What Are Black Holes

Einstein's general theory of relativity describes gravity as a curvature of spacetime caused by the presence of matter. If the curvature is fairly weak, Newton's laws of gravity can explain most of what is observed, for example, the regular motions of the planets. But very massive or dense objects generate much stronger gravity. The most compact objects imaginable are predicted by General Relativity to have such strong gravity that nothing, not even light, can escape their grip. Scientists today call such an object a black hole. Why black? Though the history of the term is interesting, the main reason is that no light can escape from inside a black hole: it has, in effect, disappeared from the visible universe. Do black holes actually exist? Most physicists believe they do, basing their views on a growing body of observations. In fact, present theories of how the cosmos began rest in part on Einstein's work and predict the existence of both singularities and the black holes that contain them. Yet Einstein himself vigorously denied their reality, believing, as did most of his contemporaries, that black holes were a mere mathematical curiosity. He died in 1955, before the term "black hole" was coined or understood and observational evidence for black holes began to mount.

A Black Hole is Born

Black holes are thought to form from stars or other massive objects if and when they collapse from their own gravity to form an object whose density is infinite: in other words, a singularity. During most of a star's lifetime, nuclear fusion in the core generates electromagnetic radiation, including photons, the particles of light. This radiation exerts an outward pressure that exactly balances the inward pull of gravity caused by the star's mass. As the nuclear fuel is exhausted, the outward forces of radiation diminish, allowing the gravitation to compress the star inward. The contraction of the core causes its temperature to rise and allows remaining nuclear material to be used as fuel. The star is saved from further collapse -- but only for a while. Eventually, all possible nuclear fuel is used up and the core collapses. How far it collapses, into what kind of object, and at what rate, is determined by the star's final mass and the remaining outward pressure that the burnt-up nuclear residue (largely iron) can muster. If the star is sufficiently massive or compressible, it may collapse to a black hole. If it is less massive or made of stiffer material, its fate is different: it may become a white dwarf or a neutron star.


The Anatomy of a Black Hole

By definition a black hole is a region where matter collapses to infinite density, and where, as a result, the curvature of spacetime is extreme. Moreover, the intense gravitational field of the black hole prevents any light or other electromagnetic radiation from escaping. But where lies the "point of no return" at which any matter or energy is doomed to disappear from the visible universe?

The Event Horizon

Observed were pulses of ultraviolet light from clumps of hot gas fade and then disappear as they swirled around a massive, compact object called Cygnus XR-1. This activity is just as it would have been expected, if the hot gas had fallen into a black hole. Astronomers are trying to establish the existence of black holes and event horizons by obtaining observational evidence, that rules out more exotic things, just as previous observations of black hole candidates have ruled out less exotic things.

Also, previous X-ray observations have offered evidence for an event horizon by surveying black hole candidates that seem to be swallowing nearly a hundred times as much energy as they radiate. Those results imply that trillion-degree gas is falling over the brink of an event horizon, like water over the edge of a waterfall. But no one has ever seen what actually happens to a piece of matter swirling into the event horizon, like water down a drain. The secret was tucked away in nearly decade-old Hubble data that took meticulous analysis.


Applying the Einstein Field Equations to collapsing stars, German astrophysicist Kurt Schwarzschild deduced the critical radius for a given mass at which matter would collapse into an infinitely dense state known as a singularity. For a black hole whose mass equals 10 suns, this radius is about 30 kilometers or 19 miles, which translates into a critical circumference of 189 kilometers or 118 miles.

The Singularity

At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature. Here it's no longer meaningful to speak of space and time, much less spacetime. Jumbled up at the singularity, space and time cease to exist as we know them.

The Limits of Physical Law

Newton and Einstein may have looked at the universe very differently, but they would have agreed on one thing: all physical laws are inherently bound up with a coherent fabric of space and time.

