Astronomers have peered with unprecedented precision into a cosmic “dragon's egg“, which could indicate one of the earliest stages of the birth of a massive star. The object is located about 16,300 light-years from Earth in a dark and cold region of the Milky Way known as the Dragon Cloud.
Using the ALMA radio telescopes in the Atacama Desert, an international team led by astronomer Ashley Thomas Barnes of the European Southern Observatory has studied the nucleus C2c1a with extremely high resolution. The new observations reach scales of about 175 to 400 astronomical units and allow scientists to literally “crack the egg“, as the title of their scientific paper says.
It turns out that the region is more complex than it seems in earlier observations. ALMA distinguishes six components, but the brightest of them, C2c1a(a), remains a compact source even at the highest resolution achieved. Its radius is estimated to be only about 87 astronomical units. For comparison, one astronomical unit is the average distance between the Earth and the Sun.
Exactly how much mass is concentrated in this “egg“, however, remains unclear. Depending on the temperature of the dust, estimates range from about 2 to 30 solar masses. This is a significant unknown, because colder material requires more mass to produce the observed radiation. The density in the interior is extreme, estimated to exceed one billion hydrogen molecules per cubic centimeter.
Even more interesting is what lies around the compact center. C2c1a(a) is immersed in a vast cold reservoir of gas and dust with a mass of about 110 solar masses in a region roughly 10,000 astronomical units in size. It is from this material that the future star could gradually accumulate a significant part of its final mass.
A star that may not yet have been born
Scientists have been looking for signs that a protostar already exists in the center, but the results have been ambiguous. They have not detected the strong molecular signals characteristic of the hot cores around young massive stars. The team has not conclusively identified a high-speed gas flow that can be unambiguously associated with C2c1a(a). Such streams are usually an important sign that a protostar is already actively consuming matter.
At the same time, the center is not completely “dead”. Its brightness temperature reaches about 40 kelvins, significantly higher than the approximately 10 kelvins measured at lower resolution in the surrounding gas. This suggests a strong temperature gradient and leaves open the possibility that an extremely young protostar is already forming in the center itself.
Therefore, astronomers do not classify the find as a newborn massive star. The more cautious conclusion is that they are observing an extremely compact core, which may represent the earliest phase of the formation of a central protostellar “nucleus“.
This makes the “dragon egg“ particularly valuable for one of the great unsolved questions in stellar astronomy: how are the most massive stars born?
In one of the main models, a large and dense protostellar core first forms, which subsequently collapses to create a massive star. Competing scenarios suggest that the initial nucleus may be relatively small and increase in mass by continuously attracting material from the much larger environment around it.
Finding convincing massive protostellar cores has proven surprisingly difficult. Some of the most promising candidates subsequently show signs of already containing protostars, while others have masses that depend heavily on the temperature estimates used. C2c1a is part of this ongoing scientific debate.
The object has been known to astronomers for years. Observations published in 2023 split the larger C2c1 structure into two main cores, C2c1a and C2c1b, and showed that the more massive C2c1a does not have obvious signs of active star formation. At the time, it was identified as one of the promising candidates for a massive protostellar core.
New observations delve even deeper into its structure. And while the “dragon egg“ still does not provide a definitive answer to how stellar giants are born, it offers astronomers a rare opportunity to observe the process almost from its very beginning.
The scientific paper Cracking open the “dragon egg”: ALMA 1.3mm observations of a compact, young high-mass core candidate at 30mas resolution has been accepted for publication in the journal Astronomy & Astrophysics.