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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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Supernovae make raw material for planets

SN 1987A

Observations with the Herschel Space Observatory show that supernova 1987A produced enough dust to make 200,000 new planets.

THE HERSCHEL SPACE OBSERVATORY has discovered that titanic stellar explosions can be excellent dust factories. In space, the dust mixes with gas to become the raw material for new stars, planets and, ultimately, life. This discovery may solve a mystery of the early Universe.

The discovery was made while Herschel was charting emission from cold dust in the Large Magellanic Cloud, a small galaxy near to the Milky Way. It is the perfect observatory for the job because cold dust radiates far-infrared light, the wavelengths Herschel is designed to detect.

Herschel saw a spot of light at the location of supernova 1987A, an exploding star first seen from Earth in February 1987, and the closest known supernova in the past 400 years.

Astronomers have been studying the remains of the explosion as its blast wave expands into its surroundings.

Planet factories

Herschel’s images are the first clear-cut far-infrared observations of SN1987A. They reveal cold dust grains at about -250 degrees C, which nevertheless emit more than 200 times the Sun’s energy.

“The supernova remnant was much brighter at infrared wavelengths than we were expecting,” says Mikako Matsuura, University College London, who is the lead author on the scientific paper detailing these results.

The remnant’s brightness was used to estimate the amount of dust. Surprisingly, there turned out to be about a thousand times more dust than astronomers had thought a supernova was capable of producing—enough to make 200,000 planets the size of Earth.

Artist's impression of the Herschel Space Observatory.

Artist's impression of the Herschel Space Observatory.

The origin of dust in the Universe is of great interest. The dust’s heavy atoms like carbon, silicon, oxygen and iron were not produced in the Big Bang and must have formed later.

Although they are only a minor part of the Universe and our Solar System, they are the main constituents of rocky planets like Earth and thus of life itself—many of the atoms we are made of were once part of the dust in the Universe.

Dust factories

However, it is not fully understood where this dust comes from, and especially where it came from in the young Universe. But scientists now have a clue.

The many old red giant stars in today’s Universe are thought to be the major dust producers, with the grains condensing like soot in a chimney as warm gases flow away from the star.

However, there were no such stars in the early Universe—yet we know there was already dust.

Now Herschel has shown that supernovae can produce enormous amounts of dust. The astronomers speculate that the dust condenses from the gaseous debris as it expands from the explosion and cools.

Since there were plenty of supernovae in the young Universe, this could help to explain the origin of dust seen at those times.

Adapted from information issued by ESA. Images courtesy ESA / C. Carreau / P. Challis (CfA).

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Are galaxies ‘see through’?

Galaxy pair AM500-620

Galaxy pair AM500-620 comprises two dusty spiral galaxies, one in front of the other.

THIS HUBBLE SPACE TELESCOPE image shows a galaxy pair known only by its catalogue number, AM0500-620. It comprises consists of a highly symmetrical spiral galaxy seen nearly face-on, partially backlit by a background galaxy.

The Hubble image shows the foreground spiral galaxy to have a number of ‘dust lanes’ between its spiral arms.

The background galaxy had originally been classified as an elliptical galaxy, but Hubble’s observations revealed it to be a dusty spiral arms and bright knots of stars.

The image was taken in order to work out how much dust is held within galaxies, and whether this dust reduces the amount of light we see from the stars within those galaxies.

By finding foreground-background galaxy pairs, astronomers were able to refine their estimates of dust in the foreground galaxies through the backlighting effect of the background galaxies.

AM0500-620 is 350 million light-years away from Earth in the direction of the constellation Dorado, the Swordfish.

Download a 1280 x 1280-pixel wallpaper image of AM0500-620.

Adapted from information issued by NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and W. Keel (University of Alabama, Tuscaloosa) / UA News Bureau.

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