Star Life Cycle

Generally speaking, there are two main life cycles for stars. The factor which determines the life cycle of the star is its mass. 1 solar mass = size of our Sun Any star less than about three solar masses will spend almost all of its existence in what is called the “main sequence”. One solar mass is the mass of our Sun. About 90% of all stars are like this.   If a star is more than three solar masses when it is “born” or formed, it will spend much less time on the main sequence, have a much shorter life span, and “die” or end violently. Once a star is born, it is set in a specific life cycle, and the outcome will not vary. Main Sequence Stars

Stellar Nebula (a star nursery) Space may seem empty, but actually it is filled with thinly spread gas, mostly hydrogen, and dust. The dust is mostly microscopic grains of carbon and silicon. In some places, this material is collected into a big cloud of dust and gas, known as a nebula. Stars form from collapsing clouds of gas and dust. All stars begin in a nebula. The dust and gas that makes a nebula comes from past exploded stars. This material is called interstellar medium.   Stars begin to form in high density areas of the nebula. Huge amounts of gas and dust condense and contract under its own gravity. Stellar Nebula (a star nursery)

Some gas and dust is pulled by gravity to the core Some gas and dust is pulled by gravity to the core. As the region of condensing matter heats up, it begins to glow. This is called a protostar. Temperature rises, and nuclear fusion begins. This is the “birth” of the star. Nuclear fusion is the atomic reaction that fuels stars. Fusion in stars is mostly converting hydrogen into helium. Stars that are up to 1.5 times the mass of the Sun are called “main sequence” stars and will burn for a long time. If there is enough matter in the core and the temperature reaches 15 million °C, fusion begins. Think of stars as giant nuclear reactors. Nuclear fusion is the atomic reaction that fuels stars.   In fusion, atom nuclei combine together to make a larger nuclei which forms a different element. The change of elements by fusion releases large amounts of energy. This energy makes the stars hot and bright. Fusion in stars is mostly converting hydrogen into helium. Stars smaller than our Sun can convert only hydrogen into helium during fusion. Medium-sized stars, like our Sun, can convert helium into oxygen and carbon, when all the hydrogen is used up. If a protostar does not reach a temperature hot enough to begin fusion, it will stay cool and dim. It is called a brown dwarf. Brown dwarfs are objects which are too large to be called planets and too small to be stars. They were first discovered in 1995. It is now thought that there might be as many brown dwarfs as there are stars. Sun-like Stars

A red giant is a large star that is reddish or orange in color. It represents the phase in a star's life when its supply of hydrogen has been exhausted and helium is being fused into carbon. This causes the star to collapse, raising the temperature in the core. The outer surface of the star expands and cools, giving it a reddish color. Red giants are very large, reaching sizes of over 100 times the star's original size. A star may remain in the main sequence until all of the hydrogen has been fused to form helium. This can take 10 billion years.   The core begins to contract and heat up. This extra heat allows helium to fuse into carbon. The outer layers of the star expand and cool. Since it is cooler, it will shine less brightly. The expanded star is now called a red giant. Remember, that something that is “white hot” is much hotter than something that is “red hot”. A red giant can exist for about 100 million years. After this time, the red giant is mostly carbon. The Sun is predicted to become a red giant in approximately 5–7.5 billion years. When our Sun expands to a red giant, its radius will be about 200 times larger than it is now. This means that it will expand through the orbits of Mercury and Venus. Earth will not be able to support life when it is located that close to the Sun. Red Giant

Planetary nebulae form when a main sequence star grows into a red giant and throws off its outer layers and the core collapses. The term "planetary" comes from the 19th century, when astronomers saw what looked like a new planet in their primitive telescopes. This was a time before people knew that there were different types of galaxies. The name has stuck ever since. The next fusion process would be to fuse the carbon into iron. The problem in this star is that there is not enough pressure in the core to do this.   Because the outward pressure of energy is no longer maintained, the core collapses and sends a shockwave outwards. This causes the star's outer layers to be cast off and form a planetary nebula. These nebulae get their mostly circular shape because the material is thrown off the star in a roughly symmetrical pattern. Planetary Nebula

