Hubble captures celestial 'hourglass'

A picture taken by the Hubble Space Telescope reveals a small, bright nebula embedded in the center of the larger one  INTERACTIVE    3-D VRML models:    • Maneuver the Hubble Space Telescope

August 25, 1999
Web posted at: 10:02 a.m. EDT (1402 GMT)

By Robin Lloyd
CNN Interactive Senior Writer

(CNN) -- Two stars on death's doorstep have exchanged gaseous matter to form a spectacular nested hourglass image that was recently captured by the Hubble Space Telescope.

The binary system, located in the Southern Crab Nebula, comprises a dying star, called a red giant, and an all-but-dead star, or white dwarf. It may have created the striking shapes as the result of two nuclear explosions, said astronomer Mario Livio of the Space Telescope Science Institute in Baltimore.

As the red giant lost mass, the white dwarf sucked it into a disc, which eventually exploded with the strength of a hydrogen bomb, Livio said.

A donut of gas surrounding the entire binary system then finished the delicate work, he said.

"Once that material blows off in the explosion, the donut of gas can act like a corset holding in the waist of that gas and keeping it relatively narrow while the material that is moving is able to blow these bubbles that form this hourglass shape," Livio said.

Astronomers already knew about the outer hourglass, which likely formed in an explosion a few thousand years ago. But Hubble returned the image of the inner, brighter hourglass in May, thought to be the result of a more recent second explosion.

Closeup of the nebula

Livio said the theory for how the nebulae formed is speculative. The results were presented at a conference that took place at the Massachusetts Institute of Technology earlier this month.

The team of astronomers that captured the image is headed up by Romano Corradi of the Instituto de Astrofisica de Canarias in Spain. The group will test its theory with calculations to simulate a similar event, Livio said.

Livio said he was fascinated by the structure and likened it to wooden Russian Matryoshka dolls that nest inside one another.

Astronomers are accustomed to unusually shaped nebulae, he said, but usually they find planetary nebulae rather than systems with a red giant still burning inside.

The Southern Crab Nebula is not visible to the naked eye but is located in the constellation Centaurus. The nebula is fairly young and at a distance of about a few thousand light years from Earth. A light year is the distance light travels in a year -- more than 9 trillion km (nearly 6 trillion miles).

The event that formed the two nesting hourglasses could happen yet again, Livio said, creating a third hourglass in the system.

"If this scenario is correct then this thing should go on for a while," he said, "and every few thousand years we would have another one of them."