Cold comfort for newborn stars - ISO unmasks the icy veil around the Eagle's fiery heart
07 September 2001This is such an unusual view of the impressive M16 nebula, also known as 'The Eagle', that even the most devoted sky-lovers will be surprised. It shows exactly what in the best known pictures of this famous nebula remains invisible: huge amounts of the cold dust that enshrouds newborn stars.
This image was taken by the European Space Agency's Infrared Space Observatory, ISO, which operated until May 1998 and which is, so far, the best infrared space telescope astronomers have ever had. As an infrared telescope ISO had the ability to see objects and material that other telescopes cannot see, for example, cold dust*. The dust in the Eagle, seen in the picture as a 'bluish fog', is at about minus 100°C. Although perhaps difficult to believe, it is inside freezing dust like this that new, hot stars are born. In this image ISO has captured a view of the ice enshrouding the fire.
The Eagle nebula is an active 'star nursery' located 7000 light-years away, in the constellation Serpens. It is a huge cloud composed mainly of gas with microscopic particles of dust. Surprising as it may seem, its cold temperature is a key requirement for star-birth to actually occur.
Stars are huge balls of hot gas that emit energy because a non-stop nuclear fusion reaction is taking place in their cores, a reaction that can only happen if the atoms in the gas become highly compressed. The cause of this compression is gravity, and in fact the formation of a new star begins when the inward force of gravity pulls together the gas and dust in the nebula. At that very moment a 'battle' between forces begins: when the gas and dust condense they also become warmer, and they start to radiate thermal (heat) energy. An outward pressure, opposed to the inward force of gravity, is therefore created. If the gas becomes too hot this outward force 'wins', and the star-birth process is interrupted because the atoms in the gas will never be compressed enough to trigger a nuclear reaction. That is why the cold environment of the nebula plays such a key role: only if the nebula is cold enough and allows the heat created in the gravitational collapse to be radiated away, can a new star be born. The whole birth process can take anything from a few thousand to tens of thousands of years for the most massive stars, and up to many millions of years for the least massive stars. A star like the Sun takes a few tens of millions of years to form.
ESA astronomer Gvran Pilbratt, who made ISO observations of the Eagle nebula, explains: "in the ISO images we can see the emission of the dust itself. The cold dust is very important if we want to unveil what is actually happening in the nebula. The process of star-formation is still not well understood, and infrared observations are adding vital pieces to the puzzle."
ISO was 'tuned' to detect dust in a certain range of temperatures. For these observations the tuning (temperature) corresponded to emissions arising under very particular conditions. Specifically, ISO was searching for evidence of organic molecules associated with very small dust grains on the outskirts of the clouds of gas and dust, and heated by a star outside of the cloud. The interior of such clouds - where the actual star-formation takes place - is much colder than these outer regions and to see this astronomers need extremely far infrared and submillimetre space telescopes (telescopes sensitive to light with wavelengths in the millimetre range). In 2007 ESA will launch the first space telescope able to detect radiation in this range; it is called the Herschel Space Observatory and is the largest imaging space telescope ever designed.
*Cold objects emit most of their energy in the infrared, and hence only infrared telescopes can see them. ISO could detect a 1 cm thick ice cube at a distance of 1000 km, solely by its heat emission.
About this image
Further analysis of these data is being carried out by Andrea Moneti (Institute d'Astrophysique de Paris, IAP) in collaboration with other members of ISOGAL.
The original observations by Pilbratt et al. were reported on in the scientific journal Astronomy and Astrophysics 333, L9-L12, (1998).
Leo Metcalfe, ISO Project Scientist
ESA Science Communication Service