An unusually "perfect" cosmic corkscrew surrounds the binary star system LL Pegasi in a new Hubble Space Telescope picture released this week.
Astronomers think the spiral's evenly spaced rings are being created because one of the stars in the binary pair is dying. Unlike more massive stars that end their lives in explosive supernovae, LL Pegasi is quietly shedding its outer layers of gas and dust to form what's called a planetary nebula.
The dying star itself is still hidden by a dusty cocoon. But it's ejecting material at about 31,000 miles (50,000 kilometers) an hour, the researchers calculate, forming a new ring in the spiral every 800 years.
"If a single star was sitting still, it would eject matter in all directions at roughly the same speed," said Raghvendra Sahai of NASA's Jet Propulsion Laboratory.
Instead the dying star is losing material as it orbits around the center of the binary system. As the star completes an orbit every 800 years, its lost matter winds around the system in a regular geometric pattern.
Image courtesy NASA/ESA
Cat's Eye Nebula
Unlike LL Pegasi, which is just starting to lose its outer layers of gassy material, the star that made the famed Cat's Eye nebula is further along in its demise: A white dwarf, the dense core of the dying star that formed the planetary nebula, is clearly visible in this 2004 Hubble picture.
The Hubble shot reveals a set of at least 11 faint rings around the white dwarf. These rings could have been made as the star ejected its matter in a series of pulses—or they could be evidence the dead star was once part of a binary system like LL Pegasi.
"Right now we see a single star, but that doesn't necessarily mean that it has always been a single star," JPL's Sahai said. "There are theories suggesting that a companion has been swallowed up by the main star."
Image courtesy NASA/ESA
Planetary Nebula NGC 2440
Not all dying stars leave such geometrically perfect remains, as evidenced by the uneven lobes of the planetary nebula NGC 2440, seen above in a 2007 Hubble picture.
The lone star that formed this nebula likely shed its mass unevenly during periodic outbursts that blasted gas and dust in many directions. Today its white dwarf—the hottest known—is giving off intense ultraviolet light that makes the surrounding stellar debris glow.
Known as Einstein rings, the perfect circles of light come from two galaxies 11 billion and 6 billion light-years away. A massive third galaxy—the bright center of the bull's eye—sits directly in front of the other two just three billion light-years from Earth.
Because of the way the three galaxies are lined up, gravity from the nearest galaxy warps and magnifies the light from the farther two, creating the ringed effect. Such visible evidence of gravity bending light in space may help scientists map otherwise invisible dark matter.