This Article From Issue
January-February 2000
Volume 88, Number 1
DOI: 10.1511/2000.15.0
The Helix Nebula, in the constellation Aquarius, is the closest planetary nebula to us, at a distance of 450 light-years. Its central star, a very hot white dwarf, ionizes an inner shell of oxygen which appears green, and an outer, pinkish shell of hydrogen.
Majestic Universe: Views from Here to Infinity
Serge Brunier
Cambridge, $39.95
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A useful analogy is to imagine space-time to be represented by an elastic sheet. In the absence of matter, space-time is like a flat sheet; a lightweight ball set rolling across such a sheet would move in a straight line. If a weight were placed on the sheet, it would cause an indentation that would deflect the ball from its straight-line path; a heavier weight (representing a greater mass) would cause a larger and deep indentation, causing greater deflection of the ball. Given the right velocity, a ball could remain "in orbit" within the indentation; similarly, a planet remains in its orbit within the curved space-time surrounding the sun.
Unfolding Our Universe
Iain Nicolson
Cambridge, $39.95
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