NASA ESA Hubble Image: Rotten Egg Nebula - Twin Celestial Jets

The object shown in these NASA/ESA Hubble Space Telescope images is a remarkable example of a star going through death throes just as it dramatically transforms itself from a normal red giant star into a planetary nebula. 

This process happens so quickly that such objects are quite rare, even though astronomers believe that most stars like the Sun will eventually go through such a phase.

Credit: NASA & ESA

Astronomers know that while large stars can end their lives as violently cataclysmic supernovae, smaller stars end up as planetary nebulae - colorful, glowing clouds of dust and gas.

In recent decades these nebulae, once thought to be mostly spherical, have been observed to often emit powerful, bipolar jets of gas and dust but how do spherical stars evolve to produce highly aspherical planetary nebulae?

In a theoretical paper published this week in the Monthly Notices of the Royal Astronomical Society, a University of Rochester professor and his undergraduate student conclude that only "strongly interacting" binary stars - or a star and a massive planet - can feasibly give rise to these powerful jets.

When these smaller stars run out of hydrogen to burn they begin to expand and become Asymptotic Giant Branch (AGB) stars.

This phase in a star's life lasts at most 100,000 years. At some point some of these AGB stars, which represent the distended last spherical stage in the lives of low mass stars, become "pre-planetary" nebula, which are aspherical.

Eric Blackman

"What happens to change these spherical AGB stars into non-spherical nebulae, with two jets shooting out in opposite directions?" asks Eric Blackman, professor of physics and astronomy at Rochester.

"We have been trying to come up with a better understanding of what happens at this stage."

For the jets in the nebulae to form, the spherical AGB stars have to somehow become non-spherical and Blackman says that astronomers believe this occurs because AGB stars are not single stars but part of a binary system.

The jets are thought to be produced by the ejection of material that is first pulled and acquired, or "accreted," from one object to the other and swirled into a so-called accretion disk.

A Hubble Space Telescope image of the Rotten Egg Nebula, a pre-planetary nebula 5000 light years away in the constellation of Puppis, and showing the bipolar jets. 

Credit: NASA/ESA and Valentin Bujarrabal (Observatorio Astronomico Nacional, Spain)

The name planetary nebulae originally came from astronomer William Herschel, who first observed them in the 1780s, and thought they were newly forming gaseous planets.

Although the name has persisted, now we know that they are in fact the end states of low mass stars, and would only involve planets if a binary companion in one of the accretion scenarios above were in fact a large planet.

"Pre-planetary" and "planetary" nebulae are different in the nature of the light they produce; pre-planetary nebulae reflect light, whereas mature planetary nebulae shine through ionization (where atoms lose or gain electrons).

Pre-planetary nebulae shoot out two jets of gas and dust, the latter forming in the jets as the outflows expand and cool. This dust reflects the light produced by the hotter core.

In planetary nebulae, thought to be the evolved stage of pre-planetary nebula, the core is exposed and the hotter radiation it emits ionizes the gas in the now weaker jets, which in turn glow.

More Information: 'Using kinematic properties of pre-planetary nebulae to constrain engine paradigms' Eric G. Blackman and Scott Lucchini - mnrasl.oxfordjournals.org/