Black Holes

The Birth and Formation of Galaxies 

Earlier in this century, Edwin Hubble's observations led to the discovery that ours is only one of many billions of galaxies that dot the universe, each galaxy home to billions of stars. 

How It All Started 

First, there was the Big Bang, the primeval explosion that brought all space and time, all matter and energy, into being. For several hundred thousand years immediately thereafter, the universe was too hot for elements to form, so it consisted of a mix of subatomic particles and radiation. As the universe cooled to the point where the matter became transparent to the radiation, the first hydrogen and helium atoms began to form. Images taken by NASA's Cosmic Background Explorer indicate that the featureless sea of cosmic particles and radiation now showed the first signs of structure. Were these subtle variations in an otherwise smooth universe the seeds that grew to form the first galaxies? We do not know; and for now, we can only hypothesize. 

The Milky Way would have formed when star clusters merged to form the galaxy's bulge, or core, which then accreted more gas and dust to form its flattened disk of spiral arms. When astronomers study the Milky Way, they can learn about the birth, life and death of its stars because they see the stars at various evolutionary stages. Detailed studies of the ages and chemical compositions of these stars suggest that the Milky Way has led a relatively quiet existence, forming stars at a rate of a few suns per year for about the last 10 billion years. But while these stars offer clues to the age of our galaxy, they present little evidence that helps to explain how the Milky Way originally formed.

Probing the mystery of galaxy birth and formation is a key objective of NASA's Origins Program. Within the first decade of the next millennium, NASA will launch the Space Infrared Telescope Facility (SIRTF) and the Next Generation Space Telescope (NGST) to probe the early galaxies and attempt to solve a mystery that holds clues to the answers to the basic questions of how the universe evolved and spawned life. 

The Search for Life in the Universe 

The millions of stars that we see in the night sky are hot gaseous bodies like our Sun. In fact, that is why they shine, which allows us to see them even though they are very far away. Life could not exist on these stars, just as it could not exist on our Sun.Then where? What if some of these stars have planets revolving around them just as our Sun has its "family" of nine planets revolving around it? If some of these planets are not too close to their sun (too hot) or too far away (too cold), and if they have the right life-sustaining properties, it is possible that we are not the only tenants of this vast universe. 

Astronomers have recently learned that stars much younger than our Sun are surrounded by huge disks of gas and dust. This finding is significant because we believe that none of our solar system's current planets existed when our Sun was born 4.6 billion years ago. Instead, the young Sun had a disk — called a protoplanetary disk — around it. Over a few million years, the gas, rocks, and particles in the protoplanetary disk bumped into and stuck to one another, building up clumps that gave birth to the current planets, moons, comets, and asteroids in our solar system. So the presumption is that some disks we see around other stars might also evolve into planetary systems similar to our own. But how many stars have such disks around them? Recent observations of hundreds of young stars show that most of them have protoplanetary disks.

While such disks might eventually evolve into planetary systems, are there any other planets existing today? Until October 1995, we could not say for sure. But then a group of Swiss astronomers announced the detection of the first planet. Since then, there has been a flurry of similar detections by other teams. So it seems, after all, that our planetary system is not unique.

Are there life-sustaining planets revolving around other stars in the solar neighborhood?

Research during the latter half of this century has led to the development of a generally accepted concept of how stars and planetary systems form. The concept suggests that objects similar in mass and composition to Earth may exist in many, perhaps most, planetary systems. Detection of such planets is a key outcome of the search for planetary systems in general. However, in order to ascertain whether any of these planets may be life-sustaining, we need to be able to investigate the composition of their atmospheres. Liquid water is a basic requirement for life as we know it, and it is the key indicator that will be used to determine whether planets revolving around other stars may be life-sustaining. These issues will only be addressed when we have conducted a survey of a statistically significant number of nearby stars for the presence of planetary systems and observed them sufficiently to infer the masses and orbital properties of the planets in those systems.

The detection of planets outside our solar system

In 1991 radio astronomers detected the first extrasolar planets orbiting none other than a dying pulsar star. This star was left over from a supernova explosion in the constellation Virgo. The pulsar's beam of radiation changed slightly due to the gravitational pull of three Earth-sized objects revolving around the host star. It was the first example of a star other than our Sun producing planets. 

In 1995 Swiss astronomers found another extra-solar planetary candidate. It was discovered by noting a slight perturbation in the position of a star in our nearby galactic neighborhood. This star, found in the constellation of Pegasus, is much more like our Sun with respect to its temperature, size, rotation speed and emitted radiation. The newly found planet orbiting the star (51 Peg) had a size comparable to Jupiter or Saturn, however, it was positioned extremely close to its parent star- at a position closer than Mercury sits from our own Sun! Although not a good candidate for a life-bearing planet, it was the first ever evidence of an extrasolar planet around a Sun-like star. 

Since then several other Jovian-sized planets have been found orbiting other Sun-like planets. Some of them are orbiting extremely close to their parent star like the 51 Peg planetary system, while others are found to be at distances comparable to where Mars and Jupiter lie in our solar system

Black Holes: What Are They?

To see why this happens, imagine throwing a tennis ball into the air. The harder you throw the tennis ball, the faster it is travelling when it leaves your hand and the higher the ball will go before turning back. If you throw it hard enough it will never return, the gravitational attraction will not be able to pull it back down. The velocity the ball must have to escape is known as the escape velocity and for the earth is about 7 miles a second.

As a body is crushed into a smaller and smaller volume, the gravitational attraction increases, and hence the escape velocity gets bigger. Things have to be thrown harder and harder to escape. Eventually a point is reached when even light, which travels at 186 thousand miles a second, is not travelling fast enough to escape. At this point, nothing can get out as nothing can travel faster than light. This is a black hole.

