Life Cycle of Stars Most astronomers agree that stars are born within nebulae, or huge clouds of dust and gases. The main factor that shapes the life cycles of a star is the star’s starting mass. Medium-sized stars pass through a red-giant stage before they become white dwarfs and die. The core of the star left behind after the supernova may become a neutron star or a black hole, depending on the star’s starting mass. The Life and Death of Stars Changes in stars may take place over a few million years or as much as several billion years – this is their life cycle. Some stars have existed almost since the origin of the universe, and others (such as our sun) have come from the matter created by the first stars. I. Protostars Galaxies contain nebulae (huge clouds of gas and dust), which is the birthplace of stars. Some of the hydrogen gas in the nebula is clumped together by gravity & the hydrogen atoms form a spinning cloud. More and more hydrogen gas is pulled into the cloud over millions of years. Collisions between hydrogen atoms cause the hydrogen gas to heat up. When the temperature of the cloud reaches about 15,000,000 degrees Celsius nuclear fusion begins. The heat produces a new star, or protostar. Because of nuclear fusion, the protostar begins to shine and give off heat & light. The life cycle for this star is now fixed because how much mass the star began with determines the life cycle. II. Medium-Sized Stars The star shines for a few billion years as its hydrogen is changed into helium by nuclear fusion in the core. When the star’s original supply of hydrogen is used up, most of the core has been changed to helium. Then the helium core begins to shrink and heat up. The outer core is still mainly hydrogen. The heat from the core causes the hydrogen shell to expand, cool, and redden. It is now a red giant. As the red giant ages, it continues to burn the hydrogen gas in the shell and the core continues to get hotter and hotter. At around two hundred million degrees Celsius, the helium atoms in the core fuse together to form carbon atoms. Most of the outer hydrogen begins to drift away and form a ring around the central core of the star. This ring is called a planetary nebula. When the last of the red giant’s helium atoms in the core have fused into carbon, the star begins to die. It slowly cools and fades. Gravity causes the last of the star’s matter to collapse inward. The matter is squeezed tightly and the star becomes a tiny white dwarf. III. White Dwarfs The matter in a white dwarf is extremely dense. A single teaspoon of matter may have a mass of several tons. White dwarfs still shine with a cool, white light. When the last of its energy is gone it becomes a dead star. How long this takes depends on the mass of the star when it was first formed. The smaller the starting mass of a star, the longer it will live. IV. Massive Stars When massive stars are formed, they usually have at least six times as much mass as the sun. Massive stars start out like medium sized stars and continue on the same life-cycle until they become red giants. Then they take a much different path. Gravity continues to pull together the carbon atoms in the core. When the core is squeezed so tightly that the heat given off reaches about 600 million degrees celsius, the carbon atoms begin to fuse together to form new elements such as oxygen and nitrogen. Fusion continues until iron forms. V. Supernovas The iron atoms in a massive star begin to absorb energy rather than release energy – after nuclear fusion stops. This energy is released in an explosion called a supernova. This explosion can appear as bright as a million suns. During the explosion, the heat is so tremendous that iron atoms in the core fuse together to form new elements, which then explode into space. A nebula is formed from the resulting cloud of dust and gases. New stars may form within the nebula. Most astronomers agree that the sun and its planets formed from a gigantic supernova many billions of years ago. VI. Neutron Stars A star that began 6 to 30 times as massive as the sun will end up as a neutron star after a supernova. A neutron star is about as massive as the sun but is often less than 16 km in diameter. Such a star is very dense – a teaspoon would have a mass of about 100 million tons. Neutron stars spin very rapidly, and give off energy in the form of radio waves as it spins. The radio waves are given off as pulses of energy (called pulsars). VII. Black Holes If a star begins with a mass 30 or more times the mass of the sun it will undergo a supernova but will not become a neutron star. The core is so dense that it is swallowed up by its own gravity. The gravity is so great that even passing light is swallowed up. The core has become a black hole: the remains of a supermassive star after a supernova. Astronomers are able to detect black holes because they often give off x-rays.