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A French proposal, in 1966, called for a satellite to map the whole sky.
The European Space Agency developed the concept in a multinational
project. Three decades after the idea was mooted the results are in,
thanks to the satellite called Hipparcos. It delivers a hundredfold
improvement. At a thousandth of a second of arc, or less than a
millionth of a degree, Hipparcos has fixed directions, distances and
motions of stars with an accuracy equivalent to observing an astronaut's
height at the distance of the Moon.
Astronomers will now re-assess all that they believed they knew about
stars and their evolution. Tycho Brahe's revolution in astrometry let
Kepler and Newton settle the argument about the nature of the Solar
System. After Hipparcos, who knows how opinions about the Universe will
change, when astronomers digest the results of this new tour de force of
Europe's space science?
3-dimensional Galaxy around us
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The brightest stars
The map above shows the stars seen most easily with the naked eye (magnitude 4
or brighter). All are treated as if they were the same size, so that a
star's width on the map gives an impression of its relative distance as
measured by Hipparcos. Numbers identify the most conspicuous stars, in
descending order of brightness.
The Warp in
the Milky Way
- Sirius, Alpha Canis Majoris A
- Canopus, Alpha Carinae
- Arcturus, Alpha Bootis
- Rigil Kentaurus, Alpha Centauri A
- Vega, Alpha Lyrae
- Capella, Alpha Aurigae
- Rigel, Beta Orionis A
- Procyon, Alpha Canis Minoris A
- Betelgeuse, Alpha Orionis
- Achernar, Alpha Eridani
- Hadar, Beta Centauri AB
- Altair, Alpha Aquilae
- Aldebaran, Alpha Tauri A
- Spica, Alpha Virginis
- Antares, Alpha Scorpii A
- Fomalhaut, Alpha Piscis Austrini A
- Pollux, Beta Geminorum
- Deneb, Alpha Cygni
- Beta Crucis
- Regulus, Alpha Leonis A
The Sun and many other stars orbit around the centre of the Milky Way
Galaxy in a flattened disk. By looking away from the centre, and
choosing distant stars from Hipparcos data, astronomers at the Turin
Observatory and Oxford University confirm that the disk is not really
flat. Outside the Sun's orbit, stars follow tilted orbits.
observations of outlying stars gave no more than an impression of their
slanting paths. Their inclinations might all have been similar. Instead,
Hipparcos data are consistent with the tilts becoming more pronounced at
increasing distances. The result is the curved shape of the disk shown
here, resembling the brim of a hat. The gravitational pull of
neighbouring galaxies, the Magellanic Clouds, may be to blame for the
warp in the Milky Way, but astronomers suspect that the distribution of
invisible Dark Matter in the halo around our own Galaxy is a more likely
For clarity's sake, the extent of the warping is
exaggerated here (X10).
Distance to the Pleiades
Hipparcos has discovered that one of the best-known star clusters, the
Pleiades (or the Seven Sisters as they are also called), is closer than
expected. The new distance determined by the Hipparcos mission is 375
light-years putting the cluster about 10 per cent closer than
The Pleiades have fascinated stargazers since ancient times. Nowadays
they are important because the stars in an open cluster were born in
the same gas cloud. They all have the same age and chemical make-up. So
they helped astronomers to develop the theory which predicts how hot
and luminous a star should be, for a given mass, composition and
age. The experts have to re-examine some assumptions, now that
Hipparcos finds that the Pleiades are less luminous than supposed.
(Photo copyright: Royal Observatory, Edinburgh, and Anglo-Australian Telescope
of the Hyades
Hipparcos data gives new insights into the distance, structure,
dynamics and age of the Hyades cluster. An overview of this study and an mpeg movie illustrating the 3-dimensional
nature of the cluster can be found in the Visual Results area.
At around 150 light-years, this star cluster makes the jaw of the bull
depicted in the Taurus constellation. Rings denote present star
positions. The tip of each line shows, from precise Hipparcos data,
where the star will be in 100,000 years' time. True members of the
Hyades will remain in company. Other stars travelling much faster or
slower than the average do not belong to the cluster, although close in
their directions and distances (out to 300 light-years).
All stars originate in groups, but many "evaporate" from their clusters
and follow an independent existence. Uneven gravity in dense regions of
the Galaxy disrupts the star clusters. Some 700 million years after the
Hyades were born, the cluster shows signs of wear and tear. The
Hipparcos data indicate that several stars are breaking away.
