Herschel captures the birth of a giant star

ESA’s Herschel telescope has snapped an embryonic star likely to turn into one of the biggest and brightest in our galaxy within the next few hundred thousand years.

The star-forming cloud RCW 120 already contains eight to ten times the mass of the Sun, and is still surrounded by an additional 2,000 solar masses of gas and dust from which it can feed further.

“This star can only grow bigger,” says Annie Zavagno of the Laboratoire d’Astrophysique de Marseille. But, she says, “According to our current understanding, you should not be able to form stars larger than eight solar masses.”


This is because the fierce light emitted by such large stars should blast away their birth clouds. Now that Herschel has observed one of these ‘impossible’ stars near the beginning of its life, astronomers can investigate why this doesn’t seem to happen.


Another new image shows stellar embryos first appearing inside filaments of glowing dust and gas draped across the galaxy. These form chains of stellar nurseries, tens of light-years long.

Herschel has also been surveying deep space in the infrared. Each galaxy appears as just a pinprick, but its brightness allows astronomers to determine the rate of star birth within it.


And Herschel has now shown that galaxies have been evolving much faster than previously thought. Astronomers had believed that galaxies have been forming stars at about the same rate for the last three billion years.

In the past, there were many more so-called ‘starburst’ galaxies forming stars at 10–15 times the rate we see in the Milky Way today. But what triggered this frantic activity is not completely understood.

Herschel has also made the first discovery in space of a new phase of water. It is electrically charged and, unlike ice, water and steam, does not occur naturally on Earth. In the birth clouds surrounding young stars, however, where ultraviolet light is pumping through the gas, this irradiation can knock an electron out of the water molecule, leaving it with an electrical charge.

“This detection of ionised water vapour came as a surprise,” says Arnold Benz of ETH Zurich. “It tells us that there are violent processes taking place during the early birth stages which lead to widespread energetic radiation throughout the cloud.”