Black Holes

Posted on May 9, 2008 by Rashid Faridi

There are many popular myths concerning black holes, many of them perpetuated by Hollywood. Television and movies have portrayed them as time-traveling tunnels to another dimension, cosmic vacuum cleaners sucking up everything in sight, and so on. It can be said that black holes are really just the evolutionary end point of massive stars. But somehow, this simple explanation makes them no easier to understand or less mysterious.

NOTE: This section is about what are called “stellar-mass black holes”. For information about black holes with the mass of billions of Suns, see Active Galaxies & Quasars .

Black Holes: What Are They?

Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a ” singularity “. As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably 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 earth’s orbit around the Sun would be unchanged. (Of course the Earth’s temperature would change, and there would be no solar wind or solar magnetic storms affecting us.) To be “sucked” into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.

The Schwarzschild radius can be calculated using the equation for escape speed.

vesc = (2GM/R)1/2
For photons, or objects with no mass, we can substitute c (the speed of light) for Vesc and find the Schwarzschild radius, R, to be

R = 2GM/c2If the Sun was replaced with a black hole that had the same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the Sun’s radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our solar system.

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

HDE 226868

Since black holes are small (only a few to a few 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. For instance, the photograph above shows the optical companion star to the (invisible) black hole candidate Cyg X-1.

However, if a black hole passes through a cloud of interstellar matter, or is close to another “normal” star, the black hole can accrete 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 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 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 or a black hole, since neutron stars always have masses of about 1.5 times the mass of the sun. 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 hundredths of a second. 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. This is one fourth of Earth’s diameter! So the region emitting the x-rays around Cygnus X-1 is rather small. Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic observations show that the spectral lines of HDE 226868 shift back and forth with a period of 5.6 days. From the mass-luminosity 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 or neutron star, it must be a black hole.

Diagram of Cygnus X-1 system

However, there are arguments against Cyg X-1 being a black hole. HDE 226868 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, which has a mass near 1.5 solar masses, 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 black hole binaries is growing rapidly.

What about all the Wormhole Stuff?

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 – the throat, or passageway, is called an Einstein-Rosen bridge. It has never been proved that worm holes exist and there is no experimental evidence for them, but it is fun to think about the possibilities their existence might create.

source:http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html

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Neither fish nor fowl: Platypus genome decoded

Posted on May 9, 2008 by Rashid Faridi

Arguably the oddest beast in Nature’s menagerie, the platypus looks as if were assembled from spare parts left over after the animal kingdom was otherwise complete.

Now scientists know why. According to a study released Wednesday, the egg-laying critter is a genetic potpourri — part bird, part reptile and part lactating mammal.

The task of laying bare the platypus genome of 2.2 billion base pairs spread across 18,500 genes has taken several years, but will do far more than satisfy the curiosity of just biologists, say the researchers.

“The platypus genome is extremely important, because it is the missing link in our understanding of how we and other mammals first evolved,” explained Oxford University‘s Chris Ponting, one of the study’s architects.

“This is our ticket back in time to when all mammals laid eggs while suckling their young on milk.”

Native to eastern Australia and Tasmania, the semi-aquatic platypus is thought to have split off from a common ancestor shared with humans approximately 170 million years ago.

The creature is so strange that when the first stuffed specimens arrived in Europe at the end of the 18th century, biologists believed they were looking at a taxidermist’s hoax, a composite stitched together from the body of a beaver and the snout of a giant duck.

But the peculiar mix of body features are clearly reflected in the animal’s DNA, the study found.

The platypus is classified as a mammal because it produces milk and is covered in coat of thick fur, once prized by hunters.

Lacking teats, the female nurses pups through the skin covering its abdomen.

But there are reptile-like attributes too: females lay eggs, and males can stab aggressors with a snake-like venom that flows from a spur tucked under its hind feet.

The bird-like qualities implied by its Latin name, Ornithorhynchus anatinus, include webbed feet, a flat bill similar to a duck’s, and the gene sequences that determine sex. Whereas humans have two sex chromosomes, platypuses have 10, the study showed.

