Among Natures most beautiful constructs are symmetrical planetary nebula, huge cocoons of gas surrounding old stars. But astronomers have not been sure why the streaming filaments of color come in myriad shapes, such as the classic hourglass, instead of being spherical.
The prime suspect has long been magnetism, and a new study provides the first conclusive proof in the case, astronomers said Friday.
An international team of astronomers has made new observations that show that the magnetic fields close to four aging stars are at least 10 times stronger than that of our own Sun. This magnetism directs the shape of the nebulae.
Planetary nebulae are mis-named because when astronomers first began to notice them with early telescopes, they looked like planets, mere smudges of light in the sky. More powerful ground-based telescopes and more recently the Hubble Space Telescope, have since brought the amazing detail and color of these objects to life.
Heres how planetary nebulae are thought to form:
When stars like our Sun reach the end of their lives, they eject a large amount of material into the space around them. This material, produced by nuclear fusion reactions in the star, forms a thick shell of gas and dust. The material flows in a turbulent manner, but is also directed by the magnetic field lines.
The ejected material, containing elements such as carbon and oxygen, in eventually recycled into new stars and planets and the building blocks of life itself.
The group, lead by Wouter Vlemmings of Leiden Observatory, observed four old stars with the U.S. National Science Foundation's Very Long Baseline Array, a network of 10 radio telescopes operated by the American National Radio Astronomy Observatory.
They detected radio emissions originating from clouds of water vapor ejected by the stars. In some circumstances, such a cloud can become a maser: the equivalent of a laser for radiation with longer wavelengths. One specific frequency of the emitted radiation, which is characteristic of water molecules, is amplified enormously, resulting in a bright, clear signal. In this signal, the group detected a so-called Zeeman-effect for the first time: subtle changes in the spectrum of the emission that can only be caused by a strong magnetic field at the location of the maser.
The effect is named after Pieter Zeeman, a 1902 Nobel Prize winner who discovered the effect of a magnetic field on the spectrum of a light source in 1896.
The water masers occur at a large distance from the star -- about twice the distance between the Sun and Pluto -- so the magnetic field strength at the surface of the star must be much higher: The researchers said it is 10 to 100 times the surface magnetic field strength of the Sun. This is sufficiently strong to play an important role in the formation of non-spherical planetary nebulae, they said.
The work is detailed in the journal Astronomy & Astrophysics.
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