Today’s Image of the Day from the European Space Agency features Westerlund 1 – a young star cluster located in the Milky Way galaxy. 

Discovered by Swedish astronomer Bengt Westerlund in 1961, the cluster is composed of thousands of stars – many of which are supergiants – and contains some of the most massive stars ever observed.

Webb image of Westerlund 1

“The open cluster Westerlund 1, showcased in this new Webb Picture of the Month, is located roughly 12,000 light-years away in the southern constellation Ara (the Altar) where it resides behind a huge interstellar cloud of gas and dust,” noted ESA.

“Westerlund 1 is an incomparable natural laboratory for the study of extreme stellar physics, helping astronomers to find out how the most massive stars in our galaxy live and die.”

What is special about Westerlund 1?

According to ESA, the unique draw of Westerlund 1 is its large, dense, and diverse population of massive stars. The star cluster is unparalleled in the known Milky Way galaxy in terms of the number of stars and the richness of spectral types and evolutionary phases. 

“All stars identified in this cluster are evolved and very massive, spanning the full range of stellar classifications including Wolf-Rayet stars, OB supergiants, yellow hypergiants (nearly as bright as a million Suns) and luminous blue variables. Because such stars have a rather short life, Westerlund 1 is very young, astronomically speaking,” said ESA.

“Astronomers estimate the cluster’s age to be somewhere between 3.5 and 5 million years (its exact age is still a matter of debate), making it a newborn cluster in our galaxy.”

Westerlund 1 is expected to gradually evolve from an open cluster to a globular cluster – a spherical collection of stars that are tightly bound together by gravity.

Star formation in the Milky Way galaxy 

The Milky Way galaxy forms roughly 1 to 2 stars per year on average. This star formation rate is relatively moderate compared to more active galaxies, which can produce dozens or even hundreds of stars annually.

The Milky Way galaxy used to produce many more stars, likely hitting its peak about 10 billion years ago. At that time, dozens (or possibly even hundreds) of stars were created in the Milky Way each year. 

“Astronomers think that most of this star formation took place in massive clusters of stars, known as ‘super star clusters.’ These are young clusters of stars that contain more than 10,000 times the mass of the Sun, packed into an unbelievably small volume. They represent the most extreme environments in which stars and planets can form,” said ESA. 

“Only a few super star clusters still exist in our galaxy – of which Westerlund 1 is one – but they offer important clues about this earlier era when most of our galaxy’s stars formed.”

Impressive example of a star cluster 

ESA noted that Westerlund 1 is an impressive example of a super star cluster because it contains hundreds of very massive stars – some that shine brilliance of almost a million suns and others that are two thousand times larger than the sun. 

“Indeed, if the Solar System was located at the heart of this remarkable cluster, our sky would be full of hundreds of stars as bright as the full Moon.”

The large population of massive stars in Westerlund 1 indicates that it will greatly influence its surroundings, noted ESA. With such a high concentration of these stars, the cluster is expected to host over 1,500 supernovae within the next 40 million years. This “super” star cluster offers astronomers a rare glimpse into one of the most extreme environments in the Universe. 

Westerlund 1 is set to play a key role in advancing our understanding of how stars, particularly massive ones, are formed, opening new doors in the ongoing quest for deeper insights.

The image was captured as part of the The Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) with Webb’s Near-InfraRed Camera (NIRCam). 

Image Credit: European Space Agency 

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