Category Astronomy

 

 

What are Cataclysmic Variables?

Two types of variable stars, called Novae and Supernovae, are classified as Cataclysmic Variables since their increase in brightness is accompanied by a spectacular flare up or explosion.

 

 

 

What are Quasars?

A quasar is the abbreviation for Quasi-stellar object (QSO). Quasars are star-like in appearance but have certain special characteristics that differentiate them from ordinary stars.

When was the first Quasar discovered?

The first Quasar was discovered in 1963 by the Mount Wilson and Palomar Observatories in USA.

How far away are Quasars?

Quasars exist at Cosmological distances, typically over fifteen billion light years away.

What are the special characteristics of Quasars?

Quasars seem to be moving at speeds faster than of any known galaxy, and at more than 90% of the speed of light. The energy emitted by a Quasar is about a hundred times more than the energy emitted by an entire galaxy. However, the diameter of Quasar is unbelievably compact. Quasars emit light which is extremely blue in colour and varying rapidly in brightness.

What is the explanation for the special characteristics of quasars?

As yet no one has given a satisfactory explanation and Quasars remain among the most enigmatic objects in the universe. They are thought to be at the centre of galaxies, their brilliance emanating from the stars and gas falling towards and immense black hole at their nucleus.

Which star other than Sun is the nearest to the Earth?

The star Proxima Centauri in the Constellation Centaurus which is at a distance of nearly 40 trillion km from Earth. It is a faint red Dwarf visible only with a telescope.

 

 

 

 

 

What are Variables?

Variables or ariables stars are stars whose brightness increases and decreases periodically.

 

 

 

What are Cepheid Variables?

An Cepheid Variable is a type of variable star. Cepheid variables vary in magnitude over a period from one to sixty days. Also, the longer the period the more luminous the star.

Why are Cepheid Variables important to astronomers?

Since the period and luminosity of a Cepheid Variable are closely related, one can obtain the absolute magnitude of the star by observing its period. Cepheid variables are therefore used by astronomers as “Standard candles” for comparing with other stars.

What is a white dwarf?

A red giant starts shrinking when its stock of hydrogen is nearing exhaustion. At a certain stage, the star becomes so dense that ever its gravity cannot matter in which shrink it any more. White dwarfs consist of degenerate matter in which gravity has packed the protons and electrons together as tightly as is physically possible so that a spoonful of it, weighs several tonnes. At this stage, the star becomes stable again though it is much smaller in size than it was at the red giant stage. The star is now called a white dwarf.

How does a white dwarf shine?

A white dwarf shines mainly by radiating away its stored internal energy, besides the fusion of any residual helium into other heavier elements. It slowly cools and darkness over millions of years.

 

 

 

What is a black dwarf?

A white dwarf shines by radiating its internal energy. When the internal energy is completely exhausted the star ceases to shine and becomes a cold, dark body called a black dwarf. 

What happens when the hydrogen in the star is completely converted to helium?

As the hydrogen gets depleted the outward force of the fusion reaction starts diminishing and the inward force caused by the star’s gravity gains the upper hand. Consequently, the star begins to shrink in size as well as increase in brightness. Its colour also changes from red to white.

How much energy is produced by the helium fusion reaction?

Almost 95% of the total energy that a star can generate through fusion reactions is generated through hydrogen fusion. The fusion of helium and heavier elements will account for just over 5% of the total energy that a star can generate.

 

How long is the fusion reaction in a star likely to go on?

Once the hydrogen and the helium fusions stop due to the exhaustion of their supply, the fusion reactions may continue to produce heavier elements but energy generated by these reactions is rather negligible. The fusion reaction will continue until all the matter in the star is completely converted to the element iron. The star will then radiate energy only by depleting its stock of accumulated internal energy.

Does the fusion reaction in a star always priced to the iron stage?

Generally, the fusion reaction in all stars will proceed to the helium fusion stage. However, the extent to which it proceeds beyond this stage depends on the amount of pressure and temperatures developed at the core of the star. The larger the star the larger will be the number of fusion stages it will go through before the fusion reaction stops.

How massive must a star be to completely convert into iron?

Foe a star to completely convert into iron, it must be at least 30 times as massive as the Sun.

Why does the fusion reaction stop when all the matter is converted into iron?

The pressure created in the cores of ordinary stars is insufficient to continue the fusion reaction beyond the element iron.

 

 

What is a red giant?

In the transition of the star to the red giant stage, its inner part or core shrinks, while its outer part or envelope expands. A red giant is a star with a relatively low effective temperature and a large radius. The low temperature makes the star glow red and the larger size makes it a giant compared to its original size.

What are the other characteristics of red giants?

Red giants are highly luminous and may have luminosities ranging from about 100 to even 10,000 times that of the Sun. They are very bright because they are so large although their surface temperature is lower than that of the Sun. The density of red giants is also extremely low ranging from 10-4 to 10-7 times the Sun’s mean density. In other words, the mean density of a typical red giant is less than that of air at sea level.

Will the Sun become a red giant?

The Sun will a red giant almost the size of Betelgeuse in about five to six billion years.

Why does a star become a red giant?

As the hydrogen fusion in the star progresses, its mass moves towards the centre of the helium increases at the star’s core, the pressure also increases.

At a certain stage, the pressure is sufficient to start the helium fusion reaction when helium fuses to from carbon and other heavier elements. Now, the outward force of the combined hydrogen and helium fusion reactions as well as the outward pressure is greater than the inward force. The star therefore starts expanding into a red giant and regains equilibrium at a larger size.

 

 

What are super giants?

Super giants are red giants having extremely large size and luminosity compared to a typical red giant. A typical red super giant would be about 100times as luminous as a typical red giant.

How big is a super giant?

If the super giant Betelgeuse occupied the position of the Sun, all the planets up to and including Mars would be contained within it. Thirty million replicas of Sun could fit inside the super giant Antares.

 

 

 

What is the main sequence?

The main sequence refers to the long period when the star shines steadily and brightly. Our Sun is now at the main sequence stage.

 

 

 

 

 

 

 

 

What is stellar evolution?

Stellar evolution refers to the stages in the evolution of a star as follows:

1. The star begins as a cloud of interstellar material perhaps a light year in radius.
2. The cloud contracts and also heats up for a few million years.
3. A spherical star shape, called a proto-star, begins to emerge. The temperature is about 20000C.
4. The proto-star begins heating up and the fusion reaction begins. The star flares up in a short interval of time and stars glowing. At this stage, the luminosity of the star may increase to several hundred times the Sun’s present luminosity.
5. The star cools down to a steady equilibrium stage. The main sequence stage is reached and continuous for some ten billion years.
6. The star starts growing and the luminosity starts decreasing.
7. The star becomes a red giant and continuous in that state for a few hundred million years.
8.  The red giant starts shrinking. The luminosity increases.
9. The star becomes a white dwarf and continuous to shine for a few billion years at the expense of its internal energy.
10. The star exhausts all its internal energy and ceases to glow. The star has now become a black dwarf and reached the final stage of stellar evolution.

What happens to the radius of the star as it passes through the ten stages of evolution?

The radius of the star continuously decreases through the various stages except at the red giant stage when it temporarily increases.