At Heart a White Star

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Hubble's Wide Field Planetary Camera 2 captured this image of planetary nebula NGC 2440 on 6th Feb 2007. The image shows a star like our Sun ending its life by casting off its outer layers of gas, which formed a cocoon around the star's remaining core. Ultraviolet light from the dying star then makes the material glow.

The burned-out star, called a white dwarf, appears as a white dot in the center. Our Milky Way Galaxy is littered with these stellar relics, called planetary nebulae.

The colours correspond to material expelled by the star. Blue corresponds to helium; blue-green to oxygen; and red to nitrogen and hydrogen.

The Colorful Demise of a Sun-like Star from hubblesite 13/02/07



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Spitzer’s infrared view of the Helix nebula. Credit: NASAs Spitzer Space Telescope

Spitzer's spectacular new view of the Helix nebula shows colours as seen in infrared. The dusty white star appears as a dot in the middle of the nebula, like a red pupil in a shimmering cloud of gas with an eerie resemblance to a giant eye.

Comets Clash at Heart of Helix Nebula Spitzer Media release 12/02/07
More on Debris Disk Around a Dead Star from Centauri Dreams.
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The bright White Light from our closest star - The Sun
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The Sun as it appears through a camera lens from the surface of Earth

The Sun has a surface temperature of approximately 5,500 K, giving it a white colour, which, because of atmospheric scattering, appears yellow. This is a subtractive effect, as the preferential scattering of blue photons (that 'colour' the sky) removes enough blue light to leave a residual reddishness that is perceived as yellow.

The high-energy photons (gamma and X-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million years to as little as 17,000 years.

After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light.

Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they very rarely interact with matter, so almost all are able to escape the Sun immediately.

For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation.
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LHC - The worlds largest microscope
by Sabine Hossenfelder @ Backreaction
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