Stars and planets are born inside swirling clouds of cosmic gas and dust that are brimming with hydrogen and other molecular ingredients. On Monday, astronomers revealed the discovery of the closest known cloud to Earth, a colossal, crescent-shaped blob of star-forming potential.

Named Eos, after the Greek goddess of the dawn, the cloud was found lurking some 300 light-years from our solar system and is as wide as 40 of Earth’s moon lined up across the sky. According to Blakesley Burkhart, an astrophysicist at Rutgers University, it is the first molecular cloud to be detected using the fluorescent nature of hydrogen.

“If you were to see this cloud on the sky, it’s enormous,” said Dr. Burkhart, who announced the discovery with colleagues in the journal Nature Astronomy. And “it is literally glowing in the dark,” she added.

Identifying and studying clouds like Eos, particularly based on their hydrogen content, could reshape astronomers’ understanding of how much material in our galaxy is available to produce planets and stars. It will also help them measure the creation and destruction rates of the fuel that can drive such formations.

“We are, for the first time, seeing this previously hidden reservoir of hydrogen that can form stars,” said Thavisha Dharmawardena, an astronomer at New York University who is an author of the study. After Eos, she said, astronomers are “hoping to find many more” such hydrogen-heavy clouds.

Molecular hydrogen, which consists of two hydrogen atoms bound together, is the most abundant material in the universe. Stellar nurseries are chock-full of it. But it is difficult to detect the molecule from the ground because it glows in far-ultraviolet wavelengths that are readily absorbed by the Earth’s atmosphere.

Easier to spot is carbon monoxide, a molecule made up of one carbon atom and one oxygen atom. Carbon monoxide radiates light in longer wavelengths that can be detected by radio observatories on Earth’s surface, a more conventional technique for identifying star-forming clouds.

Eos, as immense as it is, evaded detection for so long because it contains so little carbon monoxide.

Dr. Burkhart noticed the cloud while studying data that was about 20 years old from the Far-Ultraviolet Imaging Spectrograph, or FIMS, an instrument aboard a Korean space satellite. She spotted a structure in the molecular hydrogen data in a region of space where she believed no molecular clouds were present, and then teamed up with Dr. Dharmawardena to investigate further.

“At this point, I had known pretty much all the molecular clouds by name,” Dr. Dharmawardena said. “This structure, I didn’t know at all. I couldn’t place it.”

Dr. Dharmawardena cross-checked the find with three-dimensional maps of the interstellar dust between stars in our galaxy. Those maps were built with data from the recently retired Gaia space telescope. Eos “was very clearly outlined and visible,” she said. “It’s this gorgeous structure.”

John Black, an astronomer at Chalmers University of Technology in Sweden who was not involved in the work, commended the technique used to reveal Eos.

“It’s really wonderful to be able to see the molecular hydrogen directly, to trace out the outlines of this cloud,” Dr. Black said. Compared with carbon monoxide, the hydrogen shows “a truer picture of the shape and size” of Eos, he added.

Using the molecular hydrogen content, the astronomers estimated the mass of Eos to be about 3,400 times that of our sun. That is much higher than the estimate computed from the amount of carbon monoxide present in the cloud — as little as 20 times the mass of our sun.

Similar measurements of carbon monoxide could very well be underestimating the mass of other molecular clouds, Dr. Burkhart said. That has important implications for star formation, she added, because bigger clouds form more massive stars.

A follow-up study of Eos, which has not yet been peer-reviewed, found that the cloud had not formed stars in the past. But the question remains whether it will begin to churn out stars in the future.

Dr. Burkhart is working with a team of astronomers to conceptualize a NASA spacecraft called Eos, which also inspired the name of the newly discovered cloud. The proposed space telescope would be able to map the molecular hydrogen content of clouds across the galaxy, including its namesake.

Perhaps such a mission would find more hidden clouds or revise knowledge about the ability of known stellar mists to coalesce their material into stars and planets.

“We don’t really know how stars and planets form,” Dr. Burkhart said. “If we’re able to look at molecular hydrogen directly, we’re able to tell how the birthplaces of stars are forming — and also how they’re being destroyed.”

Federal health officials have reversed the decision to fire a few dozen scientists at the Food and Drug Administration’s food-safety labs, and say they are conducting a review to determine if other critical posts were cut.

A spokesman for the Department of Health and Human Services confirmed the rehirings and said that several employees would also be restored to the offices that deal with Freedom of Information requests, an area that was nearly wiped out.

In the last few months, roughly 3,500 F.D.A. jobs, about 20 percent, were eliminated, representing one of the largest work force reductions among all government agencies targeted by the Trump administration.

The H.H.S. spokesman said those employees called back had been inadvertently fired because of inaccurate job classification codes.

The decision to rehire specialists on outbreaks of food-related illnesses and those who study the safety of products like infant formula follows contradictory assertions made by Dr. Marty Makary, the F.D.A. commissioner, in media interviews this week.

“I can tell you there were no cuts to scientists or inspectors,” Dr. Makary said Wednesday on CNN.

In fact, scientists had been fired from several food and drug safety labs across the country, including in Puerto Rico, and from the veterinary division where bird flu safety work was underway. Scientists in the tobacco division who were dismissed in February — including some who studied the health effects of e-cigarettes — remain on paid leave and have not been tapped to return, according to employees who were put on leave.

How many fired employees will be permitted to return remained unclear.

About 40 employees at the Moffett Lab in Chicago and at a San Francisco-area lab are being offered their jobs back, the department spokesman said. Scientists in those labs studied a variety of aspects of food safety, from how chemicals and germs pass through food packaging to methods for keeping bacteria out of infant formula. Some scientists in Chicago reviewed the work and results of other labs to ensure that milk and seafood were safe.

Dr. Robert Califf, the F.D.A. commissioner under President Joseph R. Biden, said the terms “decapitated and eviscerated” seemed fitting to describe the steep loss of expertise at the agency. He said the F.D.A. was already falling behind on meetings meant to help companies develop safe products — and to design studies that give clear answers about their effectiveness.

“Most of it is really at this level of fundamental, day-to-day work that has a huge impact overall, but it’s not very controversial,” he said. “It’s just that it takes work, and they have to have people to do the work.”

Dr. Makary has also said the layoffs did not target product reviewers or inspectors. But their work has been hampered by voluntary departures, the elimination of support staff and the broader disruption at an agency where many are fleeing for the exits, according to former staff members.

Hundreds of drug and medical device reviewers, who make up about one-fourth of the agency work force, have recused themselves from key projects, Dr. Scott Gottlieb, a former agency commissioner, said on CNBC. Under F.D.A. ethics rules, staff members who are interviewing for jobs cannot do agency review work on products by companies where they are seeking employment — or for a competitor.

Dr. Gottlieb also said cuts to the office of generic drug policy wiped out employees with expertise in determining which brand-name drugs are eligible to be made as lower-cost generics, calling those job eliminations “profound.” Approving generic drugs can save consumers billions of dollars.

Support staff for inspectors investigating food and drug plants overseas were also cut, raising security concerns. Dozens of workers who lost their jobs attended to security monitoring to ensure that inspectors were safe, especially in hostile nations.