THE PLANETARY NEBULAE

From Jim Kaler's STARS

In which Hubble images are compared with Curtis's historic century-old set of Lick Observatory observations.

See the spectra of NGC 2440, the Ring Nebula in Lyra, NGC 7009, IC 418, and the original nebular spectrum, that of NGC 6543.

Planetary nebulae, some of the loveliest objects in the sky, are complex shells of gas that have been ejected by dying advanced giant stars like Mira. They were discovered as a set and given their collective name by William Herschel in the late 1700s. The story is given in the text for the "first planetary," NGC 7009. See the table that follows this introduction.

The nebulae, which can be over a light year across, are ionized and made to glow by ultraviolet radiation from their central stars, which are the stripped-down old nuclear-burning cores of stars that were once much like the Sun. In effect, the central stars are the precursors of dense white dwarfs that are basically balls of carbon and oxygen, the result of aeons of nuclear fusion. When the outer husks of the giants are finally gone via strong winds, the old cores first heat at constant luminosity (which can be thousands of times that of the Sun) to temperatures that can hit 200,000 Kelvin. See the comments that follow the table below. After nuclear fusion shuts down and most of the old hydrogen envelope is stripped away, they then cool and dim as they head for the white dwarf realm on the HR diagram. Indeed, the cooling stars are in effect already white dwarfs. All the nebulae are expanding with typical speeds of 20 -30 kilometers per second. Eventually the nebulae, some loaded with the by-products of nuclear fusion, dissipate into space (to be come fodder for new stars), leaving the lonely white dwarfs behind. Beautiful but ephemeral, the whole show is over in under 10,000 years.

The nebulae are shaped by hot winds from the stars ramming into mass lost while the stars were advanced giants. The intricate structures can be exceedingly complex, the result of double star action, rotation, the effects of stellar magnetic fields, or other causes: no one really knows.

The pages here present a contrasting picture between two great sets of planetary images made a century apart: those brought to us by Heber Doust Curtis in the Publications of the Lick Observatory, Volume 13, Part III, 1918, and those made using the Hubble Space Telescope. The first set, the first extensive compendium, was observed with Lick Observatory's Crossly Reflector. It includes nebulae north of 34 degrees south declination and us gives a sense of the visual view through the telescope. Some of the old pictures are photographic, while others are composite drawings made from photos of different exposures so as to reproduce a great dynamic range, which was impossible with the photographic emulsions of the time. The Curtis images have north always to the top; the Hubble images are then brought into the best alignment.

While the Hubble set reveals both the true natures and the beauty of these intricate and extraordinary structures, it also demonstrates the high quality and accuracy of the work of the distant past. These pages are meant to honor both. Many thanks to Lick Observatory for permission to reproduce the images and to the observers and technicians at STScI.

THE NEBULAE

• NGC (New General Catalogue) number followed by IC (Index Catalogue);
• Popular Name (if any);
• Constellation of residence
• Significant features including Messier (M) number (M 27 and M 57).

Detailed Comments

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