NASA’s Hubble Space Telescope unveiled in stunning detail this small section of the expanding remains of a massive star that exploded about 8,000 years ago, now called the Veil Nebula, February 17, 2024.
| Photo Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Astronomers may be closer to solving one of the most intriguing mysteries in science: how did water originate in the universe?
A study published in the journal Nature Astronomy on March 3 suggested that the universe’s oldest stars became founts of water as their nuclear fires were extinguished in massive explosions called supernovae.
If this finding is borne out in further research, astronomers will have to revise current theories to factor in potentially life-bearing planets being born billions of years earlier than previously thought.
The first stars
Water is the third most abundant molecule in the universe, after hydrogen and carbon monoxide, but its origins have remained obscure.
Billions of years ago, all matter and energy existed as an extremely small fleck in the void. This incredibly dense blob exploded some 13.8 billion years ago with a Big Bang to create the known universe.
The first stars were born just a few hundred million years after the Big Bang, at a time when all visible matter comprised primordial hydrogen and helium atoms. The nuclear furnaces at the cores of these early stars were powered by hydrogen, and as they shone, they heated up the surrounding intergalactic gas and dust. Hundreds of millions of years later, when the stars ran out of hydrogen to burn, they blew up as supernovae.
But by then their heat had ionised the interstellar medium around them, which set the stage for the formation of new stars and triggered a cycle of star births in perpetuity.
The third population
The longevity of a star depends on its mass. More massive stars die faster as more mass means more heat, and the hotter a star becomes, the faster its nuclear fuel is exhausted. Temperatures of millions of degrees and high densities inside a star force four hydrogen atoms to fuse into a helium atom, releasing enormous amounts of energy. Scientists have calculated that 0.7% of the mass is converted into energy, summed up by Einstein’s mass-energy equation.
Based on their age and metallicity (i.e. proportion of any element other than hydrogen and helium), astronomers divide stars into three groups. Population I stars, like the sun, are the youngest and are the most metal-rich, while population II stars are older and are less metallic.
The universe’s oldest stars form population III: massive stars composed completely of hydrogen and helium. These forerunners, the researchers of the new study have surmised, were the stellar nurseries where water must have first appeared in the cosmos.
The right conditions
Astronomer D.H. Whalen of the University of Portsmouth, England, who led the new study, said his team ran 3D simulations of population III supernovae looking for the signatures of water. They found that the conditions required to create water existed at around the same time when those first supernovae lit up the cosmos: sometime between 50 million and 1 billion years after the Big Bang.
Gigantic stars, hundreds of times more massive than our sun and with short lifespans, provided these conditions when they exploded, leaving behind hydrogen, oxygen, and other elements as their stellar remnants.
According to Whalen, the oxygen produced in these supernovae combined with hydrogen to create water, which is crucial for forming the elements necessary for life (as we know it).
The earliest stars couldn’t have possibly engendered water in the universe before they became supernovae, however. “The supernovae have to expel oxygen, which only forms during late stages of nuclear burning in massive stars that are destined to explode.”
Water in an infant universe
It may be a while before astronomers redraw their theories on the origins of water in the cosmos. “The water formation happens after the supernovae throw out most of the stellar material,” K.C. Sarkar, an associate professor of astronomy and astrophysics at the Raman Research Institute in Bengaluru said in an email interview.
“Astronomers already had an idea that the massive, metal-poor stars generated a lot of oxygen and that this oxygen would later combine with hydrogen to produce water in the universe. The current paper shows that the generation of water in early galaxies could be more efficient than [in] today’s galaxies.”
Scientists believed for decades that only traces of water were present in the early cosmos and that it became more common when newer, bigger stars exploded, yielding more of the heavier elements to an evolving universe. But the latest findings indicate that the first supernovae themselves produced enough water to drench the infant universe.
This would mean planets, a crucial refuge for water molecules, could have formed even before the first galaxies were born, and that there could have been enough water and other elements in the interstellar molecular clouds to kickstart life. If so, this pushes the timeline for potential life to have arisen in the universe way back.
Remain unchanged
There are concerns that the model used by the University of Portsmouth researchers was based on the use of indirect methods, like numerical experiments, to study population III stars. These stars are so distant that it is nearly impossible to ‘see’ them even with the most sophisticated telescopes.
However, Whalen said this challenge didn’t affect the accuracy of the study. “The important thing is to capture how ionising UV radiation from the stars heats and drives away ambient gas over their lifetimes. We have those from stellar atmosphere and evolution models that are well established in the field.”
The findings validate previous research that has shown that at least some of the earth’s water was delivered by comets early in the planet’s history. They also confirm that water molecules remain unchanged from their interstellar origins as they reach planets elsewhere in the universe.
Prakash Chandra is a science writer.
Published – March 26, 2025 05:30 am IST
