Stories from the stars

Photo: Contributed

Almost everything we have learned through our optical observations of the universe is due to starlight.

We see galaxies because of the light from the billions of stars they contain. Even when a galaxy is so distant our largest telescopes can only show it as a faint glow, that glow is due to stars. By analyzing starlight we can learn a lot about the current state and history of a star cluster, a galaxy or even the universe itself. Stars are very useful. For example a star can preserve for us a description of its environment when it was born.

When the universe finally cooled and expanded enough, the first atoms formed. By far the most common were atoms of hydrogen, with a much smaller amount of helium. Clouds of this material collapsed to form the first galaxies, and within these galaxies, the first stars. These stars were made of hydrogen and little else. Eventually their cores got hot and compressed enough for nuclear fusion to start, and the stars began to shine. The waste products of this energy production were the other elements, and when these stars ran out of fuel, they sneezed off their material in some cases, and blew themselves up in others. The waste products of their energy production got mixed into the hydrogen clouds still waiting to form into stars. Consequently, subsequent generations of stars formed from increasingly contaminated hydrogen.

Interestingly, the waste products of energy production stay confined in the cores of the stars, where the energy production takes place. The outer layers of the stars, which we can see, consist of largely unchanged material dating back to when the stars formed, telling us about their birth environment. The oldest stars have outer layers of almost pure hydrogen; in subsequent generations there are increasing amounts of waste product elements. By determining the amounts of these elements we can place stars at the right place in history. Another useful facet of stars is how their lifetimes vary with their masses. Massive stars have far shorter lives than less massive ones.

Our sun, a yellow dwarf star, will have a lifetime of about 10 billion years. A star 10 times the sun's mass, therefore, with 10 times as much fuel, will consume it at about 1,000 times the rate the sun does, and consequently, will last only around 10 million years. With their high energy production rates, these stars are hotter than the sun, far brighter, appear blue, and are referred to as blue giants or supergiants. Galaxies with a lot of bright, blue stars will appear bluish, and because these stars don't last long by cosmic standards, an ongoing supply of replacements is needed, which in turn requires lots of hydrogen. A star with about a 10th of the mass of the sun will be so miserly with energy production that it could keep shining for most of the life of the universe. It will be cooler, shine with a red light, and be very dim. These stars are known as red dwarfs. There are many of them, but their faintness makes them hard to detect.

Those bright, blue stars end their lives by swelling into red giants, before finally blowing up. Regions where the hydrogen has been used up, and no new stars are forming, end up dominated by the red giants and the growing community of long-lived red dwarfs. The core of our galaxy is such a place, dominated by old, red stars. In the surrounding disc, out where we live, there is still a lot of hydrogen and star formation continues. 

The "astro" in astronomy refers to the study of stars. Even today, with the diversification of instruments, with radio telescopes, X-ray telescopes and so on, the study of stars and what they can tell us remains at the core of this science.

• Before dawn, Jupiter and Saturn are close together in the south.
• Mars lies low in the southeast.
• The moon will be new on the 20th, which is also the date of the Summer Solstice, the day of the year the sun reaches its highest in the sky we have the longest period of daylight.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.