Murder in the Paradise

The Life and Death of Stars

Most processes occurring in the visible universe involve stars in some way. In fact, all elements, with just a few exceptions, were formed in the nuclear furnaces of stars. This includes the heavier elements such as oxygen and carbon, the main constituents of living organisms (among the exceptions we find lithium, deuterium, helium, hydrogen which were created in the big bang).

Astronomers classify stars using many schemas, including their color, brightness, size, temperature, mass, association with other stars. The system of magnitude of stars introduced by the Greek astronomer Hipparchus in the second century AD is still in use today. Hipparchus divided stars by visual brightness from magnitude 1 (brightest) to magnitude 6 (faintest). Each level represents a 2.5-fold change in brightness.

With the arrival of the telescope and later the camera, stars as faint as magnitude 30 became detectable. These stars are four billion times fainter than could be observed with the naked eye.

The modern and most meaningful way of characterizing and analyzing stars is by spectral class. We classify them by the letters OBAFGKM (the famous mnemonic for remembering this sequence is Oh, Be A Fine Girl, Kiss Me). A star’s spectral class is defined by its characteristics of temperature, size, and density. The hottest and most massive stars are in the O and B classes and typically emit blue to white light. Stars of intermediate temperature and mass range from A- to G-type and emit white to yellow light. The coolest, least massive stars are K- and M-type and emit orange to red light. In fact, two new classes have been added (L and T) to account for the discovery of very low-mass stars.

The lifetime of a star is directly related to its mass. Indeed, stars that are actively fusing hydrogen to helium, such as our Sun, are called main-sequence stars. We know that stars generally spend about 90 percent of their lives on the main sequence. And when stars exhaust their hydrogen fuel, they begin the inevitable process of stellar death.

At the point of “stellar death” in their evolution, stars begin to leave the main sequence.

Thus the most massive stars rapidly use up their fuel and may live only a few million years. This is in contrast to lower-mass stars, such as the Sun, which may enjoy a main-sequence life of over ten billion years.

As a sun-like star exhausts its hydrogen core and begins to die, a new process begins. Helium fuses to carbon and later to oxygen, which will sustain the star for a short period but at the expense of further core collapse, higher core temperatures, and continued surface expansion. The surface of the star, no longer checked by gravity, bloats and cools. At this stage, the star is referred as a red giant and the bloated diameter can exceed ten times that of our Sun. The star begins a futile cycle of further core collapse and surface expansion that can end either passively as a white dwarf surrounded by a planetary nebula or, for more massive stars, in a cataclysmic explosion known as a supernova.

Steller death is an inevitable stage in the evolution of every star. Image by © Megan Jorgensen.