Monday, 26 June 2017

M27: Her Royal Majesty the Dumbbell Nebula

M27 is the brightest of the so-called planetary nebulae in the sky. The classification "planetary" was derived from the fact that these nebulae often appear like little disks, much like a planet. In reality they're huge shells of gas that were expelled by a dying star.

Right at first glance it's obvious where this particular nebula got its "dumbbell" nickname from, isn't it? Actually, we see this nebula from its equatorial plane. When viewed from its poles it would probably appear ring-shaped, much like the Ring Nebula in Lyra. As I've explained before, it's best to imagine these older, more developed planetary nebulae as an apple without a core. In this case the bright, inner gas shells that were expelled during the collapse of the central star, have caught up with the thinner external shell that already formed before the star exhaled its dying breath. Together they're expanding at 31km/s until they'll dissolve into space. The age estimates vary greatly, between 4,000 and 15,000 years, but the most recent spectroscopic analysis suggests an age of somewhere in between: 9,800 years. There's also been a lot of controversy regarding the Dumbbell’s distance but 1,300 light-years seems to be the most recent consensus. From this distance, the nebula’s a full light-year across.

In order to find the reason why this nebula became so big and bright, we have to examine its central star which is already visible in small telescopes. This star used to be a giant containing as much as ten solar masses. It was big, but not quite big enough to explode as a supernova. Even now, the dying stellar core has a diameter of 70,000km and it still contains 60% of our Sun’s mass, making it the biggest white dwarf known. With an extremely hot surface temperature of 85,000°C, it heats up the vast gas clouds so much that they start to emit a bluish-green light.

Today, the Dumbbell’s one of the brightest and most popular objects in the northern skies, extending almost half of the diameter of a full Moon and easily visible through a pair of binoculars. It’s a privilege to be able to observe it with an 18” binoscope under a reasonably dark sky.

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