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Hidden star groups uncovered

Star clusters discovered using VISTA

Using data from the VISTA infrared survey telescope, astronomers have discovered 96 new 'open' star clusters hidden behind dust in the Milky Way, 30 of which are shown in this mosaic.

  • Almost 100 star clusters found hiding behind dust in the Milky Way
  • Uncovered using the dust-penetrating power of infrared
  • There could be 30,000 more clusters still waiting to be found

NINETY-SIX PREVIOUSLY UNKNOWN ‘open star clusters’ have been found hiding behind dust in the Milky Way.

These tiny and faint groupings were invisible to previous surveys, but they could not escape the sensitive infrared detectors VISTA—an infrared survey telescope at the European Southern Observatory’s (ESO) Paranal Observatory in Chile—which can peer through the dust.

This result comes just one year after the start of the VISTA Variables in the Via Lactea programme (VVV), one of the six surveys running on the new telescope. (‘Via Lactea’ is the Latin name for the Milky Way.)

Invisible to most telescopes

Most stars that weigh more than half as much as our Sun form in groups, called open star clusters. These clusters are the building blocks of galaxies and vital for the formation and evolution of galaxies such as our own.

However, stellar clusters form in very dusty regions that absorb most of the visible light that the young stars emit, making them invisible to most telescopes, but not to VISTA.

In order to spot the youngest star clusters, the astronomers concentrated their search towards known star-forming areas. They found that regions that looked empty in previous visible-light surveys, actually held lots of clusters.

VISTA telescope

VISTA is an infrared survey telescope at the European Southern Observatory in Chile.

No wonder they were hidden

By using carefully tuned computer software, the team was able to remove the foreground stars appearing in front of each cluster in order to count the genuine cluster members.

Afterwards, they made visual inspections of the images to measure the cluster sizes, and for the more populous clusters they made other measurements such as distance, and the age of the stars.

“We found that … the dust in front of these clusters makes them appear 10,000 to 100 million times fainter in visible light,” explains Radostin Kurtev, another member of the team. “It’s no wonder they were hidden.”

Tip of the iceberg

Only 2,500 open clusters are known so far in the Milky Way, but astronomers think there might be as many as 30,000 still hiding behind the dust and gas.

These new 96 open clusters might be only the tip of the iceberg.

“We’ve just started to use more sophisticated automatic software to search for less concentrated and older clusters,” adds Jura Borissova, lead author of the study. “I am confident that many more are coming soon.”

Adapted from information issued by ESO. Images courtesy ESO / J. Borissova / Steven Beard (UKATC).

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Hubble spots stars on the move

The star-forming region NGC 3603

The star-forming region NGC 3603 contains one of the most impressive young star clusters in the Milky Way. Surrounded by gas and dust, the cluster formed in a huge rush around a million years ago. The hot blue stars at the core are carving out the huge cavity in the gas seen to the right of the cluster. Hubble Space Telescope image.

  • Star cluster 20,000 light-years away
  • Using Hubble, scientists measure the stars’ motions
  • Surprisingly, the stars are still moving quickly

Using the Hubble Space Telescope, astronomers have taken snapshots of a star cluster taken 10 years apart to reveal the motions of the stars contained within.

Their target was the massive young star cluster in the nebula known as NGC 3603.

With a mass of more than 10,000 stars packed into a volume with a diameter of a mere three light-years, it is one of the most compact star clusters in the Milky Way and an ideal place to test theories of their formation.

By comparison, in our own immediate stellar neighbourhood, the same volume of space contains only a single star…our Sun.

A team of astronomers from the Max-Planck Institute for Astronomy (MPIA) in Heidelberg and the University of Cologne, led by Wolfgang Brandner (MPIA), wanted to track the movement of the cluster’s many stars to reveal whether the stars are in the process of drifting apart, or about to settle down.

Hubble Telescope best for the job

The cluster, formally known as the NGC 3603 Young Cluster, is about 20,000 light-years from the Sun, a distance that makes it extraordinarily difficult to measure star motions.

In order to see how the stars are moving, it was necessary to compare images that were made years or even decades apart. The telescope and camera used had to give very sharp images and be extremely stable over long periods.

The core of the star cluster in NGC 3603

The core of the star cluster in NGC 3603 is shown in great detail in this image from the Hubble Space Telescope. This is the second of two images taken 10 years apart that were used to detect the motions of individual stars within the cluster for the first time.

Brandner and his colleagues realised that the Hubble Space Telescope was the best for the job.

First, they found good data in the archives for the NGC 3603 cluster from a July 1997 observing run with the Wide Field Planetary Camera 2 (WFPC2). Then they made their own follow-up observations in September 2007, using the same camera and the same set of filters as in the original observations.

It then took the team two years of very careful analysis to extract reliable estimates for the motions of stars in the images.

Boyke Rochau (MPIA), who performed the analysis as part of his PhD work, explains: “Our measurements have a precision of 27 millionths of an arcsecond per year. This tiny angle corresponds to the apparent thickness of a human hair seen from a distance of 800 km.”

In this laborious way, they were able to measure the precise speeds of more than 800 stars. About 50 were identified as foreground stars that are unrelated to the cluster, but more than 700 cluster stars of different masses and surface temperatures remained.

Signs of unrest

The results were surprising—this star cluster has not yet settled down. Instead, the stars’ velocities still reflect conditions from the time the cluster was formed, approximately one million years ago.

Stars are born when a gigantic cloud of gas and dust collapses. In cases such as the star-forming region NGC 3603, where the cloud is unusually massive and compact, the process is particularly quick and intense. Most of the cloud’s matter ends up concentrated inside hot young stars and the cluster keeps much of its initial gravitational attraction.

In the long term such massive compact star clusters may lead to the development of the huge balls of stars known as globular star clusters, whose tightly packed stars remain held together by gravity for billions of years.

Adapted from information issued by NASA / ESA / Wolfgang Brandner (MPIA), Boyke Rochau (MPIA) and Andrea Stolte (University of Cologne) / Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration.