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“Super stars” uncovered

Artist's impression of the relative sizes of young stars

This artist's impression shows the relative sizes of young stars, from the smallest ones called "red dwarfs", with about 0.1 solar masses, through low mass "yellow dwarfs" such as the Sun and massive "blue dwarf' stars with more than 8 times the mass of the Sun, to the newly-discovered 300 solar mass star R136a1.

  • Stars found with 100+ times more mass than the Sun
  • Stellar record now stands at 320 solar masses
  • Did they form large, or did smaller stars merge?

“Super stars” born with hundreds of times the mass of our Sun, have been spotted in star clusters within our galaxy and in a neighbouring galaxy.

One of the stars, at birth, would have had over 300 times the mass of the Sun.

The UK-led team of astronomers studied two young star clusters—NGC 3603, located 22,000 light-years from Earth within our Milky Way galaxy; and R136a, located 165,000 light-years away in the Large Magellanic Cloud, a satellite galaxy of the Milky Way.

Both clusters are “star factories” where stars have formed from clouds of gas and dust.

The discovery of the giant stars might help solve a long-standing puzzle in astronomy—just how massive can stars become, and how do they get to be so massive?

Note that by “massive” the astronomers are not referring to the stars’ physical size—although the new “super stars” certainly are very large. Rather, they’re referring to how much mass they contain.

The team used archived data from the Hubble Space Telescope, plus new observations obtained with the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile.

The Tarantula Nebula (left) and star cluster R136a (right)

A region of the Large Magellanic Cloud galaxy seen with the Very Large Telescope (left), as well as a new image of the R136 cluster obtained with the MAD adaptive optics instrument on the Very Large Telescope (right). R136a contains the most massive star found so far.

Several of the stars studied have surface temperatures in excess of 40,000 degrees Celsius, which is almost 8 times hotter than the Sun. The stars are also tens of times bigger and millions of times brighter.

Computer models suggest that stars like these must have started off with masses greater than 150 times the mass of the Sun.

Stellar heavyweight champion

The heavyweight champion, in the RMC 136a cluster, is a star called R136a1. It is the most massive star known, with a current mass around 265 times that of the Sun. At birth, it must have been over 320 solar masses.

Young star cluster R136a

R136 is a cluster of young, massive and hot stars located inside one of the Milky Way's neighbouring galaxies, the Large Magellanic Cloud, 165,000 light-years away.

These kinds of hot, huge stars lose a lot of their mass by blowing off “winds” into space.

“Unlike humans, these stars are born heavy and lose weight as they age,” says astronomer Paul Crowther. “Being a little over a million years old, the most extreme star R136a1 is already ‘middle-aged’ and has undergone an intense weight loss programme, shedding a fifth of its initial mass over that time, or more than fifty solar masses.”

Two of the heavyweight stars in the NGC 3603 cluster are in orbit around each, a double star system. Using simple physics laws, the astronomers could calculate their masses at 120 and 92 times that of the Sun. At birth, they would have been 148 and 106 solar masses respectively.

Woking out how such huge stars formed in the first place is a major challenge for astronomers.

“Either they were born so big or smaller stars merged together to produce them,” says Crowther.

Story by Jonathan Nally, Editor, SpaceInfo.com.au

Images courtesy ESO / P. Crowther / C.J. Evans / M. Kornmesser.

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Stellar fireworks

A Hubble image of NGC 3603

NGC 3603 is a collection of stars surrounded by gas and dust, 20,000 light-years from Earth.

Looking like a  fireworks display, this young, glittering collection of stars  is surrounded by clouds of interstellar gas and dust—a nebula—the raw material for new stars.

The nebula is located 20,000 light-years away in the constellation Carina; the central cluster of huge, hot stars is called NGC 3603.

This environment is not as peaceful as it looks. Ultraviolet radiation and violent stellar winds have blown out an enormous cavity in the gas and dust enveloping the cluster, providing an unobstructed view of the cluster.

Most of the stars in the cluster were born around the same time but differ in size, mass, temperature, and colour. The course of a star’s life is determined by its mass, so a cluster of a given age will contain stars in various stages of their lives, giving an opportunity for detailed analyses of stellar life cycles.

NGC 3603 also contains some of the most massive stars known. These huge stars live fast and die young, burning through their hydrogen fuel quickly and ultimately ending their lives in supernova explosions.

Star clusters like NGC 3603 provide important clues to understanding the origin of massive star formation in the early, distant universe.

Astronomers also use massive clusters to study distant starbursts that occur when galaxies collide, igniting a flurry of star formation.

The proximity of NGC 3603 makes it an excellent lab for studying such distant and momentous events.

See the full-size image here (new window).

Adapted from information issued by STScI. Image credit: NASA, ESA, R. O’Connell (University of Virginia), F. Paresce (National Institute for Astrophysics, Bologna, Italy), E. Young (Universities Space Research Association/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA).

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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.