A brilliant old star
At 260 light-years the star HIP 48152 is more than twice as far as
astronomers believed before Hipparcos, and it is therefore six times
more radiant. As an ancient star formed long before the Sun, it helps in
estimating distances and ages of many farther stars. The large
correction shows why Hipparcos results have jolted star theories.
Checking distances to 900 ancient stars, astronomers at the Geneva
Observatory find that many have been understated. As a result the stars'
ages are doubtful until theories of their evolution have taken in the
new, precise information from Hipparcos.
Measuring the Universe
At 980 light-years, as newly judged by Hipparcos, a winking star called
Delta Cephei waxes and wanes every five days. The graph shows its
variations as monitored by the satellite. Delta Cephei (which appears
in the Hipparcos catalogue as HIP 110991) is the prototype of the
Cepheids, the stars that provide the yardstick for the Universe. The
more luminous a Cepheid is, the more slowly it winks.
Astronomers using the Hubble Space Telescope spot Cepheids in other
galaxies. Cepheids occur, for example, among the bright blue stars in
the Hubble image of Galaxy NGC 4639 shown here. Knowing the link between
period and luminosity, astronomers can in principle deduce the distances
of the galaxies. But their reckonings of the size and age of the
Universe have until now relied on informed guesswork about the yardstick
itself, based on estimates of how far away the Cepheids in our own
Direct measurements by Hipparcos, analysed by astronomers in Cape Town
and Cambridge, reveal that the nearest Cepheids are more distant and
more luminous than astronomers thought. If other Hipparcos data confirm
this result, all distances in the Universe at large may increase by ten
The star 47 Ursae Majoris (HIP 53721) has an unseen companion, revealed
by a wobble that American astronomers detected in 1995. Is it a planet,
or a small star? Accurate rangefinding by Hipparcos puts the visible
star at 46 light-years, allowing ESA's project scientist to set an
on the companion at 7 to 22 times the mass of Jupiter, the
Suns largest planet. The smallest true star would have a mass of 80
Jupiters, so the companion of 47 Ursae Majoris is probably a true
Hipparcos pinpoints the position of a very distant flaring
galaxy. It links, to its scheme of nearer stars, the accurate
maps of the radio astronomers and the photographic sky
surveys, of which a small extract is shown here. (Copyright of
Digitized Sky Survey: STScI, AAO, UK-PPARC and ROE).
MOTIONS OF A DOUBLE STAR IMAGED FROM HIPPARCOS DATA
Double star HIP 46706
Credit: C.F. Quist, L. Lindegren & S. Söderhjelm,
Lund Observatory, using data from Hipparcos (ESA)
A succession of three images shows two stars shifting their positions
as they orbit around each other in a double-star system and travel
together across the sky, as indicated in the diagram below. The
images are generated at Lund Observatory, Sweden, from multiple
observations by ESA's Hipparcos satellite.
The precision is astonishing. These stars are 34 light-years away and
they lie closer together than do the Sun and the planet Saturn. Their
separation in the sky is about 1/5000 degree.
The Hipparcos mission and subsequent computations have pinpointed the
positions of stars with unprecedented accuracy. Hipparcos was not
conceived as an imaging instrument, but the Lund team processes its
data by the technique of aperture synthesis, originally developed for
radio astronomy. The background ripple is a by-product of the
The double star is in the Hydra constellation. From the very precise
measurements, the astronomers compute the masses of the two stars at
0.41 and 0.42 times the mass of the Sun. They orbit around each other
every 18.3 years.
COMING OUR WAY!
At 136 light-years and far too faint for the unaided eye to see, the
star HIP 23641 is travelling towards us at 54 kilometres per second,
according to measurements by telescopes on the ground. Sailors know that
an approaching ship whose bearing does not change is on a collision
course. By taking the bearings of stars with exceptional precision,
Hipparcos can pick out those that will visit the Sun's vicinity. One
such, noted at the Royal Greenwich Observatory, is HIP 23641. It shifts
its direction by only a few millionths of a degree each year. In 750,000
years' time it will pass at 4 or 5 light-years, which is similar to the
distance of the present nearest star system, Alpha Centauri. Without
being quite as bright as Alpha Centauri is today, HIP 23641 will then
rank among the 40 brightest stars.