At the singularity, though, the laws of physics, including General Relativity, break down. Enter the strange world of quantum gravity . In this bizzare realm in which space and time are broken apart, cause and effect cannot be unraveled. Even today, there is no satisfactory theory for what happens at and beyond the singularity.

How Can We See Black Holes?

Though we cannot "see" a black hole itself (since not even light can escape the hole's gravitational field), we may see the hole's effects on nearby matter. For example, if gas from a nearby star were sucked towards the black hole, the intense gravitation al energy would heat the gas to millions of degrees. The resulting X-ray emissions could point to the presence of the black hole.

Or, if a massive black hole were surrounded by large amounts of orbiting material -- gas, dust, even stars -- their rapid motion close to the hole could be observable via shifts in the energy of the radiation they emit. Evidence along these lines is mounting, suggesting that black holes may not be that rare in the universe.

However, such evidence remains indirect and therefore inconclusive. To confirm that black holes actually exist, we'll need to be able to observe the gravitational waves they produce as they form or interact.

If scientists could build gravitational wave detectors of sufficient sensitivity, they should be able to measure the vibrations in spacetime generated by black holes as they form from a collapsing star, when they ingest large amoun ts of matter, or if they interact, even collide with a second black hole or another massive object, such as a neutron star. Certain patterns of gravitational waves emitted would reveal the "smoking gun."

So far, the wavelike disturbances in spacetime have eluded detection. In a relativistic universe, there should be no shortage of places in which to hunt for black holes. Much larger and more sensitive detectors are now under construction. With luck, soon gravitation scientists may be shouting "Eureka!"


Cygnus X-1


In the early Seventies scientists found an intensive X-Ray source in the Cygnus Constellation. They believe that this X-Ray source is a black hole.

Where is Cygnus X-1?

Cygnus X-1 is an X-ray binary in the constellation Cygnus, the swan, that was one of the first X-ray sources discovered when it was detected in 1962. It is called Cygnus X-1 because it was the first X-ray source discovered in the constellation Cygnus. The visible object HDE226868 is a 9th magnitude blue supergiant star whose radial velocity curve shows an orbital period of a little less than a week. The fact that the object is a strong X-ray emitter and that the optical and X-ray emission varies on very short time scales (as short as one one-thousandth of a second) suggest that the companion might be a black hole.

What is Cygnus X-1?

Cygnus X-1 is believed to be a black hole binary, with a 20-35 solar mass black hole and a supergiant, orbiting around with a period of 5.6 days, as companion. The mass of the unseen companion, significantly larger then 5 solar masses suggests that it is a black hole. Focused wind accretion from a primary star being extremely close to filling the Roche lobe drives the powerful source of the X-ray radiation. Cygnus X-1 is one of the brightest X-ray sources in the sky.

The Supergiant

Blue Supergiants are far more massive then our sun, and burn far hotter. A typical blue giant star has a mass perhaps 10 times that of the sun, and has a surface temperature of two to four times that of our local star.

Why are blue giants so large? It is due to the simple fact that they are large because they have a lot of material. With so much hydrogen gas to fuse in order to produce starlight, one might think that blue giant stars would have very long lifetimes. This is, in fact, not the case. The more massive a star is, the more quickly it burns it's fuel. In fact, the lifetime of a blue giant star is much shorter then that of the Sun.


This shows the relation of a supergiant, our sun and the Jupiter.

The spectrum

The intrinsic X-ray spectrum of Cygnus X-1 can be characterized as a power law of the photon index Gamma ~1.5-1.9 (Liang & Nolan 1984). This kind of the spectrum can be explained by Comptonization of soft photons, presumably coming from the optically thick accretion disk. On top of this continuum one can find an absorption edge at around 7 keV, and a broad feature above 10 keV, called high energy excess (Done et al. 1992). Recent wide-band observations show that the excess has a form of the hump extending from 10 to 200 keV, with the peak at ~30 keV (Gierlinski et al. 1995). Such a hump is considered due to reprocessing and Compton reflection of X-ray photons by an accretion disk (Lightman, White 1988, Magdziarz, Zdziarski 1995).

