NGC 7293


In Aquarius

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 7293 NGC 7293
NGC 7293
"A beautiful and remarkable object" (Curtis). The Helix Nebula, NGC 7293 in southern Aquarius, is the closest of the prominent planetary nebulae. Its accurately measured distance of 715 light years (from parallax, good to about 10 percent) allows the most detailed view of nebular structure that we have. At upper left, even the century-old Lick photograph shows a great deal of the intricate structure. But it is no match for the spectacular images at upper right and immediately above. These views, from a mosaic of ground- based (from the National Optical Astronomy Observatories) and Hubble images, show that the apparently smooth ring is actually made of myriad dense "comet-like" filaments that point away from the central star. These filaments, discovered in ground-based photos made in the 1950s at Palomar, consist of dense, dust-filled neutral knots ionized around the outside by the central star's intense ultraviolet radiation, the ionized gas then streaming away from the star. Similar structures are seen in the more-distant Dumbbell Nebula (NGC 6853). Though the Helix appears to be made of a pair of interlocking rings, it is really more of a barrel-shaped object, wherein we look down the barrel's mouth.

The nebula is huge. At its extreme, it is 19 minutes of arc across, 65 percent the angular size of the full Moon. At 715 light years, that translates into a physical diameter of four light years. The nebula would span the distance between us and Alpha Centauri. The resulting low surface brightness makes the Helix quite difficult to see in a small telescope. With a temperature around 110,000 Kelvin and a luminosity some 200 times that of the Sun (most of it radiated in the ultraviolet part of the spectrum), the hot blue thirteenth magnitude (13.4) central star is slowing dimming and cooling, and is in effect a hot white dwarf only a few times the size of Earth. Expanding at up to 25 kilometers per second, the nebula will eventually grow to invisibility, leaving its white dwarf behind.

Upper left: Image and quote by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Upper right and lower images: NASA, NOAO, ESA, the Hubble Heritage Team, M. Meixner (STScI), and T. A. Rector (NRAO).