ALMA finds evidence for two generations of stars in distant galaxy
New discoveries

ALMA finds evidence for two generations of stars in distant galaxy

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Suppose you use a telescope to study a village on a distant mountain slope. In the village, you only see young children. Apparently, the village has only recently become inhabited. But there’s a mystery. The graveyard of the village if pretty full! Now you start to wonder: maybe people have been living and dying there before!

Using ALMA, astronomers came across a similar mystery in the distant universe. They studied a very distant galaxy. Earlier observations had shown that the stars in the galaxy are just about four million years old. That’s very young for a star – comparable to a human baby. Apparently, star formation started only recently in the galaxy.

But here’s the mystery: ALMA detected huge amounts of dust in the galaxy. And dust is produced by dying stars – dust particles are the mortal remains of stars that have lived out their lives. In other words: the graveyard of the galaxy is pretty full.

It’s hard to imagine how the young stars in the galaxy could ever have produced so much dust. Instead, the astronomers began to wonder: maybe there has been an earlier generation of stars that lived and died long before the current generation of young stars were born.

It’s a surprising find, because the galaxy is so far away. The galaxy’s light took 13.2 billion years to reach us. Since the universe is only some 13.8 billion years old, we see the galaxy as it was 600 million years after the Big Bang.

If there was an even earlier generation of stars, they must have formed a mere 300 million years after the Big Bang, and lived for about 100 million years.

In the future, astronomers hope to study more distant galaxies, to see if they contain large amounts of dust, too. Eventually, the hope is to unravel origin of the very first stars in the history of the universe.


The distant galaxy studied by ALMA is known as MACS0416_Y1. It is located in the large constellation of Eridanus, the River, at such a large distance that light from the galaxy has taken 13.2 billion years to reach us. The galaxy has been studied before by the Hubble Space Telescope, the Spitzer Space Telescope, and the Very Large Telescope of the European Southern Observatory in Chile. These earlier observations revealed that the stars in the galaxy are quite young: about four million years. However, ALMA discovered large amounts of dust that were likely produced by an even earlier generation of stars.


The ALMA observations of MACS0416_Y1 were carried out by a large international team of astronomers, led by Yoichi Tamura of Nagoya University in Japan. Yoichi worked together with colleagues from Japan, Sweden, Denmark, and the United Kingdom. In addition to the dust emission, they detected submillimeter radiation from oxygen atoms. From the ALMA observations, they deduced that the galaxy must contain huge amounts of dust. The team published their results in March 2019 in The Astrophysical Journal.