“It is much more of a melange than anyone expected,” commented Ewan Birney, who led the genome analysis at the European Bioinformatics Institute in Cambridge.

The animal also possesses a feature unique to monotremes — an order including a handful of egg-laying mammals — called electroreception.

With their eyes, ears and nostrils closed, platypuses rely on sensitive electrosensory receptors tucked inside their bills to track prey underwater, detecting electrical fields generated by muscular contraction.

“By comparing the platypus genome to other mammalian genomes, we’ll be able to study genes that have been conserved throughout evolution,” said senior author Richard Wilson, a researcher at Washington University.

In captivity, platypuses have lived up to 17 years of age.

In the wild, they feed on worms, insect larvae, shrimps and crayfish, eating up to 20 percent of their body weight every day.

Males grow to a length of 50 centimetres (20 inches) and weigh about two kilos (4.5 pounds), with females about 20 percent shorter and lighter.

The genome sequenced for the study belongs to a female specimen from New South Wales nicknamed Glennie and can be accessed at www.ncbi.nih.gov/Genbank.

source:http://news.yahoo.com/s/afp/20080507/sc_afp/sciencebiologygeneticsplatypus_080507184740

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Moons in Solar System

Posted on May 9, 2008 by Rashid Faridi

A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets, and minor planets. (There are no known natural satellites of moons.)

Two hundred and forty bodies, all in the Solar System, are classified as moons. They include 166 orbiting the eight planets,[1] 4 orbiting dwarf planets, and dozens more orbiting small solar system bodies. Other stars and their planets are very likely to have natural satellites, although none have yet been observed.

The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth‘s moon: the four Galilean moons, Saturn‘s Titan, and Neptune‘s Triton. Saturn has an additional six mid-sized moons massive enough to have achieved hydrostatic equilibrium, and Uranus has five. Of the inner planets, Mercury and Venus have no moons at all; Earth has one large moon (the Moon); and Mars has two tiny moons, Phobos and Deimos. Among the dwarf planets, Ceres has no moons (though many objects in the asteroid belt do), Eris has one, Dysnomia, and Pluto has three known satellites, Nix, Hydra, and a large companion called Charon. The Pluto-Charon system is unusual in that the center of mass lies in open space between the two, a characteristic of a double planet system.

dig deeper

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Electric Currents from Space

Posted on May 8, 2008 by Rashid Faridi

This limited extent suggested that the currents which disturbed the field flowed somewhere nearby, probably near the auroral arcs. The Norwegian Kristian Birkeland, who carefully observed auroral disturbances around the turn of the century, concluded that those currents flowed parallel to the ground, along the auroral formation.

Any electric current, however, must flow in a closed circuit, and since it seemed to be caused (like that of the aurora) by processes taking place in distant space, Birkeland proposed that it came down from space at one end of the arc and returned to space at the other end. The drawing here illustrates the idea and was taken from Birkeland’s 1903 report on his expeditions to the auroral zone:

Dynamos

How can electric currents be generated in space–or, for that matter, on the Sun and in the Earth’s core?

In all those cases, some electrically conducting fluid appears to be moving through a magnetic field–plasma in space and on the Sun, molten iron (probably) in the Earth’s core. It can then be shown from the principles of physics that if a closed electric circuit exists, parts of which are moving through a magnetic field while other parts are not, an electric current will arise (additional conditions must also be satisfied). The electric energy needed to drive the current is taken from the motion, which is slowed down.

Long ago, a machine that generated electricity was named “dynamo” (today’s preferred term is “generator”). In analogy, a set-up where a conducting fluid flows through a magnetic field and creates an electric current in the above fashion, is called a “fluid dynamo,” or when a fluid process is clearly implied, simply “dynamo”.

Waterloo Bridge Experiment

    Faraday understood this very well. Knowing that water conducted electricity (because of dissolved salts), he tried in 1831 to observe such a dynamo experimentally. He strung a wire along Waterloo bridge in London and dipped its ends into the river at two separated points, so that its circuit could be completed through the flowing water. He then tried to measure the electric current generated by the river’s flow through the magnetic field.

