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Thread: Object of the Week September 6, 2015 – NGC 7009, the Saturn Nebula

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    Member Howard B's Avatar
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    Object of the Week September 6, 2015 – NGC 7009, the Saturn Nebula

    NGC 7009, Herschel’s “Curious Nebula”

    NGC 7009 is better known as the Saturn Nebula, and at magnitude 8.3 and 1.2 arc minutes in diameter is brighter but a little smaller than M57. It has a much easier to see central star (magnitude 12.7) than M57, along with a visible structure unique among the brighter planetaries - the ansae that give rise to its name.

    There are three compelling reasons to observe the Saturn Nebula:

    1. The term “planetary nebula” was coined as a result of its discovery.
    2. It was the first deep sky object discovered with a reflecting telescope.
    3. Its overall shape resembles an edge on view of Saturn and its rings.

    A Short History
    The Saturn Nebula was accidentally discovered on September 7, 1782 by William Herschel using his homemade 12 inch diameter, 20 foot long reflector. This was a year before he began his systematic sweeps of the sky with his 18.7 inch scope, also 20 feet long, so he referred to the 12 inch as the “small 20 foot”. The 12 inch was essentially mounted on a pole with block and tackle, with its bottom end on a hinge. Altitude was adjusted with the block and tackle and it appears azimuth was adjusted by nudging the bottom end around the pole – no wonder Herschel built his 18.7 inch with a much better mount.

    Nonetheless, when he stumbled across the Saturn Nebula he became the first to discover a non-stellar deep sky object with a reflecting telescope. Somewhat surprisingly, all previous discoveries had been made using refracting telescopes, but then Herschel was the first to build and use high quality, practical reflectors.

    Unfortunately, the 12 inch didn’t show the remarkable ansae coming from each end that gives NGC 7009 its unique appearance, so it was left to Lord Rosse to coin the name “Saturn Nebula” when he clearly saw its distinctive shape with his 72 inch reflector in 1849.

    Even though Herschel didn’t see the ansae, he still didn’t know what to make of this object and wrote in his observing journal:

    “A curious Nebula or what else to call it I do not know. It is of a shape somewhat oval, nearly circular. The brightness in all powers does not differ so much as if it were of a planetary nature, but seems to be of the starry kind.”

    It was from this notation that the term “planetary nebula” was born, and clearly Herschel used the word planetary to mean "disk-like”, not that these objects were related to planets. Even though M27 and M57 were already known they weren’t recognized as planetary nebulae (PN) for approximately another 80 years.

    Interestingly, Herschel didn’t directly compare the Saturn Nebula to Uranus, which he discovered the year before. But it’s difficult to imagine that the discoverer of Uranus didn’t have this in mind when he coined the term.

    Herschel had no way to determine the actual composition of the Saturn Nebula so he really didn’t know what he’d found. Spectroscopy of planetary nebulae wouldn’t come until 1864 with William Huggins groundbreaking work, but that’s another story. Nonetheless, in 1785 Herschel did speculate that planetary nebula could “be looked upon as very aged globular clusters drawing on towards a period of change, or dissolution.” He changed his mind in 1790 after observing NGC 1514, which is yet another story.

    Oddly, in none of his planetary nebula discoveries, including the Saturn Nebula, did Herschel note their sometimes vivid blue-green color. Almost as strange, he noted the blue hue of Uranus only once and was startled that it looked blue that one time. His son John Herschel was the first to comment on the color of some of the brighter planetary nebulae.

    Regardless, the blue tint of the Saturn Nebula is easy to see, especially at low powers. The color fades with increasing magnification but if the seeing is steady you’re nicely compensated with fascinating glimpses of internal detail.