When a large star has burnt all its fuel it explodes into a supernova. The stuff that is left collapses down to an extremely dense object known as a neutron star. We know that these objects exist because several have been found using radio telescopes.

If the neutron star is too large, the gravitational forces overwhelm the pressure gradients and collapse cannot be halted. The neutron star continues to shrink until it finally becomes a black hole. This mass limit is only a couple of solar masses, that is about twice the mass of our sun, and so we should expect at least a few neutron stars to have this mass. (Our sun is not particularly large; in fact it is quite small.)

A supernova occurs in our galaxy once every 300 years, and in neighbouring galaxies about 500 neutron stars have been identified. Therefore we are quite confident that there should also be some black holes.

The star eventually collapses to the point of zero volume and infinite density, creating what is known as a " singularity "----------

Singularity:The center of a black hole, where the curvature of space-time is maximal. At the singularity, the gravitational tides diverge; no solid object can even theoretically survive hitting the singularity. 

--------As the density increases, the path of light rays emitted from the star are bent and eventually wrapped permanently around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole. 

But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the only thing that would change would be the Earth's temperature. To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius------- 

Schwarzschild black hole:A black hole described by solutions to Einstein equations of general relativity worked out by Karl Schwarzschild in 1916. 

Schwarzschild radius:The radius r of the event horizon for a Schwarzschild black hole. 

------- At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape. 

If We Can't See Them, How Do We Know They're There?

Since black holes are small (only tens of kilometers in size), and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see.

However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete------ 

Accretion:Accumulation of dust and gas onto larger bodies such as stars, planets and moons

-----matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes------

Ionized Gas:Gas whose atoms have lost or gained electrons, causing them to be electrically charged. In astronomy, this term is most often used to describe the gas around hot stars where the high temperature causes atoms to lose electrons. 

-------the atoms, and when the atoms reach a few million degrees Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission. 

Binary X-ray sources are also places to find strong black hole candidates. A companion star is a perfect source of infalling material for a black hole. A binary system------- 

Binary system:Binary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process. 

------also allows the calculation of the black hole candidate's mass. Once the mass is found, it can be determined if the candidate is a neutron star------ 

Neutron Star:A particle with approximately the mass of a proton, but zero charge, commonly found in the nucleus of atoms. 

-----or a black hole. Another sign of the presence of a black hole is random variation of emitted X-rays. The infalling matter that emits X-rays does not fall into the black hole at a steady rate, but rather more sporadically, which causes an observable variation in X-ray intensity. Additionally, if the X-ray source is in a binary system, the X-rays will be periodically cut off as the source is eclipsed by the companion star. When looking for black hole candidates, all these things are taken into account. Many X-ray satellites have scanned the skies for X-ray sources that might be possible black hole candidates. 

Cygnus X-1 is the longest known of the black hole candidates. It is a highly variable and irregular source with X-ray emission that flickers in hundredth of a second bursts. Because nothing can exceed the speed of light, an object cannot flicker faster than the time required for light to travel across the object. In a hundredth of a second, light travels 3000 kilometers. Thus, Cyg X-1 must be smaller than Earth! Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic------- 

Spectroscopy:The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the chemistry that goes on in stars and in interstellar clouds. 

Spectral Lines:Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas characteristic frequencies. 

------observations show that the spectral lines of HDE 226868 shift back and forth with a period of 5.6 days. From the mass-luminosity----- 

Luminosity:The rate at which a star or other object emits energy, usually in the form of electromagnetic radiation. 

-------relation, the mass of this supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a mass of about 7 solar masses or else it would not exert enough gravitational pull to cause the wobble in the spectral lines of HDE 226868. Since 7 solar masses is too large to be a white dwarf---- 

White dwarf:A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun. This gives a white dwarf a density about 1 million times that of water!

-----or neutron star, it must be a black hole.

However, there are arguments against Cyg X-1 being a black hole. HDE 2268686 might be undermassive for its spectral type, which would make Cyg X-1 less massive than previously calculated. In addition, uncertainties in the distance to the binary system would also influence mass calculations. All of these uncertainties can make a case for Cyg X-1 having only 3 solar masses, thus allowing for the possibility that it is a neutron star. 

Nonetheless, there are now about 10 binaries for which the evidence for a black hole is much stronger than in Cygnus X-1. The first of these, an X-ray transient called A0620-00, was discovered in 1975, and the mass of the compact object was determined in the mid-1980's to be greater than 3.5 solar masses. This very clearly excludes a neutron star, even allowing for all known theoretical uncertainties. The best case for a black hole is probably V404 Cygni, whose compact star is at least 10 solar masses. With improved instrumentation, the pace of discovery has accelerated over the last five years or so, and the list of dynamically confirmed blackh hole binaries is growing rapidly. 

Evidence:Black hole in M87

M87 is an active galaxy, one in which we see interesting objects. Near its core (or centre) there is a spiral-shpaed disc of hot gas. The first picture places it in context. The second superposes spectra from opposite sides. This allows us to determine the speed of rotation of the disk and its size. From this we can weigh the size of the invisible object at the centre.

Although the object is no bigger than our solar system it weighs three billion times as much as the sun. This means that gravity is so strong that light cannot escape. We have a black hole.

Wormholes

Unfortunately, worm holes are more science fiction than they are science fact. A wormhole is a theoretical opening in space-time that one could use to travel to far away places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat called an Einstein-Rosen bridge. It has never been proven that worm holes exist and there is no experimental evidence for them.