Hi All

Johannes wrote...

J— I’m trying to understand this statement. High energy radiation contains more energy than low energy radiation? Black bodies do not emit as much high energy radiation as other frequencies? Is Nick trying to tell me that the Sun is a black body?

Answer:

Energy of any individual photon is related by E = hv - E, being energy; h being Planck's constant (6.636 x 10^-36 Js), and "v" (really a Greek nu) is the frequency. As you can see the higher the frequency the higher the energy per indivdual photon. Hence Nick's statement.

Because h is so small physicists generally use electron volts (eV - the energy of an electron accelerated one metre in a one volt field) to measure photon energy. Generally visible light has a few fractions of an eV, UV has a few eV, and x-rays and gamma-rays have "lots" of eV.

Any 'hot' object loses heat as photons over a spread of frequencies - the spectrum. The peak of photon production at a certain temperature is determined by the following equation:

peak wavelength = 0.000293 m/Temperature (in Kelvin)

and frequency is just the speed of light, c, divided by the wavelength - c being defined as 299,792,458 metres/second. Because of the way the physics works higher frequencies - shorter wavelengths - produce far fewer numbers of photons than lower frequencies. In the case of the Sun - which peaks at ~ 508 nanometres/ 5,900 terahertz/0.245 eV - about 7% of its light is UV or higher than visible frequencies, 45.5% is visible light, and 47.5% is infra-red and lower

The Sun, like other stars, glows from the photons/energy lost by the hot ionised gases that make it up. But hot gas is not the only way to make photons. Magnetic fields can accelerate ions and cause them to emit much higher energy photons than the Sun's surface temperature would lead you to believe. These magnetic explosions are called solar flares and are what make much of the x-rays and similar high-energy radiation that the Sun puts out. Such outbursts are only ever a tiny fraction of the Sun's total energy output.

Smaller stars can have flares similar in size to the Sun's but because the star's overall output is much smaller the flare is much bigger in a relative sense, causing the star to visibly brighten - hence why many red dwarf stars are called "flare stars".

Hope that helps

Adam