A planetary nebula is defined as, “A bright cloud of glowing gas and dust surrounding a highly evolved star,” according to encyclopedia.com. The term “nebula” originated in the mid-1600s, derived from the Greek term “nephele,” which refers to a cloud, fog or haze.
But defining a planetary nebula doesn’t begin to explain how this starry phenomenon came to be identified and understood by astronomers.
Ironically, “planetary nebula” is a term that is directly applicable to stars rather than planets. It was coined by astronomer Sir Frederick William Herschel in the late 1700s, shortly after he discovered the planet Uranus. Legend has it that Herschel dubbed these mysterious and colorful space entities “planetary” because they bore a resemblance to the planet Uranus. He did not realize that a star was at the heart of a planetary nebula until after he had created the term.
Even though Herschel named this illustrious space condition, he was not the first to discover it. That scientific recognition is given to noted French astronomer Charles Messier in 1764.
One of Messier’s most relevant accomplishments was his organized collection of astronomical objects. Messier, while searching for comets, catalogued other space entities that he stumbled upon. These were known as “Messier objects,” and included star clusters, nebulae and galaxies.
Once these observations spread throughout the scientific community, astrologers began to study them and began to understand some of the conditions under which a planetary nebula is formed.
To begin, it’s important to understand that a nebula is disseminated mass, or cloud, of interstellar gas or dust, or a combination of both. Nebulae are usually about 97% hydrogen and 3% helium.
A planetary nebula is comprised of a star, surrounded by a shell of ionized gases emitted by the star; the shell expands and glows. This happens to certain kinds of stars as they approach late stages of their lives. It is also known as an “emission nebula.”
These mysterious space entities have been observed in a variety of shapes, a fact which has yet to be figured out by astronomers. Some are asymmetrical, others are spherical or disk-shaped. At the center of a planetary nebula lies the star. The gaseous cloud surrounding the star continues to expand, moving farther into space, away from the dying star.
After the cloud is gone, the star is considered to be a “white dwarf.”
Because a planetary nebula’s emissions are charged gases, they are among the most breathtaking sights in outer space. In reviewing photos of them, just about every color in the rainbow can be seen.