ARC: An arc is a segment along the circumference of a circle.

ARC MINUTE (arcmin): An arc-minute is a measurement of angularity and is equal to one-sixtieth of a degree (there are 360 degrees in a circle).

ARC SECOND (arcsec): An arc-second is 1/60 th of an arc-minute.

BINARY STARS: A binary star are two stars rotating around a common center of mass. About half of the stars in the sky are binary systems.

BIT(S): A bit is a unit of information in computer memory. Think of a bit as a letter in an alphabet.

BYTE(S): A byte is group of eight consecutive bits operated on as a unit in a computer.

BLACK HOLES: A black hole is a massive object (or region) in space that is so dense that within a certain radius (the Schwarzschild radius), the gravitational field is so strong that nothing is able to escape including light.  High mass stars (those with a mass over 3 times the mass of the Sun) will evolve into red supergiants, then supernova, and then become either neutron star or a black hole. A typical black hole has a mass of roughly 10 times that of the Sun, however the range must be huge.  The words 'black hole' was coined by the physicist John Archibald Wheeler; before Wheeler, black holes were called 'frozen stars.' Astronomers think that there may be a supermassive black hole at the center of each galaxy.

CENTER OF MASS: The center of mass is the location at which the entire mass of an object (or set of objects) may be considered for purposes of calculations. It is the point of the average weighted position in space of an object (or a collection of objects).

ELECTRON-VOLTS (eV): An electron-volt is a unit of measurement of energy describing the total energy carried by a particle or photon.

ENERGY: Energy is the capacity for doing work. Energy can change from one form (heat, chemical, nuclear, potential energy) into another but is always conserved. In nuclear reactions (fission or fusion), mass can be converted into energy.

ESCAPE VELOCITY: The escape velocity is how fast an object has to be moving away from a planetary object in order to escape its gravitational field.

EVENT HORIZON: The event horizon is the radius from a black hole inside of which it is impossible to escape (a "point of no return" called the Schwarzschild radius).

FREQUENCY: Frequency is the numbers of cycles per unit time (i.e. seconds).  It's the inverse of a wavelength.

GALAXY: A galaxy is a huge group of stars and other celestial bodies bound together by gravitational forces. There are spiral, elliptical, and irregularly shaped galaxies. Our Sun and solar system are a small part of the Milky Way Galaxy.

GAMMA-RAYS: Gamma rays are very high energy electromagnetic radiation, like light or X-rays, but much higher in energy and frequency (and shorter in wavelength).

NOVAE: A novae is a white dwarf star that suddenly increases in brightness by several magnitudes and fades very slowly.

OCCULTATION: The concealment of one heavenly body by another passing between it and the observer, as of a star or planet by the moon, or of a satellite by its primary planet. It is also the concealment of a heavenly body behind the Earth. The term is commonly applied only in those cases in which the occulting body is of much greater apparent magnitude than that occulted (i.e. the partial or total eclipse). In the case of Jupiter's satellites, an eclipse takes place when a satellite passes into the planet's shadow and an occultation takes place when it passes behind the planet's disk.

PHOTON: A photon is a particle of light.

SOLAR FLARES: A solar flare is a magnetic storm on the sun, which appears to be a very bright spot, and a gaseous surface eruption. Solar flares are classified based upon their x-ray energy output at peak burst intensity.

SUNSPOTS: Sunspots are cool, dark patches on the Sun's surface. They are caused by disturbances in the sun's magnetic field which make the sunspot about 2700°F (1500°C) cooler than the surrounding area. Sunspots occur where the sun's magnetic field loops up out of the solar surface. The number of sunspots follows an 11-year solar cycle; the current cycle had already peaked in late March 2001 where there was a really big solar flare. Sunspots are visible from Earth. (WARNING: do NOT look at the sun; it can damage your eyes permanently!)

TRANSIENTS: In the dictionary, transients are defined as something as passing by or away with time; not durable or permanent; temporary, transitory; especially passing away quickly or soon, brief, momentary. In astronomical terms, transients are objects like black holes, x-ray novae, star binary outbursts, etc.

SCHWARZSCHILD RADIUS: The Schwarzschild radius is the radius of the event horizon of a black hole. This is the distance from a black hole at which nothing can escape including light because within this radius, the escape velocity from the black hole is greater than the speed of light. The size of the Schwarzschild radius may be proportional to the mass of the black hole. For a typical black hole with a mass 10 times that of the Sun, the Schwarzschild radius would be roughly 18.6 miles (30 km). The Schwarzschild radius is named for the German astronomer Karl Schwarzschild, who predicted the existence of collapsed stellar bodies that could not emit radiation, in 1916.

WAVELENGTHS: Wavelength is a characteristic of electromagnetic radiation. It is the distance between two wave crests or two troughs.

X-RAYS: X-rays are a type of electromagnetic radiation (between ultraviolet light and gamma rays in wavelength, frequency, and energy) - basically, it's light that is way past the blue-violet end of the visible spectrum - we cannot see it. They have a shorter wavelength (and higher frequency) as compared to visible light. Each photon of X-ray radiation has a lot of energy. X-rays can go through most solid objects. X-ray images of celestial objects are one way of learning about their high-energy properties. For example, the sun's corona emits X-rays, especially over sunspots. The Einstein X-ray satellite was launched in 1978 to survey celestial X-ray sources.

Soft X-Rays: Soft x-rays are lower energy x-rays.
Hard X-Rays: Hard x-rays are the highest energy x-rays.
The distinction between hard and soft x-rays is not well defined. Hard x-rays are typically those with energies greater than around 10 keV. More relevant to the distinction are the instruments required to observe them and the physical conditions under which the x-rays are produced.