Planetary nebula

From South Dublin Astronomical Society

When stars like our own Sun get old, the nuclear processes within their core start to change. The star’s core has at that stage burnt up all its hydrogen. This causes the star to slowly contract, which reduces the star’s temperature. This contraction pulls some hydrogen-rich material within the star to fall into the core, causing it to start new hydrogen-fuelled nuclear reactions resulting in an increase in temperature and a re-expansion of the star. After a while the recent influx of hydrogen into the core is all burnt out and the cycles repeats. This cyclic pulsating effect can cause the star’s outer layers to blow off, forming a shell around the star. This shell can only be seen due to the reflection of light from the associated star illuminating this shell of gas. The term “planetary” nebula dates back to 1785 when William Herschel looked at these objects through his telescope and thought they looked similar to planets. An example of a planetary nebula is M57, The Ring Nebula in Lyra.