In 2011 I started with imaging M57 with my C11 and a 12nm H-alpha filter. I then caught the first glimpse of the outer halo of M57. The ring nebula, as Messier 57 is better known, consists of three structures, the inner, bright and most familiar, nebula, which is about 86″ by 62″. This is surrounded by a second fainter halo ranging until about 156″ x 136″ from the central star. Surrounding these two there is a very faint third halo of about 3,8′ diameter. The brightnesses of these three halos differ a lot, the second ring being about 5x (2 magnitudes) fainter than the central ring, and the outermost being almost 5000x fainter than the central ring (>9 magnitudes fainter…).
Formed by a star throwing off its outer layers as it runs out of fuel, the Ring Nebula is an archetypal planetary nebula. It is both relatively close to Earth and fairly bright, and so was first recorded in the late 18th century. As is common with astronomical objects, its precise distance is not known, but it is thought to lie just over 2000 light-years from Earth.
From Earth’s perspective, the nebula looks roughly elliptical. However, astronomers have combined ground-based data with new observations using the NASA/ESA Hubble Space Telescope to observe the nebula again, hunting for clues about its structure, evolution, physical conditions and motion.
It turns out that the nebula is shaped like a distorted doughnut. We are gazing almost directly down one of the poles of this structure, with a brightly coloured barrel of material stretching away from us. Although the centre of this doughnut may look empty, it is actually full of lower density material that stretches both towards and away from us, creating a shape similar to a rugby ball slotted into the doughnut’s central gap.
The brightest part of this nebula is what we see as the colourful main ring. This is composed of gas thrown off by a dying star at the centre of the nebula. The diameter of the central ring is about 1 lightyear while the outer halo has a diameter of about 2.5 lightyears. This star is on its way to becoming a white dwarf — a very small, dense, and hot body that is the final evolutionary stage for a star like the Sun.
The central star has a temperature of about 100.000-120.000 K and sends out most of its radiation in UV. In the central ring nicely the degrading ionization of the surrounding gas can be seen. In the centre there is mostly blue-violet light, while surrounding it there is a green ring of OIII gas which needs a lower energy to transmit its light and at the outer edge of the central ring there is the low energetic red light of H-alpha.
The inner halo around M57 was only discovered in 1935 by J.C. Duncan using a 30 min image with the 2,5 m Hooker telescope. The discovery paper can be found here.
The outer most halo was only discovered when first space telescopes, like the Hubble, pointed at the Ring nebula.
The central ring has an estimated age of 5000-6000 years, while the outer most halo was probably released by the central star about 100.000 years ago, when it was still in its red giant phase.
The huge difference in brightness makes it very difficult to observe the halos. Using a C11 about 24 hours of data was used using narrowband h-alpha filters (3 and 5 nm) to lift the halo out of the background noise. Further data will be gathered in the future.
This image shows the halo as observed by the 8.2 m subaru telescope and the >8 m large binocular telescope:
This image shows the image I have taken using my Celestron C11 (27 cm) reflector from a very lightpolluted location near the city of Rotterdam in The Netherlands:
This is a clear proof of the improvements that have taken place in the technical aspects of astrophotography when you think of the fact that only 20 years ago this could only be observed from space.
H-alpha 47 x 30 min (5nm/3nm astrodon)
RGB 19 x 300 s