three levels of simulation resolution: low, medium, and high. And after plugging in starting values, their fake Suns spun for around 4,000 days to see if they ended up matching the Sun in the real world.

Because the high-res sim was able to keep track of finer details, its magnetic field was able to grow strong enough to lessen the strength of the convection. It also reduced that convective energy on large scales, as opposed to only having more local effects. Ultimately, that meant their low-res sim ended up replicating the old studies, where the resulting rotation rates were flipped.

But the high resolution one slowed the convective material enough that it created a faster-spinning equator. While there’s still work to be done, better and higher resolution models will hopefully help researchers better understand the complexity of the Sun’s interior, how that translates into things like the Sun’s 11-year sunspot cycle, and how that influences space weather. From the complexity of Earth’s Sun, we now turn to that of its galactic home.

Which turns out to also be pretty complex. Last week in the journal Nature, astronomers revealed the Milky Way isn’t as mixed up as we previously thought. When the very first stars formed in the first galaxies, the periodic table of elements would only consist of three entries: hydrogen, helium, and a teeny tiny amount of lithium.

But through the process of nuclear fusion, stars created heavier elements, both in life and in violent death. So over the 10 billion years our Milky Way has been around, its insides have been enriched with what astronomers call metals. Yes, in astronomy anything heavier than helium is a metal.

Even carbon. Even oxygen. It’s all metals.

Some of those metals hang out as gas in the interstellar medium: the stuff between the stars. Some of them condense into grains of dust. And some of them get incorporated into new stars, which also incorporate hydrogen and helium from the intergalactic medium: the stuff that’s outside the Milky Way that gets pulled into it by gravity.

While it is still a bit of a debate what exact percentage of the Sun is made of metal, it’s roughly between 1 and 2 percent. And astronomers have treated the local interstellar medium as having a similar composition. Models of the Milky Way as a whole assume that all these ingredients, intergalactic gas, enriched interstellar gas, and metallic dust, have been thoroughly mixed up and distributed equally, except in the very center, where the sheer number of stars raises the metallicity a small amount.

But a team of astronomers wanted to confirm that assumption, so they turned to the Hubble Space Telescope and Very Large Telescope in Chile to probe the local stellar region. As light from stars travels to us through the interstellar medium, gas atoms of different elements absorb different combinations of light. The resulting data, known as an absorption spectrum, reveals how much of each element is out there.

But that method cannot measure the amount of dust grains. So the team had to devise a separate and new technique to get that data, which involved looking at a bunch of different elements all at once and in relation to each other. Combining these data sets, the team revealed that the interstellar medium was not homogeneous.

Some regions had metallicities as low as 17% of the metallicity of our Sun, and the average across the 25 directions they probed was 50 to 60% the metallicity of the sun. This suggests that our galaxy probably isn’t very good at mixing new intergalactic hydrogen and helium into the interstellar medium. When blobs of it fall into the galaxy due to gravity, they just stay local, dropping the overall metallicity in those spots.

Not only does this mean that astronomers have to refine their models for how galaxies develop over time, but it means that there could be solar systems made of drastically different combinations of elements out there, just waiting to be discovered. Thanks to Omaze for sponsoring this episode. Omaze is a fundraising platform that gives everyone a chance to win unique prizes while helping to make the world a better place.

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