The collapsed core left when a red giant loses its outer layers is called a white dwarf. It is made of pure carbon that glows white hot with leftover heat from the spent fuel. It will drift in space while it slowly cools. It is the size of Earth, but very dense. A teaspoon of the material would weigh as much as an elephant. The remaining core, 80% of the original star, is now in its final life stages. The core is called a white dwarf. The core has much less mass because it has lost its outer layers.   Any planets that the star would have had revolving around it, would have done one of the following: moved to much farther orbits been completely ejected from the system been engulfed by the star in the expanded red giant phase The star eventually cools and dims. White Dwarf

A black dwarf is a white dwarf star that has cooled completely and does not glow. It will drift in space as a frozen lump of carbon. The star is considered “dead”. Black Dwarf When the star stops shining, as a result of using up all of its fuel, it is considered a dead star.

Massive Stars

Stellar Nebula (a star nursery) All stars form from collapsing clouds of gas and dust found in a nebula. Stellar Nebula (a star nursery) All stars begin the same way. They form in a stellar nebula from interstellar dust and gas,

Massive stars are stars that are between 1 Massive stars are stars that are between 1.5 to three times the mass of the Sun. A star with a much greater mass will form, live, and die more quickly than a main sequence star. Massive stars follow a similar life cycle as small and medium stars do, until they reach their main sequence stage. This occurs because the gravity squeezes the star's core and creates greater pressures, resulting in a faster fusion rate. The stars shine steadily until the hydrogen has fused to form helium. This takes billions of years in a small/medium star, but only millions of years in a massive star.   Massive stars use their fuel much faster than smaller stars do. Massive Stars

A red supergiant glows red because its outer layers have expanded, producing the same amount of energy over a larger space. The star becomes cooler. Red stars are cooler than blue or white stars. A supergiant has the pressure needed to fuse carbon into iron. This fusion process takes energy, rather than giving it off. As energy is lost, the star no longer has an outward pressure equal to gravity pushing in. Gravity wins, and the core collapses in a violent explosion. When massive stars deplete their hydrogen, the remaining helium atoms are converted into carbon and oxygen. In the next million years, a series of nuclear reactions occur, forming different elements in shells around the core. Carbon and oxygen change into neon, sodium, magnesium, sulfur, and silicon. Later, reactions transform these elements into calcium, iron, nickel, chromium, copper, and others. The core eventually becomes iron. Red Supergiant

A supernova is an explosion of a massive star at the end of its life; the star may briefly equal an entire galaxy in brightness. At this point, the mass of the star will determine which way it continues in the life cycle. The core collapses in less than a second, causing an explosion called a supernova, in which a shock wave blows off the outer layers of the star.   When these old, large stars with depleted cores explode in a supernova, they create heavy elements. Heavy elements are considered all of the natural elements heavier than iron. These elements are spewed into space by the explosion. Supernovas can be exceptionally bright. A supernova explosion on July 4, 1054 was so bright that it could be seen in broad daylight for 23 days. Supernova

Neutron Star or Black Hole? If the star is at least 1.5 but less than nine times larger than the Sun, the core left after the supernova will collapse into a neutron star. This is a star composed only of neutrons. Black Hole If the star is at least nine or more times larger than the Sun, the core will continue to collapse into a black hole, an extremely dense area with a strong gravitational pull that light can not escape. Neutron Star: At the time of supernova, the central region of the star collapses under gravity. It collapses so much that protons and electrons combine to form neutrons.   A neutron star is about 20 km in diameter and has the mass of about 1.4 times that of the Sun. A neutron star is so dense that one teaspoonful would weigh a billion tons. Because of its small size and high density, a neutron star possesses a surface gravity about 2 x 1011 times that of Earth and a magnetic field a million times stronger. Neutron stars can spin 100 times in a second. Pulsars are spinning neutron stars that have jets of particles moving almost at the speed of light, streaming out above their magnetic poles. These jets produce very powerful beams of light. They were discovered in 1967. Black Holes: If the surviving core is greater than three solar masses, it contracts to become a black hole. If a black hole passes through a cloud of interstellar matter, or is close to another “normal” star, the black hole can pull matter onto itself. As the matter is pulled towards the black hole, it gains kinetic energy, heats up, and is squeezed by forces. As it gets hotter, this matter gives off radiation that can be measured. This allows astronomers to find black holes. Neutron Star or Black Hole?

Our Sun Our Sun is a medium sized, main sequence star. It is the closest star to Earth. Our Sun