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What is a Star ?

Weekly Astro Knowledge
What is a star?

On a clear moonless night in countryside when one looks up in the sky thousands of twinkling objects are seen called as stars. But what is a star? Probably this question must have baffled mankind for ages. Scientifically a star is a ball of hydrogen and helium gas having enough amounts to start a nuclear fusion. Nuclear fusion is a process where 4 hydrogen atoms are fused to form helium. The amount of energy released in the process is in form of heat and light.


fig1
4 hydrogen atoms getting fused into a helium nuclei and the energy released in the process.
Image courtesy: opencourse.info

How a star is stable?

As the energy is released it causes a radiation pressure directed outwards and at the same time since there are large amounts of hydrogen and helium gas which causes inward gravitational collapse. When these two forces balance each other, equilibrium is established called as hydrostatic equilibrium and the star become stable.

forces
The thermal radiation pressure and the gravity balance
each other to make a star stable

Image courtesy: www.schoolphysics.co.uk

NASA’s Infrared Observatory Measures Expansion of Universe

NASA’s Infrared Observatory Measures Expansion of Universe
" H0 is around 74 +/-  0.4 (statistical)  km/(s Mpc ) "


spitzer_telescope
Spitzer Telescope

Astronomer Edwin, P. Hubble in 1920’s confirmed our Universe has been expanding.  Astronomers invested lot of effort to understand this expansion rate whether its speeding up or speeding down or remaining constant. In late 1990’s they found the expansion is speeding up. Saul Perlmutter, Brian Schmidt, and Adam Riess in 2011 were awarded Nobel Prize in Physics for their work on demonstrating positive acceleration of the expansion rate using Supernova data.


The next immediate question is to find the expansion rate. The parameter H0 known as Hubble constant gives us the expansion rate of the Universe. For most of the half of 20th century the value of H0 was believed to be around 50 – 90 km/(s Mpc ).  Data from Hubble Space Telescope, WMAP, Chandra X-ray Observatory, etc led us to conclude that the value of H0 to be around 70 – 77 km/(s Mpc ) a much more precise value reducing the uncertainty.

Leaving our Cradle - Part 2

" What goes up… must come down "
Leaving our Cradle
Part - 2

- Vikram S.Virulkar

In the earlier part of the series we discussed the beginnings of the Rockets and some early pioneers like Tsiolkovsky, Goddard and Oberth. We continue exactly where we left off, about the new refined rockets and the arch enemy of a Rocket; Gravity.

So what is this gravity? And how does it affect us? We can’t see it but we have felt it every single moment from the time we were born, it keeps our feet planted firmly on the Earth and prevents our precious Atmosphere from dissipating away into Space. Now to understand the force of gravity in depth one might attempt to seek out a true giant of science; Sir Isaac Newton. For any mildly educated person, the very mention of the word gravity brings back a picture of Newton sitting under a tree and an Apple falling on top of him, igniting his curiosity about Gravity  and the rest as they say is history.


newton-considers-gravity-as-he-observes-the-moon-and-falling-apple-peter-lloyd

Newton considers gravity as he observes the moon and falling apple peter lloyd


Although in some versions of the story, he merely saw an apple fall down, Nevertheless we owe our first attempt to understanding Gravity to the juicy fruit most of us relish. Newton started working on developing the laws of Gravity or the laws of attraction between two bodies in the Universe; He has stated detailed findings in his published work Principia (the book by Sir Isaac Newton).

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