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IC63 and IC59
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Object: IC59, 63 (Sharpless 2-185)
Type: Emission and reflection nebulae Constellation: Cassiopeia
Distance: 600 light years
Date: November 2nd, 3rd, 4th, 15-16th. 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD
Subframes: 47 x 600s Ha, 10 x 600s each for RGB (2x2 binned), flats, no darks (hot pixel removal in Astroart).IC63 (the bright, pointed object near the middle of the
image) and IC59 (the fainter bluish area above and to the upper right) are the
brightest areas of the nebulosity known as Sharpless 2-185 (Sh2-185). They
were discovered independently by Max Wolf (Dec
30, 1893) and Edward Barnard
(Feb 2, 1894).
Together, the nebulae occupy an area of space approximately 10
light-years across.
The bright, hot star Gamma Cassiopeia (seen at the bottom
right of the above image) is located only 3 or 4 light-years from the nebulae,
and it may have shed this nebulous material into the space around it. The edges
of the nebulae glow brightly from this intense radiation that is slowly
evaporating and lighting up these flowing shapes of gas and dust
Gamma Cassiopeia has a radius 14 times greater than our Sun
and is 55,000 times more luminous, 19 times more massive, and rotates at about
300 km/hour, or 150 times more rapidly than our Sun. It is known as an eruptive
blue-white sub-giant variable star. (Eruptive variable stars vary in brightness
because of violent processes and flares in their coronae and chromospheres.)
This star is an erratic variable that reached a maximum
brightness in 1937, but then unexpectedly dropped in surface temperature from
12,000°K to 8500°K. It is encircled by a surrounding gaseous disk of material
thrown off by its rapid rotation. Mass loss is apparently related to the
brightness variations.
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Stellarium map showing image field
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The constellation of Cassiopeia can be found virtually overhead during late autumn evenings (see map opposite). IC 63 (the brighter of the two and slightly closer to Gamma
Cassiopeia than IC 59) is a combination of an emission and reflection nebula.
Unlike a reflection nebula which appears blue, the glowing hydrogen gas appears
red. IC 59 is primary a refection nebula, showing much less red hydrogen, and
is appearing blue of dust reflected starlight that is passing through it.
As can be seen above, the two nebulae have very different
visual appearances. IC63 can be referred to as a "cometary cloud", is
pointing toward Gamma Cas, and is narrower and more sharply defined than IC59.
Spectral measurements suggest that IC59 is slightly cooler at 590K and less
dense than IC63 at 630K. They are not actually separate nebulae, but are part
of a much larger nebulous region surrounding Gamma Cas based upon the WHAM
(Wisconsin H-alpha Mapper) survey. IC63 has bright filaments, visible in the
image above, that are believed to be ionized fronts of gas created by Gamma
Cas, and seen by us as nearly edge-on.
Both nebulae exhibit spectroscopic evidence from the
mid-infrared of molecular hydrogen and polycyclic aromatic hydrocarbons (PAH). The
contrast in appearance between IC63 and IC59 is consistent with a difference in
actual distances from Gamma Cas and small differences in temperature and column
density..
There is some discussion in professional circles as to whether the H-alpha
signal that we pick up in our images is actual emission from the nebula, or a reflection
of the H-a emitted from Gamma Cas that is scattered by the dust in IC59 and
IC63. This light scattering and reflection is called ERE, (Extended Red Emission).
Gamma Cas is the prototype B0 IV star, emitting significant H-a. It is above
the main sequence with a more extended atmosphere. Being somewhat cooler than a
BO V star, it is only marginally capable of ionizing molecular hydrogen in its
vicinity. Thus, it is possible that the H-a we pick up in our images is a
mixture of both processes; direct H-a emission from ionization, and ERE.
H-alpha data was gathered on the bright moonlit evenings of
2-4 November. The bright (and damp!) conditions were not ideal for the imaging
of what is quite a faint nebula, but given that the previous month’s skies had
been completely clouded out, it seemed best to make do. All three evening sessions were ended at
around midnight due to high haze and mist setting in, and the resultant image
stack suffered from background noise.
Application of Starnet++ to the Ha stack allowed the
nebulosity to be stretched and smoothed, with stars from the “unstretched”
stack being pasted back in in “screen” mode to avoid star bloat.
Colour data was acquired on 15-16th November,
with the moon mercifully absent: the imaging session was still stop-start due
to clouds and was finally clouded out at around 2.00 am.
The blue reflection nebulosity was pretty faint but once
again Starnet++ allowed separation of the nebulosity from stars for processing
purposes. The Ha data was blended in
with the red channel and the blend used in an RGB composite in PSP. A partial luminance overlay of Ha was used to
sharpen the image up a bit.
References:
1. Astrodon imaging
2. NASA APOD
3. Anne’s
Astronomy News
4. https://cseligman.com/text/atlas/ic0a.htm
