blue supergiant


planetary nebulae

Helium is the heaviest element a star like the Sun can burn, and its exhaustion marks the star's death throes. As the hydrogen- and helium-burning shells move farther out, the star's instability increases, eventually causing the outer layers to blow away, creating the glowing shells of a planetary nebula—so named for their resemblance to ghostly, planet-shaped disks. Such nebulae are typically spherical, but are easily distorted by magnetic fields or by companion stars to form even more complex and beautiful shapes.


The ringlike appearance of this spherical planetary nebula is created where we look through the thickest parts of its gaseous shells.


One of the most spectacular planetaries, the Eskimo shows multiple shells of ejected matter, including cometlike outer trails. Its appearance is probably due to the interaction of the dying red giant with an unseen companion.


For a star with eight times the Sun's mass, helium burning is not the end. It can continue the fusion of successively heavier elements in its core. Eventually, the core develops an onionlike layered structure. However, the atomic structure of the different elements means that each new layer burns for a shorter time and produces less energy. Eventually, the sequence reaches iron, the first element whose fusion absorbs more energy than it releases. When the star attempts to fuse iron, its central energy source is cut off, and, with no outward pressure of radiation to hold it up, the layered core collapses under its own gravity. The rebounding shock wave tears the star apart and briefly allows fusion to form heavier elements.

outer layers of subatomic neutrinos outer layers of subatomic neutrinos

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