The most massive stars in the Milky Way contain one hundred times more mass than the Sun, even more in some cases. These stars are extremely hot, luminous, and blue, and often die in supernova explosions. Astrophysicists want to know how they get so big, and a simple household chemical might hold the answer.
Astrophysicists are puzzled by the existence of massive stars like BAT99-98, a Wolf-Rayet star with more than 200 solar masses. As young protostars grow, they exert outward radiation pressure that should theoretically halt their growth at about 30 solar masses. Massive stars also form more quickly than other stars, adding to the puzzle. Yet somehow, some stars do grow to contain immense masses.
The difficulty in understanding how massive stars grow is compounded by their environment. They usually form in regions obscured by thick gas and dust. In new research, scientists used the Very Large Array (VLA), a radio astronomy observatory, to examine the young star HW2 in Cepheus A. Cepheus A is a star formation region about 2300 light-years away. Due to its relative proximity, it’s an excellent place to study star formation.
The research, titled “Gas infall via accretion disk feeding Cepheus A HW2,” will be published in Astronomy and Astrophysics. It’s available on the pre-print site arxiv.org. The lead author is Dr. Alberto Sanna, a Senior researcher at the National Institute for Astrophysics (INAF) at the Astronomical Observatory of Cagliari, Italy.
“The star-forming region Cepheus A hosts a very young star, called HW2, that is the second closest to us, growing a dozen times more massive than our Sun,” the authors write. “The circumstellar environment surrounding HW2 has long been the subject of much debate about the presence or not of an accretion disk, whose existence is at the basis of our current paradigm of star formation.”
When a gas cloud collapses and a young star forms, a rotating disk of material called an accretion disk forms around the protostar. Material from the disk spirals into the star along magnetic field lines, and the star becomes more massive. Different mechanisms inhibit and limit the amount of material a star can accrete. Feedback mechanisms like stellar wind, jets, and outward radiation work against accretion, as do disk fragmentation and other factors.
This VLA image from previous research shows a pair of jets emitted by HW2. Not all of the material that spirals toward the star from the accretion disk is added to the star. Some is cast off in these jets. The images of the surrounding area are from the Hubble. Image Credit: Carrasco-Gonzalez et al. / Bill Saxton, NRAO, AUI & NSF / STScI.
Astronomers know that stars with dozens of solar masses or more rely on enormous reservoirs of material in their circumstellar disks to grow. These large disks can cover regions as wide as one parsec, or slightly more than three light-years. However, only the gas in the inner area of the disk, within a few hundred AU and generally referred to as the accretion disk, can accrete onto a young protostar. The barrier astronomers face is the difficulty in observing this region.
“Resolving the properties of the gas flow, as it streams in the inner hundreds of au from a very young star, has long been an observational challenge, especially for the most massive stars, which are found much further away from Earth than solar-type stars,” the authors write.
There’s no scientific consensus about what happens in the inner disk regarding massive stars. Mass infall rates would have to be extremely high for these stars. Also, an enormous gas reservoir with tens of solar masses must exist for a massive star to form. “Under these conditions, disk stability can be severely affected by, for instance, local fragmentation, tidal interactions with nearby cluster members and powerful stellar feedback, as suggested both theoretically and observationally,” the authors write.
HW2 has puzzled astronomers because they couldn’t reliably detect an accretion disk. In this work, the researchers focused on ammonia (NH3), common on Earth and in interstellar clouds. Ammonia spectral lines are valuable because ammonia transitions are excellent thermometers in cold, dense environments. Ammonia also requires high densities to be excited, making it a good way to trace density in star-forming regions and accretion disks. Its spectral lines are also good at showing gas moving toward a protostar.
Ammonia is also good at probing infall kinematics because it creates two distinct, asymmetrical peaks: blue-shifted and red-shifted.
This drawing from the research explains some of the findings. The VLA detected a column of NH3 gas infalling in front of a bright continuum emission region confined inside the inner 200 au. At larger radii, the VLA mapped emissions from NH3 gas, which is both infalling (vr) by gravity toward the central star and rotating (vϕ) at sub-Keplerian velocities. Atomic (ionized) gas is also ejected away along the general direction of the system’s angular momentum, with a lobe of blueshifted (approaching the observer) gas to the north and one of redshifted (receding) gas to the south. Image Credit: Sanna et al. 2025. Astronomy and Astrophysics.
Many ammonia transitions are visible in radio wavelengths, which penetrate dust well. VLA observations found a massive ring of ammonia around HW2 with radii between 200 and 700 astronomical units (AU). By observing the disk for ten hours in four separate epochs, the researchers found that the infall rate of gas onto HW2 is two thousandths of a solar mass per year, one of the highest rates ever observed.
“Our observations provide direct evidence that massive stars can form through disk-mediated accretion up to tens of solar masses,” lead author Sanna said in a press release. “The NSF VLA’s unparalleled radio sensitivity allowed us to resolve features on scales on the order of 100 AU only, offering unprecedented insights into this process.”
The researchers compared their observations with simulations of massive star formation and found agreement. “The results aligned closely with theoretical predictions, showing that ammonia gas near HW2 is collapsing almost at free-fall speeds while rotating at sub-Keplerian velocities—a balance dictated by gravity and centrifugal forces,” said Prof. André Oliva, a co-author who performed the detailed simulations.
The researchers found asymmetries in the structure of the disk, as well as turbulence. This suggests that external streams of gas called ‘streamers’ are delivering fresh gas to one side of the accretion disk. Astronomers have found these streamers around other star-formation sites and they could play an essential role in massive star formation by delivering fresh material.
This discovery ends years of speculation and questions about massive stars like HW2. It shows they can form the necessary accretion disk to fuel their massive and rapid growth.
“These results highlight the power of radio interferometry to probe the hidden processes behind the formation of the most influential objects in our Galaxy,” said Dr. Todd Hunter of the NRAO, “and, in ten years, the next upgraded version of the VLA will make it possible to study circumstellar ammonia at scales of our Solar system.”
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