Black Hole. An astronomical object whose immense gravitational field entraps everything, even light, that gets too close (closer than the black hole’s event horizon).
Cosmological Principle. The Cosmological Principle states that the universe is both isotropic and homogeneous. In other words, there are no preferred directions or preferred places in the universe when viewed on sufficiently large distance scales. This simply means that one part of the universe looks approximately like any other part when averaged over large enough distances.
Event Horizon. The surface surrounding a black hole with the property that any light ray emitted inside it cannot escape because of the strength of the gravitational field.
Grand Unified Theories. Classes of theories that merge all three non-gravitational forces (electromagnetism, strong and weak) into a single theoretical framework.
Graviton. Smallest bundle of gravitational force field; messenger particle for the gravitational force.
Heisenberg Uncertainty Principle. The uncertainty principle says that we cannot measure the position (x) and the momentum (p) of a particle with absolute precision. The more accurately we know one of these values, the less accurately we know the other. There is a minimum for the product of the uncertainties of these two measurements. There is likewise a minimum for the product of the uncertainties of the energy and time.
Local Group. The Local Group is the group of galaxies that includes the Milky Way among others. It comprises more than 54 galaxies, counting dwarf galaxies. Its gravitational center is located somewhere between Milky Way and Andromeda Galaxy covering a diameter of 10 mega light-years.
Neutrino. Neutrinos are electrically neutral, weakly interacting elementary subatomic particles with half-integer spin. Being electrically neutral, they are not affected by the electromagnetic forces. Neutrinos are affected only by the weak sub-atomic force, of much shorter range than electromagnetism, and gravity, which is relatively weak on the subatomic scale. Therefore a typical neutrino passes through normal matter unimpeded
Neutron Star. A dead, collapsed star consisting mostly of neutrons and is only about 20 kilometers in diameter.
Pulsar. A rapidly spinning neutron star that emits radiation while rotating.
Quark. A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Quarks are never directly observed or found in isolation; they can be found only within hadrons, such as baryons (of which protons and neutrons are examples), and mesons.
Quantum Fluctuations. The uncertainty principle in quantum mechanics leads to all allowed interactions having some probability of occurrence.
Quantum Mechanics. Quantum Mechanics is the study of subatomic particles and their interactions. On the microscopic scales of atoms and sub nuclear particles, strange phenomena such as uncertainty, quantum fluctuations, wave-particle duality, and a host of other phenomena that are unfamiliar to everyday world make their presence felt. Quantum Mechanics is the branch of physics with a framework of laws to study the subatomic world.
Singularity. Location where the fabric of space or space-time suffers a devastating rupture.
Supernova. A gigantic stellar explosion in which the star’s luminosity suddenly increases by as much as a billion times. Most of the star’s substance is blown off, leaving behind, at least in some cases, an extremely dense core which (as in the Crab Nebula) may be a neutron star.
White Dwarf. Compact star with mass less than about 1.4 solar masses, typical radius of 1000 km; supported against gravity by quantum-mechanical degeneracy pressure of electrons.