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.