The idea was sound, but the current was too weak to be measured by him. Faraday nevertheless speculated that the aurora (which he believed to be an electric phenomenon) might be powered in a similar way by the Gulf Stream flowing to the north-east through the Atlantic Ocean, beginning just offshore from the US. Of course, what Faraday forgot here was that to connect the ocean to the aurora, the current would have to cross the lower atmosphere, which is an excellent insulator and would block its flow.

An interesting dynamo of this type exists between the planet Jupiter and its innermost big moon Io (click for details). The space shuttle can also generate electric power that way (at the expense of its orbital energy), using a long wire dangling from it into space. Such a space tether experiment was conducted aboard the space shuttle Columbia on 25 February, 1996, ending unexpectedly when the tether suddenly broke.

Sir Joseph Larmor in England first proposed in 1919 that dynamos consisting entirely of fluids might explain the creation of sunspot magnetic fields. His idea was that as solar plasma flowed through magnetic fields, dynamo effects produced the very same currents which also created those magnetic fields. Because this is a process which “lifts itself by its own bootstraps” (the magnetic fields needed by the dynamo are produced by the electric currents which are its output), it must build up gradually, starting from some “seed” magnetic field of a different source.

Such a “dynamo mechanism” was also believed to be the source of the magnetic field arising inside the Earth’s molten core. It is impossible to observe that region directly, but in 1964 Stanislaw Braginsky, in Russia, mathematically demonstrated a possible class of such “dynamos.” The slow decade-by-decade variation of the Earth’s field probably comes from slow changes in the flow patterns of the core – not, as Halley once proposed, from magnetized spherical layers, one inside the other, all rotating differently.

Birkeland Currents

If the Earth has an “open” magnetosphere with field lines linked to the solar wind, it too produces a fluid dynamo of sorts. In a plasma, electricity flows easily along magnetic field lines, making them behave like copper wires. “Open” field lines from the solar wind that dip into the polar ionospere can then act like the wires which Faraday dipped into the Thames river, and they can draw an electric current, slightly slowing down the solar wind in the process.

If the bundle of open lines maps to the region inside the auroral oval, it can be shown that the “dynamo currents” from the above process flow earthward on the morning side of the magnetic pole and spaceward on the evening side. One might guess that the circuit would be completed by connecting the two flows across the polar ionosphere, from the morning side, to the evening side (drawing below; the situation is more complicated, however, because the flow also deforms the field lines).

    When in 1973 the navy satellite Triad (see history) flew through this region in a low-altitude orbit, its magnetometer indeed detected the signatures of two large sheets of electric current, one coming down on the morning side of the auroral zone, one going up on the evening side, as expected. Because Kristian Birkeland had proposed long before currents which linked Earth and space in this fashion, they were named Birkeland currents (by Schield, Dessler and Freeman, in a 1969 article predicting some of the features observed by Triad). Typically, each sheet carries a million amperes or more.

But that wasn’t all. Equatorward of each current sheet, Triad noted a parallel sheet almost as intense, flowing in the opposite direction: those field lines were no longer open, but closed inside the magnetosphere. It thus seemed that most of the electric current coming down from space (about 80%) did not choose to close through the ionosphere across the magnetic poles. Rather, it found an alternate way: it flowed in the ionosphere a few hundred miles equatorward and then headed out again to space, where the currents (presumably) found an easier path.

A 1976 study by Takesi Iijima and Tom Potemra used Triad data to map the average “footprints” of these sheets in the polar ionosphere, including their intricate overlap at midnight.

Their result is plotted below: the map is centered at the northern magnetic pole (though southern data were combined to produce this graph), midnight is at the bottom, and noon (where some minor additional currents exist) is at the top. Dark shading indicates currents flowing towards Earth, light shading, currents flowing outward into space.

source:http://www-spof.gsfc.nasa.gov/Education/wcurrent.html

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