The black hole, a special kind of celestial object discovered in the 1960s, although its existence was predicted in the 1930s, has captured the attention of today’s physicists and astronomers. But how exactly does the black hole, which radiates unimaginable levels of energy as it swallows light, come into existence and grow?

• 1. Yin and Yang
• 2. A discovery that originated in theory
• 3. Celestial objects that radiate vast levels of energy
• 4. A mystery solved and a mystery remaining

Zhu Xi (1130 - 1200), a leading light in China during the Song Dynasty, theorized that qi (energy) possesses mobility, and that li (principle) brings order to the movement of qi. He held that although qi and li are two entirely separate entities, neither can exist independently without the other. Zhu Xi’s theory applies to modern-day physics as well.

In ancient China, the taiji (supreme ultimate) diagrams, also emphasized in the Doctrines of Zhu Xi, a work on Confucianism, were used to represent the origin of the universe in terms of the yin and yang principle. Shown here is one of the taiji diagrams. This beautifully encapsulates the yin and yang principle―that yin is in yang, and yang in yin―and relates to Zhu Xi’s notion of the relationship between qi and li.

Black holes are celestial objects in space. The phrase “celestial object” probably evokes images of the sun and the moon. However, black holes differ from these objects in that their shapes cannot be observed. Even if they were visible, they would probably appear as round pitch-black holes. Capable of swallowing absolutely anything, the black hole is the final vestige of a gigantic celestial object.

When a giant star at least 20 times as big as the sun^* burns out to cause a huge explosion―a supernova explosion―it is compressed into a single point and a black hole is formed. This would be equivalent to compressing the earth into a sphere with a radius of 5 millimeters or less. The force of gravity at this point is so tremendous that nothing can escape. The black hole created in this way is called a stellar-mass black hole.

• *Opinions vary as to the size of the giant stars that are the origins of black holes. Although estimates range from 10 times to more than 30 times the mass of the sun, Professor Makishima believes the stars would have to be at least 20 times the mass of the sun.

It would seem that, due to its very nature, this black object’s location would be unknowable. However, a black hole can be detected from its glittering periphery. Why is such bright light generated at the periphery of a black hole?

A black hole exerts a strong gravitational pull, sucking in the surrounding gas and stars. In the course of this process, friction between matter causes kinetic energy to be converted into heat. As a result, the matter surrounding the black hole radiates bright light in the form of X-rays and gamma rays.

For example, the Cygnus X-1 black hole was formerly a binary star―two stars that were drawing closer to one another and circling each other’s periphery. One star, having reached the end of its life and having become a black hole, is gleaming as it draws in the gas from the other star. The gas does not fall into the black hole all in one go, but clusters around its periphery before spinning down into it. At this point enormous friction is generated, and the gas is converted into heat, which in turn gives off X-rays and gamma rays. At the same time, an astrophysical jet, a strong light emitted perpendicular to the gas’ direction of rotation, is observed.

It is through observation of this strong light emitted from the periphery of the black hole that its existence can be confirmed.

A black hole binary star, as typified by Cygnus X 1. The black hole takes in the gas from the other star and radiates strong X rays. In the process, some binary stars emit a vertical jet. (Artist’s rendition)

Since the earth’s atmosphere absorbs X-rays, however, this phenomenon cannot be observed from the earth. Thus, observation satellites are required. Successive generations of Japanese X-ray observation satellites―Hakucho, launched in 1979, Tenma in 1983, Ginga in 1987, and Asuka in 1993―have played a key role in leading the global effort to observe and understand black holes. The fifth-generation satellite, Suzaku, which was launched in 2005, is currently in orbit, as is the Chandra X-ray observation satellite, launched by NASA in 1999, and the XMM-Newton X-ray observation satellite, launched the same year by ESA.

Many discoveries have been made through the use of observation satellites and telescopes. One of these is that there is a supermassive black hole at the center of virtually every galaxy―our own included. There are also disputes that have only recently been resolved, as Professor Kazuo Makishima of the University of Tokyo notes.

One of these is the issue of gamma-ray bursts, which were first discovered in the late 1960s. Gamma rays have higher levels of energy than X-rays, and gamma-ray emissions are generated by energy transitions within atomic nuclei. It is difficult to determine the position of a gamma-ray burst, which consists of a short-lived emission of enormous levels of energy, as it is occurring, as gamma-ray bursts only last between a few milliseconds and a few minutes.

A black hole and a disk are formed after two neutron stars with differing masses have merged. It is probable that gamma-ray bursts are emitted from here.
© Yuichiro Sekiguchi (National Astronomical Observatory of Japan), Masaru Shibata (University of Tokyo)

However, in the 1990s it was realized that gamma-ray bursts can originate from any direction, that is, they are distributed in a spherically symmetrical fashion, and that bursts from further away, which appear darker, are fewer in number.

