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So, have you ever wondered why some stars are brighter than others? You might think it's because they're closer to us, but that's not the whole story. In this episode of Crash Course Kids, Sabrina chats about how stars glow and how astronomers judge their brightness. Also, she talks about a really, really, really big star.

This first series is based on 5th-grade science. We're super excited and hope you enjoy Crash Course Kids!

///Standards Used in This Video///
5-ESS1-1. Support an argument that differences in the apparent brightness of the sun compared to other stars is due to their relative distances from Earth. [Assessment Boundary: Assessment is limited to relative distances, not sizes, of stars. Assessment does not include other factors that affect apparent brightness (such as stellar masses, age, stage).]

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Producer & Editor: Nicholas Jenkins
Cinematographer & Director: Michael Aranda
Host: Sabrina Cruz
Script Supervisor: Mickie Halpern
Writer: Kay Boatner

Executive Producers: John & Hank Green
Consultant: Shelby Alinsky
Script Editor: Blake de Pastino

Thought Cafe Team:
Stephanie Bailis
Cody Brown
Suzanna Brusikiewicz
Jonathan Corbiere
Nick Counter
Kelsey Heinrichs
Jack Kenedy
Corey MacDonald
Tyler Sammy
Nikkie Stinchcombe
James Tuer
Adam Winnik

Venus Image Credit: Brocken Inaglory

(Intro music)


Let's continue our chat from last time about stars that are not you. I mean, the stars in our universe. We've already talked about what a star is - a huge ball of hot gas that makes heat and light. Basically, they glow. But how do they glow? And why do some stars seem to glow brighter than others? 

 Big Question (0:28)

Let's tackle question numero uno first: How do they glow? All stars create energy in their core through a process called nuclear fusion. Sounds super cool, right? It is. Well, super hot actually but you get the idea. It's neat. It's also way too complicated to explain in one video so I'll just give you a really, really basic overview.

The temperatures and pressures inside a star is so great that nuclear fusion, the same thing that powers hydrogen bombs, is almost constantly happening at the center of a star. Most stars are made up of mostly two kinds of gases - hydrogen and helium. During nuclear fusion, hydrogen molecules - really tiny units of hydrogen - smash into each other, or fuse together, to create helium. The enormous amount of energy that's create as a result of this fusion then travels out towards the surface of the star as heat and light, and gives the star its glow. 

All stars go through this process to get their glow but they don't all shine equally, leading us to our second question: Why do some stars glow brighter than others?

Stars have different brightnesses. There are two major things that astronomers look at when determining how bright a star is: its true brightness, and its apparent brightness. Astronomers sometimes refer to a star's true brightness as its luminosity. Luminosity is a measure of how much energy, or light, a star shines from its surface. 

Think of stars like light bulbs. Light bulbs with higher wattages, or more power, glow brighter right? Same thing with stars, the more luminous, or powerful, a star is, the brighter it glows. When measuring luminosity, astronomers are considering how bright the star is as if they were in space, standing, or I guess hovering, right in front of it. That's why they call it true brightness. It's how bright the star really is, not just how bright it might look depending on where you are. 

So, how does a star's size factor in? Some stars are really, really big and some are really, really, well, not small but smaller. So, if it's bigger, it must glow brighter, right? Not necessarily. Smaller stars sometimes have more power than bigger stars, just like smaller light bulbs can have more wattage than bigger light bulbs. Size also isn't necessarily important when measuring the second thing that astronomers look at when determining a star's brightness - its apparent brightness. 

When we talk about apparent brightness, distance is a bigger factor than size. Since astronomers can't really go into space to see the stars up close because, you know, they burn up and it would take too long to actually get to another star, they also measure how bright a star appears to us from a distance. This measure is called its apparent brightness.

So, regardless of its size or its power, how near or far a star is to Earth plays a big part in how bright it appears to us. Not convinced? Let's take a look at two stars, to see if the more luminous one also appears brighter to us Earthlings. 

 Investigation (3:14)

First, meet Betelgeuse. Betelgeuse is a star, one of the brightest stars in the night sky, in fact. Betelgeuse shines with around 100,000 times more luminosity, or power, than the Sun. So you'll need some serious shades to look at this guy up close. But how does that compare to the Sun? 

The Sun is a star we've all spotted from Earth way more times than we've seen Betelgeuse. Since we have no problem locating it, it must be bigger than Betelgeuse, right? Nope! I mean, don't get me wrong, the Sun is huge. More than a million Earths could fit inside it, but it's not so big when you compare it to Betelgeuse, because Betelgeuse is gigantic! About 400 times bigger than the Sun. But because Betelgeuse is located much farther away from us than the Sun is, it appears less bright to us. If the Sun was located that far away, you'd need a telescope to see it at all, since it doesn't have as much power as Betelgeuse. 

So it's not the Sun's size or power than makes it seem so big and bright to us on Earth, it's because it's so close to us, just 150 million kilometers away. So Betelgeuse is bigger and more luminous, or has a higher true brightness than the Sun, and you'd be able to tell that if you could look at them side by side, but the Sun has a higher apparent brightness to us on Earth because it's so much closer. So let's call this brightness battle a tie. 

 Conclusion (4:30)

To recap, stars glow as a result of the energy created in their core by nuclear fusion. That energy travels outward to the surface of the star where it's seen and felt as light and heat. All stars glow because of nuclear fusion, but they don't glow equally. The amount of power a star has - and it's distance from us - both influence its brightness. On that note, I think it's time for me to do some star-gazing of my own. Astronomer Sabrina's out!