|
|
What Are Astronomers Doing? Week of July 26
What's Inside a Strange Star?
Evolution isn’t a term used only in biology -- the heavens, seemingly enduring, are in constant flux. The "evolution" of heavenly bodies differs in some important ways from that of their earthly counterparts, but they are similar in two key ways: both are incompletely understood, and both have researchers dedicated to plumbing their mysteries.
The expression ‘stellar evolution’ refers to the changes a star goes through in its lifetime. As a star ages, the elements that compose it change in proportion. Also, the star’s size, color, and brightness change. All of these are linked to an underlying cause: changes in the fusion reactions in the star’s core.
It turns out that there are a number of different fusion reactions that power a star, each with its particular byproducts. The presence of particular elemental byproduct is closely related to particular stages in a star’s life -- but there are exceptions. Here is where researchers like Matt Shetrone step in. He and his colleagues are interested in a particular anomaly found in red giant stars. These red giants, though they shouldn’t, contain lithium as a fusion byproduct. As it happens, there is a strict order of fusion reactions – some release more energy than others. As the star ages, the fuel for the most efficient, easiest reactions gets used up, so the star starts using byproducts from earlier reactions to fuel new, less productive reactions.
Lithium is a sort of in-between element, not the best fuel, but not the worst. As a result of this, lithium builds up in a star, as an undesirable byproduct, until all the best fuel is used, at which point it becomes the fuel for further nuclear reactions. Thus, as stars reach old age, most stars -- but not all -- have destroyed the lithium they created. For unknown reasons, some old stars have been observed to contain lithium, when by all accounts they shouldn’t. Matt and other researchers are performing observations at McDonald Observatory that are designed to find stars that should contain lithium and see if they can be effectively located. This is the first step towards figuring out why some stars seem to have strange compositions.
Part of the difficulty in this project is that it isn’t easy to observe the interior of a star. Most observations and spectroscopy deals only with the star’s surface. To deal with this problem, researchers have developed models of how the contents of a star moves due to convection. Basically, the elements created in the interior of a star get mixed with the outer, observable, section of the star. Matt’s research relies in part on using such models to figure out what’s going on inside from data collected on the outside.
|
|
|
| |
|
