General Relativity and Black Holes

Stellar-mass black holes are born when the most massive stars in the Cosmos collapse in the raging, brilliant fireworks of a supernova explosion. The supernova marks the end of the very massive star’s life as a main-sequence (hydrogen-burning) star. After such a hole has been born, it can continue to gain weight by devouring its surroundings. In March 2013, astronomers studying a weird black hole system, announced that they had spotted a disk of matter circling the entire system. Such a structure has never been seen before, and it is believed that this mysterious disk is the hideous tattle-tale result of the black hole devouring its doomed binary-star sister and victim!

In the 18th century, John Michell and Pierre-Simon Laplace predicted the existence of black holes, which are gravitational monstrosities. Albert Einstein’s Theory of General Relativity also went on to describe the presence of bizarre objects with such deep gravitational wells that anything luckless enough to wander too close to its maw would be devoured. However, the real existence of such objects seemed so far-fetched that Einstein himself rejected the concept.

In 1916, Karl Schwarzschild calculated the first modern solution of General Relativity that could describe a black hole–although its interpretation as a region of Space from which nothing–not even light–could escape was not fully grasped for almost half-a-century. Such bizarre and fascinating objects were dismissively viewed as mere mathematical oddities for decades. It was not until the 1960s that theoretical calculations showed that black holes are a generic prediction of Einstein’s General Relativity.

Only the most massive stars in the Universe collapse to form black holes. A star is a gigantic ball of searing-hot, incandescent gas that is pulled in very tightly by the force of gravity. This makes the hidden heart of a star extremely dense, as well as fiery-hot. Stars are so hot that they catch fire in a process termed nuclear fusion, whereby atoms of lighter elements meld together to spin increasingly heavier and heavier elements (stellar nucleosynthesis). The fundamental fusion process begins with atoms of hydrogen. Hydrogen is the most abundant, as well as the lightest, atomic element in the Universe–and the stars fuse hydrogen in their hot, hidden hearts to form the next-lightest-of-all- atomic elements–helium.

Nuclear fusion liberates energy. This is the reason why stars shine with their brilliant glittering fire. The energy that is released exerts an outward pressure on the glowing, very hot star. This pressure creates a delicate balance, with gravity pulling in, as radiation pressure pushes out! This delicate balance between pressure and gravity continues for as long as the star “lives”. The precise balance between pressure and gravity is determined by the mass of the star. The most massive stars are squeezed most tightly. This process speeds up the nuclear reactions, which churns out more and more radiation. When a star finally burns up its nuclear fuel, gravity wins the war over pressure. The mass of the star determines just how great the victory of gravity will be over its opponent.

Small stars, like our own Sun, fuse their hydrogen blissfully for several billion years–our Sun, a middle-aged small star, is almost five billion years old, and will live for approximately an additional five billion years. When the process reaches its inevitable conclusion, the Sun–like other small stars of its kind–will experience a core collapse that results in the formation of a dense, hot little stellar corpse called a white dwarf. A white dwarf is about as massive as the Sun, but only approximately as big as our planet.

Any star with a core weighing about 1.4 solar-masses is doomed to collapse into an object even more extreme than a white dwarf. A star that–in its main-sequence life–was several times more massive than our Sun, leaves behind this hefty relic core when it blasts itself to smithereens in a supernova explosion. The massive star’s corpse is not a white dwarf, but an even denser object called a neutron star. A neutron star is a bizarre object in which gravity has squeezed out the space between the particles constituting atoms, giving rise to a sphere of solid neutrons. A neutron star is more massive than the Sun, but only about as big as Philadelphia. A teaspoon of neutron star stuff would outweigh a herd of elephants.

But the most massive stars of all die an even more extreme death, and their collapse doesn’t end with the formation of a neutron star. The cores of the most massive stars in our Universe collapse to infinite density–forming a black hole! With no more nuclear fuel to burn, any star with a mass that exceeds at least 10 to 15 solar-masses, cannot hold its own against the crush of gravity at all. The massive dead star must become the greatest of all gravitational monstrosities. All of the matter that once composed the extremely heavy star before its spectacular demise is literally crushed out of existence!

Such a star may or may not hurl its outer layers into Space. The star may be so extremely massive that the momentum of its collapse squeezes the core tightly enough to give rise to a black hole. When this occurs, the shock wave that usually hurls the outer layers away from the unfortunate star’s core may not be powerful enough to overwhelm the black hole’s immense gravity. The entire star may be devoured by the black hole! Alternatively, the star may explode, but much of the searing-hot gas may plummet back on to the black hole.

Some astronomers think that the explosion from the layers surrounding a newly born black hole may be much more energetic than a typical supernova. These explosions may be the cause of gamma-ray bursts–energetic blasts of high-energy gamma rays that can even outshine numerous galaxies sparkling with billions of dazzling stars!

Black Holes Keep Their Secrets Well

According to General Relativity, the horizon of a black hole of any size is the surface that keeps the interior of the hole separate from everything that is outside of it. It is not a material boundary; unlucky travelers tumbling into the mouth of the beast would not experience anything odd on their journey past the boundary. But once having done so, they would no longer be able to communicate with anyone on the outside. Neither would they ever be able to return. An observer on the outside would only be able to receive messages dispatched by the doomed travelers before they crossed over the horizon. In the classical Theory of General Relativity, a gravitational singularity will form occupying no more than a point when the massive star collapses to become a stellar-mass black hole.

A study published in March 2013, reported a weird and never-before-seen structure in the disk of matter encircling a black hole system. Swift J1357.2 is an x-ray binary system that regularly dispatches high-energy outbursts, and it is composed of a black hole that is slowly but surely slurping up its companion stellar-sister. Matter from the victimized star falls into the accretion disk, which encircles the voracious black hole, feeding it a banquet of gas and dust.

While observing this tragic system, a team of astronomers spotted a bizarre vertical feature moving around through the material.

“It’s the first time we can resolve such [a] structure in an accretion disk, and it might be ubiquitous in X-ray binaries during the outburst state,” Dr. Jesus Corral-Santana, lead author of the study, commented in the February 28, 2013 Dr. Corral-Santana is at the Astrophysical Institute of the Canary Islands in Spain.

The hungry black hole of this system likely weighs about three solar-masses, and is situated about 4,900 light-years from Earth in the Virgo Constellation. The unfortunate sister companion-star is small–only about 25% as massive as the Sun. This little sister star orbits the pair’s center of mass every 2.8 hours–which is one of the briefest known orbital periods for such systems.

The black hole sucks up material from the little sister star into its accretion disk, and occasionally emits bursts of X-rays that enabled the team of astronomers to discover this otherwise hard-to-observe system.

Dr. Corral-Santana and his colleagues took hundreds of optical images of Swift J1357.2, using the William Herschel and Isaac Newton Telescopes located in the Canary Islands.

The discovery can only be observed in the outer, optical region of the accretion disk, not on the inside, where the X-ray bursts are originating. The X-ray emission, which displays no periodic variation, suggests that a vertical structure is veiling the voracious black hole. Instead of appearing at a predictable, set time, the strange structure reveals itself over a steadily increasing period–indicating a wave-like movement through the accretion disk.

The paper describing this discovery,”A Black Hole Nova Obscured by an Inner Disk Torus,” J.M. Corral-Santana et al., was published in the March 1, 2013 issue of the journal Science.