|Designation|| Messier 57|
|Right ascension||18h 53m 35.079s|
|Declination||-33° 01′ 45.03″|
|Type of object||Planetary nebula|
|Magnitude|| Apparent Mag: +8.8|
Absolute Mag: -0.2
|Distance from Earth||2,300 ly|
|Radial velocity||-21 km/s|
The Ring nebula (M57, NGC 6720) is a planetary nebula in the constellation of Lyra. It is one of the four planetary nebulae in the Messier catalogue. The nebula is well known for its spectacular colours. The nebula is approaching Earth at 21 km/s
Messier 57 was first observed by Charles Messier in 1779 and was the second such object discovered. Messier was also the first to observe a planetary nebula, the first being the Dumbbell nebula. The astronomer Antoine Darquier de Pellepoix found the object a few days later, leading to long-standing error that Darquier discovered M57 as Messier described Darquier's observation as a discovery. Darquier described the nebula as having, "the apparent dimension of Jupiter," and that it, "is perfectly round and sharply limited." This comparison to a planet may be what caused William Herschel to describe the object and others like it as "planetary nebulae." William Herschel was the first to observe the nebula's ring like structure in the 1780s and believed its appearance may be due to unresolved stars. This was disproven in 1864 when spectroscopic analysis of the nebula by William Huggins produced spectra like that of fluorescent gases. The first photographs of the nebula were taken in 1896 by Jenő Gothard.
Properties and Structure
The nebula consists of gases expanding in an oval shape, with a faint star at its centre. The star is likely to be a white dwarf and has a magnitude of 14.8, making difficult to observe. The ring has a diameter of 2.4 light years and is expanding at approximately 20-30 km/year. The ultraviolet radiation emitted by the star illuminates the nebula. The nebula is thought to be shaped like a torus, similar to the Dumbbell nebula. However, from Earth the nebula is viewed from one of its poles, leading to a ring like appearance.
The ring of the nebula is composed of a variety of different elements. The mass of the gas is estimated to be 0.2 solar masses and have an average density of 10,000 ions per cubic centimeter. The composition by each element relative to the number of fluorine ions is as follows:
|Element||Number of Ions|
Each colour corresponds to the decay of an excited state to a lower energy state, with the colour depending on the difference of energy between the states. For example, the blue comes from helium, the yellow/green from oxygen and the red by nitrogen.
The central star is believed to be a white dwarf, with a mass of 0.6 solar masses and a luminosity 200 times that of the Sun. It's temperature is thought to be 125,000 kelvin, though some estimates place it higher, at 150,000 kelvin. The star appears faint (14.8 mag) as its great temperature means much of its emissions are in the ultraviolet portion of the electromagnetic spectrum.