For countless generations, stars appeared eternal and unchanging. Yet modern astronomy has revealed that stars are living objects with histories. They are born, they evolve, and eventually they die.
Stars begin their lives inside enormous clouds of gas and dust known as nebulae. Under the influence of gravity, parts of these clouds slowly contract. As matter falls inward, the central regions become increasingly dense and hot.
Eventually, temperatures rise to millions of degrees. Under these extreme conditions, hydrogen nuclei begin to fuse together, releasing enormous amounts of energy.
A star is born.
For most of its lifetime, a star remains in a state of delicate balance. Gravity continually pulls matter inward, while the energy generated by nuclear fusion pushes outward. This balance allows stars to shine steadily for millions or even billions of years.
Our Sun is currently in this stable phase of its life, known as the main sequence. It has been shining for about 4.6 billion years and is expected to continue doing so for another five billion years.
But stars are not immortal.
As hydrogen in their cores becomes depleted, their evolution depends largely on their mass.
Stars similar to the Sun gradually expand into red giants. Eventually, they shed their outer layers, producing beautiful planetary nebulae, while their cores remain behind as white dwarfs.
More massive stars experience a far more dramatic fate. After exhausting their nuclear fuel, they undergo catastrophic collapse and explode as supernovae.
These tremendous explosions create and disperse many of the heavier elements needed for planets and life.
Depending on their mass, the remnants left behind may become neutron stars or black holes.
In this way, stars are not merely sources of light.
They are cosmic furnaces.
Inside their cores, hydrogen is transformed into helium, carbon, oxygen, silicon, and many other elements. Without generations of stars living and dying, there would be no planets, no oceans, and no life.
Every atom of calcium in our bones, every atom of iron in our blood, and much of the material that makes up our world was forged inside ancient stars.
Perhaps that is one of the most remarkable discoveries of modern astronomy.
The stars we admire in the night sky are also our ancestors.
For we are not separate from the stars.
We are one of their many descendants.