Hubble Captures A Grand View Of The Birth Of "Hefty" Stars

Nolan Walborn, Space Telescope Science Institute, Baltimore, MD (Phone: 410-338-4915)

Images taken in infrared and visible light by NASA's Hubble Space Telescope recount a vivid story of the turbulent birthing process of massive stars. The pictures show that powerful radiation and high-speed material unleashed by "hefty" adult stars residing in the hub of the 30 Doradus Nebula are triggering a new burst of star birth in the surroundingsuburbs. Like their adult relatives, the fledgling stars are creating all sorts of havoc in their environment. Nascent stars embedded in columns of gas and dust, for example, are blowing away the tops of their nurseries, like a volcano blasting material into the sky. Jets of material streaming from another developing star are slamming into surrounding dust and gas in opposite directions, causing it to glow inmoving patterns.

The stellar action is happening relatively nearby, 170,000 light-yearsfrom Earth in the Large Magellanic Cloud, a satellite galaxy of theMilky Way. This backyard nursery is a great laboratory for studying thedetails of the birth and development of "hefty" stars and multiple-starsystems. "This region is larger and contains more massive stars than any similarobject in our galaxy," says astronomer Nolan R. Walborn of the SpaceTelescope Science Institute in Baltimore, Md. "In fact, it is thelargest such nebula in the entire Local Group of galaxies. We're able toclearly see objects in the Large Magellanic Cloud because their light isnot blocked by interstellar dust, which impedes our study of objects inthe Milky Way. Furthermore, it is near enough that individual stars andgaseous structures within it can be studied in detail with largetelescopes.

"Thus, 30 Doradus provides a model for interpreting even largerstarbursts in distant galaxies, for which detailed resolution is notpossible. The Hubble images of 30 Doradus reveal characteristics ofmassive star birth never before seen so clearly, and some of them not atall."

The creator of these powerful cosmic forces is R136, atwo-million-year-old central cluster of massive stars. These "heavy"stars have temperatures 10 times that of the Sun and masses up to 100times greater. Such stars emit prodigious amounts of energy throughultraviolet radiation and high-speed gas. They shed bubbles of materialat speeds of thousands of miles per second, which collide with surrounding dense clouds composed predominantly of cold, molecular hydrogen. Some of the clouds collapse, igniting a spate ofsecond-generation stars. Most of these new stars are less than a million years old.

Astronomers have not determined how many fledgling stars reside in 30Doradus, but they estimate the number to be in the thousands, which is asignificant fraction of the original generation of stars. All of thesestars are packed into a 600-light-year-wide nebula.

Ultraviolet light emitted by the hot stars that make up R136 energizesthe clouds of gas, illuminating the surrounding region and showing offthe tempestuous activity. The images of this region, taken with the WideField and Planetary Camera 2 (WFPC2) and the Near Infrared Camera andMulti-Object Spectrometer (NICMOS), show immense clouds of dust, several in the form of pillars 5 to 10 light-years tall, giant incubators where young stars and multiple star systems are forming. Resembling the towers of the Eagle Nebula, these columns of dust are oriented toward the luminous central star cluster, suggesting that the cluster's powerfulemissions are creating these stellar nurseries.

Radiation emitted by some of the fledgling stars has begun to erodetheir natal dust columns; their emergence from these columns can be seenin the visible-light images. But others are still completely immersed intheir stellar incubators. These newborn stars can be discerned only inthe infrared images, and many of them have never before been seen.Astronomers have inferred that many of these new stars, based on theirtotal brightnesses, also have very large masses.

Other new stars are shooting twin jets of material into the surroundingclouds of dust and gas, causing them to glow. These jets suggest thatthe stars have rotating disks of material surrounding them. The disksshape and aim the jets.

"The observation of bipolar outflows from massive star formationsuggests that accretion disks are essential in forming 'heavy' stars,just as they have been established to be a crucial factor in creatinglow-mass stars in our solar neighborhood," Dr. Walborn explains.

This wave of star birth will continue to migrate farther out in thenebula. In a few million years, 30 Doradus will be a giant shell of hot,glowing hydrogen, with its most massive stars around the periphery. Theonce-thriving hub of 30 Doradus will be much dimmer: the short-livedmassive stars in the central cluster (R136) will have disappeared, andonly its lower-mass members will still be present.

The WFPC2 pictures were taken in 1994 by two of the camera's designers,astronomers John Trauger of NASA's Jet Propulsion Laboratory and JamesWestphal of the California Institute of Technology, both in Pasadena,Calif. Walborn and Rodolfo Barba' of the La Plata Observatory in LaPlata, Argentina, used the visible-light images, together withlower-resol