DSO of the Month :
Caldwell 55
AKA: Saturn Nebula; NGC 7009
Position: 21 hrs 04 min 10.8 sec -11 degrees 21 min 48 sec
Due south at 22.25 (BST) on 15 September 2021
Caldwell 55 (Saturn Nebula)
Image: Les Brand, HAS member. Used with permission.
The Saturn Nebula in Aquarius has an important place in the history of astronomy. It was first planetary nebula observed by William Herschel and only the third planetary nebula to be discovered (it was preceded by the Dumbbell Nebula and the Ring Nebula), Herschel observed it from Datchet on 7 September 1782 and noted its planetary appearance (while being fully aware that it was not a planet). He described it as “a curious nebula” and remained puzzled by planetary nebulae for the rest of his life. He thought they contained a lot of solid mass like a planet, but he also believed that they were stars (which could sometimes be observed at their centres) surrounded by nebulous gas (which is of course correct). He eventually took the view that the gas was coalescing on the star and hence they were protostars, the complete opposite of our current view. He was hampered in two ways. His telescopes, even his larger telescopes, did not show the fine details of planetary nebulae which have been revealed to us by the Hubble Space Telescope. Furthermore he was not able to record the spectra which played an important role in the development of our understanding of these ghostly bodies. The examination of the Saturn nebula by large telescopes show a dazzling array of fine structures such as a halo, multiple shells, jets, filaments and knots, and ansae (handles) which explain the passing resemblance to the planet Saturn. Lying at a distance of roughly 5,000 light years, the nebula is perhaps a third to a half a light year in diameter. Its greenish-yellow colour (another resemblance to the planet) arises from the ionisation of oxygen by ultraviolet rays from the very hot white dwarf (with a temperature of 55000K) at its centre. The name Saturn Nebula was coined by Lord Rosse in 1850 when the ansae became visible in his large reflector at Birr Castle.
To observe the Saturn Nebula, first find Nu Aquarii, a 4.5 mag. star which lies below 2,9 mag. Beta Aquarii (Sadalsuud) in the east and 3.8 mag. Epsilon Aquarii (Alibali) in the west in the rather barren region between Aquarius and Capricornus. The nebula lies one degree to the west of Nu Aquarii, Although it is fairly large (the core is somewhat smaller than the full moon), at magnitude 8 it is also rather faint. In a small telescope, it will just be a small greenish-yellow disc and the colour may not be very marked. As with the Blinking Nebula (July 2021), you may see it better with an OIII filter.
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