Isotherm, in geography, a line drawn on a map connecting places that have the same average temperature for a given period of time. For example, an isotherm running through New York City, St. Louis, and Seattle would indicate that these cities, as well as any others along the line, all had the same average temperature for a specified period. Isotherms are useful in compiling and showing climate information. For example, for general farming, an average temperature of at least 60 F. (16 C.) is necessary during the warmest months. A map marked with an isotherm of 60 F. for the months of July and August would show the northern limit of the area where the temperature would permit general farming.
A black hole occurs when a massive star dies -- its enormous mass implodes and becomes so heavy that it bends space. So how do astronomers detect something that they can't see?
Every day, astronomers unravel a little more of the universe's inner workings, but the jury is still out on 95 percent of its contents.