Understanding how stars evolve is very important in understanding where Earth came from, and where the Solar System is going.
A phenomenal example of dying stars is Kohoutek 4-55 (K 4-55), a former red giant 4,600 light-years away in the constellation of Cygnus. In a picture taken by Hubble in 2009, and recently re-released by ESA, we can see one of the most fascinating events in stellar evolution: the formation of a planetary nebula.
Stars produce energy by fusing hydrogen in their core. The process lasts for many billions of years, but over time the center will be filled with helium and normal stars are not hot enough to fuse it. When the hydrogen has run out, the star first contracts on itself, bringing additional hydrogen into areas where the pressure and temperature are high enough. This causes the star's luminosity to increase by thousands of times, expanding the outer layer of the star well beyond its original size.
The Sun itself is expected to turn into a red giant in about five billion years. It will expand from its current size to engulf Mercury, Venus, and maybe even Earth. But the red giant is only the beginning of the end.
The outer layers of these aging stars are only loosely bound to the stars, so the energy produced at its center blows off gas from the outer shell and begins to lose mass. The outer shells expand far away from the star, forming the planetary nebula. The name is a misnomer: when William Herschel first observed one of these nebulae he thought they looked a bit like planets, being very round and extended.
In the case of K 4-55 we see something even more unusual, as the different elements have separated and formed a concentric ring structure. Within the bright central ring, there’s an abundance of hydrogen (in green) and oxygen (in blue). The entire system is surrounded by a large faint red halo emitted by ionized hydrogen.
Looking at a dying star is not just morbid curiosity; there are still many things we don’t know about how stars approach their doom. And dying stars have enriched the universe with the elements that make up new stars, planets, and also us.
