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| M27 (NGC 6853) - the "Dumb-bell" Nebula |
Object: Messier 27 (NGC 6853)
Type: Planetary Nebula
Constellation: Vulpecula
Distance: 1200 light years
Date(s) of imaging: September 19th, 27th and October 27th 2019
Equipment: ATIK 460EX, Skywatcher f5.5 Espirit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD
Subframes: 20 x 300s + 16 x 1200s H-alpha, 20 x 300s + 16 x 600s + 16 x 1200s OIII, 6 x 180s (2x2 binned) each for RGB star colours, no flats/darks (hot pixel removal in Astroart).
M27 was the first planetary nebula to be discovered (in July
1764, by Charles Messier) and is one of the nearest and brightest of its
type. It can be found in the dim
constellation of Vulpecula, sitting within the Summer Triangle of the three
bright stars Vega, Deneb and Altair. Its
physical diameter, (estimated to be around 1.2 light years) also makes it one
of the largest.
It gets its popular nick-name of the "Dumb-bell" nebula by its allegedly telescopic appearance to a weight-lifter's dumb-bell, the view lacking the OIII "ears" shown in the above image.
Planetary nebulae have nothing to do with planets. The term is likely derived from their often
round, planet-like shape as observed by astronomers through early telescopes,
and although the terminology is inaccurate, it is still used by astronomers
today.
To early observers with low-resolution telescopes, M27 and other
subsequently discovered planetary nebulae resembled the giant planets like Uranus,
appearing as pale blue or green discs with a similar visual diameter to the
solar system’s gas giants.
The true nature of these objects was uncertain, and Herschel
first thought the objects were stars surrounded by material that was condensing
into planets.
It wasn’t until the first spectroscopic observations were
made in the mid-19th century that the true nature of planetary nebulae became
apparent, when
William Huggins became the first scientist to
analyze the spectrum of a planetary nebula when he observed the
Cat's
Eye Nebula.
Huggins’s earlier observations of stars had shown that their
spectra consisted of a continuum of radiation with many dark lines superimposed.
He found that many nebulous objects such
as the
Andromeda Nebula (as it was then known) had
spectra that were quite similar. However,
when Huggins looked at the Cat's Eye Nebula, he found a very different
spectrum. Rather than a strong continuum
with absorption lines superimposed, the Cat's Eye Nebula and other similar
objects showed a number of
emission
lines, similar to those produced by fluorescent gases. It was clear that these nebulae were neither
planetary in nature, nor composed solely of stars.
Brightest of the emission lines was at a wavelength of 500.7nm,
which did not correspond to a line of any known element. At first, it was hypothesised that the line
might be due to an unknown element, which was named “nebulium”. A similar idea
had led to the discovery of helium, through analysis of the Sun's spectrum in
1868. While helium was isolated on Earth
soon after its discovery in the spectrum of the Sun, "nebulium" was
not.
Physicists subsequently found that in gas at extremely low
densities, electrons can occupy excited metastable energy levels in atoms and
ions that would otherwise be de-excited by collisions that would occur at
higher densities. Electron transitions
from these levels in ionised nitrogen and oxygen ions (rather than an “unknown”
element) give rise to the 500.7 nm emission line and others. These spectral lines, which can only be seen
in very low density gases, are called
forbidden
lines. Spectroscopic observations thus showed that nebulae were made of
extremely rarefied gas.
The above image shows the strong blue and red colours,
associated with the fluorescence of ionised oxygen and nitrogen respectively.
All planetary nebulae form at the end of an intermediate
massed star's lifetime. They are a relatively short-lived phenomenon, lasting
perhaps a few tens of thousands of years at the very end of a star’s life
cycle. Once all of a red giant's
atmosphere has been dissipated, energetic ultraviolet radiation from the blazingly
hot (>50,000K) exposed hot luminous stellar core ionises the ejected
material. Absorbed ultraviolet light
then energises the shell of nebulous gas around the central star, causing it to
fluoresce and give rise to the brightly-coloured light emissions of a planetary
nebula.
Only when a star has exhausted most of its nuclear fuel can
it gravitationally collapse to a small size, losing the outward radiation
pressure that supports it. Planetary nebulae came to be understood as a final
stage of stellar evolution. Spectroscopic
observations show that all planetary nebulae are expanding. This led to the
idea that planetary nebulae were caused by a star's outer layers being thrown
into space at the end of its life.
About 3000 planetary nebulae are now known to exist in our
galaxy, out of 200 billion stars. Their
very short lifetime compared to total stellar lifetime accounts for their
rarity. They are found mostly near the plane of the Milky Way, with the greatest
concentration near the galactic centre.
Only about 20% of planetary nebulae are spherically
symmetrical. A wide variety of shapes exist with some very complex forms seen. The huge variety of shapes is partially due to
the orientation of our planet to the nebula - the same nebula when viewed under
different angles will appear different. Nevertheless,
the reason for the huge variety of physical shapes is not fully understood. Gravitational interactions with companion
stars if the central stars are binary stars may be one cause. Another
possibility is that planets disrupt the flow of material away from the star as
the nebula forms. It has been determined that the more massive stars produce
more irregularly shaped nebulae.
In January 2005, astronomers announced the first detection
of magnetic fields around the central stars of two planetary nebulae, and
hypothesized that the fields might also be partly or wholly responsible for
their remarkable shapes.
This image was compiled from data collected on three different
nights, dodging autumn clouds and the moon. The
OIII and “Hydrogen Alpha” (the majority of the "red" emissions are actually from ionised nitrogen,
whose emission wavelength is close to that of H-alpha radiation), were stacked
separately in Astroart and then RGB combined in Paint Shop Pro (Red = Ha, Green
and Blue = OIII).
My first attempt was satisfactory but did not really show the faint outer regions of the nebula (see below):
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| M27, 300+600s narrowband exposures |
I was determined to try and capture the faint secondary shell surrounding the nebula, the relic of an earlier out-gassing episode. It seemed that my initial data-set just didn't have long enough exposure times to register these very faint extensions, so I took some additional 1200 second exposures at the first moon-free opportunity (this is about as long as I can go at my light-polluted location without the sky background washing everything out). The long-exposure stacks still required some aggressive selective stretching (and subsequent star reduction) to bring the "wings" out, but after a bit of trial and error in PaintShop, I was quite pleased with the final result (top image). A few binned RGB
frames were RGB combined in Astroart and the output subsequently blended with the HOO frame
to give some colours to the stars.
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| M27 - wide field including 1200s narrowband data |
The main image is a crop of the wide field original, shown above.
I had previously imaged M27 way
back in July 2005. One interesting feature of that image compared with my latest one is that it shows the presence of an additional star. The older data (to which this year's colour data was added to make visual comparisons a bit easier) was broadband as against narrowband and so the stars are inherently brighter, but the absence of the star in the later image is still apparent.
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| Old and new images of M27, showing variable star |
The variability of this star was first discovered in 1988 by Leos
Ondra, a Czech amateur astronomer, who noticed that the star appeared in some
images of M27 but not in others. He concluded that it was a long period
variable and nicknamed it “Goldilocks”.
The Goldilocks Variable was later confirmed to be a
Mira-type variable, a pulsating star going through a cycle of expansion and
contraction every 213 days. Mira variables were named after the first star observed
to have such properties, known by its Bayer designation Omicron Ceti, a red
giant star located in the constellation Cetus.
The Goldilocks Variable is not within the Dumbbell Nebula,
but is a background object much further away.
