The Astronomical Dictionary

This is a comprehensive list of scientific terms related to astronomy, accumulated into one big fat list, arranged alphabetically. About every term in this list is relatively unknown and/or misunderstood and misinterpreted by most people.

Here it goes! Enjoy!

  1. Absolute Zero : Its value is 0 Kelvin or -273.15 degrees Celsius. It is the lowest, theoretically possible temperature, at which the particles of any material have no more vibrational energy. The lowest temperature achieved in a laboratory was 0.45  nano-Kelvin in 2003 at MIT.
  2. Apogee : It is the peak or the highest point of the trajectory of an object in an elliptical orbit around the Earth.
  3. Asteroid : Asteroids are basically large or small rocks orbiting a more massive body, such as the Sun in the Solar System. Some asteroids also have no particular orbits and float around in space. Most asteroids are assumed to have been part of now destroyed planets. Asteroids are found primarily within the orbits of Mars and Jupiter.
  4. Asterisms : These are small groups of stars within constellations which do not form an entire constellation.
  5. Astronaut : Any person who travels beyond the Earth into outer space in a spacecraft.
  6. Astronomical Unit : It is the approximate distance between the Earth and the Sun and is equivalent to 1.496×10^11 meters or 9.23×10^7 miles. (Symbol : AU)
  7. Big Bang Theory : This is a theory that predicts that the universe was a singularity about 13.8 billion years ago, a highly compressed point in space with almost infinite density. This is the current prevailing theory about the birth of our universe and has been substantiated by the discovery of Cosmic Background Radiation.
  8. Black Hole : This is a singularity occurring in space-time which when created, exerts gravity so high, that it pulls matter and energy in the form of Electromagnetic radiation, into its void and does not allow it to escape. It has been hypothesized that Sagittarius A, lying at the center of our Milky Way galaxy is a Black hole.  More on this in a separate Blog post.
  9. Comets : Comets are small body made up of dust and rock floating around in space, orbiting a larger body. Comets are generally periodically visible and have extreme surface temperatures. When they come closer to the Sun, their surface frost thaws and a long squishy tail is visible. A well known example is the Halley’s Comet, seen every 76 years.
  10. Constellations : These are groups of stars that form unique patterns and shapes when viewed form the  Earth. Constellations are actually made up of stars that may be light-years away but seem to be close enough to form a shape resembling animals, humans, birds and insects.
  11. Cosmic Microwave Background Radiation : This is the electromagnetic radiation which has been found everywhere in outer space without any specific source. This background radiation is have been thought to be leftover from the energy released during the Big Bang.
  12. Cosmic Rays : Cosmic rays are high energy streams of particles which strike the Earth and orbiting objects. These rays have very high energies and are emitted mainly from bodies like stars and supernovas. They are abundance increases with altitude and require shielding to prevent radiation poisoning.
  13. Dark Matter : This is a form of matter that is considered to occupy about 98% of the entire Universe. It is however not clear whether it does exist as it is still invisible to humans and any scientific instruments. Its existence has been hypothesized on the basis of phenomenon of Gravitational Lensing of Background Radiation.
  14. Doppler Effect : This is the phenomenon of variation in the pitch of the soundwaves emitted by a moving object in relation to another moving or stationary body. The soundwaves from a source which is moving closer to another object tends to compress the soundwaves  as it moves closer. Due to this effect, the pitch increases with decrease in intermediate distance. Also, inversely, the frequency drops with increase in distance. This effect is also observed for light, which is explained in “Redshift” below.
  15. Dwarf Planet : According to the International Astronomical Union, A Dwarf Planet is defined as “A Celestial Body that orbits the Sun, has assumed a nearly spherical shape and is in hydrostatic Equilibrium, has not cleared the space surrounding itself and is not a satellite of another planet.” There are 5 officially recognised dwarf planets in our Solar System.
  16. Eclipses : When an object orbiting around a larger body, obstructs the view of a light emitting object such as the sun and casts a shadow on the surface of the larger body, the event is known as an eclipse. During a solar eclipse, the Moon obstructs the light from the Sun and casts a shadow on the surface of the Earth.
  17. Entropy : Simply put, Entropy is the measure of the amount of disorder in a system. Over time, the amount of disorder in a system always increases. A very lucid example of increase in entropy in a system would be the melting of ice into water.
  18. Escape Velocity : This is the amount of velocity with which a body must be travelling to escape the sphere of gravitational influence of a massive body. eg. To escape the Earth, this value is about 11.186 km/s.
  19. Event Horizon : This is the hypothetical boundary of a Black Hole. Any object crossing this boundary will be pulled into the Black hole. Beyond this boundary, the escape velocity for a black hole is beyond the speed of light and is thus a “point of no return”.
  20. Exoplanets : These are planets identified as existing beyond our Solar System.
  21. Flyby : A flyby is a typical space mission that enters the immediate gravitational vicinity of an object such as a planet, satellite or an asteroid but does not physically land or descend on it. A good example is the New Horizons Pluto Flyby .
  22. General Relativity : A theory published by Albert Einstein which provides a basis for the gravitational pull exerted by massive objects even over very long distances.
  23. Gravity : The pull exerted by a massive body on another massive body. According to the theory of General Relativity, gravity is in fact a side effect of the curvature of  Spacetime due to the uneven distribution of mass and energy.
