ALMA witness stellar growth spurt

Have you ever experienced a growth spurt? Three times within a year or so, you need new T-shirts or new shoes, just because you’re growing so fast. It happens with teenagers, but also with baby stars. By comparing new ALMA observations with older measurements from another observatory, astronomers have discovered that a massive proto-star experienced a huge growth spurt over the past couple of years.

Stars are born in cosmic clouds of gas and dust. The birth process may take a few hundred thousand years. During this period, ever more gas from the surrounding cloud falls into the proto-star. But apparently, the star doesn’t grow in a smooth, regular fashion. Instead, it grows in fits and starts.

A massive proto-star in the Cat’s Paw Nebula – a huge region of star formation – was observed in 2008 with the Submillimeter Array (SMA) in Hawaii. Back then, ALMA didn’t yet exist; the SMA is a relatively small precursor. In 2015 and 2016, astronomers observed the same proto-star with ALMA. To their surprise, it had become four times as bright at submillimeter wavelengths.

ALMA can’t see the protostar itself. The submillimeter radiation is emitted by the surrounding cloud of gas and dust. Apparently, the dust had become much warmer between 2008 and 2015. The only possible explanation: the embedded proto-star must have become a hundred times more luminous.

The astronomers speculate that a large clump of gas had fallen towards the baby star, ending up in its so-called accretion disk – the flat, rotating disk of material surrounding the star. Then, when too much gas piled up in the disk, it all suddenly crashed onto the star’s surface, like an avalanche.

Observations from a single-dish radio telescope in South Africa support this story. In early 2015, the radio telescope, at Hartebeesthoek, saw a burst of short radio waves from the region. This is precisely what you expect to see when a proto-star experiences a growth spurt.

By studying this kind of events, astronomers hope to learn more about the birth of stars and planets. Who knows, the proto-star in the Cat’s Paw Nebula may experience another growth spurt within the next few years. Luckily, stars don’t need new T-shirts and shoes!



The proto-star observed by ALMA is part of a small group of baby stars, embedded in a cloud of gas and dust. Known as NGC6334I, it is located in the Cat’s Paw Nebula. On photographs of the night sky, this star-forming region looks like a footprint of a cat – hence the funny name. The Cat’s Paw Nebula resembles the better-known Orion Nebula, but it’s much farther away: 5,500 light-years, in the constellation Scorpius, the Scorpion.


In 2008, Todd Hunter of the National Radio Astronomy Observatory in the United States had already observed the proto-star in the Cat’s Paw Nebula with the Submillimeter Array on Mauna Kea, a dormant volcano on the Big Island of Hawaii. In 2015 and 2016, Todd took another look at NGC6334I with ALMA. In writing the paper on the discovery (in Astrophysical Journal Letters), Todd worked together with many other astronomers, from the USA, Canada, the United Kingdom, South Africa and Nigeria.