The Helix Nebula (also known as NGC 7293), at distance of around 400 light years, is the closest planetary nebula to the Earth. It formed when a dying star threw off its outer layers which are gradually expanding into space. They are called planetary nebulae because astronomers originally believed they were new planets, due to their often spherical shapes. The Helix Nebula is around 100 times more distant than the nearest stars (excluding the Sun).

The gases are expanding into space at a rate of tens of kilometres per second. Because of this, planetary nebulae in general are short-lived celestial phenomena, lasting around 50,000 years before their expanding gases become too diffuse to be visible.



If the Helix Nebula was bright enough to be visible to the naked eye, it would cover an area of sky around half the size of the full Moon.