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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:
Attachment 1780 Attachment 1781
“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!
