The Owl and the Surfboard...

M97 and M108

Objects: M97 (NGC 3587), M108 (NGC 3556)
Type: Planetary nebula (M97) and Spiral Galaxy (M108, morphological classification SBbc)
Constellation: Ursa Major
Distance: 2000 light years (M97), 46 million light years (M108)

Date: April 15th and 19th. 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 12 x 600s H-alpha, 12 x 600s OIII, 12 x 600s luminance, 10 x 300s (2x2 binned) each for RGB, flats for all channels, bias as darks (hot pixel removal in Astroart). 

These two objects from the Messier catalogue are pictured above in the same one-degree field of view, located overhead in late Spring evenings adjacent to the star Merak (beta Ursae Majoris) in the bowl of the "Big Dipper". Though they may seem close to each other, the bluish disc of the planetary nebula M97 is a foreground object that lies a "mere" 2000 light years away within our own Milky Way galaxy, whilst M108 is a giant galaxy in its own right, 110,000 light years in diameter and nearly 46 million light years distant.

Stellarium map showing image field

The Owl Nebula is a planetary nebula named after its distinctive appearance, resembling a pair of owl-like eyes, which can be seen in larger telescopes. 

Its apparent dimensions in the sky are 3′.4 x 3′.3, or about a tenth of the diameter of the full moon. 

It was discovered by the French astronomer Pierre Méchain, Charles Messier’s friend and colleague, on February 16, 1781. Messier included the object in his catalogue on March 24, 1781.

Upon discovery, Méchain reported that the nebula was a difficult object to see.

 

Messier noted: “Nebula in the Great Bear, near Beta: It is difficult to see, reports M. Méchain, especially when one illuminates the micrometer wires: its light is faint, without a star. M. Méchain saw it the first time on Feb 16, 1781, & the position is that given by him.  Near this nebula he has seen another one which has not yet been determined, and also a third which is near Gamma of the Great Bear.”

The two “nebulae” Messier mentioned in his description of M97 are barred spiral galaxies later named Messier 108 (in the field of view above) and Messier 109 by the American astronomer Owen Gingerich, and added to Messier’s catalogue in 1960

The estimated age of the Owl Nebula is about 8,000 years. The Owl Nebula was formed when its parent star expelled its outer gaseous layers about 8,000 years ago. As the outer layers were gradually blown off over thousands of years, what was left of the original star contracted to form a hot white dwarf. The 16th magnitude central star has 55 to 60 percent of the Sun’s mass, only 4 percent of the Sun’s radius, and an estimated surface temperature of 123,000 K. The star can be seen between the Owl’s eyes. Its radiation is responsible for the nebula’s glow (see my notes on M27 for information about planetary nebulae).

Most stars that expel material to form planetary nebulae – about 80 percent of them – expel a large amount of it in two opposing directions. The jets blown off by the progenitor star of the Owl Nebula are almost aligned with our line of sight. The dust within the jets blocks enough light from the expanding nebula to create the appearance of owl-like eyes.

One of the nebula’s eyes appears darker than the other. This is the jet that is emitted in our direction,

while the fainter eye marks the jet expelled in the opposite direction, away from us.

Line of sight galaxies adjacent to M97

The nebula will gradually disperse over the next several thousand years, while the central white dwarf will cool and fade away. In about 5 billion years, our Sun will end its life in similar fashion.. 

There are several remote galaxies lying in the same line of sight as the Owl Nebula. These distant galaxies are shown in a crop of the above wide field view (see opposite).

Admiral William H. Smyth was the first to classify the object as a planetary nebula in 1844. 

M97 has about 0.13 solar masses and stretches across 0.91 light years in radius. It is expanding at an approximate speed of 27 to 39 km/s into the surrounding space.

As noted above, M108 was discovered by Pierre Méchain in 1781 three days after he discovered M97. 

