A planetary nebula is a region of cosmic gas and dust formed from the cast-off outer layers of a dying star. Despite their name, planetary nebulae have nothing to do with planets.
When stars with an intermediate mass (greater than 80% of the Sun’s mass, but less than eight times its mass) die, they expand to form red giants. The dying star will continue to expel gas, whilst simultaneously the remaining core of the star contracts and temporarily begins to radiate energy again. This energy causes the expelled gas to ionise, meaning that the atoms and molecules in the gas become charged and begin to emit light. The cast-off glowing gas is known as a planetary nebula. Therefore, planetary nebulae are classified as emission nebulae, and are entirely unrelated to planets. The misnomer came about because of a historical misclassification. 250 years ago, astronomers thought they were looking at gas planets when they observed the colourful spectacle of planetary nebulae through their less powerful telescopes. Planetary nebulae only last for about 20 000 years, making them a very short-lived part of the stellar life cycle.
Throughout the years, Hubble has studied and imaged varying shapes and colours of these intricate planetary nebulae, the different colours arising from different, often newly created, chemical elements, showing that the final stages of the lives of stars are more complex than once thought. You can explore Hubble’s beautiful collection of planetary nebula images here.
Using Hubble, astronomers caught a rare glimpse of the nebula Hen 3-1357, nicknamed the Stingray nebula, fading precipitously over just the past two decades. Even though the Universe is constantly changing, most processes are too slow to be observed within a human lifespan. However, the Stingray Nebula offered researchers a special opportunity to observe the evolution of a system in real time. Images captured by Hubble in 2016, when compared to Hubble images taken in 1996, showed a nebula that has drastically dimmed in brightness and changed shape.
To celebrate Astronomy Day in 2003, astronomers unveiled one of the largest and most detailed celestial images to date of the Helix Nebula. The Hubble Space Telescope image showed a fine web of filamentary 'bicycle-spoke' features embedded in the colourful red and blue gas ring that is one of the nearest planetary nebulae to Earth. Being so nearby, the nebula is nearly half the size of the diameter of the full Moon. Hubble astronomers took several exposures with the Advanced Camera for Surveys to capture most of it.
Hubble’s studies of a large number of planetary nebulae have also revealed that rings, such as those seen around the Cat's Eye Nebula, are much more common than previously thought and have been found in at least a third of all planetary nebulae.
The telescope also demonstrated its full range of imaging capabilities with two new images of planetary nebulae in 2020, of NGC 6302, dubbed the Butterfly Nebula, and NGC 7027. Both are among the dustiest planetary nebulae known and both contain unusually large masses of gas, which made them an interesting pair for study in parallel by researchers. The Hubble images revealed in vivid detail how both nebulae are splitting themselves apart on extremely short timescales — allowing astronomers to see changes over the past couple of decades.