SS433 is the 433rd entry in a catalogue by N. Sanduleak and C. Stephenson of stars having emission-line spectra. Such stars have a spectrum consisting of an underlying essentially black-body component — the optically-thick surface layer of a star — plus emission lines coming from an optically thin gas surrounding the star. The stars are catalogue-worthy because the gas represents some interesting interaction between the star and its neighbourhood.

The interaction causing an emission-line spectrum may have any of many causes.  Some of the stars in the SS catalogue are rotating so rapidly that centrifugal force at their equators counteracts their attractive gravitational force, so that the stars shed an equatorial disk of gas which shows as the emission-line region. Some, like SS433, are binary stars with matter flowing around and between the two stars. Since hydrogen is the most common element in the Universe, the stars in the SS catalogue typically show hydrogen emission lines, the strongest line of which is H-alpha at 6563 Å. Helium lines are also typical.

SS433 languished as one star among many hundreds in the SS catalogue until its rediscovery in 1978. Spectra taken then show the Balmer series and spectral ines from neutral helium, both arising from the circumstellar material ripped from one star by its companion. There were also present in its spectrum other lines which were unidentified, particularly because they seemed. to come and go sporadically.

It was only after several months of monitoring the spectrum of SS433 that the pattern became apparent. The unidentified lines appear in pairs, the most prominent of which shift cyclically in position about a wavelength which is somewhat to the red of the H-alpha line. The cycle has a period of 164 days, and the range over which each member of the pair shifts is more than 1000 Å. The lines are antiphased. With this clue to the interpretation of the two strongest of the unidentified lines, the other unidentified lines could then be paired off. They show similar behaviour, oscillating in antiphase about wavelengths which lie to the red of the other Balmer lines and the neutral helium lines.

Thus the emission line spectrum of SS433 consists of three components — a stationary set of Balmer and helium emission lines arising from the interaction of a double star, and two antiphased oscillating sets of moving spectral features associated with the Balmer and helium emission, and arising from a new phenomenon.

The interpretation of the moving features is in terms of a pair of equal and opposite jets of material shooting from SS433. Because of the speed of outflow of material, the spectral emissions of hydrogen and helium gases in each jet are Doppler-shifted from their rest wavelengths, one (the approaching jet) generally to the blue and the other (the receding jet) generally to the red.  The jets precess like a spinning top in a conical motion with period of 164 days and so the features move in antiphased cycles of this period.

Curiously, even at the moments when the jets are in the plane of the sky, with no component of speed towards or away from us, there is a shift.  It has its origin in the phenomenon of relativistic time dilation.  The fast-moving hydrogen atoms in the jets have 'clocks' — the natural time-scales of atomic phenomena — which are running slow with respect to ours and so emit H-alpha photons of lower frequency.  The speed of the material in the jets is an astonishing one quarter the speed of light!

Interest was focussed in SS433 because it was identified with an X-ray star centrally within a spherical radio supernova remnant called W50. This led to the suggestion that SS433 is the stellar remnant of the same supernova explosion which gave rise to W50. Perhaps SS433 contained a black hole produced in a supernova explosion on one of a pair of stars. In this model, the black hole accretes material expelled by its companion (this material gives rise to the stationary emission lines which earned SS433 its place in the SS catalogue).  As it approaches the boundary (event horizon) of the black hole, in-falling material is accreted in a.disc around the black hole and compressed by its gravitational field.  Accretion yields the X-rays which are the reason why SS433 is an X-ray star. The accretion is over the rate which would generate the Eddington luminosity of the black hole, and pressure of the radiation generated by heating of the in-falling material drives the relativistic jets, which are collimated by the holes in the accretion disk.

Not only is SS433 an X-ray star, it is also a radio star indeed. Techniques of very Long Baseline Interferometry have resolved the radio image of SS433 into a blobby, elongated structure in which the blobs move out in a cone, with transverse speeds of 0.0088 arc sec day^-1 — matching the 0.26c (light speed) jet speed at the 5 kiloparsec distance of the star.

