Gamma Ray Bursts and Colliding Stars Part-2 - Remote Sensing Application - Completely Remote Sensing, GPS, and GPS Tutorial
Gamma Ray Bursts and Colliding Stars Part-2

A Radio telescope image of this GRB taken on April 22 shows a progressive distribution of decreasing energy moving outward. This seems to confirm the "fireball" model for ejection of matter (an alternate explanation, that material is ejected in huge blobs [the "cannonball" model], is apparently not valid for this observation). The expelled material moves at nearly the speed of light.

GRB 030329, imaged at Radio frequencies, about 24 days after the initial burst of Gamma rays.

Spectrographic data showed that the initial burst of H2652 was rich in excited Silicon and Iron. These elements would be produced in a star whose mass is at least 30x that of the Sun, which would give rise to temperatures and pressures that generate nuclear reactions that fuse nuclei into Si and Fe. These are the conditions that favor a "super-Supernova", another way of referring to hyperNovae. Astronomers believe this is convincing evidence for that mode of generation of many (perhaps most) GRBs.

As knowledge of the roles of the predominant dark matter and dark energy (see pages 20-9 and 20-10) is increasing, another explanation (Louis Clavelli) - somewhat conjectural - has emerged. This holds that dark energy under the right conditions acts upon ordinary matter to convert it to dark matter. From theory, this should give off intense bursts of energy as the conversion proceeds, witnessed by us as GRBs.

Another space telescope dedicated to GRB detection is NASA's SWIFT, launched on November 20, 2004. This spacecraft has three detection systems that can be activated within minutes of the one always on that picks up the Gamma-radiation. SWIFT is capable of finding and monitoring as many as 2 GRBs per week, far more than previous instruments, and will follow the changes to the stage when Black Holes as the end product is reached. SWIFT also can "see" back to the early days of the Universe out to almost 14 billion light years away. The spacecraft and its three telescopes are shown here:

Artist's sketch of SWIFT; BAT = Burst Alert Telescope; XRT = X-ray Telescope; UVOT = Ultraviolet Optical Telescope

In January, 2005 9 GRB events were detected by SWIFT. The first detection was in December, 2004, as shown below as an energy plot and the actual image derived from the data:

Energy plot of the first SWIFT-detected GRB
The bright spot is the GRB; the blue lines are background radiation effects.

The four panels in the illustration below show sequential steps in a gamma ray burst:

X-ray images of a GRB in late January of 2009.

As mentioned above, GRBs are all short-lived, even in human terms, lasting from hundreds of seconds to a few days. One very short duration type, which releases much less energy, is known as Gamma rays flashes; this lasts for milliseconds to a few seconds. These have been observed by SWIFT and by earth-based telescopes. Their cause(s) may be Neutron star pair interactions but other mechanisms have not been ruled out. Here are several that occurred simultaneously:

Lower energy GRBs, called 'flashes', or 'baby bursts'.

Problems with explaining GRBs are compounded by observations (using EGRET, NASA's orbiting Energetic Gamma Ray Experiment Telescope, part of the Compton X-ray Observatory) of about 170 sources of continuously emitting high energy Gamma rays. Thus, these do not display short-lived bursts. This class was first discovered by the Compton Gamma Ray Observatory (page 20-3). They may be associated with clumps of supersymmetric particles (page 20-1) including a type called the neutralino.

Gamma Ray Bursts are of great significance to astrophysicists from a theoretical standpoint. But, they also should be of interest to the general public - to all of humanity, since they pose a potential threat to all life on Earth. If one should occur in this part of the Milky Way, and be pointed in the right direction, the gamma rays could be fatal both from their direct effect on living tissue and from the likelihood that they would blow away the ozone layer that protects Earth from solar irradiation. This may actually have happened at the end of the Silurian Period about 400 million years ago when a major extinction occurred, which has not yet been linked with any other cause. However, there is a very small probability that the any star near the Sun will be of the right size to produce a hypernova; but, the neutron star collision cannot be ruled out right now since such binary pairs are not easily detected.

In sum, there seems to be two entirely different mechanisms that release the huge amounts of energy associated with GRBs. Longer duration GRBs result from Hypernova explosions. Shorter GRBs are probably the final instantaneous phase of collision between two Neutron stars. Needless to say, GRBs continue to fascinate cosmologists since they represent the largest and fastest explosive events beyond that of the Big Bang itself. As they are better understood, they may reveal the action of physical processes only now being speculated upon, and suggested by particle physics experiments. The next big step in studying GRBs and the continuous types will be the launch of GLAST (Gamma-ray Large Area Space Telescope), perhaps as early as 2007. For more information about this phenomenon, consult this Wikipedia web site.

Colliding Stars

At least some of the GRBs and X-ray bursts may stem from collisions or mergers of (usually two) stars (check back at the bottom of page 20-5 for the earlier review of galaxy collisions). As recently as the 1970s astronomers considered collisions to be rare stellar events. Although an actual collision has as yet not be observed by HST or other astronomical satellites and ground telescopes, phenomena associated with certain stellar configurations have now been postulated (attributed) to either head-on or glancing encounters between stars.

As we have seen, stars in the arms of spiral galaxies or the fringes of elliptical galaxies are very widely spaced and hence the probability of collision is low. But star distributions in central cores of these two types show much closer spacing (denser).

To appreciate the significant increase in density, if one counts the stars that are about 25 light years from our Sun, the number would be about 100 but if the same 25 l.y. volume is set around the center of a globular cluster, that star number rises to an order of about 1,000,000. This crowding means that those stars are very closely packed and hence capable of numerous collisions. Three processes make collisions much more likely: 1) a process called "evaporation", in which stars approach others and some are then flinged out of the grouping which contracts to such densities as to make collisions inevitable; 2) gravitational focusing, in which approaching stars have their pathways deflected so that two stars now follow a collision course; 3) tidal capture, in which neutron stars or Black Holes latch onto nearby stars and in time draw these into the high gravity

Theoreticians have developed computer models to simulate pictorially different modes of collision. Shown here is the sequence of change as two Sunlike stars are merged:

Simulated sequence showing the stages of collision of two stars of similar size.

The end result of a collision depends on several factors: 1) whether there is a direct hit or a glancing encounter; 2) the relative size (mass) difference between colliding bodies; 3) the terminal speed of each body. The process can be as brief as an hour; or as long as days to years (this rapid time for completion is one reason while such events have yet to be observed in "real time"). Any two of the 7 density types shown near the top of page 20-5 can experience a collision. In some combinations, such as a White Dwarf striking a Red Giant, the end result is two White Dwarfs (one being the incoming member; the other [Red Giant] dispersing and losing so much of its gas by the interaction that only its core remain which quickly evolves into the new White Dwarf, Or, one star remains relatively intact as the second star is incorporated within it.

In this last case, the result is that the now coalesced star pair has gained considerable mass. This means that it now appears to be a bigger star, and since the total mass determines the rate of Hydrogen fuel consumption, the new, brighter star would appear as though it will burn its Hydrogen mass much faster and thus appears to have a shorter lifetime - hence seems younger. A specific case: if two stars, each with a mass of the Sun (5 billion years old) that has a total burn out time of 10 billion years, collide and form a single star with twice the mass, the now more luminous composite star would have a life expectancy of 800 million years. This seems to be the best explanation of "Blue Straggler" stars - much brighter than the majority of stars in a globular cluster. This is evident in this HST image of NGC 6397:

Large blue stars in globular cluster NGC6397.

Source: http://rst.gsfc.nasa.gov/