This demonstrates that, at the very least, gamma-ray bursts are being emitted by celestial objects that are outside our galaxy. If gamma-ray bursts were being generated close by, most of them would be observed to be in the plane or in the center of the galaxy in which the stars are concentrated, and the sources would thus not be distributed spherically.

As a result, the following two theories sprang up. The first theory was known as the Local Halo Theory. This theory suggested that, since enormous levels of energy were observed, the sources of gamma-ray bursts were located in a region known as the halo, which encircles our local galaxy, and which is several hundred thousand light years in diameter. This theory would explain why the sources of gamma-ray bursts are virtually evenly distributed throughout the entire sky, and―if generation of bursts were confined to this halo―would also account for why the darker bursts that are generated far away were few in number. At the same time, there was also the assumption that if the sources of the gamma-ray bursts were so close to our galaxy, such enormous levels of energy would probably not be required in order for them to be detectable.

Against this was ranged the Cosmological Theory, which held that the celestial objects that emit gamma-ray bursts were between several hundred million and several billion light years away from us. Since observing objects that are a great distance away is equivalent to observing the past, the fact that there were fewer far distant celestial objects emitting gamma-ray bursts demonstrated that it required several billion years from the beginning of the universe for stars to reach maturity. On the other hand, given the premise that gamma-ray bursts came from celestial objects scattered all over the universe but not from within our galaxy, it was initially difficult to believe that such enormous levels of energy were being emitted. Thus, for lack of any conclusive proof either way, the dispute between the Cosmological Theory and the Local Halo Theory dragged on.

In 1997, this dispute was conclusively resolved in favor of the Cosmological Theory. The X-ray observation satellite BeppoSAX, launched jointly by Italy and the Netherlands, succeeded in precisely determining the position of a single gamma-ray burst. When the position was checked by telescope, it was found to be that of an extremely distant galaxy.

The afterglow from the gamma-ray burst source GRB030329 that was observed on March 29, 2003. The source can be seen becoming extremely dark overnight.
Photograph: Kenichi Kadota, AstroArts

As the HETE-2 satellite, which observes the positions at which gamma-ray bursts are generated, was surveying gamma-ray bursts over a wide area of the sky, it detected an extremely strong burst whose position it was able to precisely determine. It then communicated this position to small-scale telescopes all over the world. Approximately one hour later, numerous telescopes detected the afterglow of a gradually darkening celestial object at the stated position. This position was then communicated to large-scale telescopes such as Subaru, and with observation conducted every night, the afterglow was seen for approximately one week before the object grew dark. It was dubbed a “hypernova.” A hypernova is considered to be generated when an extremely heavy star collapses and becomes a black hole. A hypernova emits several dozen times the level of energy emitted by an ordinary supernova.

“It was tremendously exciting,” recalls Professor Makishima. In addition to the enormous historical significance that this had in putting to rest an issue that had been the subject of an ongoing dispute, the moment of a black hole’s birth was also being witnessed.

At present, black holes can essentially be divided into two main categories. The first category is the stellar-mass black hole, such as Cygnus X-1, which ranges from several times to almost 20 times the mass of the sun. The second category is the supermassive black hole, which is situated at the center of a galaxy and unimaginably vast, ranging in size from one million solar masses to 100 million solar masses. There is a vast difference in size between these two classes of black hole. Although a stellar-mass black hole is created by a supernova explosion, nobody is sure whether a supermassive black hole is formed by the merging of stellar-mass black holes.

Professor Makishima believes that intermediate-mass black holes probably also exist, which would be of a size that is in between these two categories.

An intermediate-mass black hole could conceivably be formed when heavy stars merge violently with one another to form an extremely heavy star, which then collapses. In fact, an ultra-luminous X-ray object with a luminosity between 100 and 1,000 times that of Cygnus X-1 is known to exist in a neighboring galaxy, and Professor Makishima believes that observation by Asuka or Suzaku might well reveal it to be an intermediate-mass black hole. An intermediate-mass black hole like this might gradually fall towards the center of its galaxy, merge with other intermediate-mass black holes, and in due course form a large black hole. If this were the case, there would probably be instances in which black holes were circling each other’s periphery, about to merge with one another.

From May 2009 onwards, JAXA, RIKEN, and other organizations intend to use MAXI (or Monitor of All-sky X-ray Image), which is due to be installed on the international space station Kibo, to perform X-ray observations and search for black holes that may be on the verge of merging with one another.

Black holes have attracted widespread attention and are frequently featured in movies and novels. Probably for this reason alone, people are mesmerized by these amazing celestial objects. Nature often shows a side of herself that exceeds human imagination. As new challenges are taken up, the next great discoveries to be made are keenly anticipated.