Our understanding of the Universe is written in pencil on the pages of a loose-leaf notebook because, like many of the sciences, our knowledge of nature is constantly undergoing revision. For example, during the past few years a quiet revolution has been taking place regarding galaxies, black holes and their increasingly understood symbiotic relationship. Black holes, which were once considered scientific novelties, are now seen as fundamental forces shaping the Universe. Observations obtained since the turn of the 21st century have offered evidence that virtually all galaxies, such as M83 pictured here, harbor a king size black hole at their center- including our Milky Way!
Ask any kid about the coolest thing they’ve learned in science. Go to an elementary school and they will tell you about dinosaurs, particularly the T Rex. Go to a middle school and somewhere high on their list will also be a carnivore. But this one doesn’t have legs- it’s called a black hole.
When Einstein developed relativity theory, it took him about ten years to work out the math using a daunting from of mathematics called tensor calculus. He was only able to approximate the solutions to his own equations and the math still perplexes even the best scientific brains. However, the challenge did nothing to deter one of Einstein’s contemporary astronomers- a theoretical physicist name Karl Schwarzschild. Schwarzschild was a practical individual by nature. He pioneered new methods of studying spectra, for example. But he excelled in his abilities to deal with theoretical concepts and when Einstein’s articles on general relativity were published in 1915, Schwarzschild was one of the first to recognize their importance.
Schwarzschild was also a German patriot, so he set down his astronomical studies and enlisted in the army during World War 1. By the time he had read Einstein’s papers, he had already seen action in Belgium, France and the Russian front. Nonetheless, he was attracted to the essentialness of general relativity and began to seek exact answers for its equations. Two months after contracting a life threatening disease and being sent home to recuperate, Schwarzschild was finally able to concentrate on his calculations. Shortly before his death in 1916, Schwarzschild completed his work and it was published later the same year. Titled On the Field of Gravity of a Point Mass in the Theory of Einstein, it became one of the pillars of modern relativistic studies and in it, Schwarzschild presented his solutions to Einstein’s unfinished equations.
Significantly, it provided support for a, then, seemingly implausible situation about the effects of severely compressed matter on energy. Simply stated, Schwarzschild concluded if matter were sufficiently squeezed, it would have a gravitational force so strong that not even light would be able to escape it. He was the first to recognize that general relativity theory allows matter to be packed into an infinitely small space. Scientists now refer to this as a singularity- an object with zero-volume but all of its mass. Schwarzschild also explained that a singularity was surround by a spherical gravitational boundary that forever trapped anything that ventured within. This boundary was called the event horizon. He presented a formula that enabled the size of an event horizon to be calculated. This is now known as the Schwarzschild radius.
The formula for the Schwarzschild radius is very straightforward: 3 times M (where “M” is the mass of the sun and the result is expressed in kilometers). For example, if the Sun were shrunk to a singularity, it’s event horizon would occur at three kilometers above its surface. Interestingly, it would not disturb the orbit of our planet and we would not suddenly be sucked into oblivion! Similarly, the Schwarzschild radius of the Earth is a third of an inch: if the Earth were to be similarly compressed, the sphere of its event horizon would be about the size of a marble!
However, scientists would not grasp its significance in the role of stellar evolution for about fifty years and have only recently realized its dramatic impact on the development of the Universe, itself!
Despite the radical predictions contained in Schwarzschild’s papers, the scientific community regarded it as a curiosity rather than outrageous. Although, the idea of a singularity troubled many scientists, including Einstein, because it flew in the face of their experience- after all, the world is finite and everything can be weighed and measured. Leading thinkers of that period could not imagine conditions that would create a singularity but now we know they are common throughout the Universe. Where? In the fates of very massive stars!
Most astronomers believe that the Universe began about 15 billion years ago with the Big Bang. All the matter in the Universe, all the space and even time, itself, was released during this event.
Hundreds of millions of years followed before matter started to collect into vast clouds then coalesce and collapse, under its own weight, into the first stars. Stars are born when a massive cloud of hydrogen gas, the most abundant element in the Universe, falls in upon itself until the pressure and temperature at its center becomes so intense that a nuclear explosion is triggered. Since clouds that produce stars contain an unbelievably huge amount of material, the nuclear explosion is ceaseless and in the process, begins to change the cloud’s hydrogen into helium. The force of this ongoing, relentless release of energy pushes outward and is powerful enough to prevent the cloud from further shrinking. Once the radiation from the internal explosion reaches the cloud’s edge, it escapes into space as light and thus a star is created.
