The Galactic census project

GAIA is a Cornerstone Mission within the ESA Horizon 2000+ science plan, to fly circa 2011

GAIA will measure distances and velocities of more than a billion stars, quasars and compact galaxies

The basic GAIA data will be

For several hundred million stars, GAIA will measure 3-D positions and 3-D velocities within the Galaxy and the local group.

A complete catalog: All the stars will be detected on board during the mission, rather than being pre-specified as with Hipparcos.

Main Science Issues:





-- grey CCDs : objects are detected and approximate positions recorded

-- light grey CCDs : (x,y) positions measured accurately

-- blue and red CCDs : photometry is performed on each target as a low resolution spectrum in red and blue bands.

-- dark red CCDs : radial velocities are measured from high resolution spectra centered on the Ca triplet at 8600 Angstroms.


V mag 10 11 12 13 14 15 16 17 18 19 20 21
parallax error 4.0 4.0 4.2 6.0 9.1 14.3 23.1 38.8 69.7 138 312 1786


V mag 10 11 12 13 14 15 16 17 18 19 20 21
pm error 2.4 2.4 2.4 3.3 4.9 7.6 12.1 20.1 35.3 68.0 146. 609.

UPDATE - 20/02/2009. The astrometric accuracies now quoted at the GAIA website here are not quite as good as in the table above, with the expected astrometric accuracy for a 15th mag star having decreased by up to a factor of 2. The figure below shows the execpted accuracies as a function of colour for a 15th mag star.


Radial velocities



Major measurement objectives of GAIA

Note: the expected accuracies (Feb 2009) have been reduced by up to a factor of 2 from these targets (see figure above).

  Parameter GAIA Objectives
Astrometry  positional accuracy
parallax accuracy
proper motion accuracy 
10 microarcsecond at V=15
10 microarcsecond per year at V=15 
10 microarcsecond per year at V=15 
Photometry multi-colour 
4 broad-band + low resolution spectra (=>intermediate band colours)
accuracy about 0.01 mag throughout the mission 
Radial velocity velocity accuracy 1 km/s at V=15, limit at V=17


Measurement goals of GAIA compared with Hipparcos

  Hipparcos GAIA
Magnitude limit about 12 21
Completeness 7.3-9.0 19-20
Number of objects 120000 35 million (to V=15)
    350 million (to V=18)
    1.3 billion (to V=20)
Accuracy 1-2mas 4 microarcsec (V=10)
    10 microarcsec (V=15)
    0.2 milliarcsec (V=20)
Observing list needed a priori on-board/unbiased 


Current Status



Science Goals

Colour Magnitude of nearby stars produced by HIPPARCOS

Massive stars

Novae and nova-like variables

Planetary nebulae

Selection of Planetary Nebulae observed with the HST

Cepheids and RR Lyrae stars


Stars in Open Clusters

The open cluster M67

Globular clusters

Stellar diameters

Detection of Planetary Systems and Brown Dwarfs

Star formation

White dwarfs

Metal-poor stars and primordial nucleosynthesis

Visual and astrometric binaries

Interacting binary systems

The fraction of variable stars in the Hipparcos CMD. Stars in blue regions are rarely variable, whereas stars in the reddest regions are almost all variable. The code at the bottom of the plot shows the colour which indicates the fraction of stars which are variable.

Extragalactic Astrophysics and General Relativity

Proper motions of the Magellanic Clouds



Local Group Galaxies

  • Direct detection of the proper motion and rotation curves of local group galaxies
  • Internal kinematics of nearby dwarf galaxies
  • Galaxy orbits to give cosmological histories


Galaxies and active galactic nuclei

The role of quasars


Light-bending by the Sun and Earth

Higher-order gravitational effects



Light curve detected by the MACHO experiment of a star in the LMC,
caused by microlensing by a dark body in the Galactic halo

Microlensing by a planet around a faint star,
microlensing a bright background star.

Dynamics and Evolution of the Galaxy

Different isochrones as a function of metallicity plotted on the CMD produced by HIPPARCOS, which allow the structure of the main sequence to be determined and the age of the Galaxy to be measured.

Rotation curve of the Milky Way. The curve shows the circular rotation rate of stars and gas in the disk as a function of distance from the Galactic center. The "flatness" of this curve (rotation speed is the same at all radii) is a major piece of evidence for dark matter in the Galaxy.

Simulation of a small galaxy merging with a massive disk galaxy.


Galactic Halo

Simulation of a satellite being accreted by the Milky Way. It leaves behind a trail of stars which GAIA should be able to find, if they exist. Many such satellite accretions are expected during the early life of the Milky Way, and continue up to the present time.  Source: Johnston



Solar System Bodies


Summary of tracers



Primary reference: GAIA website