Gamma rays are the highest-energy form of electromagnetic (EM) radiation – about a billion times more powerful than the EM waves we see as visible light. When scientists observe intense bursts of gamma rays from space, it usually indicates the presence of a large-scale, high-energy cosmic event – like the sudden gravitational collapse of a star or matter streaming into the enormous gravitational sink of a black hole. Gamma-ray bursts are the most energetic events known in the universe, and, though scientists have been able to figure out a few things about them, gamma-ray bursts are still a bit of a mystery.
Gamma-ray bursts come from outside our Milky Way galaxy, usually from very, very far away. The initial wave of intense energy usually includes other types of electromagnetic radiation as well, which we can also “see” and image. The analysis of this data answers some questions, leaves some questions unanswered, and raises still other questions about the nature of the universe.
Scientists are very interested in the extreme events that produce gamma-ray bursts because such physical phenomena serve as “laboratories” whereby our understanding of space, time, and energy gets challenged, expanded, deepened, and sometimes even overturned. It’s interesting that, though interpretation of EM radiation, the very largest structures of the universe can tell us much about what happens at the very smallest atomic and subatomic levels of interaction. The most common explanation for what causes gamma-ray bursts is that, when a massive star runs out of fuel (meaning the internal fusion reactor that powers the star and is a force of expansion has burned up all of its fuel, leaving only heavier, less reactive elements with increasingly tremendous inward gravitational forces), it quickly collapses inward and creates a very dense body like a neutron star or a black hole. As it collapses, areas of the star’s magnetic field can intensify and align along two opposite poles, creating lines of magnetic force that act like “superhighways” for charged particles being ejected from the core. Astronomers refer to these superhighways as “jets.” The collision of that material into surrounding gas and matter produces the high-energy gamma-ray bursts. This isn’t the only way gamma-ray bursts can be produced, however. Scientists believe that two neutron stars colliding can also produce gamma-ray bursts. Both phenomena, though, involve the collapse of huge, dense masses.
All stars produce their energy from elements in their core. They fuse helium into hydrogen and then into successively heavier elements until the fusion process runs out of fuel. Stars still burning hydrogen to helium are called “main sequence” stars and they can be identified by the kinds and amounts of EM radiation the produce. When a star’s fuel burns out, the star begins to die. How a star lives – and dies – depends on its mass. If a star has a mass that’s less than about twice the mass of our Sun, then it will either expand into a red giant (during our Sun’s red giant phase, it will be so large that it would appear to take up a third of the sky) or condense slightly into a white dwarf (one teaspoon of a white dwarf would weigh as much as the moon). On a cosmic scale, neither of these outcomes is highly dramatic. More massive stars, on the other hand, can collapse in on themselves and form very dense neutron stars. Extremely massive stars – when they collapse – press their core into such extreme conditions that they produce black holes. Neutron stars are much more common than black holes – and red giants are much more common than white dwarfs.
Classroom Activity: Density Star Chart
Students research and chart mass, density, and age properties for proto-stars, various main sequence stars, red giants, novae, supernovae, white dwarfs, neutron stars, and black holes. Students can work individually or in groups to organize and visualize the relationships between and amongst various life stages and types of stars. Each student or group presents their scheme to the rest of the class.
Narrator: Every day or two, on average, satellites detect a massive explosion somewhere in the sky. These are gamma-ray bursts, the brightest blasts in the universe. They're thought to be caused by jets of matter moving near the speed of light associated with the births of black holes.
Gamma-ray bursts that last longer than two seconds are the most common and are thought to result from the death of a massive star. Shorter bursts proved much more elusive. In fact, even some of their basic properties were unknown until NASA's Swift satellite began work in 2004. Astronomers suspected that crashing neutron stars could explain short bursts.
A neutron star is what remains when a star several times the mass of the sun collapses and explodes. With more than the sun's mass packed in a sphere less than 18 miles across, these objects are incredibly dense. Just a sugar-cube-size piece of neutron star can weigh as much as all the water in the Great Lakes. When two orbiting neutron stars collide, they merge and form a black hole, releasing enormous amounts of energy in the process. Armed with state-of-the-art supercomputer models, scientists have shown that colliding neutron stars can produce the energetic jet required for a gamma-ray burst.
Earlier simulations demonstrated that mergers could make black holes. Others had shown that the high-speed particle jets needed to make a gamma-ray burst would continue if placed in the swirling wreckage of a recent merger. Now, the simulations reveal the middle step of the process --how the merging stars' magnetic field organizes itself into outwardly directed components capable of forming a jet. The Damiana supercomputer at Germany's Max Planck Institute for Gravitational Physics needed six weeks to reveal the details of a process that unfolds in just 35 thousandths of a second. The new simulation shows two neutron stars merging to form a black hole surrounded by super-hot plasma. On the left is a map of the density of the stars as they scramble their matter into a dense, hot cloud of swirling debris. On the right is a map of the magnetic fields, with blue representing magnetic strength a billion times greater than the sun's.
The simulation shows the same disorderly behavior of the matter and magnetic fields. Both structures gradually become more organized, but what's important here is the white magnetic field. Amidst this incredible turmoil, the white field has taken on the character of a jet, although no matter is flowing through it when the simulation ends. Showing that magnetic fields suddenly become organized as jets provides scientists with the missing link. It confirms that merging neutron stars can indeed produce short gamma-ray bursts. At this moment, somewhere across the cosmos, it's about to happen again.
Academic standards correlations on Teachers' Domain use the Achievement Standards Network (ASN) database of state and national standards, provided to NSDL projects courtesy of JES & Co.
We assign reference terms to each statement within a standards document and to each media resource, and correlations are based upon matches of these terms for a given grade band. If a particular standards document of interest to you is not displayed yet, it most likely has not yet been processed by ASN or by Teachers' Domain. We will be adding social studies and arts correlations over the coming year, and also will be increasing the specificity of alignment.