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.
(Planetary Nebula)
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.
Left Figure:
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.
Right Figure:
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.
Object Name: Ring Nebula (M57, NGC 6720)
Telescope: Subaru Telescope / Cassegrain Focus
Instrument: Suprime-Cam
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)