Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997xmm..pres...25.&link_type=abstract
XMM Press Release INFO 25-1997
Astronomy and Astrophysics
Astronomy
Scientific paper
Some of the work being presented is collected in the attached ESA Information Note N 25-97, ISO illuminates our cosmic ancestry. A set of six colour images illustrating various aspects have also been released and are available at http://www.estec.esa.nl/spdwww/iso1808.htm or in hard copy from ESA Public Relations Paris (fax:+33.1.5369.7690). These pictures cover:
1. Distant but powerful infrared galaxies 2. A scan across the milky way 3. Helix nebula: the shroud of a dead star 4. Supernova remnant Cassiopeia A 5. Trifid nebula: a dusty birthplace of stars 6. Precursors of stars and planets
The International Astronomical Union provides a forum where astronomers from all over the world can develop astronomy in all its aspects through international co-operation. General Assemblies are held every three years. It is expected that over 1600 astronomers will attend this year's meeting, which is being held in Kyoto, Japan from 18-30 August. Further information on the meeting can be found at: www.tenmon.or.jp/iau97/ .
ISO illuminates our cosmic ancestry The European Space Agency's Infrared Space Observatory, ISO, is unmatched in its ability to explore and analyse many of the universal processes that made our existence possible. We are children of the stars. Every atom in our bodies was created in cosmic space and delivered to the Sun's vicinity in time for the Earth's formation, during a ceaseless cycle of birth, death and rebirth among the stars.
The most creative places in the sky are cool and dusty, and opaque even to the Hubble Space Telescope. Infrared rays penetrating the dust reveal to ISO hidden objects, and the atoms and molecules of cosmic chemistry.
"ISO is reading Nature's recipe book," says Roger Bonnet, ESA's director of science. "As the world's only telescope capable of observing the Universe over a wide range of infrared wavelengths, ISO plays an indispensable part in astronomical discoveries that help to explain how we came to exist."
This Information Note describes several stages in our cosmic ancestry, revealed when ISO examines their counterparts still observable today.
The evolving galaxies In the beginning was hydrogen, mixed with helium and minute traces of other light atoms. These were the atomic products of the Big Bang, the hypothetical cataclysm that created the Universe more than 10 billion years ago. The primeval gas was very dull. Nature could not make dust from it, never mind a living creature. But gravity gathered the hydrogen and helium into stars, and by nuclear reactions the stars glowed. As the first stars aged, the reactions made novel chemical elements like carbon, oxygen and silicon.
Expelled into the stars' surroundings, these materials reacted with one another and with hydrogen to make the icy, tarry and stony grains of cosmic dust. The vast assemblies of stars called galaxies became crucibles where Nature could use physics and chemistry to make new materials and new stars. Rays from the most distant galaxies have taken so many billions of years to reach us that we see them as they were when they were young. The farthest galaxy observed so far by ISO is a quasar called BR 1202-0225, dating from a time when the Universe was less than one-tenth of its present age. Already it is dusty.
ISO has also observed many galaxies at about half the age of the Universe, by staring long and hard through a window in the dust of our own Milky Way Galaxy, called the Lockman Hole. In those that glow most brightly in the infrared, astronomers suspect that frantic star-making is in progress, in episodes called starbursts. In nearer galaxies, ISO's astronomers can relate strong infrared emissions to collisions and to violent eruptions in the galactic cores, which have punctuated the evolution of the galaxies.
"Having ISO in space brings special opportunities for the study of the history of the galaxies," says the Japanese astronomer Yoshiaki Taniguchi of Tohoku University. "By detecting infrared wavelengths that are hard to observe from the Earth, ISO picks out very clearly the galaxies that are evolving most rapidly, in periods of intensive starmaking. Also some sources may be infrared galaxies powered by active galactic nuclei."
The Milky Way Galaxy where we live acquired its name from the starry disk that we see edge-on as a ribbon of light. It has had a quiet history compared with some other galaxies, but the tranquillity is only relative. Violent events have made and destroyed stars throughout the Galaxy's life. The wreckage is strewn all around us.
When ISO surveys cross-sections of the Milky Way it detects old cool stars and young dusty stars glowing strongly in the infrared. But the main features of the images are thin dust clouds sprawling across the sky, made by the scattered debris of defunct stars. Here and there, thicker and more luminous dust clouds are the scenes of new star formation. It was in just such a dusty environment that the Sun and the Earth were born.
Death and rebirth among the stars The Sun is a middle-aged star. It was formed about 4.5 billion years ago, when the Universe was not much more than half its present age. Now the Sun is about half-way through its expected life-span. In the Sun, the Earth and our own bodies, all atoms heavier than than the primeval hydrogen and helium were made in stars of the Milky Way that expired before the Sun came into existence. Grains of different origins, found in meteorites and distinguished by atomic fingerprinting, confirm that many individual stellar ancestors contributed to the Solar System's stock of elements. The ashes of the ancestral stars are too dispersed to be identifiable in the Galaxy today. Astronomers can nevertheless find their analogues among more recent stars. ISO gives them special access to the stages between stellar death and rebirth.
An old star expiring scatters chemically enriched material into the interstellar medium, which concentrates again at the origin of new stars and planets. Extraordinary success in analysing the chemical composition of the gases and dust in the vicinity of old and new stars, and in comets too, has been a major contribution from ISO. As described in previous Information Notes, materials identified by their infrared signatures include carbon monoxide and water in vapour or icy form, tarry carbon-rich compounds, and minerals including olivine, which is a major constituent of the Earth's rocky mantle.
When the Sun itself grows old, it will swell and cool, and will eventually puff much its material into space. Its burnt-out core will collapse to make a white dwarf star. A star of roughly solar mass, seen in the last phase of its expiry, makes a planetary nebula -- a sphere of scattered ashes around the glowing ember of the white dwarf. ISO has examined several planetary nebulae, including the Helix Nebula, the subject of a newly released picture from ISO showing remarkable detail.
Massive stars not only burn up much more quickly than sun-like stars, but perish more spectacularly in supernova explosions. For a few weeks, an exploding star glows more brightly than a billion suns. Its interior collapses to make a neutron star far denser than a white dwarf, and the star blasts its outer layers into space. One reason why supernovae are important in the chemical scheme of the cosmos is that only they can make the heaviest elements, such as gold and uranium.
The remnant of a supernova remains discernible for thousands of years after the explosion. The most recent supernova observed in the Milky Way Galaxy occurred little more than 300 years ago and the resulting nebulous object is called Cassiopeia A. ISO has made the first detailed examination of Cassiopeia A by infrared rays unobservable from the Earth's surface. It gives direct evidence of dust formation.
"The newly cooked elements provided by the supernova have to cool before they can create fresh supplies of interstellar dust," comments Pierre-Olivier Lagage of CEA SAp at Saclay (France) who led this study of Cassiopeia A. "With ISO's camera we can pick out emissions from various elements, and we find th
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