Amateur and professional astronomers have produced a star catalog that measures the detailed characteristics of more than 42 million stars, many of which are accurately mapped for the first time in history.

The catalog is focused on variable stars, a large and diverse class of stars that change in brightness over time. Astronomers need good information about variable stars to learn details about the lives of stars – such as their mass, temperature, and internal structure – that would otherwise be difficult or impossible to determine.

The survey comes from the American Association of Variable Star Observers (AAVSO), an international nonprofit organization of star observers that has been collecting data for this map since 2009. This latest release, which was made publicly available on June 8, covers 95 percent of the northern and southern sky. The project is expected to be complete in 2014.

The survey is run from two telescopes in Chile and New Mexico and makes 8 million stellar detections per night, generating terabytes of data each year.

Because they change in brightness so slowly, variable stars need to be continuously watched to determine their long-term behavior. Most professional astronomers don’t have the time or resources for such an undertaking, which is why amateur astronomers – who can monitor and follow up with each star – are so useful.

Knowing about variable stars is valuable to professional astronomers. Some variables expand over time, pulsating and stretching due to internal forces. Understanding them allows astronomers to figure out the dynamics of stellar interiors. Other variables dim when a companion star passes in from of them, eclipsing their light. These binary stars allow astronomers to calculate the mass and size of different stars.

Another type of variable stars, known as Cepheid variables, have given astronomers clues to one of the biggest mysteries in contemporary science: dark energy. Cepheids are used to determine the distance to faraway galaxies, and using them astronomers have discovered that such galaxies are receding from us at increasing speeds, suggesting that the universe is accelerating as it expands.

The distribution of stars in the latest AAVSO sky map. Image: Edward Los.

Images: 1) The Large Magellanic Cloud, a small nearby galaxy containing many variable stars. Alson Wong and APASS. 2) The distribution of stars in the latest AAVSO sky map. Image: Edward Los.