December 1, 2011


Holiday Special

Purchase a 3-book set and receive a free DVD of your choice!


Offer ends December 15th

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 DVD's Make Great Stocking Stuffers


4 DVDs to choose from:


"Can Life be Merely an Accident? "


"Journey to the Dark Side:
Dark Matter & Dark Energy"


"Einstein for Everyone"


"Einstein & Light"




Humorous side to X-1


In 1975 Caltech Professor Kip Throne and his good friend, the iconic British astrophysicist Stephen Hawking, made a wager. Stephen bet Cygnus X-1 wasn’t a black hole and Kip bet it was. Stephen said all his theoretical work on black holes would be in vain if they didn’t actually exist, but at least he would win the bet and receive a four-year subscription to Private Eye magazine. If X-1 were shown to be a black hole, Kip would win a one-year subscription to Penthouse. As the evidence mounted, Stephen eventually conceded the bet and paid off “to the outrage of Kip’s liberated wife.”



Happy Holidays


Joan and I Wish You All a Wonderful and Safe Holiday Season!



Radio Telescopes Illuminate Oldest-Known Black Hole


New observations have yielded the most precise measurements ever made of a black hole, achieving a first-ever, complete description of one of these elusive objects. It’s fitting that the subject of this investigation is Cygnus X-1, a brilliant X-ray emitter that was discovered in 1964 and provided the first observational evidence that black holes really do exist.


We now know: that Cygnus X-1 is 6070 light-years from Earth (36,000 trillion miles); that its mass is 15 times that of our Sun; and that it spins 800 times per second at its event horizon. This makes X-1 one of the most massive “stellar” black holes yet found, but only a piker compared to the “supermassive” black holes at the centers of all major galaxies, which contain up to ten billion times our Sun’s mass.


All of a black hole’s mass is compressed into its central “singularity”, an infinitesimal ball that is probably a trillion, trillion times smaller than the smallest atom. The singularity is surrounded by an “event horizon” that marks the point of no return. The event horizon, as horizons on Earth, isn’t made of anything material but is instead a collection of locations in space. From the outside, the event horizon is the limit of what can be seen. Anything that enters the event horizon will never exit. Once inside the event horizon, nothing can travel fast enough to escape, not even light.


Our theories of black holes, based on Einstein’s General Theory of Relativity, show that they are astonishingly simple. A normal star contains a variety of elements that are spread differently throughout the star depending on their mass. Temperature and pressure vary enormously and immense columns of gas flow from its core carrying heat to its surface. All that complexity vanishes when the star collapses to become a black hole. Each black hole, theory says, is completely described by three numbers: its mass, its spin rate, and it total electric charge. Since all massive objects rapidly become electrically neutral, we really need only the other two numbers for a complete description. Astrophysicists characterize this simplicity by saying that “black holes have no hair.” Since we now know X-1’s mass and spin, we know all that can be known (without going inside).


A black hole in empty space would be extremely difficult to detect. After all, no light can exit a black hole—that’s why they’re called “black.” Astronomers can detect black holes indirectly when they influence their surroundings. X-1 has a companion star, a brilliant giant star named HDE 226868 that is about 20 times more massive and 350,000 times brighter than our Sun. The two partners orbit one another due to their mutual gravity, and because we observe the telltale motion of the bright partner, we know that the two objects are only 20 million miles apart, five times closer than Earth and the Sun, and that they complete a full orbit in 5.6 of our days.


X-1’s companion star emits an intense stellar wind—charged particles that shoot out in all directions—that carries away enough matter each year to form an Earth-sized planet. X-1 captures some of that matter, which accumulates in a thin disk that rotates around the black hole—an “accretion disk.” In effect, X-1 is slowly cannibalizing HDE 226868. See the artist’s sketch below. It is this in-falling matter from the companion star that is heated to millions of degrees and emits the X-rays that astronomers observe. Shooting out in both directions along the rotational axis of the accretion disk are two long, slender jets of charged particles. Each of Cygnus X-1’s jets carries 1000 times more power than our Sun emits.



The presence of such a massive companion star that has not yet reached the end of its life shows that the black hole cannot be more than six million years old. Additionally, VLBA data show the pair moves through the galaxy at “only” 45,000 miles per hour (Earth’s speed around the Sun is 70,000 mph). This “slow” speed seems to rule out the creation of X-1 in a supernova, as such an immense explosion would likely have given the black hole a much stronger kick.


More evidence that Cygnus X-1 is a black hole comes from observations of “dying pulse trains.” Occasionally, a substantial chunk of matter falls from the accretion disk and spirals inward to oblivion. We may then observe radiation from that matter coming to us in bursts. As the matter orbits the black hole in an ever-tighter spiral, we see a pulse of radiation each time the matter turns in our direction. As the orbit shrinks and the matter moves faster, the interval between pulses shortens and the radiation’s wavelength increases due to an ever-stronger gravitational redshift. If the in-falling matter ultimately hit a solid surface, such as that of a neutron star, we would see a final, powerful energy burst from the impact. But since a black hole’s event horizon is simply a location in space, there is no impact as the matter goes through it and hence no final, powerful burst. Observations of dying pulse trains without final bursts confirm X-1 is a black hole.



VLBA: Very Long Baseline Array


The key observations that settled these questions came from radio telescopes of the Very Long Baseline Array, “VLBA.” VLBA is a U.S. research facility of ten telescopes located along a 5000-mile path from Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands. (I can tell you there’s great scuba diving in the waters at both ends.) Each dish is 82 feet wide and weighs 240 tons. By combining observations from these telescopes, VLBA achieves a resolution equivalent to one 5000-mile-wide dish (albeit one with huge holes). VLBA has the highest imaging resolution on Earth, or in space, as good as 0.2 milliarcseconds. That’s an angle ten million times smaller than is spanned by the Moon. The dish at North Liberty, Iowa is shown below.



Guide to the Cosmos