22) One of the first (and strongest) evidences that universe is expanding is Hubble's observation of the redshift of galaxies. In fact, Hubbles studied the light coming from distant galaxies, and by analyzing the redshift of the wavelength of the light coming from those galaxies, he understood that the distance of those galaxies from us were increasing, and so that the universe is expanding.
23) The force responsible for this effect is gravity. In fact, in a planet or in a star the heavier elements experience a stronger gravitational force, therefore they are attracted to the center of the planet/star, while the lighter elements (that experience a weaker gravitational force) remain in the outer regions of the planet/star, creating a "layered structure" with higher density in the center and smaller density in the outer regions.
24) This process is called nuclear fusion. Nuclear fusion occurs mainly in stars: because of the extremely high temperature, the nuclei of lighter elements are able to overcome the electrostatic repulsion between each other, and they are able to become close enough to be in the range of the strong nuclear interaction: the two nuclei combine together into heavier elements, and the mass lost in the process is converted into energy, according to Einstein's relationship:
[tex]E=mc^2[/tex]
