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Science, Maths & Technology

Gaia: Taking the Galactic Census

Updated Wednesday, 7th August 2013

We know how many stars there are in the Milky Way, but how do we know how far they are?

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Find out in this animated video how Gaia carries out the Galactic Census of one billion stars.

Star motion The components of a star's apparent motion across the sky. (click to enlarge)
How can we measure the size and shape of the Milky Way? Counting stars on the sky is easy – but how do we know how far away they are? There is only one fundamental way to measure a star’s distance, and that is to measure its parallax.
Parallax is the small apparent movement of a nearby star compared to very distant stars caused by the motion of the Earth – and satellites in orbit with the Earth – as the Earth moves around the Sun each year. This parallax is the basis of all astronomical distances. Thus our standard distance unit is the parsec – the distance of a star which has a parallax motion of one second of arc (the apparent size of the moon is 1,800 seconds of arc). A parsec is 3.26 light years, or 30,000,000,000,000km. Yet no star has a parallax as large as one arc-second.
To map the Milky Way we must measure angles 100,000 times smaller – equivalent to a button seen on an astronaut on the moon.  This remarkable accuracy requires special spacecraft – hence, Gaia, the European Space Agency mission to deliver the first ever significant census of the stars in the Milky Way. Gaia will measure precision distances for 1,000,000,000 stars, providing us with the first ever true map of our Galaxy.

Gaia will do even more than this – by repeating the measurements for five years, Gaia will also see all those one billion stars move. That is, we will know where the stars are now, and how they are moving through space. That extra information allows us to calculate stellar orbits, determine their place of origin, and crucially, weigh the Milky Way. Available measurements tell us that there is much more to our Galaxy than just the stars and gas – Gaia will also measure the Dark Matter, which holds the Milky Way together.

Figure on the right: Top panel: the parallactic ellipse, caused by the motion of the earth around the Sun. The size of the ellipse decreases linearly as distance increases. Second panel: The star’s “proper motion”, caused by its orbit in the Milky Way. Third panel: wobbles in the motion of a star caused by the gravitational pull on the star of its planets. Fourth panel: all effects combined, illustrating the motion which Gaia will measure.

Question: Gaia will measure the size, shape and distance of the stars in the Milky Way in a five-year cycle. What else do you think we can measure in the future? Share your thoughts using our Comments facility.

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