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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.

Space telescope reveals hidden cosmos

WISE space telescope image of the Heart and Soul nebulae

Covering an area of the sky over 10 times as wide as the full Moon, the Heart and Soul nebulae form a vast star-forming complex about 6,000 light-years from Earth.

  • Infrared telescope, good for seeing cold objects
  • Making survey of the entire sky
  • Has spotted 60,000 asteroids so far

NASA’s Wide-field Infrared Survey Explorer, or WISE, has captured a huge mosaic of two bubbling clouds in space, known as the Heart and Soul nebulae.

The space telescope, which has completed about 74% of its infrared survey of the entire sky, has already captured nearly one million frames like the ones making up the newly released mosaic.

“This new image demonstrates the power of WISE to capture vast regions,” said Ned Wright, the mission’s principal investigator at UCLA. “We’re looking north, south, east and west to map the whole sky.”

The Heart nebula is named after its resemblance to a human heart; the nearby Soul nebula happens to resemble a heart too, but only the symbolic kind with two lobes.

The nebulae, which are about 6,000 light-years away, are both massive star-making factories, shown by the giant bubbles blown into surrounding dust by the radiation and “winds” from the stars.

The infrared vision of WISE allows it to see into the cooler and dustier crevices of clouds like these, where gas and dust are just beginning to collect into new stars.

The new image was captured as WISE circled over Earth’s poles, scanning strips of the sky. It is stitched together from 1,147 frames, taken with a total exposure time of three-and-a-half hours.

An artist's impression of the WISE space telescope

WISE carries an infrared telescope cooled by solid hydrogen.

WISE will complete its first map of the sky in July 2010. It will then spend the next three months surveying much of the sky a second time, before the solid-hydrogen coolant needed to chill its infrared detectors runs dry.

The first public instalment of the WISE catalogue will be released in summer 2011.

About 960,000 WISE images have been beamed down from space to date. Some show ethereal star-forming clouds, while others reveal the ancient light of very remote, powerful galaxies.

The Solar System’s rocky rubble

Many of the WISE images are speckled with little dots … asteroids in our Solar System. So far, the mission has seen more than 60,000 asteroids, most of which lie in the main belt between Mars and Jupiter.

About 11,000 of these objects are newly discovered, and about 50 of them belong to a class of near-Earth objects, which have paths that take them within about 48 million kilometres (30 million miles) of Earth’s orbit.

One goal of the WISE mission is to study asteroids throughout our Solar System and to find out more about how they vary in size and composition. Infrared helps with this task because it can get better size measurements of the space rocks than visible light.

“Infrared will help us understand more about the sizes, properties, and origins of asteroids near and far,” said Amy Mainzer, the principal investigator of NEOWISE, a program to study and catalogue asteroids seen by WISE (the acronym comes from combining near-Earth object, or NEO, with WISE).

WISE will also study the Trojans, asteroids that run along with Jupiter in its orbit around the Sun in two packs — one in front of and one behind the gas giant planet. It has seen more than 800 of these objects, and by the end of the mission, should have observed about half of all 4,500 known Trojans. The results will influence competing concepts about how the outer planets evolved.

Comets have also made their way into WISE images, with more than 72 seen so far, about a dozen of them new. WISE is taking a census of the types of orbits comets ride in. The data will help explain what kicks comets out of their original, more distant orbits, sending them in toward the Sun.

Adapted from information issued by NASA / JPL-Caltech / UCLA.