The term "black hole" does not mean empty hole. This astrophysical phenomenon is anything but empty. It is just the opposite, a great amount of matter packed into a very small area. Think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. The result is a gravitational field so strong that nothing, not even light, can escape.

In recent years, NASA instruments have painted a new picture of these strange objects that are, to many, the most fascinating objects in space.

In 1967, Princeton physicist John Wheeler coined the name, but the idea of an object in space so massive and dense that light could not escape it has been around for centuries.

Most famously, black holes were predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.

Scientists can't directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby.

If a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a black hole.

In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others.

Another interesting possibility becomes available when the black hole is in a binary star system with a compact star like a neutron star or another black hole.

When two black holes orbit each other, their accelerated masses directly create gravitational waves that stream away through space and carry information about the masses and strong fields that created them. Gravitational waves are waves of space curvature and may be detected by missions such as the Laser Interferometer Space Antenna (LISA) through the way they affect the geometry of space at the location of the detector. In a sense, a black hole is the mass it contains plus the intense gravitational field around it.

So LISA will actually be able to "see" black holes. From these observations, astronomers will be able to study the details of the gravitational field around the black hole and measure all the parameters of the black hole - its mass, its spin, and its location in the sky.

One Star's End is a Black Hole's Beginning. Most black holes form from the remnants of a large star that dies in a supernova explosion. (Smaller stars become dense neutron stars, which are not massive enough to trap light.) If the total mass of the star is large enough (about three times the mass of the Sun), it can be proven theoretically that no force can keep the star from collapsing under the influence of gravity.

However, as the star collapses, a strange thing occurs. As the surface of the star nears an imaginary surface called the "event horizon," time on the star slows relative to the time kept by observers far away. When the surface reaches the event horizon, time stands still, and the star can collapse no more - it is a frozen collapsing object.

Even bigger black holes can result from stellar collisions. Soon after its launch in December 2004, NASA's Swift telescope observed the powerful, fleeting flashes of light known as gamma ray bursts.

Chandra and NASA's Hubble Space Telescope later collected data from the event's "afterglow," and together the observations led astronomers to conclude that the powerful explosions can result when a black hole and a neutron star collide, producing another black hole.

Although the basic formation process is understood, one perennial mystery in the science of black holes is that they appear to exist on two radically different size scales. On the one end, there are the countless black holes that are the remnants of massive stars. Peppered throughout the Universe, these "stellar mass" black holes are generally 10 to 24 times as massive as the Sun.

Astronomers spot them when another star draws near enough for some of the matter surrounding it to be snared by the black hole's gravity, churning out x-rays in the process.

Most stellar black holes, however, lead isolated lives and are impossible to detect. Judging from the number of stars large enough to produce such black holes, however, scientists estimate that there are as many as ten million to a billion such black holes in the Milky Way alone.

On the other end of the size spectrum are the giants known as "supermassive" black holes, which are millions, if not billions, of times as massive as the Sun. Astronomers believe that supermassive black holes lie at the center of virtually all large galaxies, even our own Milky Way. Astronomers can detect them by watching for their effects on nearby stars and gas.

So far, there has been no direct evidence of mid-sized black holes. The question is, why not? Historically, scientists have believed simply that no such black holes exist, but recent observations have led some astronomers to think otherwise. The question of whether black holes of intermediate mass exist is a subject of much current research.

In 1997, the Hubble Space Telescope was equipped with an instrument that separates visible light into various wavelengths, the Space Telescope Imaging Spectrograph (STIS). Measurements by STIS can reveal the speed and other properties of gas as it swirls into a black hole, which, in turn, reveals certain characteristics of the black hole itself - its mass, for example, and how fast it is spinning.

It is these observations from Hubble that show that most, possibly all, large galaxies are home to a churning black hole. One black hole in the constellation Virgo, 50 million light-years away, has been calculated to have a mass equal to about three billion Suns.

Why should we care about Black Holes?
People are worried about a wayward asteroid hitting Earth and ending life as we know it. In fact, a new sentry is on guard atop the Haleakala volcano in Hawaii, scanning the skies for potentially threatening asteroids and comets.

The first of four telescopes planned for the Panoramic Survey Telescope and Rapid Response System project began a dedicated survey of the sky in May of 2010.

However, the probability of this happening is quite low. Consider a more dangerous scenario. A black hole passes near our solar system. Most, or all, of the planets and the sun could be sucked into it. There would be no escape and no hope of rebuilding our planet. Is anyone looking for wayward black holes?