M57 The search for the outer halo part 2…

This image shows the ring nebula together with its very faint halo as imaged by 11″, 12″ and 17″ reflectors.
Credit: André van der Hoeven, Terry Hancock, Fred Herrmann, Mike van den Berg and Mathijn Ippel

Like other old red giants M57, better known as the Ring nebula, has expelled most of its material in the form of hydrogen and oxygen.  At its core lies the white dwarf remnant which consists mainly of carbon.  The lighter hydrogen forms the outer reddish envelope while the heavier blue-green oxygen remains about the core.  The gases in the expanding shell are illuminated by the radiation of the central white dwarf who’s glow is still 200 times brighter than our Sun.

Normally only M57’s main core is displayed as shown in this older Hubble image: :  http://www.waid-observatory.com/images/m/m057-2012-08-12-HLA-839.jpg

When we look closely we see that M57 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…).

Observation

The huge difference in brightness makes it very difficult to observe the halos. Using data from a C11 and two 12″ RC telescopes about 40 hours of data was gathered using narrowband h-alpha filters (3 and 5 nm) to lift the halo out of the background noise during a period of over 2 years (2011-2013). The barred spiral galaxy IC 1296 is also visible in the upper part of the image.  IC 1296 distance is estimated to be 220 million light years.

This image was done as a collaboration by myself, Terry Hancock in Michigan, USA, Fred Herrmann from Alabama, USA, Mike van den Berg from the Netherlands and Mathijn Ippel from the Netherlands.

Setups:

André:  Celestron C11 with SXV-H9
Fred: Astro-tech 12″ RC with SBIG STT-8300
Terry: Astro-tech 12″ RC with QHY-9
Mathijn: Planewave 12,5″ with SBIG ST-11000
Mike: Planewave 17″ with SBIG ST-11000

Total exposures used:

H-alpha 1800s: 40x (André), 20x (Terry), 36x (Mike), 3600s: 4x (André), 5x (Fred)
RGB  Red 16x 10 min (Terry), 11x 30 min (Mathijn), Green 12 x 10 min (Terry), 11x 30 min (Mathijn), Blue 11x 10 min (Terry), 9x 30 min  (Mathijn)
Luminance 8x 15 min (Terry), 39x 30 min (Mike)

Total exposure time: 104 hours

 

The Ring nebula

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 from research 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. A follow up paper with a first image can be seen here.

The outer most halo was only discovered in 1987 using a telescope in Mexico. The discovery paper can be found here.

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.

A comparison of our image with the discovery images from 1937 and 1987 is shown here:

Here is a comparison with an image made by the Hubble and Subaru 8.2 m telescopes. It nicely shows the signals that we measured are real phenomenon in the shells:

 

 

 

 

 

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