Tuesday 7 February 2017

The eyes of the owl

In the constellation of Ursa Major - popularly known as the "big dipper" or "chariot" - resides a fascinating planetary nebula which astronomers refer to as M97. It's one of the faintest objects on Messier's list and being 3,4 arc minutes across it's pretty large for a planetary, meaning that its frail light's dispersed over a large surface. But already with an 8" telescope you'll be able to see why this nebula bears the nickname "Owl Nebula". If you look very carefully, you'll see two big, dark "eyes" in it. 

Actually, these "eyes" are only coincidental due to the angle at which we see this nebula. As I explained in many earlier posts, planetary nebula are caused by dying stars of normal size which become critically unstable and shed their atmosphere into space. The radiation from the extremely hot stellar core that's left over (surface temperatures can easily reach 100.000°C or more!) makes the gas bubble glow and so we can easily observe it. Often this gas bubble takes an hourglass or even cylindrical shape because the star's atmosphere's much thicker around its waist and therefore the gas can escape much easier from the poles. One fine example is the Ring Nebula, which isn't a ring at all but a cylinder we see face-on. In the Owl Nebula's case the nebula's quite spherical but it has two big holes, one on either pole, where gas ejected at much greater speed. An easy way to imagine this nebula is to see it as an apple from which the core's been removed with a corer (top-right to bottom-left). But Owl Nebula sounds much more intriguing than Apple-Without-Core Nebula and it does seem to stare at you, doesn't it? 

It's 8.000 years old, lies 2.000 lightyears away from us and is expanding at 30km per second. In 10.000 years it'll be gone.

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