M57-subaru-1

Title: Subtle halos about Ring Nebula

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)

Explanation:
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.

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.