Charting the Galaxy – from HIPPARCOS to Gaia – ESA

Gaia is an astrometry mission that is built as a continuation of the hugely successful HiPParCoS telescope: the High Precision Parallax Collecting Satellite (HiPParCoS). Since 2013, Gaia has been generating the largest, most precise three-dimensional map of our Galaxy by surveying more than a thousand million stars.

Gaia monitors every target star about 70 times over a five-year period recording their positions, distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission also studies about 500 000 distant quasars and will provide stringent new tests of Albert Einstein’s General Theory of Relativity.

The massive stellar census will provide the data needed to understand the origin, structure and evolutionary history of our Galaxy.

Technical Description of Galaxy-Mapping Gaia Telescope – ESA from Nature Documentaries.

Gaia will identify which stars are relics from smaller galaxies long ago ‘swallowed’ by the Milky Way. By watching for the large-scale motion of stars in our Galaxy, it will also probe the distribution of dark matter, the invisible substance thought to hold our Galaxy together.

Gaia will achieve its goals by repeatedly measuring the positions of all objects down to magnitude 20 (about 400 000 times fainter than can be seen with the naked eye).

For all objects brighter than magnitude 15 (4000 times fainter than the naked eye limit), Gaia will measure their positions to an accuracy of 24 microarcseconds. This is comparable to measuring the diameter of a human hair at a distance of 1000 km.

It will allow the nearest stars to have their distances measured to the extraordinary accuracy of 0.001%. Even stars near the Galactic center, some 30 000 light-years away, will have their distances measured to within an accuracy of 20%.

The vast catalog of celestial objects expected from Gaia’s scientific haul will not only benefit studies of our own Solar System and Galaxy, but also the fundamental physics that underpins our entire Universe.

For millennia astronomers have looked to the sky and gazed in wonder at the stars and planets. Ancient civilisations already realised that objects in the sky appeared to move in a regular manner, and many communities used the stars to determine when to plant and harvest their crops.

Thus began the oldest branch of astronomy – astrometry – that is, the study of the geometrical relationship between objects in the sky and their apparent and true motions. For many centuries, astronomers have devoted their time to the art of determining the relative position of the stars, a fundamental requirement for cataloguing the night sky.

The true pioneer of the science of astrometry was the ancient Greek astronomer Hipparchus, who in 129 BC, and with only naked eye observations and simple geometry, catalogued the relative positions of around one thousand stars. He determined their relative brightness and positions with an accuracy of about one degree (the angle equivalent to the apparent height of a person at a distance of 100 m). Systematic measurements and observations such as that of Hipparchus’s enabled construction of the state of the art contraptions like the Antikythera Mechanism.

Whilst Europe languished in the Dark Ages, astronomy flourished in Asia. Extensive observations were performed in the Chinese and Indian empires, including the compilation of stellar catalogs. Observations of the sky were accompanied by the study and translation of texts from ancient Greek scientists. Asian scholars built exquisite astronomical instruments to measure angles in the sky. They improved on the quadrant, a measuring device shaped as a quarter of a circle that was originally proposed by Ptolemy, and invented the sextant, a similar instrument in the shape of one sixth of a circle.

A catalog of 994 stars was created by Uluğ Bey (Ulugh Begh) of the Timurid dynasty in the fifteenth century. Ruling over Central Asia the astronomer and mathematician constructed an enormous sextant with a radius of 36 meters in Samarkand, located in present-day Uzbekistan. Uluğ Bey’s catalog has a precision of slightly better than one degree, comparable to that of Hipparchus’s compilation from several centuries before. Uluğ Bey and the many other astronomers that were active in Asia kept the practice of astronomy and astrometry alive, smoothly ushering them into the modern era.

Progress in the accuracy of measuring angles only accelerated in the 16th century with astronomer Tycho Brahe’s stellar observations using sextants and quadrants. He managed to fix star positions to an accuracy of about half an arcminute (an arcminute is 1/60 of a degree).

In 1609 the telescope was invented and later in the same century instruments that could be used with telescopes to determine angles in space. Finally, astronomers could determine angles to accuracies greater than the human eye can see.

By the 18th century, accuracies improved to the order of arcseconds, and by the 19th century, fractions of a second of arc. This finally opened up the ability to measure stellar parallax, the change in the apparent position of a star when viewed from two widely separated positions, for example as viewed from Earth 6 months either side of its orbit around the Sun. Using trigonometry, the parallax angle can be converted to a precise distance. But it is an extremely small quantity – even the nearest star has a parallax of less than 1 arcsecond – making the measurement accessible only to the most sensitive instruments, and only for the nearest stars.

Moreover, these early parallax measurements were limited by viewing through Earth’s atmosphere, which distorted the results and put an upper limit on their accuracy. The only way to really achieve precise measurements would be from space.

In 1989, more than 2000 years after Hipparchus first looked at the heavens, ESA launched a mission named in his honour: the High Precision Parallax Collecting Satellite (HiPParCoS). ESA’s Hipparcos was the first satellite ever devoted to astrometry, and revolutionized the field of precision astronomy, improving on accuracies achieved on the ground 100 times over, down to just 1 milliarcsecond.

Data were collected between 1989 and 1993, with the resulting Hipparcos Catalogue published in 1997. It contains the positions, distances and movements – 200 times more accurate than any previous measurement – for almost 120 000 stars. A second, larger, catalogue – the Tycho catalogue – contains data of 2.5 million stars to a lesser precision. These catalogues set the precedent on stellar positions and are continuously used in space science research and for spacecraft navigation.

Gaia continues the proud legacy of star charting. It is destined to catalog a thousand million stars, measuring each star’s position and motion 200 times more accurately than the Hipparcos mission, and producing 10 000 times more data than its predecessor. Equipped with information about each star’s position and velocity, astronomers will be able to trace the past trajectory of stars, thus ultimately deciphering the history of the Milky Way.