  24. Gravitational Lensing : It is the effect due to which a body present behind another body appears distorted and magnified. The phenomenon occurs due to the bending of the light due to the presence of the intermediate massive body and its gravity.
  25. Gravitational Time Dilation: This is the difference in passage of time for two bodies situated at separate distances from a massive body. This effect also impacts GPS satellites and has to be accounted for accuracy.
  26. Galaxy : A galaxy is a huge and massive collection of stars, planets, meteors , moons and gas and dust clouds. Galaxies are formed over millions of years with the stars in them constantly evolving. examples include our Milky Way Galaxy and the Andromeda Galaxy.
  27. Hohmann Transfer Orbit : This is an orbital maneuver that is used to increase or decrease the orbital radius of the orbiting spacecraft in the most efficient manner possible.
  28. Hypernova : A Hypernova is an ultra-energetic form of a supernova which happens in an extreme core collapse scenario. It has been hypothesized that these explosions are the most energetic bursts in the present universe after the Big Bang and are associated with formation of black holes.
  29. LEO: LEO stands for a Low Earth Orbit between 180-2000 kilometers from the surface of the Earth.
  30. Light year : Despite sounding like a unit of time, a light year is actually the distance travelled by light in a year.(9.461 times 10 to the power 15)
  31. Meteorite: Meteorites are pieces of extraterrestrial rocks that originate in outer space and land on the Earth’s surface.
  32. Moons: A moon or a natural satellite is defined as a substantially large object that orbits another object.
  33. Nebula : A Nebula is basically a large interstellar cloud made up of gases and particles clumped together.
  34. Neutron Star: A Neutron Star is a star which has collapsed due to the intense gravity of its core. This stage is reached with a very brilliant release of energy in the gamma region of the electromagnetic spectrum. The remnants of these explosions are ultra-high dense neutron masses with surface temperatures in millions of degrees, giving them the distinct name.
  35. Observable Universe : This is the portion of the entire Universe that can be seen or observed by us. The regions beyond the boundaries of the observable universe are inaccessible to us as light has not had enough time to reach us from these regions. The shape of the observable universe is assumed to be spherical with the center of this hypothetical sphere being the observer.
  36. Orbits : Orbits are the loci or the paths followed repeatedly by the objects in space under the influence of another object in space. Orbits are attained when the momentum of the orbiting object balances out with the gravitational pull of the larger object.
  37. Planet : A Planet is basically an object that is sufficiently spherical as a result of its own gravity, is not large enough to cause thermonuclear fusion and has effectively cleared its surrounding area of smaller objects.
  38. Pulsar : Pulsars are a special type of Neutron Stars which emit electromagnetic radiation in the ultraviolet, X-ray and gamma ray range, but in short pulses instead of a singular large burst of energy.
  39. Quasar : Quasars are ultra-bright objects located very far from our galaxy (Billions of lightyears!). Quasars are large and bright jets formed by heating up of the accretion discs around supermassive black holes with masses over millions of times the mass of our sun.
  40. Satellite : A satellite is any object that is orbiting around a larger object. The satellite can be natural or artificial. Artificial satellites are typically man-made and are sent into orbit by means of multistage rockets.
  41. Shapiro Time Delay : The Shapiro Time Delay is the effect of slowing down of time by the presence of a massive body. This happens as according to the General theory of Relativity, the speed of light is minutely reduced when in the presence of a massive body.
  42. Solar Winds: The Surface of the Sun in our solar system releases highly energized waves of magnetic particles at regular intervals. The streams of such particles are called solar winds and are quite harmful to humans if directly exposed. However, Earth’s Natural Magnetic Field acts as a protective shield.
  43. Space-time: Space-time is a concept theorized by Albert Einstein in his Theories of Relativity which hypothesizes the intertwined nature of space and time such that all bodies move in space and time together.
  44. Spacecraft : This is any object released intentionally beyond the Earth’s upper atmospheric levels
  45. Special Relativity : The theory proposed by Albert Einstein which states that the speed of light is the universal speed limit and is constant throughout the universe. It also states that time is not constantly flowing for everyone. The flow of time is different for each observer of an event according to his perspective. In a way, you move in space as well as time together.
  46. Sphere of Influence : The hypothetical boundary within which the gravitational influence of a body is prominent.
  47. Stars : Stars are luminous bodies made up of highly dense mixtures of hydrogen and helium. They generate heat and light by nuclear fusion reactions at their core. There are more than a billion stars in every galaxy.
  48. Superclusters : These are clumps of Galaxies located in close proximity to each other. Our Milky Way Galaxy is located within the Virgo Supercluster.
  49. Supernovae : When a star runs out of nuclear fuel, it collapses violently under its own gravity. This causes a very bright spot in the sky and may even outshine entire galaxies.
  50. Universe : The Universe is space as we know it. It contains anything and everything that we know of yet and the things unknown to us. No one knows how big the Universe actually is, how it came to be and what is its future.
  51. Wormholes: Wormholes are warping gateways which connect two relatively distant parts of the Universe. No wormhole has been discovered. Yet.
  52. Zero Gravity : Absence of gravity or more correctly very low gravity. There’s an entire page on this!


References/Further reading


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