Most astronomical research involves the minutest study of the faint remnants that survive at the Earth of the radiation from huge and exotic objects far outside our Solar System. A suite of ground-based and satellite telescopes gives us images of the sky in bands from metre-wavelength radio waves to gamma rays, and exhibits unfamiliar consequences of cosmic processes.
The naked-eye sky is dominated by nearby stars within our own Galaxy. Stars are born by the collapse of dense gas clouds, and end their lives recycling used (and rather polluted) gas back into the Galaxy, either through a wind or an explosion. The energy released at the end of a star's life can cause strong compression of other gas, perhaps initiating another cycle of star birth. At Bristol we have been working on an H-alpha study of the Galaxy. This images the Galaxy in the light emitted by hydrogen atoms as they lose energy after being hit by an explosion, or irradiated by a bright nearby star. Through the pictures that we make we can piece together a history of recent galactic events. Our study has produced images of supernova remnants such as the Veil Nebula in Vela with exquisite detail.
The strange tracery of excited hydrogen that we see is produced by the collision between the blast wave from a star that exploded about 100,000 years ago and the diffuse gas between the stars. Odd corrugated filaments in the picture may be something to do with magnetic fields in the Galaxy. In the hundreds of such pictures that we have made there is much to discover about violent processes in the Galaxy.
Lower mass stars than those that die as supernovae end their lives as shrunken white dwarf stars after ejecting much gas into a planetary nebula. The H-alpha survey has found more planetary nebulae in the past three years than had been found in the previous century of studies of the Galaxy. An example is shown below. The colours in the picture tell us about the pollutants (oxygen and nitrogen) that are mixed with the hydrogen and helium that the dying star ejected.
Looking further away, the light that we currently see from galaxy 3C351 was emitted at about the time that our Sun was born. We have been studying 3C351 using the orbiting X-ray telescope Chandra and the ground-based Very Large Array (situated in New Mexico) to try to discover what causes it to be one of the brightest radio sources in the sky.
The contours on our picture of 3C351 show its radio emission, which is bright at the centre of the source, the home of the supermassive black hole ultimately responsible for the energy output. But even brighter emission is seen from hot-spots many thousands of light-years from the black hole, and we see radio emission from an extensive region between the hot-spots and the core and to the top of the picture. The radio signal is produced when very fast (velocities within 0.1% of the speed of light) electrons spiral around magnetic fields, and implies that there is an immense reservoir of energetic particles near the galaxy.
The colours on our picture show the brightness of X-rays from 3C351. Intense X-rays come from near the supermassive black hole in the centre of the source, and bright X-rays are seen from the upper hot-spots. But we also see X-rays from all other regions that are radio-bright. These X-rays are produced in a different way, by collisions between the fast electrons and radio waves. Some of those radio waves are produced by the electrons themselves, and some are left over from the Big Bang. The brightness of the X-rays tells us how much energy 3C351 has stored up in relativistic electrons and magnetic fields: the sums show that the energetic equivalent of millions of solar masses has been ejected by the black hole and stored in this cosmic battery, probably for 100 million years. How we study the processes that made this happen is another story!
