History

Messier's number 57 is arguably the most famous planetary nebula. It was discovered in 1779 by the French astronomer Antoine Darquier of Toulouse and a short time later on 31 January 1779 by Charles Messier. He wrote: «Light clusters between γ and β Lyrae, discovered during the observation of the comet of 1779, which passed very close: it seems that this round light cluster consists of very small stars: with the best telescope it is not possible to find them to see, there remains only the suspicion that they exist. M. Messier reported about this cluster of light on the map of the comet from 1779. M. Darquier from Toulouse discovered this nebula while observing the same comet and reports: «Nebula between γ and β Lyrae. It is very uniform but sharply defined and as big as Jupiter and looks like a planet that has expanded» ^[281]

Physical Properties

The light-colored ring of M 57, which some call a donut or smoke ring, has the dimensions 80x60 arc seconds. Long-exposure photographs show that this ring is still surrounded by a weak outer shell, 170 arc seconds in diameter, which resembles a wreath of flowers (Fig. 2). Faint, parallel stripes cross the ring.

The ring shape of M 57 is actually neither a ring, nor a torus, nor a semi-transparent hollow bubble, but rather a projection of a more complicated bipolar structure due to the direction in which we are looking. Using a spectrometer of the 2.5 m Isaac Newton telescope on the Canary Islands, two clearly separated expanding gas flaps of M 57 could be distinguished. One points exactly in our direction, the other away from us.

This structure is believed to have arisen because a sun-like star had used up all of its hydrogen and expanded into a red giant. The outer layers gradually drifted off as a result of the stellar wind. The stellar wind is more concentrated in the equatorial region and thus caused the formation of a gas disk, which we see as a weak halo around M 57. Later the star experienced gusts of stellar winds, which blew away the rest of the outer atmosphere. However, since material was already present in the equatorial plane of the star, this gas escaped mainly along the axis of rotation and thus formed the two polar gas lobes (Fig. 4). However, we do not see anything of this bipolar form, since we are looking exactly perpendicular to the equatorial plane of the central star. ^[108]

It is assumed that the central star of M 57 is variable, because different measurements and observations at different times resulted in contradicting values ​​from 14.2 to over 16 magnitudes - but this may be due to the fact that the star is covered by a thin layer of gas and thus his Visibility similar to that depending on the atmospheric conditions on earth. The central star is a bluish dwarf or sub-dwarf star with a seemingly continuous spectrum. The protected surface temperature is 120'000 K and the calculated density should be a thousand times higher than that of the sun. Stars of this type produce strong ultraviolet radiation, which causes the bright fluorescence of the thin gases in the nebula. The typical bluish-greenish color of this nebula is mainly caused by the two strong emission lines (500.7 nm and 495.9 nm) of doubly ionized oxygen. The color of the ring nebula runs through all rainbow colors from blue to red from the inside to the outside. The central region only appears so dark because of the contrast of the light ring. The colors in Fig. 3 largely correspond to the natural ones. The emissions of very hot helium near the central star are coded in blue, the light from ionized oxygen a little further away appears green and the coolest gas shows ionized nitrogen glowing red. The color changes show how the gas glows, which is excited by strong UV radiation from the central star. ^{[4, 176]}

The distance from M 57 is estimated to be about 2000 light years. ^[176] According to this information, the brightest part of the nebula should have a diameter of about 50'000 AU. Spectroscopic studies have shown that the nebula expands at a speed of around 19 km/s. ^[4]