Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the Universe.
Spread among the multitude of stars that populate the sky, cloud stretches are made up largely of hydrogen, but also of dust and gas. In such interstellar clouds, or nebulae, the stars are born.
The life span of a star is so long (up to tens of thousands of millions of years) that astronomers cannot follow the life of a star throughout its entire existence. But they can observe a variety of stars at different stages of their life cycle. Thus they were able to determine how these stars live and die: from their birth in interstellar clouds, through youth and middle age, to old age and sometimes to spectacular endings.
Not all stars follow the same life cycle. It all depends on their initial composition. Most of them have a short life, but full of brilliance and an end to the spectacles. Medium-sized stars, like the Sun, for example, shine less and slowly fade away, but live longer. Small stars do not shine at all, but their lives are measured in hundreds of billions of years.
Stars are born when matter in the nebula gathers in a stage. In fact, it is not known what determines this. The stage contracts gradually, or shrinks as it collapses under its own gravity. The collapse produces energy, which heats the gas and dust, causing them to glow. The stage becomes a star prototype. It is becoming denser and hotter in the center, or core. Gradually, the temperature rises to millions of degrees. When it reaches about 10,000 degrees Celsius, nuclear gas reactions begin. Hydrogen atom nuclei begin to merge and combine, forming helium atom nuclei. These nuclear fusions release an enormous amount of energy, which materializes in the form of radiation. These radiations reach the energy layer that is emitted into space in the form of heat and light. Now the prototype has become a real star.
The radiations inside heat the surrounding gas, exerting a pressure that acts against the collapse of the star under gravity, creating a state of equilibrium. Now it has a fixed size, a fixed outside temperature and a regular shape. At this stage astronomers say that the star is on the main sequence, referring to its position in the Hertzsprung-Russell diagram. This diagram is a graph that represents the brightness of the star on one axis and the color on the other.
Small mass prototypes never become hot enough to generate nuclear reactions. They crumble into blurry red pieces, or blurred brown pieces. The first brown piece was discovered only in 1987.
The sun has a diameter of 1400000 km and an outside temperature of about 6000 degrees Celsius. He gives off a yellow light. It is believed that there are 5000 million years and there will still be more. It is typical for many stars in the Universe that have a similar mass.
Such a star uses its hydrogen “fuel” about 10,000 million years ago, living with a core made up largely of helium. Without other fuel that can be “consumed” there is not enough radiation to prevent the collapse of the core under gravity. However, this collapse releases enough energy to heat the material around it. The hydrogen in this shell produces nuclear fusion, releasing a greater amount of energy, which causes the star to shine with a stronger, but reddish light. At that moment, the star begins to expand, becoming probably ten times larger. This is called a red giant.
The core of this red giant continues to shrink, and the temperature rises to 100000000 degrees Celsius or more. Through nuclear reactions helium fuses into carbon. The energy produced causes the star to shine another 100 million years. When the helium ends, it has no more “to burn”. Thus the entire star begins to collapse under gravity, until it reaches a bit size on Earth or maybe a little larger. The energy produced by the collapse causes the star – now called the white dwarf – to shine for a while. The matter is very dense in this white dwarf – a teaspoon can weigh thousands of tons.
A star with a size of, say, five times larger than the Sun, passes through the cycles of life much faster and evolves differently. It is much brighter, the surface temperature can reach up to 25000 degrees Celsius, and remains on the main sequence only 100 million years. When it becomes a red giant, its core can reach 60,000,000 degrees Celsius. This allows carbon to fuse and form heavy elements such as iron. The energy produced causes the star to reach hundreds of times larger than its original size. At this stage it is called a supergiant.
The process of producing energy from the core of the stars stops suddenly, collapsing within seconds. A fantastic energy is released, which generates a shock wave. The star explodes in space, creating a supernova. Very rarely does a supernova occur close enough, or large enough, to be seen with the naked eye. Such an event took place in February 1987 in a neighboring galaxy, the Great Magellan Cloud. For a short time the supernova was brighter 1 billion times than the Sun.
The core of a supergiant can collapse forming bodies with diameters between 10 and 20 km, having a density so high that a teaspoon could weigh 100 million tons! They are made up of a mass of neutrons, and they are called neutron stars. A newly formed neutron star has a very strong magnetism and spins very quickly. It creates an electromagnetic field that produces radio waves or other radiation. This radiation appears in the form of extended rays from the magnetic poles of the star. The rays spread to the sky as the star spins around its axis. These stars appear as flashes of light or as pulses when detected by our radio telescopes. For these reasons they are called pulsars.
At first the pulses were detected by their radio waves. But many emit X-rays and pulses of light. The first such pulsar was discovered in the Crab Nebula, as a remnant of a supernova produced in 1054. It pulses 30 times per second. Others are much faster: PSR 1937 + 21 pulses 642 times per second.
The most massive stars, ten times larger than the Sun, also erupt like supernovae. But because of the huge mass, their collapse is even more catastrophic. Nothing can stop them. Matter crushes beyond the neutron stage, creating an area in space where ordinary matter ceases to exist. All that remains is gravity – a gravity so strong that not even light can escape it. Such an area is called the black hole. Of course, these black holes cannot be seen, but it is believed that their radiation can be detected. These radiations, known as X-rays, have been reported in different parts of the sky. For example, an X-ray source located in the constellation Cygnus X-1 is probably a black hole.