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Astronomy Terms

posted May 21, 2015, 8:34 PM by Upali Salpadoru   [ updated May 21, 2015, 8:41 PM ]


These may be described as rocks or mountains, of varying size that orbit the Earth. Majority of them are in a belt between Jupiter and Mars.  Some of them are larger than 100 km. Some even cross the path of the planets.  (The extinction of the dinosaurs has been attributed to the crashing of an asteroid.)

Fig. Ida Asteroid and the satellite captured by Galileo probe.

Ida is about 56 x 24 x 21 km. and has a period of rotation of 4 hours, 38 minutes the density is between 2.2 and 2.9 g/cm3. Galileo flew past Ida in August 1993 the greatest discovery was that Ida has a natural satellite. Dactyl is the first natural satellite of an asteroid ever.

Antimatter and Matter

Matter has mass and occupies space. They consist of atoms. Even Energy seems to have particles. ( Eg. Light to be made of Photons and Gravity to be Gravitons) Anti matter is not common in our universe. Particles in anti-matter have just the opposite properties as in matte except for the mass. For example an electron will have a positive charge. If matter and anti-matter come into contact they annihilate producing some rays.

In 1930 Paul Dirac describing the electron to comply with quantum mechanics and Special theory of Relativity predicted a particle having the same mass but a plus charge.   This was discovered in 1932 by Carl Anderson. This has been named as a positron.

It was surprise observation that anti matter has been detected to be coming from thunderclouds during lightning flashes. (Source Scientific: - American Aug.2012.)

Astronomical Unit (AU or au)

This is the average distance from Earth to the Sun, approximately 149.6 million km (93 million mi). It is used especially to measure distances within the solar system.

Big Bang

Prior to space and time ( birth of Universe) all the matter and energy now spread across the infinite space, had been cramped into a pencil point  of energy .All the forces that we know existed as a single force Such a colossal conglomeration of a mass has been termed a singularity. Just within a fraction of a millisecond it violently exploded producing sub atomic particles and cosmic rays etc, these relays still come towards us. Universe had been expanding ever since for the last 13.7 billion years. (No sound was heard as there was no matter outside to propagate sound waves and there was no one to hear either.)

Binary Stars

Our Sun is a loner; but some stars exist in pairs.  Usually a large star and a small star orbits round its centre of mass.

Black Hole

A hole is an empty place. Black is the colour we refer to an object that will absorb all the visible colours of light.  A ‘black hole ‘is neither black nor not empty. These are gigantic masses which can swallow stars by the force of gravity. They cannot be seen as they neither reflect nor emanate light. Stephen Hawkins has shown that they emit some rays which have been verified to be correct.

Extremely massive stars when they exhaust their Hydrogen for fusion, die off and remain as black holes gulping down energy and matter from the surrounding.

Fig. Stephen Hawkins.former Lucasian Professor of Mathematics at the University of Cambridge (Most recohnised physicist after Einstein)


Comets are celestial bodies, much smaller than asteroids, which follow extremely eccentric orbits with one apogee as the sun. They usually consist of ice and dust that have not been attracted to stars or planets for some reason. Most of them come from a region called the Kuiper belt near Neptune.  Shoemaker comet that visited us probably in 200 years got into Jupiter’s gravitational field remained as a satellite for a few years and exploded in due to tidal forces. Some of these fragments crashed on the planet giving our astronomers a chance to see a collision among celestial bodies.

Fig.  Eugene and Carolyn Shoemaker: Comet hunters ( Courtsey BBC)

They became world famous when in 1993 along with David Levy discovered Comet Shoemaker-Levy 9. It got torn apart by planetary tides of July 1994

First to be buried outside the Earth.

 "I don't think Gene ever dreamed his ashes would go to the Moon,"

 "He would be thrilled."

Carolyn Shoemaker said shortly before watching Lunar prospector blast-off in January 1998.


