Messier 27: The Dumbbell Nebula

Messier 27, the Dumbbell Nebula, is the brightest planetary nebula in the sky and one of the best deep-sky sights of summer. It is large and bright enough to show its shape in a small telescope, and it is a good first target for trying a nebula filter.

What it is

A planetary nebula is the shell of gas thrown off by a dying star similar to the Sun. When such a star runs out of fuel in its core, it sheds its outer layers, and the exposed hot core, a white dwarf, lights up the expanding gas with ultraviolet radiation. M27 is a nearby example: its central star lies about 1,260 light-years away, measured by the Gaia spacecraft. At that distance its 8′ by 5.7′ apparent size works out to roughly 3 light-years across the long axis. The name "planetary" is historical; these objects have nothing to do with planets, but their small round disks reminded early observers of planets. For comparison, the Ring Nebula, M57, is smaller and farther away.

When to see it

M27 is a summer object that stays in the sky well into autumn. From about 40°N it is highest around 11 p.m. daylight time in late August, close to overhead at about 73° above the southern horizon. It is visible in the evening from July to October. From the Southern Hemisphere it is lower, in the northern sky, but still reachable from most inhabited southern latitudes.

How to find it

Find Altair, the southern corner of the Summer Triangle. About 10° north of it lies the small arrow-shaped constellation Sagitta. Its brightest star, Gamma Sagittae (magnitude 3.5), marks the arrow's point at its eastern end. M27 lies about 3.2° due north of Gamma Sagittae. In binoculars, put Gamma at the bottom of the field and look for a small, grey, slightly rectangular patch near the top.

What you will see

In binoculars M27 is a small, faint glow, easy to see under dark skies and still detectable from the suburbs. A 70–100 mm telescope shows its classic "apple core" or hourglass shape: two bright lobes with fainter notches on either side. A 200 mm or larger telescope fills in the notches with fainter gas, giving the nebula a football-like outline, and at high power the faint central star can be glimpsed. An O-III or UHC filter is a big help, especially from suburban sites, making the fainter parts much clearer.

History

Charles Messier discovered M27 on July 12, 1764, making it the first planetary nebula ever found. He described it as an oval nebula without stars. The name "Dumbbell" came later, from the double-lobed shape described by 19th-century observers. Modern images show the nebula has a complex structure of knots and filaments, shaped as the gas expands from the dying star.