A possible massive-star embryo has an intriguing combination of features that includes an exceptionally compact center, a vast surrounding reservoir of cold gas and dust, and no convincing signs of energetic outflows.
Nicknamed the ‘dragon egg,’ this object could offer a rare look at how massive stars begin forming. The researchers describe it as “a promising compact high-mass core candidate” in their study.
However, astronomers still cannot tell how much mass is concentrated in its tiny core—or whether a protostar has already formed. This makes the object particularly interesting because scientists still debate how stars with at least eight times the Sun’s mass take shape.
One theory proposes that a massive core assembles first and collapses into a star. Another suggests that a smaller seed grows by drawing in material from the surrounding cloud.
By revealing the structure of a possible early-stage object, the dragon egg could help researchers investigate these competing ideas.
ALMA reveals a compact center
The dragon egg, designated C2c1a, lies in an infrared-dark cloud called Dragon Cloud, about 16,300 light-years from Earth. Its parent clump contains around 1,061 solar masses but emits little energy relative to its mass, a sign that it may be at an early stage of massive-star formation.
Earlier observations suggested that C2c1a contained 20 to 30 solar masses, but they could not establish how tightly that mass was concentrated or whether a protostar had formed.
To investigate, a team led by Ashley Thomas Barnes of the European Southern Observatory used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the region at a wavelength of 1.3 millimeters.
The new observations resolved six components: the brightest source, C2c1a(a), another prominent component, C2c1b, and four fainter structures. Unlike the surrounding region, C2c1a(a) remained a single compact source, with an estimated radius of about 87 astronomical units (AU).
One AU is the average distance between Earth and the Sun. No smaller fragments were detected within it at the observations’ resolution limit of roughly 175 AU. “The region resolves into multiple fragments, but the brightest component, C2c1a(a), remains a single compact continuum peak down to ∼ 175 au,” the study authors added.
The compact source had a brightness temperature of about 40 kelvin, compared with a gas temperature of roughly 10 kelvin inferred from earlier observations. This difference points to a strong temperature gradient, with a warmer inner region embedded in colder material.
Yet the compact source sits inside a much larger reservoir containing about 110 solar masses of gas and dust across scales of roughly 10,000 AU. Some of this material could eventually feed the developing object, although the observations do not establish how much will do so.
The missing signs of a young star
The researchers searched for emissions from hot gas that might indicate a developing star, as well as outflows—streams of material often driven away from young stars. They found no strong signals of hot gas at the center and no convincing high-velocity outflow associated with the compact source.
“A weak, deeply embedded, or unfavorably oriented outflow cannot be excluded, but the present data reveal no clear outflow signature from the compact source,” the study authors said.
Some faint molecular emissions were present, but they were spread across the region rather than concentrated at the center. The source could be cold, or thick dust could be obscuring warmer material inside.
The absence of a clear outflow also does not rule out a young star: a weak, deeply embedded or unfavorably oriented outflow could escape detection.
The core’s mass remains uncertain, too. Depending on assumptions about the dust temperature and how it emits radiation, estimates range from roughly two to 30 solar masses. It could be a massive, cold core or a warmer, less massive object surrounded by a much larger reservoir.
A promising clue, not a verdict
The compact structure and lack of clear stellar activity are consistent with models of star formation in which a massive core forms before collapsing into a star. However, the observations cannot rule out a smaller seed that grows by gathering material from its surroundings.
More detailed measurements of the dust temperature, internal structure and gas motions are needed to determine what the dragon egg contains.
For now, ALMA has exposed a remarkably compact source inside a massive cold reservoir—but the crucial question of how its central object will grow remains unanswered.
The study is published in the journal arXiv.