Gravity and Its Effects

Gravity, one or the four fundamental forces of the Universe has a devastating effect on our world. It is the force that created the stars and galaxies, and the force that destroys them. Gravity causes moons to orbit planets, that orbit stars, the orbit galaxy centers, that orbit in clusters, which orbit in superclusters. Also, it holds other types of groups like star clusters together. Examples of its effects can be seen in the end of stars' lives. A star such as our sun will expand into a red giant and shed its outer layers into space. The core the star would be left to collapse under its own gravity until its atoms are packed so tightly that a match box of material from its would weigh as much as a full-grown elephant. This remnant is called a white dwarf.

For a star whose core is more massive than 1.4 the sun's weight, a supernova occurs. This happens when a giant star runs out of hydrogen fuel. The star fuses heavier and heavier elements until the core is iron. Stellar fusion is discussed here and here. When the star attempts to fuse iron, however energy is taken in rather than released, upsetting the balance between contracting gravity and out flowing energy. Matter bounces off the core as it contracts and powered by tiny neutrinos the star rips apart. After a supernova, there are two possibilities for the star's core. One is to become a neutron star (also called a pulsar). The core collapses and atoms break under the extreme pressure. Protons and electrons combine to form neutrons and the particles are packed as tight as possible, forming a neutron star. Over a solar mass of matter is packed into a sphere only about 15 miles across. A pinhead of material from this star would weigh more than the Titanic. For the first million years of its existence, the neutron star's magnetic field is so strong (thousands of times as strong as Earth) that intense radiation beams are shot from its poles. If the beam passes Earth, a pulse is recorded earning it the name pulsar.

The other possibility for a star's core after a supernova, one weighing more than four suns, is that it collapses further to become a black hole. A black hole is gravity's greatest victory. It sucks matter in, and since its escape velocity exceeds the speed of light, no radiation is emitted, making it "black". More on Black holes can be found here, here, here and here. Everything in our Universe (almost) is orbiting another body because of gravitational pull. The Moon is orbiting the Earth, which in turn is orbiting the Sun, which is orbiting the center of the galaxy, which is orbiting the Local Cluster, orbiting the Local Supercluster. For a spherical body, the strength of the gravitational field at any point is proportional to the body's mass and inversely proportional to the square of the distance of the object. Another star interaction caused by gravity is called a nova. This is when one star of a binary system turns into a black hole. It sucks in mass from its companion, disturbing the energy-gravity balance. The star becomes unstable and eventually explodes. In other words, gravity is the force that rules our Universe.