Together, the Northern and Southern lights could be considered

one of the most beautiful sites in nature.

But these giant curtains streaking across the sky

aren't the only type of aurora out there.

There's also the much more common, but less brilliant pulsating auroras.

These can happen at any time, but we haven't

been able to actually observe what causes them.

Until now.

Thanks to research published in Nature last week,

we think we finally know how the process works.

Active auroras, like the dazzling shows you see at the poles,

have one continuous arc of light.

But pulsating auroras get their name because they, pulse.

Distinct patches of sky vary in brightness over several seconds,

and they're generally less striking than the active auroras.

These different appearances are caused by the auroras' related, but different origin stories.

They're both caused by charged particles, usually electrons,

traveling down into the atmosphere and colliding with molecules there.

The molecules' electrons gain energy from those collisions, and then release light as

they relax back to their usual state.

Now, the electrons that spawn active auroras come from

dense waves of solar material colliding with the Earth's magnetic field.

Ones that create pulsating auroras are a bit more complex,

but according to this new paper, we might have figured it out.

Basically, when the Earth's magnetic field rearranges itself, which happens all the time,

it releases a bunch of stored up energy.

That energy triggers the creation of plasma waves, or waves of charged particles.

Specifically, these ones called chorus waves.

They form high in the Earth's atmosphere near the equator,

then move north and south to more extreme latitudes.

As they go, they scatter electrons that would otherwise just

bounce around in the magnetic field.

Some of those electrons get jostled around, and ultimately

rain down in batches into the upper atmosphere.

And that, finally, creates pulsating auroras.

This hypothesis has actually been around for over half a century,

but it's only just been proven.

We had to wait until we had sensitive enough equipment to observe a clear interaction between

chorus wave plasma and the electrons causing the auroras.

Still, thanks to the Japanese ERG satellite and some aurora measurements from last March,

scientists pulled it off.

The authors still acknowledge that there could be holes in their findings, like that these

results may not be the same across different distances from Earth and geomagnetic activity.

But this new research will help scientists better understand

how these interactions affect planets' atmospheres.

This applies to any planet with a magnetic field, too, including Jupiter and Saturn,

where we've already detected chorus waves.

So this is only the beginning.

In more wide-reaching news, literally, a group of astronomers have calculated that the neighboring

Andromeda Galaxy might be a lot less massive than we thought.

Our galactic big sister seems more like a twin.

There are a lot of ways to calculate a galaxy's mass,

and they all yield slightly different results.

Most astronomers have treated our galactic neighbor as 2 to 3 times more massive than

the Milky Way, because that's what pops up in a lot of studies.

But its mass is still far from certain.

It's hard to measure the mass of a whole galaxy, okay?

But new research, published last week in the Monthly Notices of the Royal Astronomical

Society, attempts to better pin down that mass by using a completely different technique.

It requires measuring how fast an object needs to travel to escape a galaxy's gravity;

basically, its escape velocity.

Galaxies with more mass will have more gravity,

so you need to travel faster to get away from them.

You'll also need to have a higher escape velocity if you're near the galaxy's center

of mass, as opposed to at its edge.

Scientists can use all this data to work backwards.

By calculating escape velocities at different locations,

they can do some math to figure out a galaxy's mass.

In this study, the team of astronomers used velocities of 86 speedy planetary nebulas,

or the remnants of certain stars, in the Andromeda Galaxy.

By doing some math and making some estimates, they were able to use these nebulas to calculate

the escape velocity of Andromeda from different locations.

For example, at about 49,000 light-years from the galaxy's center, that escape velocity

is around 470 kilometers per second.

Then, from those escape velocity values,

they were able to derive the mass of the galaxy itself.

And it came out to be around 800 billion times the mass of our Sun, which is roughly the

same mass as the Milky Way, not twice as massive or larger, like we used to suspect.

Still, the authors did note that they had to make certain educated assumptions in their

calculations, so this mass value isn't 100% certain,

just like all the ones that came before.

There's also a chance that, although they were moving really quickly, none of the

planetary nebulas they studied were going anywhere close to the real escape velocity.

That would have also affected the mass estimate.

So we'll have to run a few more models before we're totally certain.

One way or another, pinning down the deets on Andromeda is crucial for our understanding

of galactic evolution, and the ultimate fate of the Milky Way.

After all, our galaxy and Andromeda

are on their way to a collision in several billion years.

But when that happens and how it'll look depend on,surprise, how massive they are.

Luckily, we have, you know, some time before we need to get those measurements done precisely.

Thanks for watching this episode of SciShow Space!

If you would like to keep learning about the universe with us,

you can do that at youtube.com/scishowspace,

where we have hundreds of episodes that are all very good,

and also, we just keep uploading new ones every week.

[♪ OUTRO]