Some Massive Stars Aren’t Blowing as Hard as Astronomers Expected. That Could Change Our Picture of the Early Universe

Galaxies in the early universe are weird. These stars may have some answers.

Massive stars can have tens or even hundreds of times more mass than the Sun and burn furiously hot, blasting material into space through powerful solar winds.

When stars get this big, they tend to have short lives and explode as supernovae. And when this happens, they can reshape entire neighborhoods of their galaxies. But some of the most chemically primitive massive stars appear to be blowing considerably less hard than astronomers expected.

Using the Hubble Space Telescope, researchers examined 29 massive O-type stars (extremely hot and bright) in six nearby galaxies. These galaxies contain very little of the heavier elements that help stars produce powerful stellar winds.

That part was expected. But researchers also found something surprising: in stars with less than about one-tenth of the Sun’s abundance of heavier elements, the winds appeared to weaken much more sharply than expected. This surprising fact could help us understand some of the processes taking place in the earlier times of the universe.

Looking nearby to understand the early universe

The James Webb Telescope revealed some of the oldest stars in the universe. But astronomers can’t examine individual stars in most of these galaxies directly as they’re simply too distant.

“Webb opened up a whole bunch of new questions about the evolution of these early galaxies—they’re weird,” said Grace Telford, assistant professor in the Department of Physics & Astronomy at the University of Utah and lead author of the study.

Instead, Telford and colleagues looked for nearby environments that resemble the young universe chemically.

Their program, called TEMPOS (the Treasury of Extremely Metal-Poor O Stars) targets O-type stars in small dwarf galaxies with less than about one-fifth of the Sun’s metallicity.

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In astronomy, metallicity refers to the abundance of elements heavier than hydrogen and helium, such as carbon, oxygen and iron. For example, the Sun is made up of roughly 1.3% heavy elements by mass. A metallicity of one-fifth solar corresponds to approximately 0.26% heavy elements by mass, assuming the same relative mixture of elements.

The stars Telford and colleagues studied are enormous, hot, and extremely luminous stars.

“They burn very hot, bright and fast and they end their short lives as supernova explosions that deposit a lot of energy and material into the surrounding gas,” said Telford. “They govern the evolution of their host galaxies by heating and essentially regulating the gas that’s then available to cool and form into new stars.”

But they also lose some material before exploding.

Why metals help a star blow up

Stars gradually lose some of their material by blowing gas into space. These streams of gas are called stellar winds and they determine how much mass a star loses during its lifetime.

This process is strongly influenced by the “metals” inside the star.

Radiation streaming outward from a hot star interacts with metal ions in its atmosphere. Those ions absorb some of the radiation’s momentum and help drag matter outward, accelerating gas away from the stellar surface.

So astronomers already expected stars with fewer metals to have weaker winds.

Across stars spanning roughly 5% to 50% of the Sun’s metallicity, the overall trend was clear: lower metallicity corresponded to lower maximum wind speeds. But the study reports tentative evidence that the decline becomes considerably steeper below around 10% solar metallicity.

It’s a relatively small survey with only a handful of observed stars, but individual stars outside the Milky Way are very faint and require many hours of observation. This is basically the best dataset of its type.

“It’s a sample of 29 stars, which doesn’t sound like a lot, but when each one costs up to 35 hours of Hubble time to observe, it gets really expensive,” Telford said.

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So what does this mean?

A weaker wind means a massive star may hold onto more of its material throughout its life, which could translate into a more explosive ending. It could make massive stars more likely to turn into black holes, although this paper doesn’t analyze all the plausible consequences.

But this matters a lot now because JWST has shown us that the early universe is stranger than many astronomers expected.

JWST observations have shown that some young galaxies were unexpectedly bright and chemically unusual, forcing researchers to reconsider how the stars in these galaxies formed and evolved.

TEMPOS isn’t a perfect solution. The stars it observed are nearby analogues, not surviving stars from the dawn of the universe. But they provide something astronomers badly need: direct measurements under chemical conditions closer to those that prevailed billions of years ago.

The team is now combining Hubble and Keck observations to better understand how these stars lose mass. Ultimately, if they pan out, these findings could reshape our picture of the early universe.

And if the universe’s earliest massive stars kept substantially more of their mass until death, astronomers may have to rethink not only how those stars lived, but how they helped build the first generations of galaxies.

The study was published in The Astrophysical Journal Supplement Series.