The Brightness Of Stars

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  • 0:01 Relative Brightness
  • 0:53 Luminosity and…
  • 3:26 Distance and the…
  • 4:18 Words of Caution
  • 5:52 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will define and describe important terms related to a star's brightness, including luminosity, intrinsic brightness, apparent brightness, and more.

Relative Brightness

I recently visited a relative's home. Sitting in his garage, I spotted some sort of large object. As I came closer, I realized it was a gigantic handheld flashlight. Seriously, this thing was huge! Just the one end of it, where the reflector and bulb sit, had a diameter larger than a basketball. I joked that he could probably light up the neighborhood if the power went out.

And you know what? It was as bright as it was large. Of course, I exaggerated that it could light up the entire neighborhood because, relatively speaking, the flashlight isn't very bright at all from far away, despite its blinding nature in the garage. You'll understand why that's the case by the end of this lesson as we define concepts like luminosity, apparent brightness, and their differences.

Luminosity and Apparent Brightness

The property of brightness of a flashlight is similar to that of stars out in space. Brightness shouldn't be confused with luminosity, which technically refers to the total energy a star radiates in one second. Luminosity is how truly bright a star actually is. Or to put it another way, luminosity is the star's intrinsic brightness.

This is different from the apparent brightness of a star. Meaning, how bright it appears to us here on Earth or to grey aliens living on another planet looking at the same star. If you're confused, don't be.

Think of the flashlight I found in the garage. If you're staring into the huge flashlight from a foot away, you'll be blinded by it. If you take the same flashlight and put it three miles away, you might barely see the light emitted by it. That's apparent brightness for you.

But luminosity is different. I mean, think of our flashlight compared to a military ship's searchlight, with a bulb that obviously has a larger luminosity. If you're 20 feet away from both, and both are turned on, our once huge and bright flashlight will appear quite faint compared to the searchlight because the flashlight's luminosity is much smaller. Therefore, brightness depends on the distance you are away from a star, whereas luminosity does not.

The intrinsic brightness of a star is sometimes referred to as absolute visual magnitude and the brightness of a star as seen from Earth as apparent visual magnitude. To help remember which term refers to what, think about the fact that a star 'app'ears to be a certain brightness to us ('app'arent visual magnitude), and by default the absolute visual magnitude is therefore the star's intrinsic brightness.

More technically, the absolute visual magnitude of a star is what its apparent visual magnitude would be if the star was only 10 pc away. A pc (parsec) is a unit of distance used for calculations in astronomy, and it is equal to 2.06 * 10^5 AU, which is 3.26 light years.

Distance and the Brightness of Stars

To find the absolute visual magnitude, we need to first find out its apparent visual magnitude by way of precise instruments, like solid state detectors, in a process called photometry. Once you find out the apparent visual magnitude of a star using photometry, all you need is the distance to the star. Measuring the distance to a star is covered in another lesson.

Thereafter, you can use a simple formula to find out the absolute visual magnitude of a star by plugging in the apparent visual magnitude and distance. This formula is called the magnitude-distance formula.


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