The universe is everything that exists. It contains many galaxies (ours is the Milky Way) that are clusters of stars and planets. Within galaxies, there are many solar systems. These contain one star, the planets that orbit this star, the planetary satellites (such as moons or artificially launched satellites), asteroids, and comets.
According to Kepler's first law, all objects have elliptical orbits. However, planets tend to be more on the circular side, whereas comets have intensely elliptical orbits.
Life Cycle of Stars
Formation of Stars
Nebulae - Stars form when nebulae (large clouds of gas and interstellar dust) slowly contract under gravitational forces.
Protostars - When the cloud of dust gets dense enough, it splits up into protostars. The loss in gravitational potential energy in bringing all the dust/gas together causes the protostar to heat up.
Fusion - Eventually the protostar becomes hot enough for fusion to occur (a few million degrees), where hydrogen nuclei combine to form helium.
Main Sequence - The process of fusion releases a vast amount energy, keeping the core of the star hot until it has burnt through all the hydrogen fuel. This phase is relatively stable, because while the gravitational force acts to compress the star further, the radiation pressure from the emitted photons and the pressure of the gas push outwards.
The lifetime of a star depends on its mass: a larger star burns through its fuel much faster than smaller stars. The threshold for small and large stars is the Chandrasekhar limit (around 1.4 solar masses) - it is the maximum mass for which electron degeneracy pressure (that keeps the star going) can counteract the gravitational force (that tries to collapse the star).
Low Mass Stars have a core mass of up to 1.4 solar masses (the mass of the sun), and a lifetime of five to ten billion years.
Massive Stars have cores heavier than 1.4 solar masses and a lifespan of just a few hundred-thousand years.
After the main sequence, the star enters the final phase. Again, this is dependent on the size of a star.