In the February installment of Roanoke Skies, www.roanoke.com/columnists/goss/, the ages of certain stars in our February evening sky were described using a timeline. How do stars form in the first place?
Astronomers and astrophysicists have studied the topic of stellar evolution for two hundred years, eventually arriving at a model that explains their observations. It really boils down to understanding in depth the processes of gravity, thermodynamics, and thermonuclear fusion.
The basic model of stellar formation begins with a nebula composed of gas that is many tens of light-years wide. The nebula is cold and contracts under its own very weak gravity. Perhaps the gravitational field of passing star or a shock wave from nearby hot stars initiates the collapse.
After a few hundred thousand years, the collapsed nebula forms a dense ball. As that ball contracts further, it heats. Once its core temperature reaches a million degrees or so, it begins thermonuclear reaction, i.e., it starts to fuse hydrogen into helium. A star is born.
The physics of stellar evolution is now largely well understood. Astronomers can determine a star's distance, motion through space, mass, diameter, rotational rate, intrinsic luminosity, surface temperature, and chemical composition. Most of this is accomplished through spectroscopy, which measures the star's spectrum, much as a prism reveals our sun's rainbow-like spectrum. This is simply amazing since the stars lie so far away.
Scientists developed their model by studying a great many stars. Then, they were able to deduce specific characteristics which occur at specific stellar ages. If they know a star's mass, distance, luminosity and composition, they can determine its approximate age.
Astronomers have found that high mass stars are very luminous and age very quickly living only a few million years. Rigel falls into that category. Lower mass stars aren't as luminous or as hot and don't age quickly, living 10 billion years. The sun falls into that group. Very low mass stars, the so-called red dwarfs, are not very hot (as stars go), and are not very luminous. They age extremely slowly, living one hundred billion of years. None of them can be seen with the unaided eye simply because of their low intrinsic luminosity, being about 1/100 as bright as that of the sun. Isn't it strange that every red dwarf that has ever formed is still dimly shining today because, compared to the lifetime of a red dwarf, the universe is relatively young at 13.7 billion years old.
Such is the view from our Earth...
