Subtle halos about Ring Nebula
September 16, 1999
Low Res. (87 KB)
High Res. (345 KB)
Object Name: Ring Nebula (M57, NGC 6720)
Telescope: Subaru Telescope / Cassegrain Focus
Filter: H alpha(0.65micron), B(0.45micron), V(0.55micron)
Color: Blue (B), Green (V), Red (H alpha)
Date: UT1999 May 14, 23; June 15
Exposure: 25 min (H alpha), 6 min (V), 40 min (B)
Field of View: 3 x 4 arcmins
Orientation: North up, east left
Position: RA(J2000.0)=18h53m36s, Dec(J2000.0)=+33d02m00s (Lyra)
The planetary nebula M57 (NGC 6720), or the Ring Nebula as it is most commonly referred to for obvious reasons, lies about 1600 light years away from the Earth. The name "planetary nebula" given to this class of objects is due to the fact that many of them appear disc-like, similar to how the planets in our Solar System appear when viewed through a telescope. In reality, planetary nebulae are stars at the point of death.
The bright ring of M57 is composed of a doughnut-shaped cloud of gas illuminated by a very hot central star. Past observations have revealed that the bright ring is surrounded by a faint outer halo. Because the halo is so faint, previous observations have in general been insufficient to allow us to develop a detailed understanding of its nature.
Recent observations obtained using Suprime-Cam attached to the Subaru Telescope have successfully imaged in great detail both the bright inner portion of the nebula and the faint outer halo of M57. It is expected that these observations will improve our understanding of how the Ring Nebula came to be, including insight into the gas flow from the aging star at the center of the nebula when it was in its "red giant" phase.
The behavior of a star at the end of its life depends on the mass of the star. Stars with a mass from 0.8 to 8 times that of the Sun become huge red giants after burning all the hydrogen at their cores. During this phase, most of the gas towards the star's surface expands outward. As the surface gases become rarefied, the central part of the star contracts, becoming a high density "white dwarf". The contraction raises the surface temperature of the white dwarf to several tens of thousands of degrees. At such high temperatures, the star emits high energy ultraviolet light. A planetary nebula appears when the expanding gas released during a star's red giant phase is illuminated by the ultraviolet light emitted by the central white dwarf. The ultraviolet light heats and ionizes the gas, causing it to glow. The shape of a planetary nebula depends on the distribution of the gases that were released, the strength of the ultraviolet radiation from the white dwarf, and the particular view we have of the nebula from our vantage-point here on the Earth. This is the reason why planetary nebulae come in a wide variety of shapes.
This false-color image shows an observation made in the light given off by hydrogen atoms (centered on the "H_alpha" line at a wavelength of 6563 Angstroms). We see that the bright inner ring is not uniform and in addition there is a complex extended outer structure or "halo". The major-axis of the bright ring measures about 0.7 light years. There are two bright stars seen within the ring: the central star is the white dwarf that illuminates the Ring Nebula; the other star is an unrelated object along the same line of sight.
This is the first time the outer halo associated with M57 has been observed so clearly. The figure shows that there are two components to the outer halo: a brighter inner part within which there are many loops; and a fainter detached outer part. While the ring and the inner halo appear oval, the outer halo is almost circular. The major-axis of the inner halo measures about 1.2 light years and the diameter of the outer halo is about 1.8 light years. Besides the loops and filaments within the inner halo, we also see many small clumps called "knots" within both the inner and outer halos.
Planetary nebulae like the Ring Nebula are often described as having a fairly simple structure, generalized as an elliptical shell. We clearly see from the new Subaru Telescope observations of the outer double halo that their true structure is considerably more complex.
This figure is composed of three separate images, each taken through a different color filter and combined to recreate the scene in color. The original image was then processed using a "Maximum Entropy" method to enhance the image sharpness. The process makes the outer halos become faint but reveals a great wealth of delicate structure within the bright ring.