Leaving aside the question of how black holes generate the energy in gamma-ray bursts-cosmic explosions called GRBs-their sheer brilliance has posed a dilemma. So much radiation emanates from the GRB's compact space Line that astronomers would like to conclude that the photons are too densely packed (5) to allow each other to escape, but in that event, how can GRBs be observed? The resolution of this conundrum, developed over the past several years, is that the gammas are not emitted immediately instead the initial energy release of the explosion is stored in the kinetic energy of a shell of particles (including photons as well as electrons and their antimatter counterpart, positrons). This (10) produces a fireball moving at a speed close to that of light and expanding to a diameter of 10 billion to 100 billion kilometers, then converting some of its kinetic energy into electromagnetic radiation, yielding a GRB. The initial gamma-ray emission is most likely the result of internal shock waves within the expanding fireball, shocks which arise when faster blobs in the (15) expanding material overtake slower blobs. Because the fireball is expanding so close to the speed of light, the principles of relativity dictate that the timescale witnessed by an external observer is vastly compressed, such that over a few seconds the observer may witnesses a burst of gamma rays that required a day to produce. The fireball continues to expand, eventually encountering and (20) sweeping up surrounding gas, then subsequently generates another shock wave at the boundary between the fireball and the external medium that persists as the fireball slows down and shrinks. This external shock nicely accounts for the GRB afterglow emission and the gradual degradation of this emission from gamma rays to x-rays to visible light, then, finally, to radio waves. (25) Although the fireball can transform. the explosive energy into the observed radiation, astronomers have yet to agree on what generates the energy to begin with. One family of models, referred to as hypernovae, involves stars born with masses greater than about 20 to 30 times that of our sun simulations show that the central core of such a star eventually collapses to form. a rapidly rotating (30) black hole encircled by a disk of leftover material. Nevertheless, compact-star coalescence could still have a place in the big picture and a mechanism may account for the poorly understood short-duration GRBs. Moreover, additional models for GRBs are still in the running, one of which credits the birth of the fireball to the extraction of energy from an electrically charged black hole. This (35) model suggests that both the immediate and the afterglow emissions are consequences of the fireball sweeping up the external medium. Astronomers have come a long way in understanding gamma-ray bursts, but they still do not know precisely what causes these explosions, and they know little about the rich variety and subclasses of bursts. Which of the following best expresses the main idea of the passage?