Source: LG-1 998-08-011 -GSFC
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NASA's Rossi X-ray Timing Explorer (RXTE) probes the physics of cosmic X-ray sources by making sensitive measurements of how they change over time scales ranging from milliseconds (1/1000 of a second) to years. X-rays are a highly energetic form of electromagnetic radiation (light). X-radiation is produced under extreme conditions, such as when matter is fused inside exploding stars, accelerated to nearly the speed of light by the rotating magnetic field of a pulsar, or rubbed out in the intense gravitational field of a black hole.
Studying how cosmic X-radiation changes over time reveals how such violent processes work. It also allows astronomers to probe the structure of some of the strangest objects in the universe, such as white dwarf stars, neutron stars, pulsars, and black holes. All these objects are produced during the end stages of stars' lives. Which object is formed depends on how massive the original star was. When a star runs out of fuel for nuclear fusion, its core contracts under its own gravity and heats up. This causes the outer layers of the star to expand greatly and is called the red giant phase.
Stars about the same mass of our Sun enter the red giant phase and blow off material in their outer layers with strong stellar winds. After the red giant phase, they leave behind a hot, dense core about the size of the Earth, called a white dwarf star. If it were possible to bring some to Earth, a marble-sized piece of white dwarf matter would weigh nearly a ton.
A more dramatic fate awaits stars with more than six times the mass of our Sun. The final stage of a star like this is marked by a cataclysmic explosion called a supernova. For a few days, a single star shines with the light of an entire galaxy. The gravitationally collapsed cores of these stars form some of the most exotic objects known: neutron stars and black holes.
Neutron stars are extremely dense, city-sized stars that contain about as much material as our Sun. They are so dense that subatomic particles called neutrons are the primary form of matter found inside the star, earning the name neutron star. A marble-sized piece of neutron star matter would weigh approximately 100 million tons. Neutron stars have intense magnetic fields, typically one trillion times greater than the Earth's magnetic field. A magnetic field is an invisible force field generated by moving electric charges. We experience this force field in everyday life every time we use an electric motor or a magnetic compass. Some rotating neutron stars emit beams of electromagnetic radiation from the poles of their magnetic fields. This radiation can be detected on Earth as regular pulses as the beam sweeps by like a lighthouse beacon; these neutron stars are called pulsars.
If the remaining core of a dying star is greater than about three solar masses, nothing can halt its collapse. It becomes crushed to a super- hot, super-dense state called a singularity, forming a black hole. This tremendous density gives the black hole an intense gravitational field; near it, nothing, not even light, can escape - hence the name. Because of this, black holes can't actually be seen. However, they reveal their presence by their effects on nearby matter, like the hot gas in the accretion disk surrounding the "Old Faithful" black hole.
The images on the front are taken from a computer animation sequence that depicts the periodic disruption of a disk of matter surrounding a black hole in our galaxy.
Figure 1: the black hole, called GRS 1915+105, is located in the center of the multi colored disk. The black hole is orbiting a massive "companion" star, depicted as a red sphere on the left. The black hole's powerful gravity pulls hot gas from the surface of the companion star. This hot gas forms a disk as it orbits the black hole, much like soap suds swirling down a bathtub drain. Called an accretion disk, it is represented by a multi-colored disk to the right of the companion star. As gas falls into the black hole, it is compressed and heated to millions of degrees, emitting light of various colors, which correspond to different temperatures. The hottest material, depicted as a blue/white area in the center of the multicolored disk, is closest to the black hole and emits ultraviolet light and X-rays. Light of these types is actually not visible to the human eye.Figure 2: a disruption of some kind, which is not well understood at this time, is transmitted through the gas in the disk.
Figure 3: eventually, the disruptions become so severe that they cause the gas in the disk to be ejected in opposite directions from the black hole, in jets at nearly the speed of light (approximately 650 million miles per hour).
Figure 4: after the ejection, the center of the disk is empty, and the black hole, represented here by a marble-like object in the center of the disk, begins to draw more gas toward itself again.
Figure 5: the entire process repeats every half hour, forming jet-like structures when seen from a distance.
The amount of gas ejected in each cycle has a mass of about 100 trillion tons. The ejection of this much matter at such a high velocity requires an amount of energy approximately equal to six trillion times the annual U.S. energy consumption.
Observing these objects lets astrophysicists study matter and energy generation under extreme conditions unattainable in laboratories on Earth. This may lead to theoretical insights that can be applied to power generation on Earth.
The 6,600 pound RXTE satellite was launched on 30 December 1995 atop a Delta 11 rocket into lowearth orbit (about 375 miles high with 23 degrees inclination to the equator). RXTE is part of NASAs Structure and Evolution of the Universe (SEU) theme for space science. The SEU theme has three fundamental scientific quests: to explain the structure of the universe and forecast our cosmic destiny, to explore the cycles of matter and energy in the evolving universe, and to examine the ultimate limits of gravity and energy in the universe.
For more information of RXTE, visit: http://heasarc.gsfc.nasa.gov/docs/xte/XTE.html
For more information on black holes, neutron stars, pulsars,
and white dwarf stars, visit:
http://imagine.gsfc.nasa.gov/docs/science/science.html
For more information on the Structure and Evolution of the Universe theme, visit: http://universe.gsfc.nasa.gov/
(Photo Credit: Walt Feimer/NASA Goddard Space Flight
Center)
(http://pao.gsfc.nasa.gov/gsfc/spacesci/pictures/rxte/bh.jpg)
FOR THE CLASSROOM:
To help students understand such tightly compressed objects such as neutron stars, have students push on a ball of cotton. As it gets smaller in volume, it becomes more dense, yet the mass stays the same. If you have a scale in the classroom, try weighing a pillow full of feathers. Then squash the pillow as small as it will go and weigh it again. It will weigh the same, both fluffed and compressed. A nerf sponge ball is another example of a compressible object.
To mimic how matter speeds up as it spirals into a black hole, securely tie a small nut onto a string. Have the students whirl it around a finger, winding up the string. The nut travels faster and faster as it approaches the finger. Finally it bangs into the finger. The closer it gets to the central object, the faster it orbits around. This illustrates one of the many physical processes happening in an accretion disk.
From NASA Educational Brief: Black Holes