Open Yale courses ASTRO 160 - Frontiers and Controversies in Astrophysics
Professor Charles Bailyn
The second installment of the online course is about agujeros negros and relativity.
* definition
According Comenzamos by the definition which is a agujero negro as an objet escape velocity is greater than the speed of light, we reach the Schwarzschild radius.
Rs = 2.GM / c ^ 2 (1)
That is, an object is a black hole when its mass is concentrated in a radius smaller than the radius of Schwarzshild.
this ratio can be calculated for any object, but not as "simple" as a compact object, usually outweigh other forces of gravity. Taking the example of a star, the gas pressure and radiation (nuclear fusion in its core) modules will retain a balance that prevents the gravitational collapse.
* Formation of black holes in final stages of stellar evolution
is known from observations and theoretical models by the end of the stable life of a star, is "running out" nuclear fuel, and it loses its outer layers, exposing the small, bright core (white dwarf ). Degeneracy pressure (Pauli exclusion principle) can restore the balance of forces. Chandrasekhar
in the '30s showed that if the remaining core was greater than
M> 1.5 x Mo (2) Limit
Chandrasehkhar
(Mo = mass of sun = 1.98x10 ^ 30 kg) degeneracy pressure would be insufficient. This leads to the formation of stars neutron. Hydrogen atoms collapse; proton (p +) and electron (e-) combine to form a neutron (n), emitting a neutrino (nu), through the reaction
e-+ p + -> n + nu (3)
Stars of this type were tested experimentally in the 60s (pulsars)
Knowing the very high densities that exist in a neutron star, we can calculate the Schwarzschild radius (Rs). It shows that a remnant of
M> 3 * Mo (3)
The radio is less than its Rs ... therefore with masses larger than the stellar remnant ends up collapsing a black hole .
A sphere of radius Rs is the so-called event horizon "What happens inside the horizon is difficult to predict and impossible to verify as being the escape velocity greater than light can we never get inside information . Following the postulates of the theory of relativity of Einstein (the "best" we have so far), all the mass collapses into a point of infinite density (singularity).
* experimental confirmations of the theory of relativity
Bailyn explains in the following classes the most important concepts of relativity, such as the constancy of the speed light regardless of the speed observer and the source station, the famous mass and energy equal
E = mc ^ 2 (4)
well as the concept of space-time and relativistic effects on mass, time and space when reaching speeds approaching c, and relativistic metrics and geometry. Do not go into details here, because it would exceed the intention of the post widely. Here the wiki summary of relativity special and general .
What if comment are some of the largest demonstrations were made so far on the theory, and the best "laboratory" in the relativistic conditions are reached is the observational astronomy.
1) Rotation of the perihelion of Mercury
was known at the time of publication of the theory in an unusual move óribta Mercury. Despite being small (571 arcseconds / century) was measurable, and discounting the influence of the planets known, subtract 43 arcseconds unexplained. Efforts to find an inner planet Mercury to explain the deviation as Newton did not succeed. In contrast, the theory of relativity explained this deviation as a relativistic effect without additional planets.
2) deviation of starlight in the gravitational field of the Sun
The CONFIRMATION that earned worldwide fame Einstein was measuring positions of stars near the sun made by Eddington during a solar eclipse in 1919 (only possible to measure the influence of the sun during an eclipse, otherwise the Sun's brightness makes it impossible to detect anything else ...).
As a result of the deflection of light, the apparent position of stars reslta modified by the gravitational field of the Sun (only in the vicinity of the effect is noticeable).
The gravitational lensing effect is an extension first detected effect on the Sun, in astronomical objects much farther away, but also much more massive. In recent years, particularly since the beginning of the telescopes in orbit, there were several lenses in which the deformed object is multiplied several times, adopting the ring-shaped or similar deformations.
Abell 2218 3) gravitational redshift
Escaping from a gravitational field, the light loses energy. However, its speed is constant. The prosperity of energy manifests instead as an extension of the wavelength Δλ
/ λ = (1 / (1 - Rs / R) ^ 0.5) - 1 (5) Where
Rs is the Schwarzschild radius (1)
Δλ is the change in observed wavelength and the emitted λ R is the distance
issued a beam of light of a wavelength determind in two directions, a tangent to the surface, and other, perpendicular. Experimentally verified in the beam perpendicular a change in wavelength due to the Earth's gravitational field. In contrast, there is no change in the tangent bundle. In 1959 Pound and Rebka conducted this experiment in a tower at Harvard University.
4) Gravitational waves The theory posits that the deformation that generates a massive object in spacetime, is transmitted to the gravitational wave speed of light. The energy of these waves come from the orbit, which means that the orbits will gradually decline. Although not yet detected these waves directly, indirectly confirmed in a binary pulsar PSR1913 +16, discovered by Hulse and Taylor in the 70's and observed in detail thereafter, the radio pulse typical pulsars allowed check over the years, the reduction of the orbit predicted by theory.
* In search of black holes
So, What we're going to talk about next time is how Might one go about testing relativity strong field. This is a subject close to my heart Because It's What I do for a living .
recommend 32 minutes from where their experience in search of black holes (have to let it load ...)
Knowing their theoretical properties and their possible formation as an effect of the life cycle of a star, it was a challenge
"Finding a black hole
-Identify if it behaves as predicted by the theory of general relativity for example - have an event horizon rather than surface?
In the 60 identified in the sky very powerful sources of X rays, emitting thousands of times the Sun's energy combination of temperature, light and rapid change, determined that its source was much smaller than a star normal, ie should be addressed at the beginning of a neutron star.
The theoretical model explaining radiation, proposed a joint normal star orbiting a compact object. The latter draws material from the outer layers of its companion, forming an accretion disk. The rotation generates gas rotation and reaching speeds up to the object, reaching temperatures of millions of degrees, emitting X-rays
(Image: Astronomical Illustrations and Space Art by Fahad Sulehria) The key was then to observe the companion star óribta and determine the mass of the compact object. If the object has a mass greater than 3 solar masses, then we have found a black hole.
Recipe to find a black hole "
1-detect new X-ray source
2-Wait ...
3-X-rays decreased in intensity
4-Measure the mass of the object
5-If more than 3 solar masses, bingo!
Image - Prof James Bailyn - Yale You've Probably Never Heard of the constellation of Musca - Musca the Fly. Yeah. There's Two Reasons You Have not Heard of it. One is it's in the southern hemisphere. You can not see it from here. But The Other reason is it's a pretty pathetic excuse for a constellation. It's one, lousy, fourth magnitude star. That's Why They Call It the Fly, right? But They Had to Call That Part of the sky something, and it's now my favorite constellation Because it has this object in it. 
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