From the time the human evolved star gazing may have been very popular pastime. The position of stars helped them to find directions at night and the changing of seasons.  So they devised a way of remembering the star maps in the night sky.  They connected a group of stars as they saw them and completed a picture of an animal or a familiar object.  These apparent clusters of stars came to be known as constellations.  Astronomers now have accepted over 80, a few of which are as follows.

Andromeda (The Princess), Orion (The Hunter) Sagittarius (The Archer) Cassiopeia (The Queen) Cancer (The Crab) Taurus (The Bull) Aquarius (The Water Bearer), Gemini (The Twins) Scorpius (The Scorpion) Leo (The Lion)


This is atmosphere of the Sun where the matter exist in a plasma state extending to millions of km above the surface..  While the temperature of the surface is about 5800 K the corona temperature is in the millions of degrees. The corona has been carefully observed during total eclipses.

Cosmic Rays

It may surprise you to know that these are not rays but sub-atomic particles. 90% of them are protons, which are the nuclei of Hydrogen atoms. Among others there are charged particles of matter and anti-matter. They come at a very high speed that the scientists have been baffled as to how they achieve that. As the name suggests, they come from outside our solar system. There are some coming from the sun and the other stars in our milkyway. Cosmic ray collisions with the molecules in the upper atmosphere have given us the first Hadron Collider. Here the particles have thousand times more energy than the Large Hadron Collider in Geneva.

Dark Matter

The visible bodies in a galaxy are held together by the force of gravity.  This force is directly proportional to the mass of the bodies concerned and inversely proportional to the square of the distance. According to the magnitude of the force scientists have calculated the mass of matter that has to be present in a galaxy. It has been discovered that the visible matter and the invisible atoms and all the energy does not provide the sufficient mass. So they have introduced the hypothetical substance ‘Dark matter’ that should be pervading the space in between.

Double Star

Same as Binary stars.

Dwarf Planet

Fig. The orbits of the dwarf planets Pluto and Eris

A celestial body that orbits the sun, smaller than planets but has sufficient gravity to maintain spherical shape. (unlike asteroids) There are 5 recognised at the moment Ceres, Pluto, Haumea, Makemake, and Eris. All these are in the Kuiper belt.

Galactic Halo

A spherical region of dust-free space surrounding a galaxy.  Unlike the central region, which has bright and new stars, Halo contains very old feint stars and the mysterious ‘Dark matter’..

Galactic Nucleus

The central part of a galaxy which has a very high density of new stars. Usually a black hole is present here.


A very large conglomeration of stars. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars.

Hawking radiation

A theory first proposed by British physicist Stephen Hawking, those energy fluctuations from the vacuum causes the generation of particle-antiparticle pairs near the event horizon of the black hole. One of the particles falls into the black hole while the other escapes, before they have an opportunity to annihilate each other. The net result is that, to someone viewing the black hole, it would appear that a particle had been emitted.

Hubble's Law

In 1920 Edwin Hubble found that the rate of receding a galaxy is directly proportional to the distance.

Light Year

An astronomical unit of measure equal to the distance light travels in a year; it’s about 9.5 trillion km., which is 9,500,000,000,000 kilometers.

Magellanic Clouds

Fig. Our neighboring galaxies seen as clouds.

The two irregular galaxies and Large Megallenic cloud and Small Magellanic Cloud, found just outside Milky Way; just visible in the southern skies. They are considered as recent visitors to our Milkyway.

Meteor ( Shooting stars )

A celestial object smaller than an asteroid. The gravitational force of the Earth sometimes capture  objects shooting in free space at high speed. As they enter the atmosphere they get heated due to friction and usually burn and evaporate. Very rarely some of them crash on the earth and end up as meteorites.

Meteoroid .

A celestial object smaller than an asteroid.


Mainly Hydrogen gas present between the stars, which get ionized by the UV light from the stars. In an emission nebula, electrons re-combine energy is released. They aggregate to form stars. A reflection nebula is visible by reflected light.

