Abell 30
A born-again planetary nebula with hydrogen-depleted knots.
Abell 30 is a planetary nebula in the constellation Cancer, about 5,500 light-years away. It belongs to the rare class of born-again planetary nebulae, where the star reignited after the initial nebula had already formed.
George O. Abell discovered it on photographic plates from the Palomar Sky Survey, and his findings were published in 1966 as part of a list of 86 old, faint planetary nebulae—now called the Abell Catalog of Planetary Nebulae. In 1979, astronomers noticed that the center of Abell 30 (along with Abell 78) was unusually hot and bright compared to the rest of the nebula, and it contained knots invisible in H-alpha light. The process behind these central knots was described in 1983.
A planetary nebula forms late in the life of a sun-like star. After billions of years fusing hydrogen into helium in its core, the star runs low on fuel, its core contracts, and it expands a hundredfold into a red giant. Eventually, the red giant’s outer envelope is ejected, drifting away at less than 100,000 miles per hour. The star itself shrinks into a hot, compact white dwarf, which emits intense ultraviolet radiation and a fast wind of particles moving at about 6 million miles per hour. The UV radiation and fast wind interact with the ejected envelope, creating a large spherical shell—the planetary nebula. The outer shell of Abell 30 is about 12,500 years old.
In rare cases, the star’s helium shell reaches a critical mass and fuses into carbon and oxygen around the core, heating the outer envelope so much that the star temporarily becomes a red giant again. This is called a very late thermal pulse. The same sequence—envelope ejection followed by a fast stellar wind—repeats, but much faster than before, creating a small, inner planetary nebula inside the original one. In effect, the nebula is reborn. This renewed activity produced the knots in the inner region. Observations by the Hubble Space Telescope over 20 years show these knots expanding at rates that suggest they are between 610 and 950 years old.
Abell 30 has an outer shell, a faint cloverleaf pattern, and a central star surrounded by knots. The outer shell has a radius of 63 arcseconds and gradually brightens to a sharp edge. The cloverleaf pattern is more pronounced toward the southeast and southwest, extending 26 arcseconds from the center—almost halfway to the edge.
- Distance
- 5,500 light years
- Constellation
- Cancer
- Outer shell radius
- 63 arcseconds
- Outer shell age
- 12,500 years
- Knots age range
- 610 to 950 years
- Central star temperature
- 110,000 K
- Central star variability period
- 1.06 days
Lore & Background
The planetary nebula was discovered by George O. Abell in photographic plates obtained during the Palomar Sky Survey, published in 1966 in a list of 86 old faint planetary nebulae known as the Abell Catalog of Planetary Nebulae. In 1979 it was found that the centre of Abell 30 was noticeably hotter and brighter than the rest of the nebula and features knots not visible in H-alpha. The mechanism that led to the creation of the central knots was described in 1983.
Abell 30 consists of an outer shell, a faint cloverleaf pattern, and a central star surrounded by knots. The outer shell has a radius of 63 arcseconds and brightens gradually up to a sharp edge. The cloverleaf pattern reaches 26 arcseconds from the centre. The brightest knots lie within 10 arcseconds of the central star; originally four knots were identified (J1 to J4), but higher resolution images revealed they are composed of smaller cometary knots and filaments. Two bright polar knots lie about seven arcseconds from the central star, each featuring a bow shock towards the star. The other knots are arranged in a disk like spokes of a wheel seen at an inclination of 60 degrees.
The infrared morphology shows smooth disk-like emission along a northeast to southwest axis, passing through the central star and knot J4, while no enhancement is seen at knots J1 and J3. X-ray emission consists of a point source at the central star and diffuse emission associated with the knots and cloverleaf structure, possibly from material heated into plasma by the fast stellar wind. The central star has a spectral type between a carbon-rich Wolf–Rayet star and a PG 1159 star, categorised as a [WC]-PG 1159 star, with an estimated temperature of 110,000 K and a variability period of 1.06 days, possibly due to a colder, dimmer companion.
Reader's Guide
Abell 30 is significant as a rare born-again planetary nebula, illustrating the phenomenon of a very late thermal pulse where the helium shell of the star reaches critical mass and fuses into carbon and oxygen, temporarily making the star a red giant again. This sequence of envelope ejection followed by a fast stellar wind is repeated on a much faster scale, creating a small-scale planetary nebula inside the original one. The knots in the inner part of the nebula are a direct result of this rebirth, with expansion rates observed by the Hubble Space Telescope over 20 years indicating ages of 610 to 950 years. The chemical composition of the knots shows notable segregation: polar knots have a helium-to-hydrogen ratio of 7, while other knots have a ratio of about 4, and polar knots are depleted in oxygen and nitrogen compared to typical planetary nebulae. The detection of carbon dust around the central star, with a total dust mass estimated at 3.20+3.21−2.06×10−3 M☉, and the finding that there is more carbon than oxygen in the ejecta, supports the very late thermal pulse model. The nebula's legacy lies in its detailed study of knot chemistry and morphology, providing insights into the late-stage evolution of sun-like stars.
Did You Know?
- Abell 30 was discovered by George O. Abell in photographic plates from the Palomar Sky Survey and published in 1966.
- The central star of Abell 30 has a temperature estimated at 110,000 K and varies with a period of 1.06 days.
- The knots in Abell 30 have an age of 610 to 950 years, determined from expansion rates observed by the Hubble Space Telescope over 20 years.
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