Gaia: ESA's billion star surveyor

In a recent post I talked about the different methods astronomers use to measure the distances to the stars, and how the parallax method is probably the most important of all of these as it is one of the few true measures of distance.

Parallax relies on being able to measure the precise positions of the stars on multiple occasions so that their changing positions can be measured as the Earth orbits the Sun. Measuring the positions of stars is known as astrometry, and represents an entire branch of astrophysics. Making precise position measurements is incredibly difficult and is possible using only the most advanced telescopes on Earth, and this is only possible for stars in small areas of the sky at a time.

To measure parallaxes for stars across the entire sky requires a dedicated space telescope designed to make the most precise positional measurements of as many stars as possible. The telescope designed to do this is Gaia, the European Space Agency's (ESA) current flagship mission.

An artist's impression of ESA's Gaia Satellite
(Credit: ESA)

Gaia has been 20 years in development, planned since the final days of it's predecessor, the Hipparcos satellite, which measured the positions of the brightest 100,000 stars in the sky. It represented a giant leap forward in astrophysics, providing accurate distances for a large number of stars for the first time, but in some respects it barely scratched the surface of our galaxy.

Our Galaxy is approximately 100,000 light years across and contains roughly 100 billion stars. While Hipparcos was revolutionary, it observed only a fraction of the stars in our galaxy out to distances of only about 3000 light years. Gaia's goal is to surpass this and provide the first detailed, structural map of our entire galaxy.

Gaia will achieve this by imaging the entire sky repeatedly, approximately 70 times over the 5 year mission of the satellite. With each scan the satellite will record the positions of all the stars it observes, allowing scientists on Earth to measures the parallaxes and therefore the distances to all these stars.

But Gaia doesn't just measure the parallax towards these stars but also their motion across the sky, known as their proper motion. The stars in the sky are not fixed, but constantly moving and Gaia can measure these movements using the images it takes over the satellite's lifetime.

In fact Gaia needs to measure both the proper motion and the parallax of the stars because when the two are combined they cause the stars to follow a unique apparent motion across the sky. The figure below shows this. On the left you can see the positions of the stars that you might see from a single image of the night sky. Add in their motions across the sky (their proper motions) and the stars will follow straight paths (shown in the central panel), but then add in the parallax effect and the stars will appear to trace out loops across the sky (see the right-hand panel).

The apparent motions of the stars built up from their positions (left), proper motions (centre) and parallaxes (right)
(Credit: Wikipedia)

The complicated paths traced by the stars are the reason Gaia needs to perform so many astrometric measurements, allowing scientists to separate the motions due to parallax and proper motion.

The Gaia satellite was launched in December 2013 on a Soyuz rocket from ESA's launch site in French Guiana, the Guiana Space Centre. After a successful launch the satellite was manoeuvred to its designated orbital position, known as L2.

Gaia's launch aboard a Soyuz rocket (Credit: Japan Times)

Once the satellite's mission is over scientists will be able to determine the parallaxes and proper motions of approximately 1 billion objects, as well as other useful information such as their colours and some spectroscopic information.

This detailed and important information, for so many stars will revolutionise astronomy. For the first time we will be able to map out the 3-dimensional structure of our Galaxy and we'll finally know the true distances to so many interesting astronomical objects (including star clusters!), allowing us to know where they are in our Galaxy, how they're moving, and how luminous they are.

I'm sure I'll be posting more news and information about Gaia in the future, so stay tuned!

How far away is that star?

One of the simplest and yet most important questions in astronomy relates to how far away the objects we study are. This question is relevant to all astronomical objects, from stars to galaxies and beyond. It's important to understand how far away these objects are because that's how we know how large or how luminous they are, and knowing these characteristics is necessary to build up our model of the Universe.

Despite this, measuring distances in astronomy is incredibly difficult, because sometimes all that can be resolved about an object is a single dot of light. For this reason astronomers have built up a series of methods for estimating the distances to objects, each of which is used for different types of object at different distances, and with each method calibrated using one of the other methods. We refer to this as the distance ladder.

The Parallax Effect
(Credit: Wikipedia)

The most fundamental method to determine distance, and the most important step on the distance ladder, is known as parallax. Parallax is the effect by which objects at different distances change their apparent position based on your vantage point.

In astronomy this is possible because the Earth changes it's position throughout the year as it orbits the Sun. Because of this the apparent positions of stars relative to each other change throughout the year.

This diagram shows an example of this. When the Earth is on the opposite side of the Sun the line of sight to a nearby star will change relative to more distant stars. The apparent shift in the position of the nearby star is known as the parallax angle and is directly related to the distance to the star - the nearer the star is, the larger the parallax angle will be.

You can simulate a small-scale version of this process for yourself using your two eyes as the two different vantage points. Hold out your hand in front of your face with a single finger pointing vertically upwards. Close one eye and look at the scene in front of you. Then switch the eye that is closed and see how the scene in front of you changes. You should see that the position of your finger changes relative to the background scene it is projected against.

In this example your finger is the nearby star and the background scene is the background stars. If you try moving your finger closer or further away from your face you should see that the apparent shift in your finger's position when you switch your closed eye changes - does the shift get larger when your finger is closer or further away from you?

Because parallax is such an important method of distance determination it has led to the most commonly used unit of distance in astronomy, the parsec. A parsec (1 pc) is the distance at which an object's apparent position shifts by 1 second of arc (1/3600 of a degree) as the Earth orbits the Sun. It's a very small shift, but then a parsec is a very large distance - approximately 30,000,000,000,000 km!

Despite how big the parsec is, all the stars in the sky are actually more distant than a parsec, and many are much much further than this. Because of this astronomers need very precise instruments and telescopes to be able to measure the tiny changes that result from the parallax effect. One of the most famous such telescopes was the Hipparcos space telescope, which measured parallaxes for thousands of stars out to distances of several hundred parsecs. The Hipparcos telescope was one of the most important telescopes in astrophysics, simply because of the unprecedented accuracy with which it measured the distance to so many stars. The successor to Hipparcos, the Gaia space telescope, was launched about a year ago, and is continuing this mission as we speak.

The European Space Agency's Hipparcos satellite (Credit: CNES)

For very distant objects where the parallax method is not feasible the only way to determine distances is to estimate how intrinsically bright the object is and then determine its distance based on how bright it appears to us. To do this we need to use objects with a known, or predictable, brightness, often referred to as standard candles. Examples of this including pulsating stars such as Cepheid variables, which Edwin Hubble used to determine distances to other galaxies. This method is most commonly applied to distant galaxies that are too far away to use parallax, but close enough to resolve and study their individual stars.