One interesting question to which the Isaac Newton Telescope on was put in June 1985 was to attempt to link the optical jets directly with the radio ejection process. The star was followed on 14 consecutive nights to show the evolution of the moving features, at the same time that the radio star was observed across the European VLBI network.  The optical data show two events (the clearer of which appears in the graphs with this article), in which the moving features break up and re-form at different wavelengths. On night A of the accompanying spectra, obtained with the IPCS and the Intermediate Dispersion Spectrograph, the two jets are represented by emission lines at 6790 and 6880 Å. On night B the jets had changed orientation just a little and the emission lines had moved to 6795 and 6860 Å. On night C, faint emission lines can still be seen at 6790 and 6860 Å, but the main lines are at 6695 and 6900 Å. By night D the original jets are only faintly present and the main jets have opened to 6660 and 6930 Å.

The picture behind this sequence is that the jets of SS433 are somewhat like tracer bullets in the sweep of a machine gun. Once fired, the bullet flies in its trajectory, decoupled from the subsequent behaviour of the gun, which is revealed by later bullets.  Between nights B and C therefore, on this interpretation, the machine gun had shifted to a different position, and a bullet was fired, with an orientation in the sky defined, through the projected radial velocity, by the wavelengths of the main emission lines on night C. The bullet is predicted to appear in the VLBI observations at that orientation after the appropriate time delay, and to coast outwards from the central radio image. The prediction is currently being put to the test by analysis of the huge volume of VLBI data.
 

In 1982, an international research team, with leading members from the RGO, weighed the centre of a quasar, and the discovery strengthened the theory that the immense and concentrated power of a quasar is due to gas swirling around a very massive black hole in the centre of the host galaxy, NGC 4151.

The team have been studying the galaxy since 1978. To qeigh its centre, they investigated gas clouds very close to the galaxy's core. They found that the clouds are moving at speeds up to 14,000 km/sec. In a crucial new step, they also ascertained the distances of the clouds from the core by finding the time it took for a brightening of the core to 'light up' the clouds.

It truns out that the slower-moving gas clouds lie farther from the core -just as the slowest planets in the Solar System are those that are farthest from the Sun. This means that there is some very massive object at the centre of the galaxy NGC 4151 (analogous to the Sun at the centre of the Solar System). The speeds and distances of the gas clouds show that this object is 1,000 million times heavier than the Sun - and the only kind of object which can be so massive, yet sufficiently small, is a black hole.

In May and June 1979, astronomers were particularly lucky to pick up a major outburst in the galaxy which was the studied by the IUE satellite. In 1984 another exciting result, made possible by the fading of NGC 4151, followed the first spectra taken with the resited Isaac Newton Telescope. These showed an enormous change in NGC 4151 compared to spectra taken ten years previously when the INT was at Herstmonceux. In 1974, the spectra showed clear signs of the rapid motions in the centre of the galaxy mentioned above. In 1984 these had all but disappeared and the spectral classification of the galaxy had changed from Seyfert Type 1 (broad spectral lines, indicating rapid motions) to Type 2 (strong narrow emission lines but no evidence of rapid motions). This result the suggested that other Type 2 Seyfert galaxies might all be presenting fossilized evidence of previous quasar-type activity in their centres. In the Seyfert Type 2 galaxies, the emission lines were known to come from volumes hundreds or thousands of light years in size, and so of course cannot change rapidly even if the original activity in the core completely stops. The fossil activity would continue for hundreds or thousands of years. 
 

Quasi-stellar objects, or quasars, are the most luminous sources in the universe. Most astronomers now share the view that quasars are the nuclei of distant galaxies where energetic processes, such as accretion of gas by a massive black hole, produce more light than the total amount emitted by all the stars in the galaxy (a typical galaxy contains about one hundred billion stars as luminous as our Sun). Because of the still mysterious nature of their central 'power-houses', quasars are objects of great interest at present. However, in recent years astronomers have devoted large efforts to the study of quasar spectra for an entirely different reason.