Courtesy:Hubble site


Could you believe in one second over sixty billion neutrinos pass through 1 cm2 of your body exposed to direct sunlight. They traverse through the planet Earth just like a beam of light passing through a pane of glass. This is a fundamental particle produced in massive numbers by the nuclear reactions in stars

"Neuitrino is the most tiny quantity of reality ever imagined by a human being" said Frederick Reines  who  discovered the particle along with Clyde Cowen.

Neutron star

Imagine compressing all the human beings in to the size of a sugar cube. That’s the type of pressure and density (about 1014 g/cm3) inside a neutron star. An average n star has a mass of 1.4 times the Sun and a diameter of about 10 km. It is believed that these are dead carcases of supernova formed in a similar manner to black holes. Unlike black holes these emit pulses of energy.

Intense pressure pushes the orbiting electrons in atoms into the nucleus neutralising the positive charge of protons.

Proton + electron = neutron + neutrino

This explains why they are predominantly packed with neutrons.  The outer 1 km. is solid in spite of the temperature being in region of millions of degrees.

These stars also have the strongest magnetic fields in the known universe. The strongest inferred neutron star fields are nearly a hundred trillion times stronger than Earth's fields, and even the feeblest neutron star magnetic fields are a hundred million times Ear  The covering of this layer is made of an isotope of iron 56Fe polymerised due to the intense magnetic field. There are no atoms present the whole mass can be compared to a single nucleus.

Oort Cloud.

This is a  hypothetical sphere surrounding the solar system starting at a distance of one light year from us extending to another light year The comets that visit us from this region give us valuable information as to the conditions there.


The stars have been formed in the form of clusters.  


The path of a celestial body as it moves through space.


A parsec equals 3.26 light years.


The point in the orbit of the satellite at which it is closest to the main body.


The point in the orbit of a planet or other body where it is closest to the Sun.


A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity is not massive enough to be a star…

Planet Nebula

When Sir Frederick Wilhelm Herschel, (1738 – 1822) observed cloud like formations through his telescope, he believed them to be formed around planets.  Later only he realised that they were not planets but stars. When a medium sized star consumes the Hydrogen necessary for fusion, force of gravity overrides the force due to thermal expansion. This results in a white dwarf and continues to glow due to the high temperature. It is said that not a single such a star has sufficient time to cool during the last 13 and a half years. Light gases spread as far as a light year and diffuse the rays coming from the star giving the appearance of a cloud.

NGC 6039 Discovered by Hershel .5 Billion yrs old.Color image Hubble Space Telescope .Courtesy NASA

Quasar (Quasi-Stellar Object)

Brighter than a trillion suns; yet you cannot see them unaided. They are so far away; billions and billions of light years away.  They are black holes surrounded by galaxy forming matter which emanate large doses of energy…

In these we see almost the beginning of the Universe. It had taken 12 billion light years to get the light year while the age of the universe is 13 billion.

Red Giant

The stars derive most of the energy by fusing hydrogen nuclei into helium nuclei at the central core. When hydrogen gets depleted there is a setback in supplying the pressure against the matter collapsing due to force of gravity.  Collapsing material develops a very high temperature and starts to fuse the Hydrogen in the outer region This increases the luminosity hundredfold and the hot atmosphere expand swallowing up any planets in the system It has been estimated that when the time comes our sun will extend up to Jupiter The red colour of these giants is due to the weakening of rays passing through the gigantic atmosphere.

Solar Flares

The activity of the sun seems to peak in 11 year cycles which we may experience in next year. Due to the changes in the magnetic field of the sun the hot plasma gets ejected from the corona. These are called solar flares and reduce the temperature at the base causing sun spots. These explosions are million times more violent than volcanic eruptions.

Courtsey  Christian Monitor.


Red flashes that last only for a few miliseconds have been observed  about 80 km above sea level It is believed that these  are produced by fasts moving stray electrons produced during lightening  Source:- Sc Am  aug 2012.


Upali Salpadoru