Close-up of M108 from wide-field image above

M108 (known by some as the "Surfboard Galaxy") is classified as a barred spiral galaxy with loosely wound spiral arms, but visually there is little evidence of a well-defined spiral pattern in the galaxy as the spiral is inclined towards us only 15 degrees from edge-on. It is one of the larger members of the Ursa Major galactic cluster, being approximately 100,000 year years across. 

Observations show young star clusters exposed against dark dust lanes and bright emission regions. M108 also contains "super-shells", which are shells of gas driven by bursts of star formation and resulting supernova explosions. The super-shells could also be driven by stellar jets or an in-fall of gas from outside the galaxy.

At the centre of M108 is a supermassive black hole estimated to be 24 million times as massive as the Sun. The Chandra X-ray Observatory discovered multiple X-ray sources in M108, with the brightest X-ray source suspected to be an intermediately sized black hole that is actively accreting material.

I took the LRGB data on the cold, frosty evening of the 15th. There was a bit of a haze and the stars were a bit bloated. Once I had acquired the Ha and OIII data I wanted for M97, I made an RGB frame to add the star colours (in "Lighten" mode) to an HOO image, and then used the luminance data just to brighten up and add detail to M108.  Although the image scale is small, the Ha regions in both objects can definitely be seen.

The Pumpkin Patch in Auriga...

Sh2-232, Sh2-235, Sh2-231, Sh2-233

Objects: Sharpless 2-232 (Pumpkin Nebula), Sh2-235, Sh2-231 and Sh2-233
Type: Emission Nebulae 
Constellation: Auriga
Distance: 5870 light years

Dates: 11th, 13th and 16th. February 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 12 x Red (300s), 12 x Green (300s), 12 x Blue (300s), 20 x 600s H-alpha, 12 x 600s OIII (2x2 binned), no flats/darks (hot pixel removal in Astroart).

Stellarium map showing location of Sh2-232

Between Theta Aurigae and Iota Aurigae (the two southernmost stars of the “kite” of the constellation of Auriga) lies a string of emission nebulae. The above image shows four faint areas of nebulosity lying just to the east of the brighter and better known nebulae IC 410 and IC 405.  Of these, Sh2-235 is the most central and the brightest nebula of an H II region known as G174 + 2.5; it is observed in the direction of the northern part of the OB Aur OB1 association and includes the nebulae catalogued as Sh2-231, Sh2-232, Sh2-233 and Sh2-235, identified in the 1959 Sharpless catalogue of H II regions.

Although in optical images they appear as distinct nebulae, in reality they all belong to a single giant molecular cloud, some parts of which appear illuminated by young and hot stars. The cloud is found in the Milky Way spiral Arm of Perseus at a galactic latitude which places it slightly off-centre with respect to the centre of the galactic disc.

Sh2-232 is very faint in comparison to Sh2-235 and much larger, covering an area slightly greater than that of the full moon. Its photographic appearance has given it the nickname of the Pumpkin Nebula.  Although it appears in the above image as similar in brightness to its companions, this is an artefact of image processing: it has been selectively “stretched” to bring out detail.  The annotated unstreched hydrogen alpha image (below) shows the true relative brightnesses of the nebulae.

Annotated H-alpha frame showing true relative brightnesses

There is very little OIII detail to be had, although it did highlight the little planetary nebula lying just off-centre in Sh2-232 (circled opposite). The OIII stack was stretched and combined with the blue filter stack, with the H alpha data being combined with the red filter stack. The Ha and OIII data were combined as layers in “multiply” mode and then combined with the green filter stack. The three colour channels were then RGB combined and a partial Ha layer added over it in luminance mode, with some smoothing and star reduction plus other selective boosts to colour saturation, sharpness and brightness to give the final image.

Sky conditions were not great on any of the three nights I collected data. Attempts to boost the brightness of Sh2-232 have resulted in a rather noisy image, but it is a rather more colourful rendition of this area than is usually seen and I quite like it.

Messier 27 - The Dumb-bell Nebula...

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):

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.

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.

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.