Being so luminous, quasars can be observed over much larger distances in the universe than any other class of object. We now know that the universe we live in is expanding in a manner analogous to a balloon being inflated - the further two points are on the surface of the balloon, or two galaxies in the universe, the faster the distance between them increases with the expansion, and therefore the faster they recede from one another. Proof of the extremely high velocities of recession of quasars is readily provided by the very high red-shifts of their spectra. As an example of this, Fig. 1 shows the optical spectrum of a quasar identified by Boksenberg and Sargent with the Isaac Newton Telescope  (the object was recognized as a quasar candidate by C. Hazard and R.  McMahon.) The most obvious feature in the spectrum is the broad emission line of neutral hydrogen labelled Lyman-alpha. The rest wavelength of this line - that is the wavelength at which it would be recorded in objects at rest relative to an observer on Earth - is 1216 Å, in the far ultraviolet.  In this quasar, the same spectral line is instead observed at 5691 Å implying that, since the time when the quasar light was emitted, the universe has increased in linear size in the same proportion as the wavelength of the Ly-alpha photon, that is by a factor (5691/1216) = 4.68 = (1 + z).  At a redshift z=3.68, this quasar is one of the most distant objects in the universe known to date.

Because of the tremendous distance over which the quasar photons have travelled, the time taken to reach the Earth is a large proportion (about 9/10) of the total time since the expansion of the universe began.  Furthermore, in its journey to the INT on La Palma, the light from the quasar has passed through intervening matter in the universe, matter which is too distant to be observed directly, but which has left its characteristic signature in the quasar spectrum in the form of absorption lines. It is easy to see now the importance of quasars as 'cosmic beacons': their spectra offer a unique oportunity to view the universe at much earlier times and provide clues as to how its properties have evolved over a significant fraction of its history.

Over the last five years there have been several major steps forward in our understanding of quasar absorption spectra, made possible in large part by the availability of a new type of detector - the Image Photon Counting System (IPCS), developed jointly by University College London and RGO.  The IPCS is particularly efficient at detecting faint light signals with the maximum possible accuracy; this is especially important for QSO absorption line work, since both high-spectral resolution and photometric accuracy are necessary to register the weak and narrow absorption lines seen in the spectra of generally faint quasars.

It now appears that there are at least two kinds of absorption lines, formed in physically distinct intervening regions. In a typical quasar spectrum most of the lines at wavelengths longer than the emission Ly-alpha line can be readily grouped in well-defined absorption systems, with all the lines within a system appearing at the same redshift, generally lower than that of the quasar itself. These lines can be convincingly identified with those of the most abundant elements - such as H, C, N, O and heavier species, up to and including Fe and Zn - in the stages of ionization prevalent in the interstellar medium of our Galaxy.  Furthermore, the metal-line systems are clustered in redshift in a manner consistent with the present-day clustering of nearby galaxies. Thus, it appears that they are most likely formed in intervening haloes of galaxies randomly distributed in line of sight to the quasars. This in turn implies that galaxies possess tenuous gaseous haloes extending far beyond their optical dimensions, a conclusion supported by recent ultraviolet observations of the halo of our own Galaxy. Another important result deduced from studies of the quasar metal-line systems is that in a few cases where it has been measured with the required accuracy, the chemical composition of the interstellar gas in these distant galaxies has been found to be similar to that of the interstellar medium near our own Sun. This indicates that even when the universe was only 1/10 of its present age, at least some galaxies had already undergone a significant amount of metal enrichment via stellar nucleosynthesis.

The second class of quasar absorption lines is found only at wavelengths shorter than the emission Ly-alpha and consists of single Ly-alpha absorption lines, with no obvious associated heavy-element lines. These Ly-alpha lines are far more numerous, by a factor of about 50, than the metal-line systems. Furthermore, they are not clustered like galaxies, leading to the suggestion that they represent an intergalactic population of primordial clouds. Much effort is currently being directed towards determining stringent upper limits to the metallicity of the Ly-alpha clouds. If this is indeed pristine gas, which has not condensed to form stars, it should still bear the imprint of the original composition determined  by nucleosynthesis in the early universe and can thus serve as a powerful test of the predictions of the hot Big-Bang model. One result which has now been established beyond doubt is that the comoving density of the Ly-alpha clouds exhibits a marked cosmological evolution, in the sense that the clouds became progressively less abundant as the universe expanded. This result has important implications for a problem of much current interest: how structure, in the form of galaxies, clusters and superclusters, formed and evolved from an initially smooth universe.