There are tens of trillions of stars in our Milky Way galaxy and billions of galaxies in deep space, each with trillions of stars. Taken in total, there are likely more stars in the universe than grains of sand on all beaches on Earth.
That’s truly a lot of stars, yet most are seething balls of complexity driven by exploding energy kept in check by gravity.
Astrophysicists classify the many different kinds of stars according to their size and temperature. Sizes range from a few hundred miles across to 1,000 or more times the size of our sun. The largest have diameters larger than the orbits of Mars.
Most luminous stars lie on a plot of luminosity and temperature known as the “main sequence.” Almost all stars spend their active lives as main sequence stars, to which astrophysicists assign designations of O, B, A, F, G, K and M, with O being the hottest and brightest.
Our sun is a rather average G-class star approximately halfway through its predicted life of 10 billion years.
The mass of a star is the primary factor in determining where it falls in the main sequence. The more massive the star, the greater the pressure at its center, the faster it burns its nuclear fuel and the hotter and brighter it is. Its temperature determines its color from the hottest blue to the coolest red.
Every star on the main sequence represents a delicate balance between the outward pressure of a nuclear explosion and the inward pressure of gravity.
In order to maintain the nuclear fusion reactions that power a star, the atomic nuclei must be compressed enough to overcome the electrical repulsion between positive charges to initiate and maintain fusion. Gravitational pressure must be maintained in order to keep the star from blowing itself apart in a thermonuclear explosion, and the explosive pressure must be great enough to keep gravity from crushing it.
Stars on the main sequence fuse hydrogen into helium, which creates a slow, steadily increasing proportion of helium in the core. Eventually, helium content predominates, and hydrogen fusion ceases at the core.
If a star is more than 0.4 solar mass, hydrogen fusion will continue in a slowly expanding shell around the helium core.
This ever-expanding shell causes the star to gradually grow until it reaches the red giant phase. Stars with at least half the mass of the sun begin to fuse helium at the core, while more massive stars fuse heavier elements along a series of concentric shells.
Once a star like the sun exhausts its nuclear fuel, its core collapses into a dense white dwarf and ejects the outer layers as a planetary nebula.
Stars of 10 or more solar masses explode in a supernova as their cores collapse into a neutron star or a black hole.
Astronomers cannot understand stellar evolution by studying a single star, as changes occur far too slowly. The process is similar to the way an intelligent extraterrestrial might study human growth and development by studying humans at various stages of maturity and then logically determining a sequence from birth to old age.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
