What happens if a star is even hotter than blue Sirius? In such cases Planck's law still applies, but the resulting glow will be of a color beyond the range to which our eyes or ordinary telescopes are sensitive. In particular, objects much hotter than Sirius will glow in ultraviolet or X-ray light. Different temperatures, and their connection to color through the law of black body radiation, reveal that seemingly distinct phenomena such as ultraviolet light and X-rays are really just parts of the broad electromagnetic spectrum. The electromagnetic spectrum describes a whole range of different colors, well beyond the sliver of light that we can see with our eyes.
So white dwarfs are buried deep within their planetary nebulas, and are so hot that they don't emit much visible light, but instead radiate mainly in the ultraviolet and X-ray parts of the spectrum. It's thus not too surprising that the superheated star at the center of the Red Spider Nebula remained unseen for many decades. That situation finally ended in 2005, when Mikako Matsuura and colleagues used the powerful Hubble Space Telescope, located in orbit above the Earth's atmosphere, to identify a tiny speck of light corresponding to the white dwarf at the heart of the Red Spider. In this and subsequent studies, astronomers have been able to make a precision measurement of the star’s color, and then have used Planck's law of black body radiation to calculate its temperature.
The results are astonishing—the surface temperature of the star at the center of the Red Spider Nebula is an incredible 540,000 degrees F, more than 50 times hotter than the Sun, and 30 times hotter than mighty Sirius.
This amazing star, with its extreme temperature and the spectacular glowing nebula that surrounds it, is of more than mere academic interest. For in gazing at the Red Spider, we are seeing our future fate. Around 5 billion years from now, the Sun too will run out of fuel, and will similarly shed its outer layers. All that will remain of our star and its solar system will be a beautiful planetary nebula, illuminated by an intensely hot white dwarf at its center.
Reprinted from Extreme Cosmos by Bryan Gaensler by arrangement with Perigee, a member of Penguin Group (USA) Inc., Copyright (c) 2011 by Bryan Gaensler.