Abell 30

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.

History

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.

Formation

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.

Morphology

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.