One of the issues that motivates astronomers concerns star formation. There are many unanswered questions about this fundamental process, including if it has always worked the same throughout the Universe's long history. One of the reasons the JWST was built and launched is to address this question, a testament to curiosity about the subject.
A prime difference between the modern and ancient Universe concerns metallicity. In astronomy, elements heavier than hydrogen and helium are called metals. These heavier elements are produced by massive stars, while the Big Bang produced only hydrogen and helium, for the most part. So a generation of stars had to live and die before the Universe contained more metals.
Astronomers often seek out lower-metallicity star forming regions in order to understand what star formation in the early Universe might have been like compared to how it works in the present day. One of the lower-metallicity environments is the outer Milky Way. Most spiral galaxies have a negative metallicity gradient, meaning the further a region is from the galactic center the lower its overall metallicity is. That's for several reasons, including the fact that the galactic center is more densely packed with stars. That means there are more massive, metal-producing stars there than in the outer galaxy.
This figure from a 2023 paper illustrates the reverse metallicity gradient in the Milky Way. It's binned by groups of stars with different ages. Image Credit: Lian et al. 2023 NatAstr
A team of Japanese astronomers has been working with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the Milky Way's outer star-forming regions. They found protostellar jets very similar to the jets around stars well inside of the galaxy. Their results are published in The Astrophysical Journal, titled "The Detection of Spatially Resolved Protostellar Outflows and Episodic Jets in the Outer Galaxy." The lead author is Toki Ikeda from Niigata University in Japan.
The researchers examined a single star-forming region containing multiple protostellar candidates. Within that region, they focused on a single one of the protostellar candidates called Sh 2-283-1a SMM1. It's about 26,000 light years from the Sun, and about 51,000 light years from the center of the Milky Way. This region contains only about 33% of the heavy elements that are found near the Sun.
This figure from the research shows the positions of the target star-forming region in the Galaxy and the near-infrared color composite images of the targets. (a) The target positions are represented as the yellow rectangles. The background is an artist’s conception of the Milky Way (R. Hurt/NASA/JPL-Caltech/ESO). (b) The positions of the target protostellar candidates are indicated by the red circles. The numbers of sources are labeled when multiple sources are enclosed. Image Credit: Ikeda et al. 2025. ApJ
"We present the first detection of spatially resolved protostellar outflows and jets in the outer Galaxy," the authors write.
The jets and outflows are both collimated, and they found the jets have multiple bullet structures. They also found two other characteristics that attracted their attention. One is the flow velocity, which "linearly increases with the position offset from the core center," the authors explain. They also found "the continuous velocity components of the periodical flows (spine-like structures), which may indicate episodic mass ejection events." The intervals between these ejection events is between 900 and 4,000 years.
Episodic mass ejections are a critical part of the growth of protostars. They regulate stellar mass, they clear out surrounding material near the star, and they shape the stellar environment. The events are likely caused by instability in the protostar's accretion disk, which can inject large amounts of material at once into the star. The rapid jump in accretion creates more powerful magnetic fields around the star that triggers the jets and outflows. As far as astrophysicists know, these events are integral to the star formation process, so finding them in low metallicity environments similar to the ancient Universe draws a strong parallel between the modern and ancient Universe.
This figure from the research explains some of the findings. (a) shows the red-shifted jet travelling away from us, and (b) shows the blue-shifted jet travelling toward us. The white crosses are the "bullets" in the jets that represent episodic mass ejection events. These events are integral to star formation. Image Credit: Ikeda et al. 2025. ApJ
“By resolving jets and outflows in a protostar so far out in the Galaxy, we can see that the same physics shaping stars near the Sun also operates in low-metallicity environments. This discovery unlocks a unique opportunity to fundamentally advance our understanding of how stars are born across diverse cosmic environments,” said lead author Ikeda in a press release.
Since this is the first time these jets and outflows have been detected at such a large galactocentric distance, this is the first time the parallel between the modern, higher metallicity Universe and the ancient, lower metallicity Universe can be drawn. "These characteristics align with those of nearby protostellar systems, indicating that early star formation in low-metallicity environments, such as the outer Galaxy, resembles that in the inner Galaxy," the researchers explain.
While the physical features of the jets and outflows are similar to protostars in higher metallicity environments nearer the galactic center, the chemistry is different. "In contrast to the physical similarities, the N(SiO)/N(CO) ratio in the jet bullet appears to be lower than that measured in the low-mass protostellar sources in the inner Galaxy," the authors write.
That indicates that the shock chemistry is different in that part of the Universe, as well as the properties of the dust. When high velocity gas in the jets collides with other matter, the energy can drive chemical reactions. SiO is one of the results of those shock chemistry reactions, since silicate dust is common and abundant in space. The N(SiO)/N(CO) ratio measures how much SiO there is in the line of sight compared to CO. SiO and CO are complementary ways of tracing shocks in the jets.
“Finding such a clean jet structure in the outer Galaxy was unexpected,” said Takashi Shimonishi, a co-author from Niigata University. “Even more exciting, the protostar was found to harbor complex organic molecules, opening up new opportunities to study star formation in more primitive environments from both physical and chemical perspectives.” The researchers found CH3OH (methanol) CH3OCH3 (dimethyl ether) and other organic molecules like formaldehyde.
The chemical fingerprints the team found could apply to star formation conditions in the early Universe, just like the jets and outflows do.
"The present results would indicate that the earliest star formation processes are not significantly different, at least physically, in a low-metallicity environment of the outer Galaxy," the researchers conclude.
