Hipparcos 3D Stereo Images of the Sky

These 3d Stereo images were created using data from The Hipparcos and Tycho Catalogues. The relative distances are calculated from the Hipparcos parallax measurements and the star colours are generated using the V magnitude and B-V values from the Catalogues. The following star fields are available (in sets of 3):

The images can be viewed on the screen, or printed and viewed on paper copies. The effect will work for black and white prints, but is better with colour copies. In each of the following examples, the target object is at the centre of the field, which is about 6 × 6°². The target object is projected to lie in the plane of the page or screen. Only objects from the Hipparcos Catalogue are displayed.

Each field comprises 3 images which can be viewed in two distinct ways:

• Cover the leftmost of the set of three images, and use the rightmost pair for "fused" free-eye imaging: view the page from a distance of about 30 to 50 cm under good and uniform lighting conditions. Focus on the images, but "relax" the eyes so that they converge at infinity (imagine that you are staring through the paper at a distant point, so that the left eye observes and focuses on the left (middle) image, while the right eye observes and focuses on the right image). Try to fix on a particular object until the depth effect appears: when it does, the results are rather dramatic, and you can roam across the field, examining the relative distances of the various stars in it. Unfortunately, many people seem to be either unable, or not patient enough, to find the effect. In this case, you might try the second method

• Cover the rightmost of the set of three images, and place a good quality mirror, with a height of about 10 cm, midway between the leftmost pair, perpendicular to the page, and with the reflecting surface facing the leftmost image. With your head a few cm above the top edge of the mirror, look at the right (middle) image with the right eye, and look at the left (inverted) image, in the mirror, with the left eye. You may need to experiment with the positioning of the mirror, and your head, such that you have an unobstructed view of the right image with your right eye, and of the full mirror image with the left eye. Again, fix on a particular object until the depth effect appears. The method is simpler, if slightly less dramatic, than the fused method; it works because in focusing on the page the eyes naturally try to converge, so that viewing the left image in the mirror requires no particular muscular contortions.

The first accurate three-dimensional mapping of the celestial sphere by Hipparcos

The scientific motivation underlining ESA's Hipparcos space astrometry mission has been to establish an extremely precise stellar reference frame and, in the process, to determine the physical parameters of a very large number of stars, in particular, those lying relatively near (in Galactic terms) to our Solar System. The accurate measurement of stellar distances leads to the possibility of transforming observed quantities - such as apparent magnitude, angular diameter, and angular motion projected onto the celestial sphere - into absolute physical quantities - such as absolute luminosity, stellar radius, and the star's velocity through the Galaxy. Accurate determination of these quantities is of great importance in advancing present theories of stellar evolution, in gaining further understanding of stellar structure, and unravelling the kinematic and dynamic complexities of our Galaxy's structure and motion.

The only rigorous way to determine the distance to a star is through the measurement of its trigonometric parallax. This utilizes the angular displacement of the stellar image as observed from widely separated points, such as the different positions of the Earth in its orbit around the Sun. Observational difficulties associated with the measurement of this tiny effect through the Earth's atmosphere have hitherto limited the use of the parallax to the several hundred stars lying within a few tens of light years from the Sun. One of the goals of Hipparcos has been to drastically expand the space in which accurate trigonometric distances are measured.

The extremely accurate positions, motions, and stellar distances determined from the Hipparcos data have resulted in an enormous stellar catalogue of unprecedented accuracy and scientific importance, which will place ESA's contribution to the measurement of star positions in a remarkable historical context. Over the last two thousand years, stellar positions have been compiled into catalogues furnishing increasingly more accurate two-dimensional coordinates projected onto the celestial sphere. Depiction of the heavens has also evolved through the ages, and has included the celestial globe, the astrolabe (commonly used during Medieval times to help locate celestial objects and solve practical problems in astronomy and navigation), the armillary sphere (an open framework emphasising the principal coordinate circles), and the star charts, devoted to celestial cartography, or uranography.