    Observing Notes
    Here are my observing sketch and observing notes from last year’s Oregon Star Party, using my 28 inch f/4:

    SaturnNebula_2014OSP_crop.jpg SaturnNebula_2014OSP_cropinvert.jpg

    “Yowza, now this is a great view, best ever if memory is correct. The central star is visible about 80% of the time as a small, sharp dot and the internal detail snapped into remarkable detail several times – wonderful! The sketch above also shows the elongated bright areas in both ansae, as well as a bright spot in the circular halo. I could still see a turquoise-green hue in the brighter parts of the nebula, which is pretty remarkable at 695x, 21.52 SQM, no filter.”

    Filters don't improve the view because they don't highlight unseen structures, and they hide the beautiful color.

    The blue-green color of the bright central area is most evident at low powers, making the nebula easy to locate by its color rather than its shape. But however you find it, you’ll be rewarded with a fascinating view of Herschel’s Curious Nebula - which, I propose, is a great alternative name for this wonderful object.

    GIVE IT A GO AND LET US KNOW!
    Last edited by Howard B; September 7th, 2015 at 04:08 AM.
    Howard
    30-inch f/2.7 alt-az Newtonian
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    Member Ivan Maly's Avatar
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    Seeing is a huge factor with this object. Here is what I have in my notes, with 16":

    "At 450x (AII-4) it looks like a flattened Saturn in horrific seeing. E and W appendages are thin but not sharp. There are persistent hints of separation of the W appendage from the oval body of the nebula. The slight color is preserved in the body."

    Can one say with confidence that Messier or Mechain did not discover any objects with reflectors? Messier used reflectors for his observations in the Catalog but never specified the discovery instrument.

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    Member Howard B's Avatar
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    Hi Ivan, my source for the Saturn Nebula being the first non-stellar deep space object discovered with a reflector is Wolfgang Steinicke's book, "Observing and Cataloging Nebulae and Star Clusters", page 19. Given Wolfgang's normally impeccable research I'll bet he's right.

    There's also engravings of the small and large 20 foot telescopes on the same page.
    Howard
    30-inch f/2.7 alt-az Newtonian
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    Contributing Editor, Sky & Telescope magazine

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    Member Ivan Maly's Avatar
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    That is a great book (that I've actually read cover to cover... hmm). There is little doubt that NGC 7009 is the first discovery with a reflector that is documented in extant sources. My understanding, however, is that no record remains to determine if Messier used his reflectors for reobservation only.

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    My view of NGC 7009 changed dramatically when I learned to observe it under very good seeing conditions.

    That means concrete:
    - structures or different morphologies of the both ansae
    - inner structure is a thin parallelogram
    - within the inner structure another thin shell crossing the parallelogram similar to NGC 6543

    Frank Richardsen could see even more detail as in the below showed sketch between 1500x-2500x with his 20" telescope. Sounds crazy or impossible but at the best seeing places + best seeing conditions it could be possible.

    My sketch was made under nearly perfect seeing condition on one of the best place in Europe. Although still low altitude from Austria (32° above horizon) needless to see that the view was more than impressive. Under theses conditions perhaps the most beautiful PN in the sky.

    27", 1172x, no filter, Seeing I, Nelm 7m+
    NGC7009.jpg
    Clear Skies, uwe
    http://www.deepsky-visuell.de
    Germany

    27" f/4,2

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    Member Howard B's Avatar
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    Fantastic drawing Uwe, what wonderful detail you saw - the view in Frank's 20" scope must have been mind-blowing! Great seeing brings out so much great detail in some planetaries, and this is certainly one of them.

    Ivan - I've read Wolfgang's book cover to cover too, it's a fascinating read and one I highly recommend to everyone interested in the history of the NGC.
    Howard
    30-inch f/2.7 alt-az Newtonian
    https://sites.google.com/site/howardbanichhomepage/
    https://sites.google.com/site/sprays...pemirrors/home
    Contributing Editor, Sky & Telescope magazine

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    Member akarsh's Avatar
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    Uwe, I am thoroughly amazed by the amount of detail you have seen. I have seen that kind of detail only with Jimi's 48" telescope on this object!

    In my scope, I do see the two things at the end, and the "ring", but nothing more.

    Regards
    Akarsh
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