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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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Rosy glow of young stars

NGC 371

The red glow of ionised hydrogen gas surrounds a group of young stars, which together make up the object known as NGC 371.

THE VIVID RED CLOUD in this new image from the European Southern Observatory’s (ESO) Very Large Telescope is a region of glowing gas surrounding the star cluster NGC 371.

NGC 371 is an example of a mature ‘star birth’ region. The red cloud is made of ionised hydrogen—an HII region in astronomers’ jargon—surrounding a place that has experienced high rates of recent star birth, leading to the formation of an ‘open star cluster’.

An open star cluster is group of stars that formed together and are all in the same vicinity, but with a random scattering (unlike ‘globular’ star clusters, which form into a ball shape).

Stars in open clusters all originate from the same diffuse HII region. Over time the majority of the hydrogen is used up to form the stars, leaving behind just a leftover shell of hydrogen such as the one in this image.

NGC 371’s home, the Small Magellanic Cloud (SMC), is a dwarf galaxy a mere 200,000 light-years away, which makes it one of the closest galaxies to the Milky Way.

Watch a video that zooms into the SMC and NGC 371:

The SMC contains stars at all stages of their evolution…from the highly luminous young stars found in NGC 371 to supernova remnants of dead stars.

The energetic young stars emit copious amounts of ultraviolet radiation, making the surrounding gas light up with a colourful glow that extends for hundreds of light-years in every direction.

NGC 371 is of particular interest due to the unexpectedly large number of variable stars it contains. These are stars that change in brightness over time.

A particularly interesting type of variable star, known as slowly pulsating B stars, can also be used to study the internal working of stars through a technique known as asteroseismology, and several of these have been confirmed in this cluster.

Variable stars play a pivotal role in astronomy—some types are invaluable for determining distances to far-off galaxies and the age of the Universe.

The data for this image were selected from the ESO archive by Manu Mejias as part of the Hidden Treasures competition.

Adapted from information issued by ESO / Manu Mejias.

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