Baby star grows thanks to magnetic fields in jets

Baby star grows thanks to magnetic fields in jets

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Have you ever played with a magnet? It’s like magic. A magnet has an invisible force field. You can only see it indirectly. For instance by sprinkling iron filings on a piece of cardboard, and putting the magnet beneath the cardboard.

Magnetic fields not only show up in the distribution of iron filings. They also show up in light that passes through. In the presence of a magnetic field, light becomes polarized. In polarized light, the light waves no longer vibrate equally in all possible directions. Instead, the vibrations are stronger in one direction than in others.

With special equipment, it is possible to detect whether or not light is polarized. Taiwanese astronomers have now detected polarization in the radiation from the jets of a newborn star.

A baby star forms from a contracting cloud of gas and dust. In most cases, the baby star is surrounded by a flat, spinning disk of material. Very often, the star also spews out narrow jets of gas in two opposite directions, perpendicular to the disk.

Molecules of silicon monoxide (SiO) in the jets from the star emit radiation at millimeter wavelengths. The astronomers used ALMA to study this radiation. They found the radiation to be polarized. That can only mean one thing: there must be a magnetic field in the jets.

Astronomers have long assumed that the jets of baby stars must be threaded by magnetic fields. Otherwise, it is hard to understand why the two jets remain so narrow, even at large distances from the star. But this is the first time that there is solid evidence for the presence of a magnetic field.

The find is important, because it helps astronomers to understand the formation of stars. Normally, it would be hard for material from the contracting cloud to fall into the newly forming star. That’s because of the rapid rotation of the central parts of the disk and of the baby star itself. Because of the fast rotation, material is more likely to be flung away from the star.

Somehow, the two jets carry away a lot of this rotational energy. Thanks to the jets, the star’s rotation slows down, and it is able to grow by attracting more gas. The new discovery by ALMA now shows that this is due to magnetic fields. Without magnetic fields, the jets wouldn’t remain narrow enough to carry away much of the star’s rotational energy.


ALMA was trained at a young baby star (also called a proto-star) at a distance of about a thousand light-years in the constellation Perseus. The baby star, known as HH-211, is less than ten thousand years old – extremely young for a star. At present, it has only one-twentieth of the mass of our own sun, but it is expected to grow more massive in the near future. The star is blowing two long, narrow jets of gas into space. ALMA was able to measure the millimeter radiation of silicon monoxide molecules in those jets, as close to the star as a hundred billion kilometers.


The ALMA observations of HH-211 were carried out by a team of Taiwanese astronomers. The team was led by Chin-Fei Lee of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA). Chin-Fei worked together with colleagues from Taiwan and from the United States. The new results have been published in the November 2018 issue of Nature Communications.