Our Milky Way
Formation of Stars
The large mosaic of 15 Hubble images showing the central part of the Orion complex is one of the most detailed images of a star-forming region ever made. It shows a very young star cluster blowing a 'bubble' in its remnant parent cloud of glowing gas so that the stars start to be seen in visible light - like the smoke in a forest fire being driven away by the heat.
Hubble's high resolution has been crucial in the investigation of the dust discs, dubbed proplyds, around the newly born stars in the Orion Nebula. The 'proplyds' may very well be young planetary systems in the early phases of their creation. The details that are revealed are far finer than than can be seen with ground-based instruments and, thanks to Hubble, we today have visual proof that dusty discs around young stars are common.
Since star birth always seems to take place in dusty environments, Hubble's infrared capabilities have been a very important factor. The infrared instrument NICMOS can peer through much of the dust and reveal the complex processes taking place in star-forming regions. Otherwise invisible close double and multiple stars have been discovered, as well as faint substellar brown dwarf companions. With NICMOS and its visual counterpart WFPC2, Hubble has observed giant jets of material spewing out from infant stars surrounded by large discs of dust, giving a fantastic view into the dramatic first steps in the lives of newly born stars.
Hans Zinnecker
Astrophysicist, Astrophysical Institute Potsdam
"To astronomers and laymen alike the topic of star formation has always been a particularly appealing one. Important clues about our genesis lie hidden behind the veil of the dusty, and often very beautiful, star- forming molecular clouds. Our Earth and the Solar System were born 4.6 billion years ago and our knowledge of the event is sparse. Astronomers turn their eyes to the birth of other stars and stellar systems in neighbouring stellar 'maternity wards' and use these as a time machine to see a replay of the events that created our own Solar System."
"Hubble has had a major impact in two areas in the field of star formation. Firstly it has studied the formation of stars like our Sun and has literally seen dusty discs which may end up as planetary systems around those stars. Secondly Hubble has made an impact in the area one could call 'cosmological star formation', that is, the formation of stars all over the Universe. The Hubble Deep Field North for instance opened up the box and allowed us to follow the history of star formation through the entire Universe and in this way enabled us to study the 'cosmic evolution' of the stars."
Stellar Evolution
Most of the light and radiation we can observe in the Universe originates in stars - individual stars, clusters of stars, nebulae lit by stars and galaxies composed of billions of stars. Stars are spheres of glowing hydrogen and other chemical elements which produce their prodigious energy output by converting lighter elements to heavier ones through nuclear processes similar to those in hydrogen bombs. Like human beings they are born, mature and eventually die, but their lifetimes are vastly longer than our own.
Hubble has gone beyond what can be achieved by other observatories by linking together studies of the births, lives and deaths of individual stars with theories of stellar evolution. In particular Hubble's ability to probe stars in other galaxies enables scientists to investigate the influence of different environments on the lives of stars. This is crucial in order to be able to complement our understanding of the Milky Way galaxy with that of other galaxies.
Hubble was the first telescope to directly observe white dwarfs in globular star clusters. White dwarfs are stellar remnants and provide a 'fossil' record of their progenitor stars which shone so brightly that they long ago exhausted their nuclear fuel. Through these measurements it is possible to determine the ages of these ancient clusters which is an important cosmological tool.
Another area where Hubble's work has been widely acknowledged is the linking of star formation (also see pages 28-29) with stellar evolution. Hubble's infrared instrument, NICMOS, is capable of looking through the dust surrounding newly born stars. Some of the most surprising discoveries so far have come about by peering through the clouds of dust surrounding the centre of our Milky Way. Astronomers found that this centre, which was thought to be a calm and almost 'dead' region, is in fact populated with massive infant stars gathered into clusters.
The last phases of solar-like stars have been investigated through observations of planetary nebulae and proto-planetary nebulae. These are colourful shells of gas expelled into space by dying stars. The varying shapes and colours of these intricate structures with different colours tracing different, often newly created, chemical elements, have shown that the final stages of the lives of stars are more complex than once thought.
Gerard Gilmore
Astronomer, University of Cambridge
"Hubble has in my view revolutionised the study of globular clusters - especially those in other galaxies. These objects are so dense and the stars so tightly packed together that it is almost impossible to separate the stars from each other with ground-based telescopes. We have been able to measure what kind of stars they are composed of, how they evolve and how gravity works in these complex systems."
