Introduction to the Universe


The universe is vast and amazing. There are many millions of trillions of stars in the sky, most of them being bigger than our sun. Stars account for 98% of the matter in a galaxy. The rest of 2% consists of interstellar or galactic gas & dust in a very attenuated form. The normal density of interstellar gas throughout the galaxy is about one tenth of a hydrogen atom per cm³ volume. Stars tend to form groups.

Major types of Stars according to their grouping

lone star
Lone Star . ©: NASA

Lone Stars: Being alone, i.e., no planets around, and going on their own, lone stars are exceptional. They do not follow the condition to be in a galaxy and hence they live outside of a galaxy.

Single Stars: These are found in galaxy & are single but with planets around (for example Our Sun). These do not number more than 25% of Stellar (Star) Population.

Binary Stars or Double Stars: These exist as pair of stars (e.g., Antares in Scorpio is actually ‘two stars’ combination). They are some 33% of stellar population.

Multiple Stars live among many stars around them. Capella & Alpha Centauri comprise 3 stars each, while Castor consists of 6 stars.

Note that, stars which appear single to the naked eyes are sometimes double stars. There are two stars revolving around a common center of gravity. They are also found in orbital motion round each other, in periods varying from about one year to many thousands of years.

Vivid types of Stars according to their nature

Red Giants

Red Giant

When the hydrogen, the main element in a star, is depleted, its outer regions swell and redden. This is the first sign of age. Such stars are called Red Giants.

Our Star, the Sun, is expected to turn into a red giant in another 5 billion years. Red giants are dying stars that has expanded greatly from its original size and gives off red light. They have gigantic dimensions.

Black Dwarf

black dwarf
via Wikipedia

It is the blackened corpse of a star. Ultimately it disappears into the blackness of the space.

White Dwarf

White Dwarf
via Wikipedia

It is a tiny, dense, hot star, representing a late stage in the life of a star. The matter in it is so incredibly dense that a single teaspoonful of it would weigh several tonnes.


universe, Image, Gaurav Tiwari

These are huge stars, with all their hydrogen fuel used up in their core, but continue to expand hundred of times bigger than its original size before they finally die.

Novae & SuperNovae

universe, Image, Gaurav Tiwari
Artistic imagination

These are kind of stars, whose brightness increases suddenly by 10 to 20 times are more and then fades gradually into normal brightness. The sudden increase in brightness is attributed to a partial or outright explosion. In Nova, it seems that only the outer shell explodes, whereas in SuperNova the entire star explodes.

Variable Stars

There are stars that show varying degrees of luminosity.


universe, Image, Gaurav Tiwari

These are variable stars and are powerful quasi stellar sources of radio radiations.


universe, Image, Gaurav Tiwari

These are also variable stars which emit regular pulses of electromagnetic waves of very short duration.

Solar System

solarsystem Introduction to the Universe: Part II: Systems of Stars: Solar System and Our Sun
Solar system Imaginated

The Solar System means system of the Sun. All bodies under the gravitational influence of our local star, the Sun, together with the Sun, form the Solar System.

Bodies? What kind of bodies?

The largest bodies, orbiting the Sun, including Earth are called planets.

Often smaller cool bodies called satellites or moons, orbiting a planet.

Bodies smaller than the planets that orbit the sun are classed as asteroids if they are rocky or metallic, comets if they are mostly ice and dust, and meteoroids if they are very small. Most comets release gases as they near the heat of sun, producing a luminous cloud called coma & often a long tail.

A meteoroid that burns in Earth’s atmosphere is a meteor, while one that reaches Earth without burning completely becomes a meteorite.

After the exclusion of Pluto from the planet category, a new category is formed: Dwarf Planet.

Elements of Solar System

Stars: (1) The Sun
Planets: (8) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Dwarf Planets: (4) Pluto, Charon, Eris, Ceres- along with the pk-heading-numbered numerous satellites that travels around most of the planets.
Others: Asteroids, Interplanetary Dust, Plasma.

The Sun

animated sun 2 Introduction to Universe: Part III: Our Sun and Its Structure

Fun facts about the Sun:

• Sun is one of more than 100 billion stars in the giant spiral galaxy called the Milky Way.

• Sun is the center of the solar system. Its mass is about 740 times as much as that of all the planets combined.

