How heavy are the stars?

R136a1 is currently the heaviest known star. It weighs as much as 265 suns. Most stars, however, are much smaller and lighter – down to about one tenth of the mass of the sun. Celestial bodies that have accumulated too little gas cannot ignite nuclear fusion and remain brown dwarfs. Supergiants like R136a1, on the other hand, glow intensely but die young. How heavy are the stars of the universe on average? This is described by the so-called primordial mass function. It says that heavy stars are much rarer than light ones. So the universe is likely to be dominated by lightweights. This is important for our understanding of the evolution of galaxies.

However, the original mass function was established primarily based on measurements from our cosmic environment. Stars in other galaxies can only be observed when they are particularly bright, so there would be a discrepancy here. The assumption is therefore that the Milky Way already will not be such a special galaxy and therefore the conditions here can be transferred well to the whole universe. But in this, astronomers seem to have been wrong so far, according to a new study published in The Astrophysical Journal by a team of researchers at the University of Copenhagen’s Niels Bohr Institute.

Using observations from 140,000 galaxies across the universe and a wide range of advanced models, the team tested whether the same distribution of stars seen in the Milky Way holds elsewhere. The answer is no. Stars in distant galaxies tend to be more massive than those in our local neighborhood. “The mass of stars tells us astronomers a lot. If you change the mass, you change the number of supernovae and black holes that form from massive stars. So our result means we have to revise many of our previous assumptions, because distant galaxies look very different from our own,” said Albert Sneppen, a doctoral student at the Niels Bohr Institute and lead author of the study.

According to the researchers, the new discovery will have a variety of implications. For example, it remains unclear why galaxies die and stop forming new stars. The new finding suggests that this could be explained by a simple trend. “Now that we are better able to decipher the mass of stars, we can see a new pattern: The less massive galaxies continue to form stars, while the more massive galaxies stop giving birth to new stars. This suggests a remarkably universal trend in the death of galaxies,” Sneppen says.

Artist’s impression of R136a1 (Image: Sephirohq, CC-BY 3.0).


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  • BrandonQMorris
  • Brandon Q. Morris is a physicist and space specialist. He has long been concerned with space issues, both professionally and privately and while he wanted to become an astronaut, he had to stay on Earth for a variety of reasons. He is particularly fascinated by the “what if” and through his books he aims to share compelling hard science fiction stories that could actually happen, and someday may happen. Morris is the author of several best-selling science fiction novels, including The Enceladus Series.

    Brandon is a proud member of the Science Fiction and Fantasy Writers of America and of the Mars Society.