A massive star on the distant outskirts of our Milky Way galaxy is seen blowing a powerful cosmic blowtorch in a new image courtesy of James Webb Webb Space Telescope’s Near-Infrared Camera.
The James Webb Space Telescope (JWST) caught the two jets slamming into the interstellar medium around them, forming the nebulous structure known as Sharpless 2-284, or Sh2-284 for short. The jets stretch across eight light-years total as they expand at a rate of hundreds of thousands of miles per hour. What’s more, their very existence is proof of the process by which the most massive stars in the universe form.
“Once we found a massive star launching these jets, we realized we could use the Webb observations to test theories of massive-star formation,” Jonathan Tan of the University of Virginia in Charlottesville and Chalmers University of Technology in Gothenburg said in a statement.
Stellar jets are frequently seen being launched by lower mass stars as those stars form. The jets are fueled by material, mostly hydrogen gas, falling onto the growing star. This material bunches up into a disk around the young protostar. Some of the material in the disk is absorbed by the star, increasing its mass, but if too much material bunches up in the disk, some of the excess is flung away by tightly wound magnetic fields that beam the material out in two jets along the young star’s axis.
Low-mass stars form in relatively orderly fashion, but one theory of the formation of more massive stars — the type of stars that go supernova — is that their accretion of infalling gas is more chaotic. If this were the case, it would result in the star and its accretion disk wobbling about, and the jets moving, twisting and spraying across a larger area.
However, the JWST image of Sh2-284 shows no evidence of the jets having moved, with them being straight and pointing almost 180 degrees opposite one another. This implies the star’s formation has not been chaotic at all.
Models of star formation suggest the size of the jets scales with the size of the star producing them.
“These models imply that the star is about ten times the mass of the sun and is still growing and has been powering this outflow,” said Tan.
How energetic these jets are, how straight, how narrowly collimated, and their ages can all help astronomers better understand the environment in which stars sourcing those jets form, as well as the intrinsic properties of such stars.
“We didn’t really know there was a massive star with this kind of super-jet out there before the observation,” Yu Cheng of the National Astronomical Observatory of Japan, who led the JWST observations, said in the statement. “Such a spectacular outflow of molecular hydrogen from a massive star is rare in other regions of our galaxy.”
Cheng alludes to Sh2-284’s location in our galaxy. This young star is located 15,000 light-years from Earth on the very outskirts of the Milky Way’s spiral disk, where the abundance of elements heavier than hydrogen and helium is low. That’s because such elements are formed within stars, and star-formation on the outskirts of the galaxy just hasn’t been vigorous enough to produce many of these elements, which astronomers (somewhat confusingly) collectively refer to as “metals,” even though they are not all metals in the conventional sense.
To find a star forming in this low metallicity environment is fortunate for astronomers though, because these conditions mimic those found in the early universe.
“Our discovery is shedding light on the formation mechanism of massive stars in low metallicity environments, so we can use this massive star as a laboratory to study what was going on in earlier cosmic history,” said Cheng.
More recent is Sh2-284’s history. The tips of the jets are the oldest part, with the young star’s life chronicled in the long extent of the jets.
“Originally the material [in the tips] was close into the star, but over 100,000 years the tips were propagating out, and then the stuff behind is a younger outflow,” said Tan.
The growing star is not visible directly. The bright stars with the diffraction spikes are closer to us in the foreground. The JWST is however able to pick out the structure of Sh2-284, with filaments, knots, bow shocks and linear chains of clumpy material all arising out of the jets’ interactions with the surrounding interstellar medium.
Yet, as powerful as these jets and the nebulosity they produce are, they are only transitory. Eventually, the star will emerge from its cocoon, fully grown with possibly tens of times the mass of our sun. Its lifetime will be limited to a few million years. Then, it will explode, creating a whole new nebula, one of star-death, not birth, but its legacy will be to enrich space with the metals that it forged.
And the cycle of stars will continue.
The observations of Sh2-284 were reported on Wednesday (Sept. 10) in The Astrophysical Journal.
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