A star is a self-luminous celestial body consisting of a mass of gas held together by its own gravity. Stars exist in a balance—their internal energy generated by nuclear fusion reactions results in an outflow of energy to the star's surface. This outward flow of directed gas and radiation pressures is balanced by the inward-directed gravitational forces.
Since ancient times, astronomers have been charting stars into constellations—recognizable grouping of starts that appear in the night sky and move with the seasons as the Earth orbits the Sun (Figure 1.43). Although stars in constellation often appear in association by appearance, they may be large distances apart and very greatly in brightness (intensity). In addition, stars exist in a wide range of colors, most obvious when observed through telescopes or from space (Figure 1.44). Many stars are clustered together, often sharing a common stellar history (Figure 1.45). Some stars orbit each other relatively close to one another as binary systems (Figure 1.46). Some star systems have multiples stars in orbit around each other.
Among the millions of stars observable in our galaxy, astronomers have been classifying them by size, color, and brightness (intensity) . Most stars in our galaxy fall into a class called the main sequence of which our Sun belongs (Figure 1.47). Astronomers have developed theories about star formation and the life cycle of stars in their different classes. With years of observation, abundance of knowledge has been gained about the life cycle of stars (Figure 1.48).
Life Cycle of Stars
Stars form in giant molecular clouds called nebula. A nebula is an interstellar cloud within a galaxy consisting of gas and dust, typically glowing from radiant energy from stars nearby within them (Figures 1-49 to 1-54). Nebulae are the birth place of both stars and other objects within solar systems. Nebula can form from the explosion of stars at the end of their life cycle, resulting the creation of a new generation of stars and solar systems.
As stars form, gravity draws material in (mostly gas) and it mass increases until the internal heat and pressure is enough to start nuclear fusion reactions (converting hydrogen into helium). As stars age, they consume their fuel and eventually run out of nuclear fuel. Stars like the Sun may take billions of years to consume their nuclear fuel. When the fuel runs out, stars collapse under the weight of their own gravity. However, the fate of a star depends upon its mass.
Stars up to about seven times the mass of the sun fall within the "main sequence" grouping of stars. These go through stages as they consume their fuel. Young stars fuse hydrogen into helium. When stars run out of their hydrogen, the force of gravity causes them to collapse, which increases the heat and pressure within its core. During this phase of a star's life it will expand and become a red giant. Once the helium in the core of a star is consumed, stars in the main sequence will shed much of their mass into space (creating nebula), and the remaining core will shrink and cool and shrink to become a hot remnant called a white dwarf.
Fate of Supergiant Stars
Stars with masses greater than about seven times the mass of the Sun experience a more spectacular fate. More massive stars will burn through their fuel much faster than stars of the main sequence because their cores are hotter and under greater pressure. One these massive stars burn through their hydrogen and helium, this increase in heat and pressure allows fusion to convert helium into carbon, then carbon into neon, and then into iron. As the star continues to burn through it's fuel it eventually shuts down because it the fusion process of creating iron actually consumes more energy than it produces and the star looses it balance and collapses under it own gravity. The collapsing core reaches temperatures in the range of 100 billion degree and the core recoils as a massive explosion called a nova. Great star collapses produce supernova where a star may shed the majority of it mass into space. What happens to the core depends on the mass of the star. Stars about 7 to 20 times the mass of the Sun produce massively dense objects called neutron stars (their density is so great that electrons and protons collapse to form a great mass of neutrons). Stars with masses greater than about 20 times the mass of the Sun collapse to form black holes. Black holes of so dense that their gravity prevents light from escaping from within their "event horizons" where matter is pulled into an inner space where nothing escapes.