Stars go through changes over the course of their existence. The rapidity and violence of those changes depends upon the mass of the star. Low mass stars are slow to change. High mass stars change quickly. Perhaps a case of Live Fast, Die Young.
This post is intended for those readers who have wondered what happens during the lifetime of a star. It is not intended to be an exhaustive description of the life-cycles of stars. Some of the numbers used here represent a range of values assumed for these events. Sources will differ some. There are many parts of star behavior that are complicated and not understood well by the experts. General concepts are presented here to make the processes more understandable.
All stars involve two types of processes that oppose each other. Gravity pulls the star components inward and tries to reduce the star’s volume. Nuclear fusion exerts outward forces and tries to increase the star’s volume. This interplay of opposing forces can create equilibrium. Change in strength of the processes will cause the star to either expand or contract in size. Since the mass of the star is quite constant, the inward pull of gravity is constant. The outward forces can change in strength as nuclear fusion processes change.
Stars form from dense regions of gas and dust called stellar nurseries. These regions have a large amount of hydrogen. If the collection of hydrogen has enough mass it will collapse gravitationally. The closest region to Earth where this process occurs is in the Orion nebula. Another region is the Eagle nebula. Images of the Orion and Eagle nebulae show small dense regions where new star formation is happening. See this post for images of the Eagle nebula. These star forming regions are called Bok Globules.
When gravitational collapse proceeds, the density increases. Collisions between hydrogen nuclei are more frequent and energetic. The nuclei may begin to fuse together into helium nuclei if they collide with enough kinetic energy, releasing an enormous amount of energy in the process. This energy generates much heat and light. The process causes an increase in outward directed force to oppose the inward force of gravity. The mass of the star forming regions determines the energy and the lifetime of the resulting stars. How do we know this about stars? One star cannot be watched continuously over its billions of years of existence. It is a data sampling problem. Scientists have observed many similar stars at various points in time of their existence and pieced together the sequence of events. This video will shed some light on how that process works.
Star masses can be divided into general categories. Very low mass objects don’t have enough mass to trigger star formation. Low to average mass stars, such as our Sun or smaller, will end up as a white dwarf star surrounded by a debris cloud called a nebula. There are many examples. I’ve written about several in the series Astro-Images. Large mass stars end their existence as neutron stars. Very large mass stars end up as black holes.
Very Low Mass Objects
Brown Dwarfs are objects from stellar nurseries with masses less than 0.08 times the Sun’s mass. That compares to 13-80 times the mass of Jupiter. They do not have enough density and temperature in their core to trigger nuclear fusion. They don’t become stars. Discovered in 1995, they are abundant but difficult to detect. Their internal structure is stable and long lasting causing them to cool very slowly and glow dimly in the infrared. They may last trillions of years.
Red Dwarfs are between 0.08 and 0.8 times the mass of the Sun. Some amount of fusion takes place in their core at their start. But, the convection process stirs the contents of their core continually mixing it. The fusion process gradually runs out of hydrogen fuel and stops. Red Dwarfs are the most abundant stars and very long-lasting.
Low to Average Mass Stars
In the graphic above, stars with masses between 0.8 and 8 times the mass of the Sun have a life-span of about 10 billion years. They eventually reach a stage where helium combines by fusion forming carbon and oxygen nuclei. This more energetic fusion causes an increase in the outward forces and expands the volume of the star to a Red Giant phase. Our Sun will eventually do that and engulf the Earth. These strong outward forces blow gases and dust outward and form a nebula cloud. What eventually remains at the core is a tremendously dense object more than 200,000 times more dense than the Sun called a White Dwarf.
Large Mass Stars
Stars more that about 8 times that of the Sun have greater inward pull due to gravity. The fusion process first combines hydrogen nuclei into helium nuclei. The lifetime of these stars is in the range of 30 million years. The mass and strength of gravity is strong enough to cause the fusion of helium nuclei into carbon. The fusion process can continue between several different nuclei of increasing mass. It forms a series of layers of increasing mass closer to the center. The most massive nucleus to form in this fusion process is that of iron. The central core of the star can be pictured like an onion layered deeper and deeper.
The core of iron grows as the fusions in the outer layers continue. When the iron core reaches 1.4 solar masses, the strength of gravity forces electrons and protons to combine into neutrons in a very rapid explosive event. In less than a second, a tremendous release of energy takes place as the core collapses from 8000 km down to 19 km. It releases more energy than 100 Suns over their lifetime of 10 billion years. It blasts the outer layers of this cosmic onion apart in an event called a supernova.
The force of the supernova fuses nuclei together to form nuclei much more massive than iron. A mix of the heavier elements are formed around the supernova event. Many of the elements we find in our world and within our bodies are made of the debris of a supernova. We are all literally star material.
The dense core of the star remains as a spinning ball of neutrons. There are stars with cores more than 1.4 solar masses.
Very Large Mass Stars
Some stars are much more massive than those described above. Processes of fusion take place rapidly. Lifetimes are on the order of 3 million years or less. If their iron core exceeds about 3 solar masses, the intense strength of the gravity field can further collapse the neutron star formed during supernova to a black hole. No physical events such as nuclear fusion have enough outward force to overcome the intense inward pull of gravity. Nothing is able to go fast enough to escape the region. Even light itself does not have sufficient speed to escape.
Thanks go to the NASA Chandra X-Ray Observatory for the details presented here. The graphic below from Chandra summarizes the range of star masses and events over time I’ve discussed. It is a large image. Much more detail is available to the willing reader in Chandra’s accompanying text at this link.