When you look up at the night sky, it's easy to think that all the stars up there

are basically the same.

After all, for the most part, they do all look alike from your point of view.

But get this: Some of the brightest, most easily visible stars in the sky also happen

to be among the most important to astronomy.

You've been looking at some really weird stars, and you didn't even know it!

Here are three of them.

First, take the Big Dipper.

It's visible in most parts of the world, and it's made up of four stars in the “cup”

and three in the “handle.”

But if you look at them really closely, they're not all what they seem.

Next time you're outside, take a peek at the middle star in the handle.

Look closely, and you'll see that it's actually two stars, called Mizar and Alcor.

The pair is about 80 light-years away, and we've known about it for thousands of years.

In fact, they probably represent the first known binary stars: stars that orbit one another.

But that's not where the fun stops.

If you point a small telescope at Mizar, you'll see that it isn't just one star, either.

It's also a binary, with two stars now called Mizar A and B.

In 1617, it became the first known telescopic binary, or a pair of stars orbiting each other

so closely that you need a telescope to split them.

Then, in 1890, we found even more stars.

The use of a new tool called a spectrograph, which measures light patterns, revealed that

Mizar A was itself a binary.

In fact, in the years since, we've actually realized that Mizar A, B,

and Alcor are all binary stars.

So what looks like one star in the handle of the Big Dipper is actually an intricate

sextuple star system!

It's binaries all the way down.

The Big Dipper is one of the most common and well-known figures in the sky, but it's

also an easy reminder that everything isn't always what it seems.

Sometimes, it's a lot cooler.

During summer in the northern hemisphere, there's another weird star you can spot.

Just look straight up.

Even in light-polluted skies, you should be able to see an

enormous trio of stars called the summer triangle.

The brightest of those three stars is Vega,

and it's long been one of the key reference stars in astronomy.

Among other things, astronomy's magnitude system for describing brightness was anchored

by Vega, the so-called magnitude-zero star, for decades.

But here's the thing: Vega might not have been the world's

best choice for a standard star.

Normally, stars are actually pretty simple things,

and every one of a given type should have roughly the same size and brightness.

Except for Vega.

It seems to be a lot brighter than its type would suggest.

And over the last couple of decades, astronomers have finally figured out why.

The fact that stars should follow patterns like these hinges on a critical assumption:

that they're all basically spheres.

And this makes sense.

In the simplest sense, stars are just big collections of hot gas.

All that hot gas wants to spread out as much as possible,

but gravity is constantly tugging it back together.

And since gravity pulls with the same strength in every direction, voila, a sphere.

But Vega is an exception.

It turns out that this star is spinning really, really quickly,

something like 70-90% of its breakup speed.

And just like when you're riding a rollercoaster,

that spinning causes an apparent force that counteracts gravity.

Since the equator spins faster than the poles,

gravity is weakest there, and Vega bulges out.

This isn't a subtle effect, either:

Its radius is 19% larger at its equator than the pole.

It looks so bright to us on Earth because, through random chance, our view looks at Vega's

pole, meaning we're seeing the largest possible cross-section of the star.

And with the maximum amount of area emitting light towards us, it's no surprise that

Vega seems brighter than it should be.

And, finally, Vega's got some brightness troubles,

but no star in the sky has it worse than Mira.

Like the stars in the Mizar system, this one is also a binary pair.

It's made of Mira A, a huge, red giant, and Mira B, a little white dwarf.

You can't spot Mira B with the naked eye,

but you can see the red giant in the constellation Cetus.

It's visible from basically anywhere on Earth,

but it's best seen near the end of the year.

Well, when you can see it at all, that is.

Some days, it's one of the brightest stars in the night sky; other times, it's so dim

you can't see it without a telescope.

And that is a sure-fire sign something weird is going on.

Mira A, usually just called Mira, is probably the earliest-known example of a variable star,

or one whose brightness changes substantially back and forth over time.

This happens because of big swings in the star's temperature and even its size.

Today, we know of a few kinds of variable stars, including other red giants, and we

generally understand how they work.

But we just can't figure out Mira.

See, stars enter the red giant phase after exhausting the hydrogen fuel in their cores

and starting to burn heavier elements like helium.

Helium burning isn't nearly as steady, so in other kinds of variable stars, it sort

of sputters and causes the star to brighten and dim.

But that doesn't seem to be the whole story with Mira's big swings.

Instead, its variation probably has to do with convection, the process that transfers

heat from a star's core to its surface.

For instance, if the amount of heat being transported outward was changing over time,

that would result in the star shining with different brightnesses.

But we're still figuring it out.

One thing is sure, though: Mira isn't alone in this.

We know of hundreds of so-called “Miras,”

most of which are just too far away to be easily seen.

But if you're lucky, you can spot the first one with the naked eye.

Oh, and as a bonus?

You won't be able to see it without a fancy ultraviolet telescope, but researchers also

discovered in 2007 that Mira has a tail!

It's a whopping 13 light-years long, and it's made of elements like carbon and oxygen

being shed by the star as it zooms through space.

Just in case this object wasn't already weird enough.

Some days, modern astronomy can feel remote and unknowable, because a lot of the discoveries

we make happen light-years away or on time scales too large to comprehend.

But there's a lot visible from your own backyard, too.

So, sometime, go outside and look up!

You might just be looking at something really incredible.

Thanks for watching this episode of SciShow Space!

While you're out stargazing, you might also notice another object in the sky: the moon.

Sometimes, it looks way bigger on the horizon than it should,

and it's all thanks to a cool optical illusion.

You can learn all about it in another one of our episodes.

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