Before you were born, your body developed in your mother’s womb. At first you were an “it.” Your genetics contain the information to form you as either a male or a female, but the genetic code that is original to your cells told hormones called androgens to turn on during development, causing your body to form male hormones and parts or female hormones and parts. Your brain and spinal cord developed first, and then your cells differentiated (changed) into different organs and parts. By the time your mother was four months along in her pregnancy, an ultrasound could determine if your heart was functioning normally, if you were a male or female, and if there were any major problems in your development.
After your mother delivered you, you needed tender care as an infant, but you grew and developed rather quickly. Humans typically double their birth weight by age one. Growth rates slow, until the teenage “growth spurt” associated with the onset of puberty. Throughout adolescence, the body and the brain develop into what will be the human adult. Adults do not generally experience dramatic physiological changes throughout their adulthood. Adulthood is a time of maintaining balance, and when the balance of human health is altered by disease or trauma, humans decline with failing health until finally they pass on.
Stars follow a similar pattern of life. They develop in a nebula instead of a womb, but it is a time of rapid change and growth. A gravitational center caused atoms to “clump” together, adding more mass at faster and faster rates. This is similar to human gestation. Typically, the amount of mass babies gain in the first two trimesters is NOTHING compared to the mass babies gain in the last trimester of pregnancy. Stars add mass similarly, until they are a large “clump” of spinning mass.
This spinning mass is near to “being born” when the outer edges of the growing “clump” are either added into the central mass or blown off into space by the spinning motion of the “clump.” Gravity pulls the atoms tighter and tighter into the center, causing atomic collisions and thus the temperature at the center of the clump to increase. As temperature increases, gas pressure pushing out from the center of the developing star increases. When gas pressure and temperature reach equilibrium at a temperature that can sustain hydrogen fusion, a star is born.
From the birth of a star through most of adulthood, the star burns (fuses) hydrogen in order to maintain equilibrium. There are small adjustments made to maintain Main Sequence status, but these changes are only to maintain balance between gravity and gas pressure.
A star spends most of its life in this phase of early growth through adulthood. When its hydrogen fuel is all spent, and remember that originally the star was 97% hydrogen so it makes sense this is the longest phase of life for a star, then the star must either find another fuel to keep up its core temperature or die. If the star doesn’t die, then it has found a way to burn helium and keep equilibrium. Once a star begins burning helium, the geriatric phase (old age phase) of the star’s life has begun.
If the star achieves helium burning, it will continue to do this until it is again out of fuel. Once the helium is gone, the star can die or find another fuel to burn. If the star does not die, then it will burn carbon and fuse that into iron.
Unfortunately, this is the beginning of the end. It is similar to older populations of humans – one health problem can be dealt with. Several health problems will lower the quality of life, but there can still be enjoyment and fulfillment. But there comes a point where the saying becomes true: If you don’t have your health, you don’t have anything.
It’s the same with stars: If you don’t have equilibrium, you don’t have anything. After carbon fusion, there is no fuel to burn once the core of a star has been converted to iron. Iron cannot fuse and lose energy at the same time. Therefore, once the carbon is gone the temperature of the star’s core will drop. This causes the gas pressure to decrease. Then gravity has the upper hand, and the star will collapse upon itself until it dies, exploding in a supernova event that creates all of the heavier elements found in the universe.