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You know about Newton’s law of universal gravitation…the law states that all masses everywhere attract each other with a force proportional to the product of their masses and inversely proportional to the square of their distance.

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Presentation Transcript
slide1

You know about Newton’s law of universal gravitation…the law states that all masses everywhere attract each other with a force proportional to the product of their masses and inversely proportional to the square of their distance.

slide2

Well then…why don’t truly massive objects like stars, including our Sun, just IMPLODE owing to the huge gravitational forces inward toward the center?

Would you believe it… Conservation of Momentum

comes to our rescue?!

Read on to see how…

slide3

OK…so the problem is…

Immense gravitational

forces act inward

causing the star to

tend to implode. Something must be

counteracting this

huge force of attraction!

But……what?

slide4

To find the answer

…look closer…

look at one

atom inside

the star…

Atom

slide5

All the atoms in the starare continually

absorbing energy (heat, light, whatever)

and emitting energy (usually as light).

Thus each atom is either in an

excited state (higher Bohr level) just after

absorbing energy, or in a ground state

(lowest Bohr energy level) just after

emitting energy.

Atom

slide6

Consider an atom sitting at a particular instant in an excited state (higher Bohr level) just after absorbing energy. The atom now emits this energy in the form

of a photon of light….that’s the little red guy below…

Photon is Emitted

Atom

But…hang on…Conservation of Momentum demands that the atom, after shooting a photon to the right with some momentum, MUST recoil to the left with an equal and opposite momentum! Like so…

Photon is Emitted

Atom

Atomic Recoil

slide7

The fun begins when this emitted photon happens to collide with a neighboring atom which happens at that particular instant to be sitting in the ground state. The second atom absorbs this photon, like so...

Photon emitted by

first atom is absorbed

by the second atom

Atom

Atom

Atomic Recoil

But…hang on…Conservation of Momentum demands that the second atom, after absorbing the photon, MUST recoil to the right! As shown below…

Second Atom

Recoils Also

Photon emitted by

first atom is absorbed

by the second atom

Atom

Atom

Atomic Recoil

slide8

So we have a process where both atoms end up recoiling away from each other, i.e., repel each other!!

This interatomic repulsive force, generated by conserving momentum during the process of photon emission and absorption, is what counteracts the colossal imploding force of gravity!!!

BTW, the process by which the photon emitted by one atom is absorbed by a neighboring atom is called

RADIATION TRAPPING.

Photon is Emitted

Photon is Absorbed

Atom

Atom

Atomic Recoil

Second Atom Recoils Also

slide9

Once a star dies, i.e., stops producing light, the huge gravitational forces win and the star implodes, forming either a dense white dwarf (a 110 lb person on this object would weigh 400,000 tons!), or a neutron star ( a 110 lb person on a neutron star would weigh 340,000,000,000 tons!), or – the ultimate – a black hole.