ESA Science & Technology - About Star Mapper

The ESA Star Mapper visualisation is an exploration of some central aspects of astrometric star catalogues, using data from ESA's Hipparcos mission.
 

Astrometry

Astronomy was one of the first natural sciences developed by early civilisations across the globe, and astrometry – the science of charting the sky – one of the oldest branches of astronomy.

Over the course of more than twenty centuries, star mapping has been transformed by the developments in precision instruments and the arrival of the space age.

The first space astrometry mission was ESA's Hipparcos, which operated from 1989 to 1993. Hipparcos' precision measurements of the positions, motions and distances of more than 100 000 stars have had a major impact on many areas of astronomy research.

The next great breakthrough in this field will come with ESA's Gaia mission, launched in 2013. Gaia will make a census of more than one billion stars – roughly one percent of the content of our Galaxy – of such superb precision and detail that it will revolutionise astronomy again.

Contents of Star Mapper

The ESA Star Mapper visualisation shows 59 921 stars from the Hipparcos Catalogue. This subset of the entire catalogue was selected as follows:

• we include stars with the best parallaxes – the criterion is parallax/parallax error > 5. Parallaxes give a measure of the distance to these stars;
• we do not include stars that have no catalogue record of the magnitude or colour information;
• we include all stars from the constellations, even if their parallax values do not satisfy the criterion above. For those stars we assign an artificial parallax value of 1 mas. In practice, this is needed only for one of the stars in Orion's belt (Alnilam);
• for the 27 435 stars for which there is no radial velocity measurement from the XHIP catalogue, a random value between -30 and 30 kilometres per second.

Features of this visualisation

Click on Start to begin your exploration. The tour is divided into five sections: Apparent Magnitude, Absolute Magnitude, Hertzsprung-Russell, Motion, and Explore.

Each section displays a short explanation of the current view; click on the cross to hide the explanation, or on the horizontal lines to open the explanation panel.

To move around the sky use scroll to zoom and drag to rotate the view.

In the Apparent Magnitude section you can drag the slider to set the limit on what stars are displayed.

In the Absolute Magnitude section toggle between the Apparent Magnitude and Absolute Magnitude buttons to how the brightness of the stars differs with distance.

The Hertzsprung-Russell diagram can be shown with stars plotted in white or colour.

The Motion section lets you see how stars move through space. To help orient yourself you can toggle Star names, the outline of the Constellations, or a Graticule of grid lines.

The Explore section lets you apply the different filters together and to change the projection.

Apparent and Absolute Magnitudes

The magnitude scale is a logarithmic scale used to indicate the brightness of stars. An increase or decrease of 1 magnitude is a change by a factor of 2.512, with larger magnitudes corresponding to fainter stars. As an example, a magnitude 20 star is about 400 million times fainter than the brightest star in our sky, Sirius, which has an apparent magnitude of -1.46.

The apparent magnitude is a measure of the brightness of a star or celestial object as seen from Earth or a telescope in space (near Earth). The value depends on the object's true brightness (its luminosity), its distance, and the amount of light that is absorbed between the star and the viewer. The unaided human eye in a dark-sky location can see stars to about magnitude 6, while with the aid of binoculars we can reach magnitude 9. Telescopes can detect much fainter stars. Hipparcos detected stars as faint as magnitude 13.3, and the Gaia mission will map stars down to magnitude 20.7.

The absolute magnitude of a star or celestial object is the apparent magnitude it would have if placed at a distance of 10 parsecs (32.6 light years) from Earth. Absolute magnitude is used to compare the true brightness of celestial objects, regardless of their distance from us.

Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram is used by astronomers to study how stars evolve. The colour of stars, which is an indication of their surface temperature, is plotted on the horizontal axis, and their absolute magnitude on the vertical axis. The graph has a distinctive shape and the location of a star indicates what stage of its life cycle the star is in; for example, stars spend most of their lives on the main-sequence – the diagonal branch that runs from upper left to lower right. Stars appear in different parts of the main sequence depending on their mass, with the most massive, brightest stars in the top left and lower-mass, fainter stars towards the lower right. While on the main sequence stars burn hydrogen into helium in their cores, giant and supergiant stars (those which have used up most of their supply of hydrogen) lie above the main sequence, and white dwarf stars (the final evolutionary stage of low-mass stars like the Sun) are found below it.

Star colours

Depending on their surface temperature, stars have different colours; these range from blue stars with surface temperatures around 10 000 K (or warmer) to red stars with surface temperatures around 1000 K (or cooler). The colour-coding in this representation of stars is based on the Harvard Spectral classification OBAFGKM. The filters applied the data used in this visualisation resulted in no O stars in the subset used. B stars (28 000 K to 10 000 K) are coloured blue; A stars 10 000 K - 7500 K) are cyan; F stars (7500 K to 6000 K) are white; G stars (6000 K to 5000 K ) are light yellow; K stars (5000 K to 3500 K) are dark yellow and M stars (less than 3500 K) are red.

Named stars

Stellar motion

Despite appearances, the stars do move, but by such tiny amounts that we cannot perceive this on human timescales without the aid of precision instruments. A star's velocity in space, relative to the Sun, is a combination of its proper motion, the motion of a star seen on the plane of the sky, and its radial velocity, measured along the line of sight. The proper motion can be obtained by monitoring the change in a star's position over time, and radial velocity comes from the red- or blue-shift of its spectrum of light.

Background image

The background image is the Milky Way panorama (credit: ESO/S. Brunier). This 360-degree panoramic image, covering the entire southern and northern celestial sphere is made with images collected from two exceptional astronomical sites: the Atacama Desert in the southern hemisphere and the Caldeira de Taburiente in the Canary Islands in the northern hemisphere. The images were recorded during 2008 and 2009. While recording the views, some of our Solar System planets passed across the field of view. Jupiter is especially prominent, visible as a bright blue object with multiple spokes.

Requirements

This interactive 3D visualisation is supported only by WebGL compatible browsers and graphics cards. Some combinations of browser and operating system may give less than optimal performance. Further information about WebGL is available here: http://get.webgl.org/.

Production credits

ESA's Star Mapper visualisation was developed for the European Space Agency by Jan Willem Tulp (TULP interactive) with support from Jos de Bruijne (ESA), Karen O’Flaherty (EJR-Quartz for ESA) and Claudia Mignone (Vitrociset Belgium for ESA).

ESA Star Mapper

V1.0; release date 7 September 2016