BACKGROUND INFORMATION: HUBBLE SPACE TELESCOPE'S WIDE FIELD CAMERA REVEALS SPLENDOR OF 'SUPERGIANT' NEBULA
The 30 Doradus Nebula is the largest object of its kind in the Local Group of galaxies, which includes Andromeda (M31), Triangulum (M33), our Milky Way, and numerous smaller systems. The nebula is relatively nearby, located in a small satellite galaxy of the Milky Way called the Large Magellanic Cloud, 170,000 light-years from Earth. (A light-year is the distance light travels in one year at a speed of 186,000 miles per second, or about 6 trillion miles.)
Such nebulae, which astronomers call "ionized hydrogen regions," are the "signposts" of recent star birth. High-energy ultraviolet radiation from young, short-lived, massive, hot stars causes the surrounding gaseous material to glow by fluorescent processes. In keeping with its premier status, the most spectacular cluster of massive stars in the Local Group powers 30 Doradus. This compact cluster is called R136, so named because of its designation in an early catalogue of the brightest stars in the Magellanic Clouds, compiled at the Radcliffe Observatory in South Africa. Hubble telescope observations have shown that R136 contains several dozen of the most massive stars known, each about 100 times the mass of Sun and about 10 times as hot. The cluster also harbors many thousands of smaller stars. For many years it defied analysis from ground-based observations, and was once even suggested to be a single "superstar," about 3,000 times the mass of the Sun!
30 Doradus is a "Rosetta Stone" for understanding regions of intense star formation, because it is near enough to Earth for its stellar contents and nebular structures to be studied in detail by the Hubble telescope. Even larger clusters and nebulae, called starbursts, exist in the more distant universe, but they cannot be resolved in comparable detail. Such objects are vital contributors to the evolution of galaxies and even life, because massive stars synthesize many of the heavier chemical elements in their nuclear furnaces and final supernova explosions. The explosions disperse the heavy elements to the surrounding interstellar medium, where new stars and planetary systems form from the enriched material.
This image of 30 Doradus is a mosaic of five overlapping pointings taken by the Hubble Telescope's Wide Field Camera. Astronomers combined several color filters to display different stellar and nebular features. The picture covers a field of 200 by 150 light-years and for the first time shows the full structure of the inner 30 Doradus Nebula at Hubble's resolution.
The Hubble telescope has provided at least two major advances in our understanding of giant nebulae. First, it has shown that they are not uniformly filled with glowing gas as previously thought. Rather, the brightest emission occurs at narrow interfaces between a central cavity, where massive stars are propelling gas outward, and the surrounding dense dust and gas clouds. The radiation produced by massive stars is so intense that it blows off the stars' outer layers in "stellar winds." These winds, in turn, push surrounding gas outward from the stellar cluster, generating compression zones at the inner faces of the clouds. This pressure can trigger the collapse of parts of the clouds, producing a new generation of star formation around the original cluster's periphery.
The second Hubble telescope revelation about such regions is that this process of "triggered" star formation often, or perhaps always, involves massive dust and gas pillars oriented toward the central cluster. Such pillars form when a particularly dense dust and gas concentration shields material behind it from evaporation and dissipation by the energy coming from the stars. Observations at radio and infrared wavelengths, which can penetrate the dust, show that the same process creates new stars in the heads of the pillars. The new image of 30 Doradus reveals the full extent of the interfaces surrounding the central cluster and shows numerous dust and gas pillars oriented toward the cluster on or near the interfaces. Newborn stars within several of the pillars already have been discovered in images taken by the Hubble telescope's infrared camera, the Near Infrared Camera and Multi-Object Spectrometer.
Hubble telescope observations of other nebulae, together with those of 30 Doradus, provide a time sequence of the gestation and birth of new stars within pillars surrounding the initial cluster. The story begins with the famous Eagle Nebula (M16) pillars. They contain few bright infrared sources, but radio astronomers subsequently discovered luminous sources in the heads of the pillars. These bright objects are almost certainly protostars, which are still too cool to emit at infrared wavelengths. But they will continue to collapse and heat up until they do. Hubble telescope images of NGC 3603, the largest optically visible nebula in the Milky Way, show two massive pillars pointing toward its central cluster. Although smaller, this cluster bears a striking resemblance to R136.
Observations by the Hubble telescope's infrared camera have found luminous infrared sources within several of the 30 Doradus pillars. Moreover, images taken by the Hubble telescope's Wide Field and Planetary Camera 2 also reveal one young triple-stellar system that was just born by blowing off the top of its natal pillar with its own energetic radiation and winds. These new stars are probably just a few hundred thousand years old, whereas R136 is about 2 million years old. (For comparison, the Sun is 4.5 billion years old, and its total lifespan will be 10 billion years.)
Hubble telescope pictures of other objects also allow astronomers to trace the overall evolution of the giant nebulae, and on even larger scales, that of entire starburst galaxies, which are often composed of multiple giant nebulae and clusters. Central regions that are completely devoid of gas characterize a class of nebulae called giant shells. Only an older cluster – about 4 million years old – remains there. The most massive stars in this central cluster already have disappeared, since the lifetimes of stars are inversely related to their masses. The glowing gas is located around the periphery and is powered by a younger generation of more massive stars. An example is NGC 604 in M33.
It is easy to foresee that 30 Doradus will become a giant shell nebula in another 2 million years, when the most massive stars will be gone from the central cluster. However, the new generation of massive stars around the periphery will be in full bloom. Still later, all of the most massive stars and nebulosity will have disappeared from the region. Only older stellar clusters without nebulae will remain.
Moreover, Hubble telescope images of the global structure of starburst galaxies like NGC 4214 and the Antennae show that different regions within them are composed of giant nebulae and clusters of different ages, so that the temporal progression of star formation episodes can be traced throughout. But detailed studies of nearby objects, such as 30 Doradus, are essential to support such inferences about the more distant ones, in which individual stars and nebular structures cannot be resolved.
