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.
What Will We Learn from Gaia?
Astronomers believe that our Galaxy has a flattened disk shape with spiral arms and a bulge toward the center, similar to what we see in the Andromeda galaxy. But we didn’t always know this to be the case and even in the recent past, our view of what we thought our Galaxy looked like was quite different. The challenge arises from the fact that we are, obviously, sitting inside the Milky Way, so getting an overview perspective on our Galaxy is akin to trying to get a picture of your house … while sitting inside your bedroom. Mapping out as many stars as possible is thus key to getting the best picture that we can. Of the 1 billion stars that Gaia will continue to survey, a predicted 400 million will be observed for the first time.
An important component of the map will be its 3D nature. Astronomers cannot tell the distances to objects just by measuring their observed brightness with a camera, no matter how many billions of pixels that camera has, without some knowledge of the objects’ intrinsic brightness. In other words, we can’t always tell if a source appears faint because it is actually intrinsically faint or because it is just far away.
Gaia will use a technique known as parallax to measure distances to ~2 million stars to very high precision. A star’s parallax is a measure of the shift in the star’s position relative to background sources when the star is viewed from different perspectives. In the case of the Gaia mission, those different perspectives will be provided by observations from different points in Gaia’s orbit. The measured shift can then be used to judge distance, similar to how we use the two eyes in our head to provide depth perception.
Parallax observations are challenging because the shift in the star as observed from these different perspectives is extremely small and thus difficult to measure. The closest star to our solar system, and thus the star with the largest measurable parallax, Proxima Centauri, has a shift of less than 1 arcsecond.
For those who don’t think in arcseconds every day, a shift of 1 arcsecond is the angle subtended by a dime viewed from 2 miles away. And that’s the for the closest star – that means that’s the best case scenario! Gaia will measure these shifts with an accuracy of 24 microarseconds for stars that are ~4000 times fainter than what our eyes can see. That’s roughly the equivalent of being able to make out the diameter of human hair sitting in New York City … from Cincinnati.
The more detailed look at the size and shape of our Galaxy we will gain from the incredible detail of the Gaia map will help us to better understand how our Galaxy formed and where it might be headed. Knowing the positions of so many stars can also help map out where the dark matter is in the Galaxy due to its gravitational influence and even provide tests of Einstein’s theory of general relativity.
And that is just the stars! ESA astronomers also expect that the high sensitivity maps will discover hundreds of thousands of asteroids and comets within our Solar System, 7,000 planets around other stars, 20,000 supernova (stars in their death throes undergoing intense explosions), and hundreds of thousands of quasars (very bright, distant galaxies with active super massive black holes).
The nominal mission for Gaia is planned for 5.5 years, but without any unexpected fuel issues, the mission could technically go as long as 2023. The satellite has a few other things to worry about as well, including potential damage to its cameras should it encounter a solar flare.
The Gaia mission to map 1 billion stars is a huge step forward in our understanding of the universe. However, 1 billion stars is still only 1% of the stars expected to be in just our Galaxy, let alone the billions of other galaxies that call our universe home.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
Image courtesy of ESA.
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