What Are Sunspots?

What Are Sunspots?
Coming up next: Why Do Sunspots Occur?

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  • 0:01 Sunspots & Active Regions
  • 1:21 Penumbra & Umbra
  • 2:21 Differential Rotation…
  • 4:24 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will describe for you what sunspots are, where they appear, what parts they have, and why they're dark. It also discusses what differential rotation and a sunspot cycle are.

Sunspots and Active Regions

On a daily basis I see some commercial claiming their new product is the one to get rid of dark spots and blemishes on your skin, those that come with hormones or aging. The sun, surprisingly enough, has some cosmetic blemishes of its own to deal with. They're called sunspots. A sunspot is a relatively dark, irregularly shaped region of the photosphere with a lower temperature than the other parts of the photosphere. The photosphere is the bright visible surface of the sun, lying underneath the chromosphere and corona.

The region of the sun's surface, where large magnetic fields are found and sunspots, prominences, and flares occur preferentially, is called an active region. I certainly had lots of active regions where things flared up on my face when I was a teen.

Like blemishes on our skin, some sunspots are found alone on the sun, but more often than not, they're found in sunspot groups. Their sizes vary from less than 2,500 km across to as large as 50,000 km across, but a typical sunspot has a diameter the size of Earth and lasts anywhere from less than an hour to a few months. Oh, the horror our sun must face when waking up in the morning and looking in the mirror!

Penumbra and Umbra

Each sunspot has an umbra, the dark central core of a sunspot, sort of like a blackhead, and a penumbra, the brighter border of a sunspot. The penumbra has radial features that give the sunspot an appearance of a flower of sorts.

The reason the sunspot is, overall, darker than the surrounding photosphere is because the temperature in the sunspot is lower than the surrounding photosphere. The average photospheric temperature is about 5,800 K. The penumbra is 5,000 K, while the umbra is around 4,300 K.

But don't be fooled! The sunspots are dark only when seen against the much brighter parts of the photosphere. Had you put a sunspot against the dark background of space, the sunspot would be 10 times brighter than the full moon! It's like placing a blindingly bright flashlight next to a much punier one. The puny one will only appear bright if it's alone in a really dark room.

Differential Rotation and the Sunspot Cycle

Blinding brightness reminds me to tell you that you should never, ever, look at the sun to try and see sunspots, even if they can be seen with the naked eye. Without a proper solar filter for our eyes or the telescope you're using, the sun can permanently blind you if you look directly at it.

Which makes it all the more surprising that the first sunspots to be observed in detail were those seen long ago by Galileo. Galileo didn't go blind because of this due to… Well, let's face it, he was a genius and knew better. He just used his telescope to project the image of the sun onto a screen to safely study it instead of looking at it directly.

Galileo discovered that the sunspots near the equator take 25 days to circle the sun, while those farther north or south take longer, up to 36 days all the way near the poles. This is known as differential rotation, a kind of rotation where the rotation period of a body differs with latitude.

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