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Star Color
A star's surface temperature is associated with a specific spectral pattern. The temperatures are given in degrees using the Kelvin scale and classified into 7 main types by the Morgan-Keenan spectral classification system. The hottest stars are blue and have surface temperatures ranging from 10,000 to 60,000 degrees Kelvin. White and yellow-white stars are the next hottest group, ranging from 6,000 to 10,000 degrees Kelvin. The Sun is a yellow star, with surface temperature between 5,000 and 6,000 degrees Kelvin. Cooler stars with surface temperatures below 5,000 degrees Kelvin are orange and red, with the red stars being coolest.
Star Luminosity
Luminosity of a star is determined by algebraic calculations using its brightness and distance from earth and is given in terms of magnitude. The Sun is used as a baseline for magnitude since its distance and brightness is well-known. Magnitude is graded in numbers with lower numbers being brighter. The Sun's magnitude is -26.7. The naked human eye can see stars to about magnitude +6. Aided by binoculars, this increases to +10. The Hubble Space Telescope can see stars of magnitude +30.
Main Sequence Stars
Stars form out of clouds of cold molecular gas that collapse together due to their gravitational force. As it collapses, the stellar material increases in temperature; eventually, its core will reach a point where fusion can begin. In this process, the star convert protons of hydrogen into helium, releasing tremendous amounts of energy. Stars remain in the main sequence phase for various lengths of time, determined by their mass. Small stars such as red dwarfs can stay in main sequence for hundreds of billions of years. Larger stars such as the Sun will produce energy for 10 to 15 billion years. The most massive stars have relatively short lives, and some will only burn for a few million years.
Stars Past Main Sequence
Stars exit the main sequence phase of their lifespans when their store of hydrogen fuel is exhausted. At this point, the star begins to contract inward due to gravity. As it collapses, a shell of hydrogen around the core is heated and again ignites fusion, with the star becoming thousands of times brighter than in its main sequence. This causes the outer layers of the star to expand outward, forming a red giant star. The Sun will one day expand in diameter to reach the Earth's orbit. Eventually, the star collapses again, leaving only the degenerated core or, in rare instances, a black hole. Giant stars much larger that the Sun can end their lives in explosions and form supernovas.