Planetary Nebulae
Overview
A planetary nebula is a glowing shell of gas ejected from a red giant star late in its life. The term planetary nebula emerged because early astronomers observed them to be planetlike in appearance. However, we know now they have no relation to planets.
Origin
As an intermediate-mass star exhausts its hydrogen fuel, its helium core contracts and heats to meet the energy needs of the star. The core contraction releases gravitational energy, which has two consequences. First, hydrogen just outside the core begins to burn, producing a more massive helium core over time. Second, the star’s outer layers begin to expand away from the core. Ultimately, the star transforms into a red giant.
For stars between one and eight solar masses, the core continues to condense until the conditions (temperature and density) become sufficient to burn helium into carbon. The ignition of helium occurs rapidly, producing a flash of light. The star’s outer shells expand, leaving a bright core that soon becomes a white dwarf. The expanding shells of gas, still illuminated by radiation, become a planetary nebula.
An assortment of planetary nebulae seen by the Hubble Space Telescope. Credit: H. Bond (STSci), B. Balick (University of Washington), NASA
Shape
Planetary nebulae are occasionally spherical and symmetric, and roughly a light year across. As the gas expands away from the star, it sweeps up cooler gas like a snowplow. The gas glows because of the ultraviolet light from the stellar remnant at its center.
Planetary nebulae represented as triangles in the night sky toward Orion.
Location
The Milky Way Galaxy consists of two major star populations: an older halo population spherically distributed around the Galactic center, and a younger population present in the Milky Way’s disk. Because the planetary nebula phase of a star’s evolution is relatively short (on the order of thousands of years), when we see a planetary nebula we know it’s formation is relatively recent, cosmically speaking.
Because of this, we expect to see planetary nebulae where star formation and evolution is ongoing—in the disk of the Galaxy. The stars that will evolve into planetary nebulae have relatively eccentric orbits around the Galaxy and; therefore, a wider range of distances above and below the Galactic disk when compared to younger objects like open star clusters or HII regions. So, we see a wider dispersion above and below the disk with planetary nebulae when compared to other objects like HII regions.
Periwinkle triangles represent planetary nebulae shown here around the Sun. We are looking toward the center of the Milky Way Galaxy. The lines connecting the stars of the constellations are shown which, from this perspective, appear to all point back to the Sun located at the focal point of those lines.
Population
Due to their short lifetimes, on the order of thousands of years, astronomers expect roughly 20,000 planetary nebulae in the Milky Way. Some are spherical in nature, but others are elliptical or even bipolar. They are notoriously difficult objects to obtain accurate distances for, so we are limited to those in our part of the Galaxy. And, you will notice an abundance as we look toward the center of the Galaxy. It’s safe to assume, like the stellar distance uncertainty, that these positions have a large uncertainty associated with them.
Planetary nebulae as seen from outside the Milky Way Galaxy. Most are located relatively close to the Sun, and you can see an overdensity as we look toward the Galactic center.
Profiles
Dossier
Census: |
1,657 planetary nebulae |
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Asset File: |
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OpenSpace Version: |
3 |
Reference: |
Planetary nebulae in Gaia EDR3 Central star identification, properties and binarity |
Prepared by: |
Brian Abbott, Zack Reeves (AMNH) |
Source Version: |
3.09 |
License: |