How Many Stars Are in the Sky?
Want to know how many stars are in the sky?
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Like the majority of the world’s population, I have lived most of my life in cities where the light from street lamps, apartments, and even the searchlights of LA Live drowned out the stars in the night sky. But if we can escape the city lights, our unaided eyes can see roughly a few thousand of the brightest stars in the sky. Even an amateur telescope can increase the number of visible stars to about a million. But how many more stars are out there?
Astronomers estimate that our galaxy, the Milky Way, is home to 100-200 billion stars. And that is just our galaxy! We also expect there to be billions of other galaxies hosting their own sets of billions of stars.
The Gaia Mission
Last week the European Space Agency released the most detailed map to date of the stars in our galaxy as well as stars in some of our galaxy’s nearest neighbors. The map is the first data release from the space-based Gaia mission. In December 2013, Gaia was launched into orbit about 1.5 million kilometers (or just over 930,000 miles) away with the goal of creating “the largest and most accurate three-dimensional map of the Galaxy ever obtained.” To do so, Gaia plans to scan the entire sky and observe the positions (and brightnesses) of over 1 billion stars with 70 visits to each star.
The Gaia mission will observe everything visible from Earth down to a brightness level of ~1 million times fainter than what can be seen with the naked eye. Such a daunting task requires no less than the largest digital camera in the solar system.
Gaia employs 170 individual cameras, also called charge coupled devices or CCDs, that work together as a mosaic. Each CCD has ~9 million pixels a piece which adds up to a camera with more than a billion pixels. (For comparison, cell phone cameras have on the order of 10 million pixels.)
Astronomers expect Gaia to collect all of this data in only five years, but the volumes of data produced by the mission will be a source for scientific discovery for decades. After five years of observations, Gaia will have collected more than 1 petabyte of data. (Remember that “peta” implies 15 zeroes!) Around 2,000 person years worth of effort will go into all of the data processing.