Star cluster study questions stellar evolution

The theory that the stars in a cluster are of the same age and origin is fundamental to our understanding of stellar evolution. But a new study is calling that assumption into question.

Iain Todd

Published: March 7, 2017 at 12:00 pm

One of astronomy’s vital models for learning how the Universe and its stars evolved may have to be revisited, following a new discovery. Astronomers have discovered a star cluster that contains a younger generation of newly-formed stars amongst older ones.

Star clusters are groups of stars held together in clumps by gravity, and current models of stellar evolution work on the basis that all the stars in a cluster formed from the same material at the same time, more or less.

This discovery of younger generations of stars in an older cluster could force astronomers to get back to the drawing board.

A study led by the International Centre for Radio Astronomy Research in Perth, Australia, looked at star clusters in one of our neighbouring galaxies, the Large Magellanic Cloud.

A total of 15 candidates were found for stars that may be younger than those found elsewhere in their home clusters.

Initially, the team suspected this may be explained by cosmic gas entering the cluster and causing a new burst of star formation, but radio telescope observations revealed no relationship between hydrogen gas and the star clusters’ locations.

This vibrant image from NASA's Spitzer Space Telescope shows the Large Magellanic Cloud, a satellite galaxy to our own Milky Way galaxy.The infrared image, a mosaic of 300,000 individual tiles, offers astronomers a unique chance to study the lifecycle of stars and dust in a single galaxy. Nearly one million objects are revealed for the first time in this Spitzer view, which represents about a 1,000-fold improvement in sensitivity over previous space-based missions. Most of the new objects are dusty stars of various ages populating the Large Magellanic Cloud; the rest are thought to be background galaxies. The blue color in the picture, seen most prominently in the central bar, represents starlight from older stars. The chaotic, bright regions outside this bar are filled with hot, massive stars buried in thick blankets of dust. The red color around these bright regions is from dust heated by stars, while the red dots scattered throughout the picture are either dusty, old stars or more distant galaxies. The greenish clouds contain cooler interstellar gas and molecular-sized dust grains illuminated by ambient starlight. Astronomers say this image allows them to quantify the process by which space dust the same stuff that makes up planets and even people is recycled in a galaxy. The picture shows dust at its three main cosmic hangouts: around the young stars, where it is being consumed (red-tinted, bright clouds); scattered about in the space between stars (greenish clouds); and in expelled shells of material from old stars (randomly-spaced red dots). The Large Magellanic Cloud, located 160,000 light-years from Earth, is one of a handful of dwarf galaxies that orbit our own Milky Way. It is approximately one-third as wide as the Milky Way, and, if it could be seen in its entirety, would cover the same amount of sky as a grid of about 480 full moons. About one-third of the entire galaxy can be seen in the Spitzer image. This picture is a composite of infrared li — Close-up, peering into NASA's Spitzer Space Telescope image of the Large Magellanic Cloud

“We believe the younger stars have actually been created out of the matter ejected from older stars as they die, which would mean we have discovered multiple generations of stars belonging to the same cluster," says Dr Kenji Bekki, co-author of the study.

Currently, the stars are covered in gas and dust and can only be seen in infrared by using the NASA Spitzer Space Telescope and ESA's Herschel space observatory.

But the authors of the study hope that as the stars grow, the gas and dust will blow away, meaning the powerful Hubble Space Telescope, which observes in optical light, should be able to observe the young and old stars and confirm whether the team’s discovery is correct.

Science journalist

Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.

An image of the Large Magellanic Cloud by NASA’s Spitzer Space Telescope. Astronomers searched this galaxy for star clusters and made a discovery that could turn current models of stellar evolution on their head. Credit: NASA/JPL-Caltech/M. Meixner (STScI) & the SAGE Legacy Team The infrared image, a mosaic of 300,000 individual tiles, offers astronomers a unique chance to study the lifecycle of stars and dust in a single galaxy. Nearly one million objects are revealed for the first time in this Spitzer view, which represents about a 1,000-fold improvement in sensitivity over previous space-based missions. Most of the new objects are dusty stars of various ages populating the Large Magellanic Cloud; the rest are thought to be background galaxies. The blue color in the picture, seen most prominently in the central bar, represents starlight from older stars. The chaotic, bright regions outside this bar are filled with hot, massive stars buried in thick blankets of dust. The red color around these bright regions is from dust heated by stars, while the red dots scattered throughout the picture are either dusty, old stars or more distant galaxies. The greenish clouds contain cooler interstellar gas and molecular-sized dust grains illuminated by ambient starlight. Astronomers say this image allows them to quantify the process by which space dust the same stuff that makes up planets and even people is recycled in a galaxy. The picture shows dust at its three main cosmic hangouts: around the young stars, where it is being consumed (red-tinted, bright clouds); scattered about in the space between stars (greenish clouds); and in expelled shells of material from old stars (randomly-spaced red dots). The Large Magellanic Cloud, located 160,000 light-years from Earth, is one of a handful of dwarf galaxies that orbit our own Milky Way. It is approximately one-third as wide as the Milky Way, and, if it could be seen in its entirety, would cover the same amount of sky as a grid of about 480 full moons. About one-third of the entire galaxy can be seen in the Spitzer image. This picture is a composite of infrared li