Stars

The stars we see in the night sky are just a small part of the billions of stars in our galaxy.

Stars are formed when the gas molecules in the interstellar (between stars) medium are pulled towards one another due to gravitational attraction. Thus the density and pressure increases and so does the temperature. As the temperature increases the hydrogen molecule breaks into atoms and as the core gets hotter and hotter fusion starts. The star does not shine as soon as the fusion starts. Only when the photons make their way through the interior of the star they come out and then the star shines in visible light.

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Proton-Proton cycle in sun

The temperature of our sun’s core is 15 million (or 15 Lacs) degree Celsius while its surface temperature is almost 5500 degree Celsius. Due to such high temperature sun is full of highly ionized gas. It’s a burning ball of plasma in which fusion takes place.

Alpha Centauri A and B
Alpha Centauri A and B Stars : www.nasa.gov

The cross shape around stars is because of the diffraction effect. The light has to go through a cross shaped thing that holds the secondary mirror of Hubble Space Telescope and thus it gets diffracted. This does not happen in images of galaxies or nebulae because their light is not as concentrated as stars’.

My mommy once showed me the cross shape around the flame, while watching it from the door which had lots of small rectangular grids. It was also due to the diffraction effect.

So now we know that stars are mostly made of hydrogen atom and during fusion they release energy. But as the hydrogen fuses to helium hydrogen starts depleting. And now it fuses helium to form carbon and oxygen in its core. The outer layer of star move outward and the star turns into a red giant. Now if the star does not have enough temperature to fuse the atoms further, the carbon and oxygen atoms will remain in the centre. Its outer layers explode outwards and form a planetary nebula. The core that is left is called white dwarf. It’s still not known exactly how the implosion of star becomes an explosion.

IDL TIFF file
Helix Nebula (Nearest Planetary Nebula) : www.nasa.gov

We didn’t know what is in a star’s mind i.e. whether it will become a white dwarf or not until a 19 year old student revealed their plan. Subrahmanyam Chandrashekhar gave a formula to evaluate how mass of star affects its death.

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Chandrashekhar mass limit formula

Chandrashekhar limit is 1.4 solar masses.

So if a star has a mass less than the chandrashekhar limit, the remaining core will become a white dwarf. The basic idea behind is that when the core collapses the atoms get more and more close to each other and due to repulsion between electrons they cannot get more close. Thus the star remains stable due to this repulsion. Just like in the process of formation of white dwarf, where carbon and oxygen remain in the center and cannot fuse further. This is because the temperature is not hot enough and thus the star remains intact due to repulsion between electrons.

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Brightest star in the night sky – Sirius and its tiny companion Sirius-b white dwarf : hubblesite.org

Depending on the temperature of white dwarf it can be made up of

  1. Carbon-Oxygen
  2. Oxygen-Neon-Magnesium
  3. And if a star cannot fuse helium a Helium white dwarf is formed.

This year 19 October was more special because we celebrated Deepawali and the birthday of Subrahmanyam Chandrashekhar on the same day.

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Rangoli made by me and my sister

Fire or the beautiful flames in this picture are also plasma. But fire is not as hot as sun’s center, so it cannot fuse atoms!

Happy Deepawali!

 


UPDATE : Astronomy picture of the day (APOD) – 24th October 2017

All of the atoms in the periodic table were created and are constantly being created in many different conditions. Today’s APOD shows “humanity’s best guess to the nuclear origin of all known atoms”.

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Where Your Elements Came From : APOD

 

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