These first stars were huge, hot and powerful. They consumed vast amounts of material and therefore only shined for a few hundred million years. For comparison, our Sun has been beaming for over four and a half billion years.
When the non-stop internal nuclear explosion that powers a star has converted all of its hydrogen into helium, the star inflates and begins a new round of energy release by converting its helium into even heavier elements. Over time, the nuclear furnace inside a star will only contain iron, and because iron cannot be used as a nuclear fuel, the star will stop releasing enough energy to prevent it from beginning to rapidly collapse again. If a star contains less that two or three times as much material as our Sun, then the force of its sudden inward rush will rip it apart in an titanic explosion called a supernova. The explosion exposes the star’s core- a dense, city-sized ball of material made only of atomic neutrons– and it slowly begins to cool.
According to current thinking, if the star contains more than three times as much material as our Sun, then it will also explode when it runs out of nuclear fuel. But, following this explosion the star’s core will continue to shrink until it becomes a singularity- something fantastically dense with the weight of a planet per thimbleful! This also produces intense gravity- so strong that anything coming near is pulled toward its event horizon then further inward where nothing escapes, not even light. Thus, the star becomes a practically invisible, black, bottomless pit, just a fraction of its original size. Astronomers call this a black hole!
When the nuclear fuel powering the first stars was exhausted, the explosion that followed provided the material for the next generation. Because of their size, the cores of the first stars continued to compress becoming massive black holes, perhaps 200 times the weight of our Sun or more. Black holes can weigh as little as a few Suns. But these initial versions might have been 100 times as massive. During the tens and hundreds of millions of years that followed, new stars were born from the detritus of the first parents. Locally denser regions of gas contracted and new stars arose in clusters and these were attracted to other stellar associations. Eventually, congregations of many thousands of stars developed and began to look and behave like something that could be called a sub-galaxy with an enormous black hole purring at its center.
Most galaxies are located in a group with other galaxies and even these are organized into larger associations called super clusters. They are formed at the junction of large gravitational bubbles that seem to fill the Universe. Galaxies are in constant motion within their cluster and, over time, they may approach, collide and combine. This is the way galaxies grow and evolve- most galaxies have interacted with others at one time or another since they were formed. Mergers are thought to have contributed significantly to the growth of galaxies- the early universe was much smaller and incredibly crowded therefore galaxies were more likely to collide.
If two galaxies merge, so should their central black holes. Recent computer modeling speculates the event would be violent, unleashing tremendous light as trapped gas rushes between the two black holes.
Galactic mergers take millions of years to complete but there are plenty of examples for astronomers to study. For example, the Chandra X-ray space telescope recently identified a pair of super massive black holes near the center of a merged galaxy.
Our galaxy is a member of a cluster called the Local Group and M-31, our nearest large galactic neighbor, is approaching the Milky Way at about 120 kilometers per second. About three billion years in the remote future, M31 and the Milky Way will meet. Computer simulations indicate that this will cause tremendous chaos. The chance of individual stars impacting each other is extremely low because the space between stars is incredibly large. However, the space between the stars is not empty- it’s filled with gas and dust remnants from exploded stars. This material will interact when the two galaxies begin to blend. This gas and dust in one will begin to tug and pull on the other and this will result in a disruption of their shape. Friction will also induce shock waves that trigger new stars to form.
Some stars and dust will be spewed into intergalactic space. It will also drive dust and gas toward the central regions of the galaxies. This will also produce new stars and feed the black holes that existed in each, enabling them to grow. As material grows closer to a black hole, it gains incredible speed so that some of it is able to avoid being sucked into the black hole’s event horizon. The portion that escapes, however, is flung far into outer space as a powerful jet of material that is thousands of light-years in length. Most galaxies do not have active jets at any given moment, but all galaxies probably have had jets at one time or another.
Eventually, the central black holes will merge and our galaxy, combined with M31, may, for a period of time, resemble a quasar.
The central region of spiral galaxy M-83, pictured here, lies beyond our local group of galaxies and is located approximately 15 million light years in the distance toward the southern constellation of Hydra. It is about eight times farther than M-31. I produced this image during the summer of 2006 using my remote controlled 20 inch Ritchey-Chretien telescope with a SBIG STL-11000 eleven mega-pixel astronomical camera. The total exposure lasted around twelve hours and a wider view can be seen here. I have avoided (and will continue to do so) using my own images to illustrate article discussions, but believe this picture represents an decent amateur-astronomer peek at the lair of a super massive black hole in a near-by galaxy.
Written by R. Jay GaBany