Physical Properties

When stars with about 0.8 to 8 times the solar mass of the hydrogen driving nuclear fusion are used up, they collapse to form dense, hot, white dwarf stars. During this process, the dying star sheds its outer layers of material and forms a cloud of gas and dust known as the planetary nebula (PN). This name is historical and misleading. It stems from the fact that the first planetary nebulae discovered in telescopes were mostly spherical and looked like distant gas planets.

However, some PNs like Abell 78 are the result of what is known as a «reborn» star. Although the star's core has stopped burning hydrogen and helium, a thermonuclear runaway is ejecting material at its surface at high speed. This ejection shocks the material of the old nebula and sweeps it away, creating filaments and the irregular shell around the central star. The faint outer halo is predominantly hydrogen, while the inner elliptical ring is predominantly helium. This confirms that inside stars hydrogen is converted to helium, which can be ejected back into the galactic gas pool to create future enriched stars. ^{[334, 335]}

Finder Chart

The planetary nebula Abell 78 is located in the constellation Cygnus (Swan). The best observation time is March to December. On 13 August the PN is in opposition with the Sun.