Wednesday, January 4, 2023


The definition of "planetary nebula" may lead us in error: these objects have nothing to do with the planets of the Solar System. This name is due to the fact that if you look through a small and not very powerful telescope, as those of 1700, these objects have a circular shape that somehow resembles the disk of a planet.

Planetary nebulae are actually the outer layers of gas ejected from a star of solar-like mass at the end of its life.

Among the most beautiful planetary nebulae in the sky is M27, the Dumbbell Nebula, first observed by Charles Messier in 1764. M27 being relatively bright (apparent magnitude 7.5) is quite easy to find in the sky. It is located in the direction of the constellation of Volpetta and can be detected already through a binoculars 10x50 or a small telescope.

The nebula is located quite close to the Earth, only 1200 light years, and therefore its structure is studied in great detail. 

For example, having determined the rate of expansion of the gases (30 km/s) and its actual dimensions (just over a light year in diameter) it was possible to derive its kinematic age, about 10 thousand years. If the rate of expansion of its gases is supposed to have remained constant over time then the kinematic age tells us how long ago the nebula formed. 

In principle, however, there is no reason why the speed of the gas has remained constant over time, but it is more likely that it has declined progressively. Studying the rate of expansion, however, remains a very important tool that lets us easily derive a lower age limit of the nebula.

Credit: ESO.

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