Hubble has photographed one of the most famous nebulae in the southern sky, NGC 3918. The structure, with its distinct eye-like shape, is a brilliant cloud of colorful gas hiding inside a dying star. NGC 3918 is located 4,900 light-years from Earth in the constellation of Centaurus.
The object is a planetary nebula. Despite the misnomer, the nebula is not about planets, its all about its central star and the material it has ejected over the last few thousand years. The star is an old red giant. It hasn’t got enough mass to go supernova, so its last hurrah is to eject as much of its other layer as possible before it emerges as a white dwarf. The layers of material are showered in a powerful ultraviolet light, which makes them glow. The clouds of NGC 3918 easily outshine the central star.
The shape of planetary nebulae is also an interesting subject. NGC 3918 looks like a piercing celestial eye. It has a bright inner shell and a more diffuse outer shell. It might at first appear like these were two different ejections but scientists have confirmed that they formed at the same time. They were simply thrown out at different speeds. Some of the jets of material shot by the star move at the staggering speed of 350,000 kilometers (217,500 miles) per hour.
The mechanism that shapes these nebulae is little understood. It is partly due to the properties of the star or stars at the center, but also the stellar winds and magnetic fields play a role. The Hubble Space Telescope observations have been key to furthering our understanding. The term planetary nebula was used because they looked round-ish, like a planet, in the earliest observations. Hubble showed that there is a variety of morphologies out there, some actually quite complex.
Planetary nebulae are short-lived phenomena lasting at most a few tens of thousands of years. Although a blink in term of a stellar-lifetime, these nebulae tell us about the progenitor stars and their death. They might also play an important role in enriching interstellar gas. The material thrown in the gas shells often contains heavier elements that might then be used to form planets.
Planetary nebulae also give us a glimpse into the distant future: It may be how our Sun will end.