• It continuously gives off energy in several forms- visible light; invisible infrared, ultra-violet, X-rays and $\gamma$ -rays, cosmic rays, radio waves and plasma.

•The Sun generally move in almost circular orbits around the galactic center at an average speed of about 250 km per second.

•It takes 250 million years to complete one revolution round the center. This period is called a Cosmic Year.

•It’s energy is generated by nuclear fusion in its interior. It is calculated that the Sun consumes about 4 million tonnes of hydrogen every second. At this rate, it is expected to burn out its stock of hydrogen in about 5 billion years and turn into a red giant.

Solar Statistics

Absolute Visual Magnitude: 4.75
Diameter: 1,384,000 km
Time of one Rotation as seen from the Earth: 25.38 days (at equator) to 33 days (at poles).
Chemical Composition:

  • Hydrogen 71%
  • Helium 26.5%
  • Other Elements 2.5%

Age: 4.5 billion years, estimated.
Expected lifetime: 10 billion years.
Mean distance from Earth 8.2 light seconds i.e., about 150 million km.

Structure of the Sun

sunlayers Introduction to Universe: Part III: Our Sun and Its Structure

We may divide the internals of the Sun into following major parts:

• Corona
• Chromosphere
• Photosphere
• Solar Envelope
• The Core


corona Introduction to Universe: Part III: Our Sun and Its Structure

Corona is the outermost part of the sun & you may see it when Full Solar Eclipses occur. The temperature of corona is about 2.7 million °C, which is hot enough to emit ultraviolet and X-rays. The corona extends millions of kilometers into space above the photosphere.


In a solar eclipse, a red circle around the outside of the Sun can be seen sometime. This is the chromosphere. The chromosphere is made up of the gases that extend away from the photosphere.Chromosphere is of red color, caused by the abundance of hydrogen. It has a greater (than Photosphere) temperature of about 10000°C. The Chromosphere merges into Corona & Photosphere.


The photosphere is the zone from where the sunlight we see is emitted. The photosphere is a layer of low pressure gasses surrounding the envelope. It is 400 km thick, with a temperature of 4500°C to 6000°C.

The Core:

The innermost layer of the sun is the core with a density of 160g/cm³ (10 times that of lead). The core might be expected to be solid. However, the core’s temperature of 15 million°C keeps it in a gaseous state. In the core, fusion reactions produce energy in the form of $\gamma$ rays and neutrinos.

From the photosphere of the sun to the chromosphere and to the Corona , the temperature increases, while the same procedure follows up from the photosphere to the core of the sun (I mean temperature increases). Thus you may say in other words that the photosphere is the coolest place in the sun.

Spots in Sun? — SunSpots

 Introduction to Universe: Part III: Our Sun and Its Structure

The sun has enormous organized magnetic fields that reach from pole to pole. Loops of the magnetic field oppose convection in the convective envelope and stop the flow of energy to the surface. This results in cool spots (i.e. SUNSPOTS) at the surface which produce less light than the warmer part.

Sunspots are dark spots on the photosphere, typically with the same diameter as the Earth.

sunspots2 Introduction to Universe: Part III: Our Sun and Its Structure

Sunspots have even lower temperatures than the photosphere. The center of a spot is called the umbra, looks dark gray if heavily filtered & is only 4200°C (as compared to the photosphere at 6000°C). Penumbra is the portion around the umbra, which looks lighter gray (if filtered).

Sunspots come in cycles, increasing sharply (in numbers) & then decreasing sharply. The period of this solar cycle is about eleven years. The largest spot ever measured (April 1974) covered 18130 million km² i.e., 0.7% of the Sun’s visible surface. The life periods of these spots also vary—from a few hours to many weeks.

Polar Auroras: The Beauty of Sunlight on Earth

polarauroras Introduction to Universe: Part III: Our Sun and Its Structure

Polar auroras appear only on the poles and since there are two poles on the Earth, there exist two types of auroras, Aurora Borealis or northern Lights on the north pole and the Aurora Australis or Southern Lights on south pole. These are lights that sweep across the sky in waves or streamers or folds. They are very often multicolored and provide one of the finest spectacles in nature. They occur in Arctic and the Antarctic regions (respectively), but the Northern lights can be seen as far south as New Orleans in America and the Southern lights as far north as Australia.

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