question about ultraviolet light?

Question

When photographs are taken of the Sun in ultraciolet light, the Sun appears to be slightly larger in diameter than it appears in visible light photographs.Why is this

Answer 1

Human vision is limited to a narrow band of the radiation spectrum. We don't see even 1/4 the energy emitted by the sun; especially the most energetic emissions.

The ultraviolet spectrum is just beyond violet, which is the most energetic color that we can see. Ultraviolet rays are still photons; they just have more energy in them and vibrate at a higher frequency than we can see. All light rays move at the same speed, but the particles vibrate at different rates. Photons with a higher vibration rate vibrate at a higher frequency; they literally shake more. So it isn't surprising that the sun appears larger when you look for more energetic particles.

The sun's gravity is very powerful and a star is capable of bending light rays, because of gravity. It takes more energy to bend the path of a more energetic particle (one with a higher vibration i.e. higher frequency). So photons with a higher frequency are able to escape the sun quicker.

Not all of the light escapes the sun; some is bent back or even pulled back. The sun's surface is not a quiet place. It is more like a hurricane tossed sea; with swells that could easily swallow the moon. So some radiation is expelled, as well as very small particles; the solar wind. The sun has convection currents so a particle on the surface is pulled into the core, where it heats up and gains more energy so it can move further away from the core. When these particles are moving fast enough they can escape the sun; photons are one of these particles.

According to Wikipedia: http://en.wikipedia.org/wiki/Sun
“The Sun's radius is measured from its center to the edge of the photosphere. This is simply the layer below which the gases are thick enough to be opaque but above which they are transparent; the photosphere is the surface most readily visible to the naked eye. Most of the Sun's mass lies within about 0.7 radii of the center.”

Therefore the photosphere is the edge of the solar atmosphere where the solar mass is thin enough that we can see the surface (in our visible spectrum). With a higher frequency radiation then this limit would increase. So the surface of the sun in ultraviolet would be further from the core. Since we are looking in the ultraviolet spectrum we see the photons are a higher rate so the sun’s surface, it’s photosphere, is extended. We are seeing faster particles that can get farther from the sun before they are pulled back into it. This is explained in the next quote

“The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is because of the decreasing overall particle density: the photosphere is actually tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K (10,340°F / 5,727 °C), interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023 m−3 (this is about 1% of the particle density of Earth's atmosphere at sea level).”

The surface of a planet doesn’t change, because most of the particles are pulled in by gravity and the gravity is stong enough to hold them on its surface. On Earth the only particles that are moving fast enough to escape the surface are gases. The atmosphere has an edge because most of the gases in it can’t get past that point. Remember that gas is the third state of matter, and it is reached by heating an object past it’s vaporization point. Another words you are putting more energy into it. That is why when you heat a substance into a gas it takes up more space. The gas pressure (caused by the particle vibration) is higher. If you were able to cool an object so much that the particles stop vibrating then you would reach absolute zero. The fourth state of matter is even more energetic than gas, the particles in the gas cloud are vibrating at a higher rate; a higher frequency. Most of the sun is plasma, because it is so hot.

If you look at this picture: http://en.wikipedia.org/wiki/Image:I_screenimage_30579.jpg
Then you can get an idea of what I am talking about. The 4 pictures are taken at a higher frequency (in clockwise order). The last two pictures are at such a high frequency that the picture scale had to be changed. The white circle in the middle of these two images is the photosphere. The idea is to show how a sunspot can turn into a solar flare, and it is taken at higher frequencies (that are made visible to us by using false colors) with each shot as the time elapses. They have to keep increasing the spectrum so you can see the more energetic particles. If you saw the solar flare at the same spectrum then it would thin out to the point of invisiblilty.

Answer 2

I think it is because the ultraviolet light is refracted by the sun's gravity more than visible light since UV quanta is greater than Visible quanta. Just a guess. It could also be because the corona emits UV.

Answer 3

http://en.wikipedia.org/wiki/Sun#Atmosphere
The reason is that the visible portion, the photosphere of the sun is closer to its surface. The regions of the atmosphere above the photosphere are 20 to 1000 times hotter. The hotter regions peak wavelengths are in the UV and below.

Answer 4

Uv is invisible to the eye. It can be captured on special film, or with a UV sensitive video or digital camera. Most security cameras are somewhat uv sensitive , as are some web cams. Exploding stars emit all frequencies of light ,including UV and X rays.