Copyright: ESA/Gaia – CC BY-SA 3.0 IGO
Stellar density map - Annotaded
This image, based on housekeeping data from ESA’s Gaia satellite, is no ordinary depiction of the heavens. While the image portrays the outline of our Galaxy, the Milky Way, and of its neighbouring Magellanic Clouds, it was obtained in a rather unusual way.
As Gaia scans the sky to measure positions and velocities of a billion
stars with unprecedented accuracy, for some stars it also determines
their speed across the camera’s sensor. This information is used in real
time by the attitude and orbit control system to ensure the satellite’s
orientation is maintained with the desired precision.
These speed statistics are routinely sent to Earth, along with the
science data, in the form of housekeeping data. They include the total
number of stars, used in the attitude-control loop, that is detected
every second in each of Gaia’s fields of view.
It is the latter – which is basically an indication of the density of
stars across the sky – that was used to produce this uncommon
visualisation of the celestial sphere. Brighter regions indicate higher
concentrations of stars, while darker regions correspond to patches of
the sky where fewer stars are observed.
The plane of the Milky Way, where most of the Galaxy’s stars reside, is
evidently the brightest portion of this image, running horizontally and
especially bright at the centre. Darker regions across this broad strip
of stars, known as the Galactic Plane, correspond to dense, interstellar
clouds of gas and dust that absorb starlight along the line of sight.
The Galactic Plane is the projection on the sky of the Galactic disc, a
flattened structure with a diameter of about 100 000 light-years and a
vertical height of only 1000 light-years.
Beyond the plane, only a few objects are visible, most notably the Large
and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way,
which stand out in the lower right part of the image.
A few globular clusters – large assemblies up to millions of stars held
together by their mutual gravity – are also sprinkled around the
Galactic Plane. Globular clusters, the oldest population of stars in the
Galaxy, sit mainly in a spherical halo extending up to 100 000
light-years from the centre of the Milky Way.
The globular cluster NGC 104 is easily visible in the image, to the
immediate left of the Small Magellanic Cloud. Other globular clusters
are highlighted in an annotated version of this image.
Interestingly, the majority of bright stars that are visible to the
naked eye and that form the familiar constellations of the sky are not
accounted for in this image because they are too bright to be used by
Gaia’s control system. Similarly, the Andromeda galaxy – the largest
galactic neighbour of the Milky Way – also does not stand out here.
Counterintuitively, while Gaia carries a billion-pixel camera, it is not
a mission aimed at imaging the sky: it is making the largest, most
precise 3D map of our Galaxy, providing a crucial tool for studying the
formation and evolution of the Milky Way.
Source: ESA
About Gaia
Gaia is an ESA mission to survey one billion stars in our Galaxy and
local galactic neighbourhood in order to build the most precise 3D map
of the Milky Way and answer questions about its origin and evolution.
Gaia’s scientific operations begun on 25 July 2014 with the special
scanning through a narrow region in the sky, while the normal scanning
procedure was switched on a month later, on 25 August.
The mission’s primary scientific product will be a catalogue with the
position, motion, brightness and colour of the surveyed stars. An
intermediate version of the catalogue will be released in 2016. In the
meantime, Gaia's observing strategy, with repeated scans of the entire
sky, will allow the discovery and measurement of transient events across
the sky.
Acknowledgement: this image was prepared by Edmund Serpell, a Gaia
Operations Engineer working in the Mission Operations Centre at ESA’s
European Space Operations Centre in Darmstadt, Germany.
This image is licenced under the Creative Commons Attribution-ShareAlike 3.0 IGO (CC BY-SA 3.0 IGO) licence.
