The Goldilocks zone around three different type of stars
The Goldilocks Zone. The above image is a great illustration of the relative size of the habitable zone around different types of star, with stars like our Sun at the bottom.
Even very dim M class dwarf stars (pictured top) could harbour planets with liquid water – the planets would just need to be situated much closer in. These stars can have very active magnetic fields however, frequently throwing harmful radiation out towards any orbiting planets. M class stars are also extremely stable, some destined to burn for over 100 billions years, much longer than our Sun which has around 4 billion years of fuel left.
In the middle we see the K class dwarf stars. These will also out live our Sun (by a factor of 4), have nice wide zones of habitation, and much less magnetic activity than the M class stars. Potentially these K class stars are the ideal incubators for the slow evolution of life, and there’s plenty of them. Nearly 13% of stars in our galaxy are K class red dwarfs. That’s approximately 26 billion in our galaxy alone!
An artist’s impression of a rocky world orbiting a red dwarf star, like the M and K class stars mentioned above.
The term planetary nebula is highly erroneous, as these emission nebula have nothing whatsoever to do with planets. Perhaps the most famous of these is the beautiful ring nebula in Lyra, not far from the brilliant star Vega, although many other planetary nebula are scattered around our night skies, and can be observed comfortably in larger telescopes.
The following video by ESA is a fantastic 3D model of the Ring nebula. In essence the ring nebula is the remnants of a dying Sun like star beyond its red giant phase. As the star enters its final stages its outer layers are shed in great expanding waves, and the residual hot white dwarf star at the centre ionises these gases into beautiful coloured shells.
This ionisation process is very similar to the mechanism that produces Earth based aurora. Electrons are recaptured within the host atoms (often hydrogen, helium, oxygen and nitrogen) and the drop to lower energy levels releases light of a specific frequency, governed by the simple equation we all learn in physics, E = hf.