. Once the nebula dissipates, this leaves behind a White Dwarf (WD). White dwarf stars are very hot, but with such a small radius that their luminosity is very low, placing them on the lower left of the H-R diagram. The excitation and ionization of the gas in the surrounding PN makes it shine with an emission line spectrum, with the wavelength-specific emission 20.5 Observations of stellar remnants 117 Figure 20.4 Left: Optical image of the Crab Nebula, showing the remnant from a core-collapse supernova whose explosion was observed by Chinese astronomers in 1054. Right: A composite zoomed-in image of the central region Crab Nebula, showing the optical (red) image superimposed with an X-ray (blue) image made by NASAs Chandra X-ray observatory. The bright star at the nebular center is the Crab pulsar, a rapidly rotating neutron star that was left over from the supernova explosion. Images courtesy of NASA/Hubble Space Telescope. Figure 20.5 Left: M57, known at the Ring Nebula, provides a

The central hot star is the remnant of the stellar core, and after the nebula dissipates, it will be left as a White Dwarf star. The annotations indicate the spectral lines responsible for the various colors, and the lines show the scale and compass orientation of the image. Right: A gallery of planetary nebulae, showing the remarkable variety of shapes that probably stem from interaction of the stellar ejecta with a binary companion, or perhaps even with the original stars planetary system. Images courtesy of NASA/Hubble Space Telescope. of various ion species giving it range of vivid colors or hues. Figure 20.5 shows that these PN can thus be visually quite striking, with spherical emission 118 Post-Main-Sequence Evolution: High-Mass Stars nebula from single stars (left), or very complex geometric forms (right) for stars in binary systems. 2. Neutron stars and Pulsars A star with initial masses in the range 8M

< M . 30M ends its life as a core collapse supernova that leaves behind a neutron star with mass 1.4M < Mns < 2.1M . The conservation of angular momentum during the collapse to such a small size ( 10 km) makes them rotate very rapidly, often many times a second! This also generates a strong magnetic field, and when the polar axis of this field points toward Earth, it emit a strong pulse of beamed radiation in the radio to optical to even X-rays. This is observed as a pulsar. One of the best known examples is the Crab pulsar, which lies at the center of the Crab Nebula, the remnant from a core-collapse supernova that was observed by Chinese astronomers in 1054 AD. Figure 20.4 shows images of this Crab nebula in the optical region (left) and in a composite of images (right) in the optical (red) and X-ray (blue) wavebands. 3. Black holes and X-ray binary systems Finally, stars with initial masses M & 30M

1M