X Rays from stars

Some stars, such as our Sun, emit X-rays only weakly. Others are powerful X-ray sources. Recent research shows that those that emit strorigly are the scenes of violent activity.

If a camera sensitive to X-rays, or an X-ray telescope, is sent more than 150 km above the ground and pointed towards the Sun, it will record the fact that the Sun does emit some X-rays. A careful examination of photographs has shown that the X-rays do not come from some source behind the Sun. We are not looking at an X-ray picture of the Sun similar to the kind of X-ray shadow picture we get of the human body in a hospital. What astronomers see is that the Sun's thin outer atmosphere or corona is emitting X-rays. These rays come from patches in the corona which are extremely hot, reaching perhaps four million degrees K. Observations from Skylab, which took simultaneous pictures of the Sun at many wavelengths, show that these hot spots are associated with sunspots and solar flares.

These observations show us that some X-rays are emitted by the Sun and presumably, therefore, by other stars which have 'sunspots' and flares. However, these are the longer wavelength or 'soft' X-rays. which are not so energetic, and X-ray telescopes used in space at the present time are not likely to detect them. On the other hand, such telescopes have observed a considerable number of cosmic X-ray sources that are far more powerful. Some of these are stars, others are associated with clusters of galaxies. Such a source has been found in the Virgo cluster, another in the cluster known as Abell 1060.

The X-ray sources in distant clusters - the Virgo cluster is at about 21 million parsecs (68 million lightyears), the Abell one at 46 million parsecs (150 million light-years) - do not seem to be collections of stars. They come from the central region of the cluster and seem to be generated by very hot, thin gas in which the whole cluster is embedded and which may well be at a temperature of ten million degrees. In one sense, then, the X-rays from the clusters of galaxies have one thing in common with those of the Sun, they both originate from hot gas, though the gas in the cluster is billions of times more extensive and far hotter; its Xrays are 'harder' (of much shorter wavelength). However, we can hardly call them X-ray stars, and astronomers usually refer to them by the vaguer term of 'X-ray sources'.

What of X-ray stars themselves? Many have been observed as intense sources in the harder X-ray region of the spectrum, yet from studies of the Sun, it is clear that such radiation is not from stellar coronas; it would be weaker and of different wavelength. The sources are in fact found to be of two main kinds - the remnants of a supernova explosion or a binary star in which one of the components is either a white dwarf, a neutron star, or even a black hole.

That a supernova remnant can be an X-ray emitter is shown by the famous Crab Nebula in Taurus. In recent years observations in optical and radio wavelengths have shown that at the centre of the residue of a supernova explosion is a pulsar; the original star must therefore have collapsed down to a neutron star. However, it is also evident that there is a vast amount of swirling ionized gas and also an intense magnetic field which, of course, we should expect the neutron star to have.

What seems to be happening is that electrons from the ionized gas are moving in spiral paths through this magnetic field. Some are moving so very fast that they emit X-rays by the process known as 'synchrotron' radiation. Named after the synchrotron machine used by nuclear physicists for accelerating nuclear particles to high velocities, the radiation was first discovered during experiments with it. Physicists found that when electrons were accelerated to high speeds in the magnetic field of the machine, they emitted radiation.

The wavelength of this radiation depends on the speed of the electrons; the faster they move the shorter the wavelength becomes. The fact then, that X-rays are received from the Crab Nebula means that astronomers have more details about what happens after a supernova explosion, namely that electrons are caused to move very fast through a strong magnetic field. This is important for developing further ideas about the way in which stars behave.

Binary stars act as X-ray sources when one of the pair is a white dwarf, neutron star or black hole, because of the transfer of material within the system. The small, very dense white dwarf or neutron star attracts material from its companion, provided the two components are close enough. When this gaseous material moves over to the very dense star, it suddenly becomes heated strongly enough to emit X-rays. The heating will be sudden and intense, but will not last for long. As is the case with many novae it may shine brightly for longer at optical wavelengths. That is why many binary X-ray sources flare up in less than one second and then fade down in a matter of minutes.

Such Xray flashes or X-ray 'bursters' may occur a number of times within a few days. On some occasions, though, a binary develops less rapidly into an X-ray emitter, taking days to come up to full brilliance, and then may fade over a period of months. Then nothing else seems to happen. In such a case, it appears that the transfer of material is not regular: one large transfer may occur, to be followed by a long quiet period.

If one component of the binary is a black hole, then gas and electrons will stream into it from its companion star. In this case the material will swirl very fast round the hole before it falls in - another instance of the X-rays being synchrotron radiation.