ESA Science & Technology - INFO 25-1997: ISO Results Presented at International Astronomical Union

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. "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."

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 that clumps of hot material flung out from the star evolve directly into corresponding clumps of dust."

In contrast the Trifid Nebula is region of rebirth, where massive stars of a new generation are forming. Seen by visible light, hot young stars light up a large cloud of gas. It is criss-crossed by dark dust clouds which divide the bright nebula and give it its name. An infrared image from ISO shows a remarkable change in appearance. The dark clouds become luminous and the bright regions are dark. By its trick of penetrating the dust, ISO reveals dense regions inside the obscuring clouds, where new stars are forming.

The Cosmic Egg from which a Star will Hatch

One prize sought by ISO astronomers has been the detection of the earliest stages of star formation. Pre-stellar cores are egg-like objects hidden within a larger dust cloud. A cold, thick shell of dust obscures the interior, where gas collapses under gravity to make an embryonic star. By the time the dust has dispersed, and the object inside has hatched as a plainly visible star, the main event of star formation is complete. In the earliest stages, only radio waves and far-infrared rays can escape from the dust cloud, allowing us to observe the real origins of the stars.

Derek Ward-Thompson of the Royal Observatory Edinburgh (UK) and his colleagues at Cambridge University and in France, first detected the pre-stellar core L1689B, using observations at sub-millimetre radio wavelengths, in the constellation Ophiuchus. It is a very young pre-stellar core, on the brink of collapsing to form a new star. Now the team has used ISO to make the first infrared images of L1689B using the photometer ISOPHOT at long infrared wavelengths, up to its limit of 200 microns. The shell of dust is so cold, at roughly minus 260 degrees Centigrade (or 13 K), it is undetectable even at short or medium infrared wavelengths.

Astronomers can now combine ISO's results with observations of the same object and others like it, at sub-millimetre radio wavelengths, to build up a detailed picture of the earliest stages of star formation.

Ward-Thompson says: "Our fellow-astronomers thought we had no chance of detecting pre-stellar cores with any instruments available today. Now that we've done it, with the radio telescopes on the ground and with ISO in space, a new chapter in the study of star formation can begin. Already our results contradict the theory that a pre-stellar core should spin rapidly. It doesn't. In addition, our observations have shown us that the manner in which a newly-forming star first collapses is different from that which was previously predicted."

Looking for the Origin of Planets

Our immediate cosmic mother was the Solar Nebula, the cloud of gas and dust that supposedly swirled around the Sun at its birth about 4.5 billion years ago. Gravity flattened the gas and dust into a disk, like a giant version of the rings of Saturn. Stony and icy grains of dust congealed to make the Sun's family of planets, including the Earth. The comets are relics of the construction of the Solar System, and ISO has investigated their chemical composition. Yet the concept of planet-making in the dusty disk of the Solar Nebula was just a theory until the advent of infrared space astronomy.

One of the more time-intensive programmes of ISO deals with the existence of disks of dust particles around normal stars. The Sun still has a disk, visible as the Zodiacal Light seen close to the horizon after sunset in spring or before sunrise in autumn. The dust is, however, too sparse to be detected, if one were looking for a similar feature in the surroundings of another star.

Therefore, it came as a big surprise when ISO's predecessor, the Dutch-US-UK Infrared Astronomical Satellite (1983), detected similar dust disks around a few nearby stars, notably Vega and Beta Pictoris, with much more material than the Sun's dust disk. However, the material is much colder than the zodiacal cloud and thus farther away from the stars. In our solar system this would be beyond Neptune.

The disks fascinate astronomers because they show the presence of material around stars that is left over from the time of their formation. This suggests that many stars other than the Sun may have a "solar system" of planets, asteroids and comets around them. Although there are some other ways to get more information about such "solar system" disks, it falls to ISO to take the next big steps. These are to observe many more stars at much higher sensitivity, to establish how often such disks occur, and to see for how long how they can survive the natural processes that tend to destroy them.

The first results show that ISO detects weak disks in some cases. In a few others it sets upper limits to possible dust. Some disks are detected at quite long wavelengths, indicating that they extend to fairly far away from the star. Data-reduction of observations of many more stars is in progress. A preliminary conclusion is that the existence of a disk is a property of many but clearly not all stars.

Background on ISO

Advanced technology created ISO's extremely cold telescope capable of observing cool regions of the Universe. Multinational teams, with leaders in France, Germany, the Netherlands and the United Kingdom, developed the special scientific instruments. A European Ariane 44P launcher put ISO into orbit on 17 November 1995. Requests from the world's astronomers for observations with ISO have always far exceeded the available operating time, even though the spacecraft's controllers at ESA Villafranca supervise an average of 45 astronomical observations every day.