A planetary nebula is a type of emission nebula that consists of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The name “planetary nebula” is a bit misleading as these nebulae actually have nothing to do with planets. The name originates from the planet-like, round shape of the nebulae. Which was observed by early astronomers observing the Solar System. The term was first used by William Herschel.
All planetary nebulae form at the end of the life of a progenitor star of intermediate mass and low mass stars, normally with main sequence masses of 1-8 solar masses. They are a relatively short-lived phenomenon, lasting perhaps a few tens of thousands of years, compared to considerably longer phases of stellar evolution.
Once all of the red giant’s atmosphere has been dissipated, energetic ultraviolet radiation from the exposed hot luminous core, called a planetary nebula nucleus, ionizes the ejected material. This absorbed ultraviolet light then energizes the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.
A typical planetary nebula is around one light year across. Young planetary nebulae have the highest densities, and, as they age, their expansion causes their density to decrease. The central stars of planetary nebulae are very hot. These nebulae came to be understood as a final stage of stellar evolution. Spectroscopic observations show that all planetary nebulae are expanding, which led to the idea that planetary nebulae were caused by a star’s outer layers being thrown into space at the end of its life.
Starting from the 1990s, NASA’s Hubble Space Telescope images revealed that many planetary nebulae have extremely complex and varied morphologies. Only around 20% of planetary nebulae are spherically symmetric and the variety of the shapes is partially the projection effect
— the same nebula when viewed under different angles will appear different.
However, the reason for the huge variety of physical shapes is not fully understood. The main shapes are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar, quadrupolar and other shapes.
Notable Planetary Nebulae
- Cat’s Eye Nebula
- Little Gem Nebula
- Ring Nebula
- Helix Nebula
- Lion Nebula
- Dumbell Nebula
- Saturn Nebula
- Stingray Nebula
- Little Ghost Nebula
- Butterfly Nebula
- Red Spider Nebula
- Box Nebula
- NGC 2818
- NGC 6543
How is a planetary nebula formed?
A planetary nebula is formed at the end of the life of a star with a mass of between 1-8 solar masses. Gas and dust is ejected from the star and energetic ultraviolet radiation from the exposed hot luminous core, called a planetary nebula nucleus, ionizes the ejected material. This causes a shell of nebulous gas around the central star to glow brightly.
How big is a planetary nebula?
They usually measure around one light year across. The density decreases as they age and expand.
What causes a planetary nebula to glow?
The glow is due to the ionization of the ejected atmosphere by the ultraviolet photons, causing the dust and gas to shine.
How many planetary nebula are there in our galaxy?
There are believed to be about 20,000 objects called planetary nebulae in the Milky Way Galaxy, but, due obscuration of dust in the galaxy, only about 1,800 planetary nebulae have been cataloged.
What happens after the planetary nebula phase?
The lifespan of this type of nebula is around 30,000 years. As a planetary nebula phase ends, the star becomes a white dwarf, and the expanding gas cloud becomes invisible to us. This gives us an indication as to what will happen to our own Sun when it gets to the end of its life.