Extreme Astronomy

HUNTING BLACK HOLES, EXPLODING STARS AND MORE

The Universe is full of action and excitement, however peaceful it is to look up into a starry sky. In far away galaxies, whole star systems are swallowed by gigantic black holes that are millions of times heavier than the Sun. The biggest stars live fast, die young and leave a good looking corpse (a neutron star or a black hole), but only after a spectacular explosion that releases more energy in a few moments than the Sun will emit in its whole life. Most of the time, this cosmic violence results in the production of lots of high-energy radiation (e.g. X-rays and gamma rays) and exotic particles like neutrinos, but not much that we can readily see here on Earth. Most of the time.

The X-ray view of the heart of the Crab Nebula, the remains of a supernova that was seen night and day 950 years ago.
(NASA/ CXC/ASU/ J. Hester et al.)

Nearly 1000 years ago, in May 1006, a new star suddenly appeared, and in a few days became the brightest star ever recorded - in fact it was a supernova, and for about three months was visible even in daylight. It was visible to the naked eye at night for another three years after its dramatic appearance.

In all, there have been seven historical supernovae over the last 2000 years that were close enough for everyone to see. You might remember the last one that exploded in 1987 if you were lucky enough to be in the Southern Hemisphere at the time. All were easily seen at night for months, and the brightest few were also visible during the day. However, only a tiny fraction of the supernova energy is radiated as visible light, and most of the energy goes into ejecting the gases in the outer layers of the star. The gas is blown out at up to 15,000 km/s (about 30 million m.p.h.!) and is heated to over one million degrees as it plunges into the gas in the environment, producing a huge amount of X-rays.

COMPACT BINARIES - THE MOST EFFICIENT POWER HOUSES IN THE UNIVERSE?
After stars die, the kind of corpse they leave behind depends on the mass of the original star. From lightest to heaviest the possibilities are: white dwarf, neutron star or black hole. In a binary system (one that has two stars circling each other) an interesting event occurs if one of the two stars dies. If they remain in position after the explosion, then a compact binary could be formed, allowing the corpse star to feed on the companion star, like a cosmic vampire! They are powered by the gravitational energy gained by the gas while "falling" from the companion to the compact star, often via an accretion disc. This is formed when the material from the companion star overshoots the dead star and forms a graceful curve as it is sucked in.

An apple falling onto a neutron star would make a bigger bang than 500 million Hiroshima bombs - not something Isaac Newton would want to fall on his head! Neutron stars are about twelve miles across and about one and a half times heavier than the Sun, while white dwarfs are much bigger, about 7500 miles across and half as heavy as the Sun. This means that the gravity at the surface of a neutron star is lots more than on a white dwarf.

Compact binaries with neutron stars or black holes are called X-ray binaries because the gas becomes X-ray hot as it falls in and most of the energy is produced as X-rays - an X-ray binary with a combined mass of only two times that of the Sun can be up to 500,000 times more powerful. Compact binaries with white dwarfs produce mostly ultraviolet and visible light and are called cataclysmic variables (CVs).

FIND YOUR OWN CV!
Cataclysmic Variables have a whole repertoire of cool ways to change their appearance, and best of all, you can see them do it with an ordinary telescope, unlike X-ray binaries. One of their favourite tricks is to let the accreted matter accumulate and explode it all off at once. Some of them blow off a little and often, having an outburst every few months where the brightness increases by a factor of about 10 for a few days: these are called dwarf novae. Then there are the classical novae, that are thought to explode only once every 3000 to 10000 years but become up to 100 million times brighter. Amateur astronomers discover many new dwarf novae and classical novae each year. The real fireworks happen when the white dwarf piles on so much extra mass that it collapses into a neutron star - the result is a second type of supernova, like the brightest one ever in 1006.

HOW DO WE X-RAY THE UNIVERSE?
X-rays and Gamma rays are very high frequency light that we cannot see - extremely dangerous to life. Luckily for us, the Earth’s atmosphere protects us. But unfortunately for astronomers, it does mean that we need to put telescopes into space if we want to study them. Early X-ray telescopes were sent up on rockets and balloons for a couple of hours at a time, but nowadays we use hi-tech satellites such as the European Space Agency’s X-ray observatory XMM-Newton (see picture) that run for several years. X-ray images such as the one shown on page one are taken with CCD cameras, like very advanced and expensive versions of the digital cameras available today.

IF BLACK HOLES ARE BLACK, HOW CAN WE SEE THEM?
If you were to take any star in the sky, and squash it into a small enough space, you would get a black hole. Once you get close enough to a black hole, the gravity is so strong that nothing, not even light, can escape. This point of no return is called the event horizon; for our Sun the event horizon is about 2 miles, some 460,000 miles below its surface. We know of two kinds of black hole. One kind is millions of times heavier than the Sun, and is found snacking on stars at the heart of many galaxies, while the other kind is "only" about 10-100 times heavier than the Sun and is formed by the supernova death of the very heaviest stars.

Although we can’t see black holes themselves, we can see their effects on surrounding objects. Most stars exist in pairs, so if one turns into a black hole it can feed off the other one: the gravity of the black hole pulls gas from the companion, forming an accretion disc of material that spirals into the black hole, feeding the monster. X-ray binaries with black holes in them can be many times more powerful than ones with neutron stars because the black holes are 10-100 times heavier: they can be up to 50 million times more powerful than the Sun! The giant black holes at the centres of galaxies perform the same trick, swallowing nearby gases and stars. In galaxies like ours, the beast is docile, but in others, the black holes are VERY active, spewing out great jets of material at a respectable fraction of the speed of light.

An active galaxy with huge jets of material being ejected by a black hole.
(STScI/NASA)

These Active Galactic Nuclei (AGN) produce more energy than the rest of the galaxy put together! They often look like stars because some are so bright that they can be seen from billions of light years away. They are bright in X-rays, visible light and radio band. The X-rays in AGNs change the most - we reckon that the quickest variability is 50 seconds per million Suns of mass in the black hole, so for a black hole as heavy as 100 million Suns, we’d expect to see changes over 1-2 hours.

WHAT ELSE CAN WE X-RAY?
The brightest thing in the X-ray sky is the Sun, because it is so close. The X-rays are produced by gas that is more than a million degrees Celsius in the corona of the star (the bit you can see during total eclipse- as seen in the picture). The gas is heated by the ever-changing magnetic fields of the star. Sometimes a whole lot of energy can be released in one go as stellar flares; the star can become hundreds of times brighter in X-rays during a flare! We can only see X-rays from ordinary stars if they are relatively close (a few hundred light years - still a long way.) For example the X-ray power of the Sun is the same as about 50 power stations per person on Earth - and stellar flares can be 10,000 times more than this!

An X-ray view of the Sun.
(ISAS/Yokoh team/Lockheed)

The picture above shows an X-ray view of the Sun: you can see a huge amount of detail as the X-ray hot gases in the Sun’s corona follow the fantastically complicated magnetic fields. However, the Sun is about a million times fainter in X-rays than in visible light; so for us to see other star systems in X-rays they have to be doing something amazing. When X-ray astronomy took off - literally - in the 1960s , we found the universe to be more dynamic and extreme than we’d ever imagined!