A Hint from the Webb Telescope that the First Stars Were Massive

“How did the first stars and galaxies form?” was the title of my first book, published in 2010 in anticipation of the launch of the Webb telescope — which took another dozen years to materialize. Together with my follow-up textbook “The First Galaxies in the Universe”, published in 2013 with my former student Steve Furlanetto — currently a tenured professor at UCLA, these books framed theoretical expectations for the scientific details of the verse in Genesis 1:3 — “Let there be Light.” Our local source of natural light is the Sun. The biblical story relates the appearance of light to the first day of the Universe.

But extensive data on the cosmic microwave background — left over from the Big Bang, implies that the temperature of the cosmic soup of elementary particles was higher than the core temperature of the Sun, 15 million degrees, during the first day after the Big Bang. The Universe was hotter and denser than stars in the first day after the Big Bang, and so starlight must have formed later.

As the Universe expanded, it cooled to temperatures below the surface temperature of the Sun, 5,800 degrees Kelvin, a quarter of a million years after the Big Bang. The first galaxies with less than a millionth of the mass of the Milky Way condensed about a hundred million years later and gave birth to the first stars.

The primordial gas contained mostly hydrogen with a trace amount of helium. Cooling by molecular hydrogen was inefficient and resulted in fragmentation into stars that are at least ten times more massive than the Sun. The theoretically predicted population of the first stars with initially no heavy elements, is labeled “Population III stars”, to differentiate it from the familiar stellar Populations I and II which formed later and are known to contain heavy elements. What are the observable characteristics of the first stars?

In 2001, I wrote a paper forecasting the radiation spectrum from the first stars, in collaboration with my former postdoc, Volker Bromm, who currently chairs the Astronomy department in the University of Texas at Austin, and with Rolf Kudritzki, then the director of the Institute for Astronomy at the University of Hawaii. We predicted that the surface temperature of massive Population III stars is about a hundred thousand degrees, making them copious factories of ultraviolet radiation.

The emitted ultraviolet light was capable of breaking the cosmic hydrogen atoms into their constituent electrons and protons in a process called “reionization”. The bubbles of broken hydrogen grew over time and eventually overlapped a billion years after the Big Bang. A new generation of radio observatories aims to map the reionization history by using the spectral line of hydrogen at a wavelength of 21-centimeter, as reviewed in a paper I wrote in 2011 with my former postdoc, Jonathan Pritchard, currently a tenured professor at Imperial College London. The 21-centimeter line was predicted theoretically in a 1945 paper by H. C. van de Hulst and discovered in 1951 in the Milky Way galaxy by the Nobel Laureate Ed Purcell and his graduate student Doc Ewen, using a horn antenna placed through an office window in the Lyman Laboratory of Physics at Harvard University.

The heavy elements produced by nuclear fusion in the interiors of the first stars were later dispersed into the surrounding gas by explosions known as “pair-instability supernovae”. A few million years after formation, the cores of the first massive stars consumed their nuclear fuel and contracted, reaching temperatures high enough to produce electron-positron pairs. This led to catastrophic explosions with no remnants left behind. The heavy elements were then dispersed into the surrounding gas that made subsequent generations of stars.

Rocky planets could not have existed around the first stars because of the lack of heavy elements. However, they could have formed near the second generation of stars that were made from the material enriched by the first supernovae. The delay was merely a few million years, insignificant relative to the age of the Universe at that time. The cosmic microwave background warmed the surface of the first planets to about a hundred degrees above absolute zero, between the surface temperatures of Saturn and Uranus, irrespective of how far the early planets were from their host star. This temperature is still nearly two hundred degrees below the freezing temperature of water, and so extra warming by starlight was needed to enable liquid water and the chemistry of the first life in the Universe, as I discussed in a 2016 review paper. The biological clock of Earth would have resulted only in microbes at these early times, with no astronomers to witness the initial cosmic fireworks.

Is there any hint for these theoretical expectations regarding the first stars in the new data from the Webb telescope? Remarkably, the first deep images revealed galaxies at redshifts 9–15, which formed their stars hundreds of millions of years after the Big Bang (see the papers here and here), and are far brighter than expected from the familiar populations of stars within the earliest galaxies. The data is consistent with the possibility that massive bright stars are abundant in these galaxies, as expected theoretically for Population III stars.

The colors of the farthest galaxies detected by the Webb telescope indicate that their stellar population is particularly devoid of heavy elements and could contain Population III stars. Future spectroscopic observations from Webb and ground-based telescopes will tell whether the effective temperature of the farthest stars is indeed close to a hundred thousand degrees, nearly twenty times hotter than the surface of the Sun. Such an inference would imply that the first galaxies were indeed full of Population III stars as forecasted theoretically in past decades.

My biggest reward in the coming years would be to witness our theoretical predictions validated by the Webb telescope. As Henry David Thoreau noted in his book Walden: “Heaven is under our feet as well as over our head.”


Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, is scheduled for publication in June 2023.



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Avi Loeb

Avi Loeb


Avi Loeb is the Frank B. Baird Jr Professor of Science and Institute director at Harvard University and is the bestselling author of “Extraterrestrial”.