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All that is meant by an 'excited' atom is that one or more of the atoms electrons in it's shells, are then excited into shells that are in higher energy states that they were in before.

F...

At short distances between the nucleons at roughly 3 femtometers or so, the strong nuclear force between two nucleons begins to overcome the electrostatic repulsion (if both or one nucleon is a proton)...

There are 3 different outcomes for the fate of our universe:if the average density is smaller than what is known as the critical value, then the universe will carry on expanding fo...

The centre of gravity (or centre of mass) of an object is the point through which an object's weight will act through. In other words, the object will always balance around this point.

Olbers' Paradox is a famous problem which baffled many scientists in the early 19th century. 

The German astronomer Olbers (and many of his colleagues) made assumptions that the universe ...