Iron line
Barr, White, and Page (1985) reported a wide (equivalent width ~120 eV) emission iron K-alpha line at ~6.2 keV. Fabian et al. (1989) attributed this line to the fluorescence of the inner part of the accretion disk, at a few Schwartzschild radii. Such an emission should exhibit a characteristic, double-wing profile. More recent work (Ebisawa et al. 1995) show that the observed feature is consistent rather with the narrow gaussian line (1 sigma < 0.2 keV), suggesting that it can come from the outer part of the disk. The weakness of the line (equivalent width ~20 eV), inconsistent with theoretical predictions (George, Fabian 1991), is not understand well.

At higher energies (E > 100 keV) observed flux falls down exponentially. In most cases gamma-rays from Cygnus X-1 can be approximated by a power law with exponential cutoff. Haardt et al. (1993) showed, using Monte Carlo methods, that inverse Compton continuum from the optically thin (tau=0.3) and hot (kT=150 keV) plasma is consistent with SIGMA and OSSE observations.

Something around 1 MeV
Several authors have reported a hardening of the spectrum in the region around 1 MeV (Ling et al. 1987, Bassani et al. 1989), though many other observations have found no evidence for such emission (e.g. McConnel et al. 1994). It suggests that this spectral feature must be transient. Liang and Dermer (1988) proposed explanation of the 1 MeV bump in context of the hot (kT ~ 400 keV) electron-positron plasma in the inner region of the accretion disk.

Emission variability
X-ray states
X-ray emission of Cygnus X-1 exhibits strong variability
at time scales from milliseconds to years. Generally,
the X-ray emission falls into one of the two distinct
states, named "low" and "high"(Liang & Nolan 1984).
Cygnus X-1 spends most of its time (>80%) in the low
state, where flux in soft X-ray range (2-20 keV) is
lower than in the high state. Ling et al. (1987) have
divided the hard X-ray luminosity (45- 140 keV) into
three states: gamma1, gamma2, and gamma3. Recent
observations (Phlips et al. 1995) suggest that
variability does not seem to be between discrete
states but rather among continuous range of possible flux values.

Intensity dips
Another temporal feature of Cygnus X-1 are intensity dips
(Li & Clark 1974) that preferentially occur near the time
of superior conjunction. The duration of the dip may vary
from minutes to hours. The flux during the dip decreases
in soft X-ray range (E<10 keV), and absorption is generally
complex. The partial covering model is consistent with the
observational data (Kitamoto et al. 1984). The absorber
seems to form a dense blobs of matter, intervening the
X-ray source (Pravdo et al. 1980).

Is there really a black hole in Cygnus?

Scientists don't know if this is really a black hole. It could be a small star, too faint to see in optical wavelengths, or possibly a planet sized hunk of rock. But the Object is too small for a star. A better explanation is that the object is a neutron star or a white dwarf. Neutron Stars usually have very regular and distinct pulses. Cygnus X-1's emmissions, however, show no regularity or periodicity. They seem to have no repeating patterns, and vary on short and long timescales equally.

Another argument deals with the mass of the object.

Subrahmanyan Chandrasekhar first determined the upper limit to the mass of a white dwarf as 1.4 solar masses. This value, called the Chandrasekhar mass limit, is still used today. Later, J.R. Oppenheimer and G.M. Volkoff determined the upper mass of a neutron star. It is called the Oppenheimer-Volkoff mass, and has been recalculated many times since. Because we are dealing with degenerate neutron gas, which we can only make educated guesses about the exact properties of, we cannot truly determine pecisely what this limit is. It is usually said to be about 2 to 3 solar masses, and generally stays well below 4 or 5.

The Spac250 homepage explains.

So if the lower limit for the mass of a black hole is higher than the lower limit of a neutron star, then the object in Cyg X-1 have to be a black hole.

sources:, the spac250 homepage
Last edited: 18.10.2002