Black Holes

Black Holes

Don't let the name fool you: a black hole is anything but empty space. Rather, it is 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.

Universe Black Holes-1 Spitzer. This star-studded infrared image from NASA's Spitzer Space Telescope shows the Milky Way's churning center. In this false-color image, old, cool stars appear blue, and the dust near hot, massive stars is red. Astronomers believe there is a supermassive black hole in the galaxy's core, visible here as a bright white spot. Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech).

Although the term was not coined until 1967 by Princeton physicist John Wheeler, 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.

Universe Black Holes-2 Double Burst. When a massive star runs out of fuel, as this animation shows, the core collapses and forms a black hole.Scientists long thought that the explosion would be followed by an afterglow of dying embers, but new evidence from the Swift telescope indicates that a newborn black hole somehow re-energizes the explosion again and again, creating multiple bursts of energy in a few minutes. Credit: NASA/GSFC/Dana Berry.

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 lightknown 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.

Babies and Giants

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.

Universe Black Holes-3 Globular Clusters. For decades, most researchers have believed that black holes came in two sizes: the mass of a few stars, or the mass of a million stars or more. These two previously undiscovered black holes provide an important link that sheds light on the way in which black holes grow. The black hole in M15 (left) is 4,000 times more massive than our Sun. G1 (right), a much larger globular cluster, harbors a heftier black hole, about 20,000 times more massive than our Sun.

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 achurning 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.

Universe Black Holes-4 Merging Galaxies. This illustration shows two merging galaxies, an event that triggers a burst of star formation and provides fuel for the supermassive black holes in each galaxy's center. The inset shows a Chandra image of two central black holes -- about 70,000 light years apart - in merging galaxies. The varying colors represent differences in X-ray absorption by gas and dust around the black holes. Credit: CXC/M.Weiss.

The matter surrounding a stellar black hole — known as the accretion disk — consists of gas and dust. Around a supermassive black hole in the middle of a galaxy, this disk can include stars as well. In 2004, data from Chandra offered scientists their first-ever glimpse of a black hole shredding a nearby star.

Later that year, Chandra spotted two supermassive black holes orbiting in the same galaxy — and therefore doomed to collide. And in October 2005, Chandra revealed a series of stars thought to have been spawned by the supermassive black hole at the center of the Milky Way.