20120711

Presentation: photosphere

Consider this ancient Egyptian representation of the sun--with wings, and if that weren't awesome enough--fire-spitting cobras wearing hats! (To top it off, the hats the cobras are wearing are decorated with cobras.) What's up with that?

But now look at an actual photo of the sun during a total solar eclipse--do you see the wings? And the fire-spitting cobras? (Their pink tongues are darting in and out from behind the moon.) The sun does actually look like it has wings and fire-spitting cobras, but these features correspond to the corona and the chromosphere, which are very faint and can only be seen with the naked eye during a total solar eclipse. Today we know that the sun doesn't literally have wings and fire-spitting cobras, but the ancient Egyptians were merely trying to explain what they saw with what they knew: wings and fire-spitting cobras.

This is the approach we are going to take with the brightest portion of the sun--the photosphere, or what we perceive as its "surface." While this didn't actually happen in Sunshine (Twentieth Century Fox, 2007), imagine what would happen if you had to land on the sun. Yes, you would probably die, because, science. But would there be a spot on the sun that you would be able to survive just a moment longer, because again, science?

So like the ancient Egyptians, who explained what they saw with what they knew--wings and fire-spitting cobras, let's explain what we see on the photosphere of the sun--granules and sunspots--with what we know.

Here is an hour-long time-lapse of sunspots--the dark spots--and granules on the photosphere of the sun. The sunspots are temporary dark features on the sun, and usually in pairs, while the granules are always present and in motion. (Video link: "Orange Sun Oozing.")

First, explaining what we see--granules--with what we know--Lava LampsTM, and miso soup.

A Lava LampTM is an excellent model of a cross-section of the sun. The lamp below represents the hot core, where energy is produced from fusion. This energy heats up wax blobs, which expand and decrease in density relative to the clear oil. The wax blobs become more buoyant, and rise. At the top, the wax blobs cool off, contracting and increasing in density, such that they begin to sink. As long as heat is supplied from the bottom, the wax blobs continuously rise and sink, creating convection currents. (Video link: "080913-1050512.")

Imagine what these convection currents would look like as seen from above, looking down. This is what miso soup does, if it is served piping hot. The soup deep down in the bowl is hot, causing the miso particles to rise up to the surface, where it cools off, causing the particles to sink, where they warm up again. Heat from below, cooling off at the top. (What does miso soup look like after it has been cooled off for a while? And if you have never had miso soup before, make sure to order it when you are at a Japanese restaurant, and enjoy your sun-surface soup.) Keep in mind that the sun is not literally a gigantic Lava LampTM and bowl of miso soup, but it is useful to understand how it behaves like a gigantic Lava LampTM and bowl of miso soup. (Video link: "080729-1040612.")

Second, explaining what we see--sunspots--with what we know--magnets and TVs, and light bulb filaments. (You may be last generation of students that will actually understand these analogies!)

The reason why sunspots come in pairs is that they are temporary magnetic regions, with both north and south poles. Here a strong magnet below exerts forces on iron filings on a sheet of paper. However, the sun is not made of iron filings.

But the sun does have lots of circulating gas, which is electrically charged. Have any of you held a magnet up to the screen of a old-school cathode-ray television? If you have never done this, or have suppressed that traumatic childhood memory, we'll sweep a strong magnet across the screen of a TV. (Note that unlike the bare black and white TV glass screen, the metal mask just behind a color TV glass screen will be permanently magnetized.) Normally electrons stream down the TV to hit the screen at certain places to build up an image, but the magnet distorts the electron stream, especially near the poles. This is what is happening in sunspot regions--the strong magnetic fields there disrupt and stagnate convection currents there, making a sunspot cooler than the hotter circulating granule areas. (Video link: "080913-1050522.")

Why do sunspots--these regions of cooler, trapped, non-circulating gas--appear black? Let's look at a very dim light bulb against a very hot, brighter background. This is our model of the photosphere, which is normally very hot and bright, and the filament represents a sunspot, which is cooler and dimmer. The light bulb filament appears black, but if we turn off the hot, bright background, we see that the light bulb filament is actually a dim orange color. If you could scoop up a sunspot with super-sized oven mitts and look at it away from the sun, the sunspot would have this dim orange color. Keep in mind that the sun is not literally a gigantic TV with a magnet, but it is useful to understand how it behaves like a a gigantic TV with a magnet. (Video link: "080913-1050513.")

So if you ever had to land on the sun, well, yes, you would die. But aim for a sunspot, and though you will still die, you may survive just a bit longer.

Remember that the photosphere is just the surface of the sun, and a lot is going on there.

But looking at the photosphere is literally just scratching the surface of the sun. In a later presentation, we will look at the juicy insides of the sun, specifically the core, where fusion takes place.

1 comment:

Sal Arredondo said...

I love the analogies in this one! They truly help.