The Hubble space telescope has captured a brilliant picture of OH 231.8+04.2, also known as the Calabash Nebula because of its similar shape to the southern African gourd.
The highly detailed image shows the consequences of an old star losing its outer layers, transforming from a red giant star into a planetary nebula. The ejected material is being propelled at the incredible speed of 1 million kilometers (620,000 miles) per hour.
The Calabash nebula is located 5,000 light-years away in the constellation of Puppis, in the southern hemisphere. It is also known as the Rotten Egg nebula because scientists discovered that the nebula is rich in sulphur, which is one of the elements responsible for the characteristic smell of a rotten egg. But don't worry, the density of Earth's atmosphere is a thousand trillion times the density of a planetary nebula, so even if you were there, you wouldn’t be smelling much.
This shot is an incredible view of a fleeting phenomenon, in astronomical timescales. Planetary nebulae form in just a few thousand years, which is a blink compared to stars' lifetimes of several billions of years.
It is also a window into the future of the Sun and the many low-mass stars that inhabit the universe. Only stars that are heavier than eight of our Suns can become supernovae. The others evolve into red giants and then turn into planetary nebulae.
These red giants have a dense, incredibly hot core (temperatures of many millions of degrees) and they swell up, with the outer layer loosely bound by gravity. The energetic photons from the core push the gas away and excite it, turning the lost layers into a brightly shining nebula.
Eventually, the star loses its cover and cools down. The star becomes a white dwarf and the outer layers, without the hot photons, stop shining.
The name planetary nebula is a misnomer. They are not related to planets at all. But the first astronomer to see one, William Hershel, thought they looked a bit like a planet, being somewhat round, and for better or for worse the name stuck.
