The term ‘planetary nebulae’ was coined in the 18th century by William Herschel, because, when viewed through the telescopes of the time, these objects often appeared as pale blue or greenish orbs, not unlike the planets Neptune and Uranus. Despite the inaccuracy, the name has stuck. Both planetary nebulae and supernova remnants result from the death of stars: the former when stars of up to around eight times the mass of our sun explode, sending the star’s outer layers out into space as a glowing, ionised ball of stellar material; the latter when larger stars explode.
Planetary nebulae have really captured my imagination lately, and I’ve spent a fair amount of time looking at the Ring nebula (M57) in Lyra, the Saturn neb in Aquarius, and the Blue Snowball in Andromeda. I’ve also spent a lot of time searching for (and failing to find) the Little Gem nebula in Sagittarius. I love the names as much as anything, and I think that’s part of the attraction. Other examples include the Lemon Slice in Camelopardalis, the Little Ghost in Ophiuchus, the Medusa in Gemini, the Skull in Cetus, the Helix in Aquarius, the Phantom Streak in Aquila, and the Soap Bubble in Cygnus …
As well as the intriguing names, there’s also a great sense of achievement in finding them. Planetaries can be very tricky to find. The normal approach to finding objects with a telescope is to start with lower magnification, and a wide field of view. Once you find the object, you can increase the magnification. At low magnification many planetaries are hard to distinguish from the background stars, so you often have to go with higher magnification, but a smaller field of view, to start with. Unless you’re very lucky and are looking in the right place (or have a GoTo setup), then finding them can be a struggle.
Thankfully, not all planetaries are tricky to find. The Ring nebula, for instance, is relatively easy, as are the Cat’s Eye in Draco, the Eskimo in Gemini, and the Owl in Ursa Major (Messier 97). Amongst those that I have been viewing recently, my favourite has to be the Blue Snowball. It’s hard to spot at low mag, but once you get your eye in, find the target and up the magnification (to at least 80x or so), the planetary neb is revealed. I’ve gazed at it for ages, entranced by its colour, and the way in which it seems to just hang there in space; a soft blue ball against the dark sky and the sparkling background stars.
Another of my favourites, not visible until later in the year, is NGC 2438, a planetary nebula in the constellation of Puppis. It’s directly in the line of sight of the 500 or so stars that make up the open cluster Messier 46. The combination of the two objects really is a fantastic sight.
Another fascinating aspect to observing planetaries is the sense of scale and perspective one gets. For example, both the Blue Snowball and the Ring nebulae are approximately 1 light year in diameter. They look so small in the vastness of space, and yet it would take light around a year to travel from one side to the other…
Any discussion of supernova remnants at this time of year has to include Cygnus loop (also known as the Veil and Witches broom nebulae). I have seen this with my 20×80 binoculars, but it really looks its best at low magnification (x30-50) and with an OIII (that’s pronounced ‘O3’) filter fitted to the eyepiece. It’s possible to trace the loop of stellar material all the way round, taking in the filamentary structure and translucent billowing sheets of ionised gas and dust, the remains of a star that exploded around 5000-8000 years ago. I often find myself wondering whether it was visible from Earth. It would have appeared suddenly, gradually increased in brightness over a few weeks, before fading and disappearing completely. What would the early residents of Maiden Castle, for example, have thought of it I wonder …
The Crab nebula, in Taurus, is another supernova remnant, and it will be visible in the evening sky once we get into October. Chinese astronomers recorded seeing the original supernova in 1054AD. This is a much dimmer object than the Cygnus loop, but is still worth a look on a dark and clear night.
Planetary nebulae and supernovae play a crucial role in what’s called galactic enrichment, and, therefore, in the existence of life. In fact, it’s only by seeding the galaxy with Oxygen, Carbon, Iron, Calcium, etc, all of which are formed within the dying star, is life possible. As Carl Sagan once said, “We’re made of star stuff …” And it’s from planetary nebulae and supernova remnants that we come to be. One day, distant astronomers may view the planetary nebula that was our sun. I wonder what they’ll call it.
What’s up?
The moon is currently dominating the evening skies: full on the 18th, with the next new moon occurring on the 1st of September. Saturn is still well placed (unlike Mars, which is well past its best now), but won’t be around for much longer, as it’s slipping lower in the west every night. By mid-September it’ll be gone from the evening sky until next June.
When darker evening skies return, why not have a look for a few planetary nebulae. The Owl nebula lies just to the left of the lower right-hand star of the pan of the Big Dipper (Merak), and should be visible at magnifications of 20 times and above. There’s a nice galaxy nearby too (the Surfboard, Messier 108), and this should be visible in 10×50 binoculars. The Ring nebula is almost right overhead at the moment, and is visible at magnifications from x20 and up. A star map or smartphone app will get you there.
Speaking of easy binocular targets, the Andromeda and Triangulum galaxies are ideally placed in the evenings.
Clear skies!
Kevin Quinn is an amateur astronomer based in Cerne Abbas. He is the proud owner of a ten-inch reflector, a small refractor, a case of eyepieces, and a couple of pairs of binoculars. He tweets via @CerneAstro, and blogs (occasionally) via theastroguy.wordpress.com.
©Kevin Quinn