These tremendous works combined remarkable scientific, artistic, and decorative skills, and aimed to assist in the visualisation of the arrangement and extent of the ancient constellations, written descriptions of which date back to the works of Ptolemy in his classic Almagest of the Second century A.D. Famous examples of these star charts include the Uranometria atlas of Johann Bayer (1603), the Prodromus Astronomiae of Johannes Hevelius (1690), the Atlas Coelestis of John Flamsteed (1729), the Uranographia atlas of Johann Bode (1801), and the Uranometria Nova of Friedrich Argelander (1843). Huge and numerous celestial mapping programmes of considerable scientific importance have continued since then, all of them concentrating on the determination of the two-dimensional coordinates of the stars, with increasing accuracy, increasing numbers of stars, or increasing limiting magnitude.

A dramatic indication of both the scientific objectives and the remarkable success of the Hipparcos mission are the accompanying three-dimensional sky images, which are being published here for the first time, based on the global solution of the first 30-months of the mission data. A non-linear transformation between real and apparent distance has been used for these images, partly to enhance and facilitate the depth effect, and partly to limit the scientific information conveyed in these images in advance of catalogue completion. The three-dimensional images correspond to the view of the celestial sphere that would be obtained with a separation of about 2 light years between the left and the right eye (rather than the 0.00003 light year used by Hipparcos, viz., the diameter of the Earth's orbit). Stellar images are shown with a size proportional to their apparent brightness, and with stars fainter than about 9 mag all shown the same size. The accompanying black and white images are the corresponding regions of the sky, as they appear to the naked eye in which, of course, depth information is entirely absent.

It should be stressed that the stellar distribution perpendicular to the plane of the sky is, for the vast majority of stars, and for the very first time, using the tremendous depth and precision of the Hipparcos data, a precise reflection of the true stellar distribution in space.

Instructions for viewing

The following images, which appeared in ESA Bulletin 77 with coloured glasses for viewing, should be viewed using red/green coloured glasses, with the red filter covering the left eye, and the green filter covering the right eye. The images should be viewed in good lighting conditions, from a distance of some 50-100 cm, with the line joining the two eyes parallel to the horizontal sides of the image. As the two separate images for the two eyes 'fuse' into a single image, the depth of the stellar distribution should become clear. It may take several minutes of viewing for this effect to become fully apparent. It may be worth experimenting with different viewing distances, and with and without spectacles if these are usually worn.

[Please note that the orientation of the displayed images depends on the type and version of browser. On some systems, the images may appear rotated by 90 degrees. For correct viewing, the red/green images of the same star should be separated horizontally.]

The images were prepared by L. Lindegren, Lund Observatory.

Ursa Major

Figures:

• The black and white image corresponding to Ursa Major now  [.GIF image]
• The colour image of Ursa Major now  [.GIF image]
• Ursa Major, as it will appear 60 000 years from now [.GIF image]

Cygnus and Lyra

Figures:

• The black and white image corresponding to the cover picture [.GIF image]
• The colour image of Cygnus and Lyra  [.GIF image]

Cassiopeia

The illustration covers the constellation of Cassiopeia (centred at roughly 1 hour in right ascension and +60 degrees in declination, and covering a region of about 31 x 26 degrees). The distinctive "W" formed by the five brightest stars in this well-known constellation are evident in the black and white image, showing how the field appears to the naked eye.

The ancients regarded the heavens as a ceiling or dome over the Earth, and later as a sphere surrounding it. The constellations have little practical relevance to modern astronomers or astrophysicists, and are not physical associations of stars. Rather, they were used to represent the pattern of stars in the sky using depictions of imaginary people, creatures or objects.

Figures:

• The black and white image corresponding to Cassiopeia  [.GIF image]
• The colour image of Cassiopeia  [.GIF image]

Scorpius

This illustration is of the constellation Scorpius, visible from the Southern Hemisphere. It is centred at roughly 17 hours in right ascension and -30 degrees in declination, and covers a region of approximately 42 x 36 degrees. In the naked eye representation the constellation extends from the cluster of bright stars at the bottom left of the field, and sweeps up vertically through the line of bright stars, including the brightest in the constellation - the first magnitude star, Antares - to the collection of slightly fainter stars to the top right of the field.

The four 3-d images here together cover some one-tenth of the entire sky, all of which has been mapped by the Hipparcos satellite. The Scorpius region, typical of the celestial sphere mapped by the astrometry satellite, contains some 5000 stars included in the main Hipparcos observing programme, and nearly 50 000 measured astrometrically and photometrically with the Tycho experiment.

Figures:

• The black and white image corresponding to Scorpius [.GIF image]
• The colour image of Scorpius [.GIF image]