How to find the Blue Snowball Planetary Nebula (NGC 7662)

This article examines a spectacular type of object created by a star's final act, a planetary nebula, or more specifically, the Blue Snowball Nebula. Other names that are commonly used for this nebula include NGC 7662, Caldwell 22, and Snowball Nebula.

The nebula is located about 2,500 light-years away from Earth, in the Andromeda constellation. Nebulae such as these represent a stage of a star's evolution when its fuel runs out. A star is a nuclear furnace that emits helium by fusing hydrogen fuel. As medium-mass stars exhaust their fuel, they grow into red giants, unlike massive stars which explode as supernovae.

After billions of years of nuclear fusion, the star starts to die. Gravity, no longer balancing nuclear fusion's outward pressure, compresses the star's core as it cools. As gas escapes from a star's outer layers, it creates a planetary nebula such as the Dumbbell Nebula. Called a planetary nebula because these objects often resemble planets when viewed through a small telescope. A small white dwarf lies at its centre, the remains of the original star's compressed core.

And this is what has happened for the snowball nebula however this is also one of the brightest planetary nebulas in the sky with an apparent magnitude of +8.6. It is best seen from the Northern Hemisphere in the months of October, November and December.

What do you need to see it?

While the nebula is visible with binoculars it only appears to be a star. In contrast, an 80mm telescope at high power shows the object to be slightly elliptical and green in colour. The star appears as an out of focus fuzzy green star. The planetary nebula spans 32 x 28 arc seconds of the sky.

With a 150mm (6-inch) telescope and a magnification of at least 100x, the nebula appears as a fuzzy blue disk with an elliptical outline.

If you use a 200mm (8-inch) or larger telescope with a magnification of at least 250x, you will be able to see the dark centre of NGC 7662. Blue Snowball Nebula is a structurally complex planetary nebula with a double-ringed structure, which means that its bright central ring of gas is surrounded by a much larger, dimmer and hazier envelope.

Large amateur telescopes of at least 16 inches are needed to see this structure and the other halo as well as the dim central star (mag. +13).

The object can also be magnified if you have good seeing and a steady mount, with amateur astronomers regularly using 500x to 1000x magnification in large aperture telescopes.

How to find Caldwell 22?

Click to enlarge Blue Snowball Nebula star chart

To find the Blue Snowball Nebula you need to first find the Andromeda Constellation. The best way to find this is to look for Cassiopeia, which is a bright constellation even in light-polluted skies. Then follow the V of Cassiopeia to Andromeda. Once you are at Andromeda, follow the leg of Andromeda down until you reach the 6th-magnitude star 13 Andromedae. You should be able to see the nebula within the eyepiece's field of view, half a degree southwest of the star.

To see what the planetary nebula looks like through the eyepiece and more in-depth star charts, please watch the episode below.

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