Uncloaking a Dark Molecular Cloud
An overdense region of the interstellar medium will gradually condense under its own gravity. If it’s massive enough, its eventual collapse begets stars and planets. Before it reaches that point, the cloud becomes so cold and dense that its constituent atoms form molecules. Unfortunately for astronomers, those conditions render the most-abundant molecule, hydrogen (H2), invisible, as too little ambient energy is available to excite the molecule’s rotational and vibrational transitions. However, the cloud’s margins, buffeted by their hotter surroundings, can become warm enough that H2 fluoresces in the far ultraviolet (FUV). Now a team led by Blakesley Burkhart of Rutgers University, New Jersey, and Thavisha Dharmawardena of New York University has found and characterized a nearby molecular cloud in an FUV survey of the Milky Way [1]. Dubbed Eos, the cloud is bathed in so much FUV that it could be fated to evaporate before it collapses. Even so, Eos is providing new insights into how previous rounds of star formation influence current rounds.
Launched in 2003, South Korea’s STSat-1 orbiter mapped the Milky Way in the FUV by scanning the sky once a day in 1°-wide strips. Looking at the archived map, Burkhart noticed a fuzzy patch high above the Galactic plane. The FUV emission suggested H2 fluorescence from a molecular cloud, yet most ongoing star formation takes place in the Galactic plane. Each round of star and planet formation leaves behind dust, which ends up in molecular clouds. To help identify the fuzzy patch she’d found, Burkhart turned to Dharmawardena, who creates 3D maps of Galactic dust. Together they found that the fuzzy patch of H2 coincided with a fuzzy patch of dust.
From Dharmawardena’s dust maps, the researchers determined that Eos is 307 light years from the Sun. The distance places it just outside the “Local Bubble,” a volume 1000 light years across that encompasses the Sun. Supernovae, the death throes of a previous generation of stars, expelled gas and dust from the Local Bubble leaving it devoid of material to make new stars.
Eos also appears close in the sky to the North Polar Spur, a giant arc of glowing plasma that rises high above the Galactic plane. Although the distance and origin of the spur are uncertain, evidence suggests that Eos and one part of the spur, Loop I, are neighbors. The spur is likely made up of gas and dust swept up by the expanding shock waves of earlier supernovae. Eos fits into an emerging picture in which star formation takes place in regions seeded and plowed by previous generations of stars.
However, there’s a twist. Burkhart, Dharmawardena, and their collaborators estimated that the mass of Eos is now below the threshold for gravitational collapse. What’s more, their calculations suggest that x-ray radiation from Loop I is intense enough to photodissociate the H2 molecules in Eos, which could cause the cloud to evaporate in 5.7 million years. As to whether a larger Eos might once have birthed stars, the answer appears to be no. Researchers looked at the positions and velocities of nearby stars determined by the Gaia spacecraft. They found no unusual spatial or kinematic clustering of stars near Eos that might suggest a parental association [2].
Although Eos has neither formed stars nor seems likely too, its discovery has implications for studying and understanding star formation. Astronomers typically map molecular clouds using the next-most-abundant molecule to H2, carbon monoxide (CO). Burkhart, Dharmawardena, and their collaborators found that Eos is almost completely dark in CO emission. Astronomer Michael Busch of the University of California, San Diego, observes molecular clouds via emission from their hydroxide (OH) molecules. “This discovery is remarkable due to the fact that it is using H2 emission directly to map a molecular cloud,” he says.
Unfortunately, although FUV emission from H2 is efficient, the photons are readily absorbed by the interstellar medium. A search for other molecular clouds in the STSat-1 data will likely be confined to a few kiloparsecs (kpc) around the Sun (the Milky Way is 30 kpc across). Still, Dharmawardena points out that theory predicts about 50% of molecular clouds could be CO dark. “There should be an abundance of these clouds waiting to be discovered,” she says.
But perhaps Eos’s biggest implication concerns the efficiency of star formation. Eos appears to be evaporating faster than the local star-formation rate, which is consistent with the known star-formation inefficiency of the Milky Way. “It seems to be quite hard for the Galaxy to form stars out of all of its molecular gas,” says Busch.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- B. Burkhart et al., “A nearby dark molecular cloud in the Local Bubble revealed via H2 fluorescence,” Nat. Astron. (2025).
- S. Saxena et al., “Searching for star formation towards the Eos molecular cloud,” Mon. Not. R. Astron. Soc.: Lett. slaf044 (2025).