Star Formation

Stars begin their lives in molecular clouds, vast reservoirs of gas and dust of which there are many in our galaxy. In the past two decades we have learnt that the formation of stars involves not only the accretion, or inflow, of matter but outflow as well. Outflows from young stars take many forms and are visible right across the electromagnetic spectrum from radio waves to the ultraviolet. When the Sun was only a million years old and before the Earth formed, it too would have produced such jets of matter stretching for vast distances of several light years.

Understanding how a star is born is important not only in itself but because it tells us about the conditions that give rise to planetary systems like our own. The study of star formation has made enormous strides in the past few decades for a number of reasons. New instrumentation is allowing us to peer into stellar nurseries, the dark dusty clouds that permeate the Milky Way, and revealing for the first time the various stages involved. At the same time increased computational power has allowed us to realistically simulate stellar birth. Both observational and theoretical work on star formation is pursued in the School of Cosmic Physics.

On the observational side, our efforts have concentrated on understanding the phenomenon of outflows from young stars. Since star formation involves the gravitational collapse of clouds of gas and dust, it is of course associated with the “inflow” of matter. Paradoxically, however, it seems that such inflows cannot occur without the expulsion, i.e. outflow, of material as well. How outflows are generated is not known but they may be a means of removing angular momentum from the system allowing further material to be accreted onto the new born star. Outflows are visible at a range of wavelengths and are found to have molecular, atomic and ionized components. They are most dramatic in the earliest phases of the star formation process, when a star like our Sun is only 100,000 years old (compared this to the current age of the Sun of 5 billion years). Then the young star is seen to eject enormous jets of gas that stretch for several light years and signal its birth.

Often disks are observed to surround new stars, disks that we think in many cases will eventually form planetary systems like our own. Since disks and jets appear to be inextricably linked, current theories concentrate on understanding how such disks can generate jets.

A Hubble Space Telescope image of twin jets from a young star (T. Ray, Dublin Institute for Advanced Studies). The two cusp-shaped blue nebulae are caused by scattered light from the star and the dark lane between them is the disk. The uppermost jet is brighter because it is coming out of the dark cloud from which the star formed whereas the lower one is dimmer because it is moving in.

Theoretical work in the School of Cosmic Physics in the area of star formation has concentrated on understanding how jets from young stars propagate and how they interact with their surroundings. This involves carrying out simulations of virtual jets using highly sophisticated computer codes developed by ourselves. The codes are run on state-of-the-art machines such as the Dublin Institute for Advanced Studies Beowulf Cluster or the Queens University Belfast/Trinity College Dublin IBM SP2. Although such simulations may take weeks to run on one of these machines, they represent the evolution of a jet over thousands of years.

A computer simulation of a jet from a young star (upper half of the jet only). As the jet propagates supersonically into its surroundings two shocks are formed near the front. The furthest one from the source is the bow shock where material surrounding the jet is accelerated and the other is at the end of the jet where jet material is slowed down. By varying the flow through the jet, knots are formed inside which resemble the knots seen in real jets.

This is an infrared image (showing emission from shocked molecular hydrogen) of a real outflow from a young star to compare with the simulation above. Note the multiple bow shocks that are probably caused by large-scale variations in the flow. The source is optically hidden behind enormous amounts of dust. Its position is marked with a cross.

More recently we have being applying diagnostic techniques to jets, based on the strength of the emission from different elements, to determine their basic physical parameters. For example, we have calculated the amount of matter that flows away from the star via these jets and this would appear to be substantial fraction of the mass of the star over the lifetime of these outflows.