These days, most people live in cities. Little wonder then that most people are also born in cities. But if you want to know where exactly in those cities most people were born, it’s not so easy to find out.

With stars, it’s the same. Almost all stars are part of a galaxy. (For instance, our own Sun is part of our Milky Way galaxy.) So astronomers know that stars are also born in galaxies. But where exactly?

Today, new stars are mainly born in relatively small regions. Even for active star-forming galaxies (galaxies in which many new stars are born), the stars do not form all over the galaxy. Instead, star-birth occurs in certain regions. It’s as if some neighborhoods of a big city have a much larger population of newborn babies than others.

Using ALMA, astronomers have now discovered that the situation was very different when the Universe was young. Some ten billion years ago, when the birth rate of stars was at its peak level, stars were born all over their host galaxies.

By looking at very distant galaxies with the Hubble Space Telescope, astronomers also look back in time. After all, light from those galaxies has taken billions of years to reach Earth, so we see them as they were billions of years ago.

With the Very Large Array radio telescope in the United States, astronomers already had discovered which of those distant, early galaxies produce a lot of new stars. But since the galaxies also contain a lot of dust, it wasn’t possible to see where exactly the new stars are born.

However, the millimeter and submillimeter radiation observed by ALMA is not absorbed by dust. It is produced by large clouds of cold gas from which new stars are born. With its ultra-sharp vision, ALMA was able to see where those star-forming clouds are located in the distant galaxies: not in just a few regions, but all over the place.

Hopefully, the new result will help astronomers to better understand the history of the Universe, especially during the time when most of the current stars were born.



The Very Large Array radio telescope in the United States and the ALMA observatory in Chile carried out detailed observations of galaxies in the Hubble Ultra Deep Field – a region of sky studied by the Hubble Space Telescope in 2003. Most of the galaxies in this region are between 9 and 11 billion light-years away. As a result, astronomers see those galaxies as they were 9 to 11 billion years ago. Because of their large distance, the radio waves from the galaxies are extremely weak: a cell phone signal from Pluto would be easier to pick up here on Earth.


The study of the distant galaxies was led by Wiphu Rujopakam. Wiphu is an astronomer at the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo. He also works at Chulalongkorn University in Bangkok, Thailand. He worked together with a large team of colleagues from Japan, Europe, the United States and Canada. The results were published in The Astrophysical Journal, a professional astronomy magazine.