Re: At what frequency do large stars emit their light?

The light from a star obeys many of the same laws as the light from a glowing coal. One of these laws is a fairly simple mathematical relation that describes the wavelength near which most of the star's light is emitted, known as Wien's Law. In words, it states that, if you pass starlight (or light from a glowing coal or light bulb for that matter) through a prism, the wavelength near which most of the energy is emitted varies in inverse proportion to the temperature of the star (or coal or light bulb). This is really just a mathematical way of saying that hotter objects appear bluer.

Our Sun has a temperature of about 6000 K. That's 6000 degrees on the Kelvin scale, which measures temperatures above absolute zero. Absolute zero is -273 C. The Sun emits most of its light near a wavelength of about 5000 Angstroms (500 nanometers), in the yellow-green region of the spectrum. Using Wien's Law, then, we see that a star's spectrum will peak in the ultraviolet region (which starts below about 3500 Angstroms), or at even shorter wavelengths, if its temperature is greater than about 9000 K.

There are actually many stars hotter than this. The star Vega (the brightest star in the constellation Lyra) for example, has a temperature of about 9500 K. The brightness and temperature of a star increase very quickly with the star's mass. Because of this, Vega isn't even all that massive, being only a bit over two times more massive than the Sun. Stars more than 10 times more massive than the Sun have temperatures in excess of 30,000 K. These stars are responsible for the emission nebulae which we see as glowing gas clouds in interstellar space.

What Stephen says is entirely correct if one takes "large stars" to mean very massive stars. If one is only concerned about physical dimension (i.e. the diameter), there is an additional category of stars to consider: the red giants.

At the end of its life (after having burned all its hydrogen in the core), pretty much every star goes through a stage during which it expands by a factor of a 100 or more. At this time, the major part of energy production of the star takes place in a burning shell rather than in the central core. Due to some funky behaviour of the "transparency" of the envelope of the star, the absorbs energy and expands tremendously. The expansion decreases the envelop temperature, and the surface of the star turns red. Hence the name "red giant", to be contrasted with the "blue giants" described by Stephen which are much smaller in size, but much more massive and which have a much higher surface temperature.

This red giant phase will occur in about 5 billion years in the case of our Sun. At that time, the Sun will swell up to a size beyond the current radius of the orbit of the Earth.

marc herant, madsci astro moderator.