Its position in the sky can be easily identified by using alignments between the stars of the Lira, but, since its brightness is close to the ninth apparent magnitude, the nebula can only be observed through telescopes. In this image M57 was immortalized by the Hubble Space Telescope. The colors indicate the different temperatures at which the gases are found: in blue they are the ones closest to the central white dwarf and therefore the warmest, while in red they are the outermost and the coldest.
M57 is a perfect example of an axially symmetrical bipolar planetary nebula. These nebulae usually appear as the Handlebar Nebula, but in case they are observed by pure coincidence from Earth along the major axis of symmetry, as in this case or the Helix Nebula, then have a ring shape with concentric material distribution.
Planetary nebulae represent the last phase of the evolution of stars of mass similar to that of the Sun. When a Sun-like star terminates the hydrogen at its disposal, the nuclear reactions at its center stop and the star becomes a red giant. This change causes the combustion of helium in the nucleus and, depending on the mass of the star, even heavier elements. When this combustion also comes to an end, the star is no longer able to produce energy to counteract the gravitational collapse: the surface layers are expelled into space forming the nebula, while the nucleus collapses forming a white dwarf.
In the case of the Ring Nebula it is estimated that the central white dwarf has a mass of 0.6 solar masses (its original mass was around 1.2 solar masses), a surface temperature of 125 thousand K and a brightness 200 times that of the Sun.
Credit: The Hubble Heritage Team (AURA/STScI/NASA).
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