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Hipparcos: Filling In the Galactic Map

From the Cape of Good Hope, Alpha Centauri is a beacon in the sky, the third brightest star after Sirius and Canopus. The combined light of Centauri A and B (and Proxima as well, though at 11th magnitude, its contribution is minimal) caught the eye of Scottish lawyer Thomas Henderson, who in the course of a varied career found himself director of the Royal Observatory in South Africa. Cursed with poor eyesight, Henderson fixed on a mathematical approach to astronomy and chose to subject the Centauri stars to distance measurements, observing the system from both sides of Earth’s orbit to look for apparent motion.

And find it he did, a movement of three quarters of a second of arc that, using some basic math, gave him a distance of 41 trillion kilometers. This stellar parallax method has since been used on countless stars, but it’s really suitable only within 200 light years or so. Which is why older astronomy texts show such variation in stellar distances. One estimate of the distance of Deneb, for example, places it between 1470 and 1600 light years away, whereas JPL’s estimate runs to 3228 light years.

Spiral galaxy like our own

Star catalogs produced from space measurements tighten up the numbers considerably, which is why the news about Hipparcos is so interesting. This ESA mission, the High Precision Parallax Collecting Satellite, was launched in 1989 with a goal of measuring the position and distance to 100,000 stars with an accuracy exceeding ground-based measurements by a factor of ten to 100. Two data collections, the Hipparcos and Tycho catalogs, resulted from all this, with the first Tycho catalog covering about a million stars.

Image: A spiral galaxy not so different from our own. Credit: ESA/NASA.

A new version of the Hipparcos catalog is now available (in print form for now, with the online version appearing next year), the result of re-processing of the original data with upgraded technology. We’re seeing the benefits of re-examining old datasets — in many respects, our data collection capabilities have out-run our ability to analyze the data in short periods of time. Hipparcos data now produce a revised catalog improved by a factor of two on the original. A revised Tycho catalog appearing in 2000 swelled its own stellar ranks to 2.5 million.

ESA’s Gaia astrometry mission will launch in 2011, its catalog intended for release in 2020. Ultimately, let’s start thinking about extending the baseline for parallax enormously. An outer Solar System observatory could extend the baseline to hundreds of AUs. And imagine the potential benefit of an observatory in the Alpha Centauri system, pushing the baseline to 4.34 light years and making it possible to measure stellar distances accurately out to a range of almost four million light years.

What we’re doing, of course, is continually fine-tuning our idea of where we fit into the celestial scheme of things, a process that Thomas Henderson and other early star-distance measurers like Friedrich Georg Wilhelm von Struve and Friedrich Wilhelm Bessel began in the 19th Century (Bessel beat Henderson into print with a measurement of the distance to 61 Cygni, somewhat eclipsing the latter’s fine work). Mapping the Milky Way itself is a grand challenge that is continuing to bring our own city of stars — and our place in it — into far sharper focus.

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  • ljk January 25, 2008, 10:48

    Hipparcos distances of Ophiuchus and Lupus cloud complexes

    Authors: M. Lombardi, C. Lada, J. Alves

    (Submitted on 22 Jan 2008 (v1), last revised 24 Jan 2008 (this version, v2))

    Abstract: We combine extinction maps from the Two Micron All Sky Survey (2MASS) with Hipparcos and Tycho parallaxes to obtain reliable and high-precision estimates of the distance to the Ophiuchus and Lupus dark complexes.

    Our analysis, based on a rigorous maximum-likelihood approach, shows that the rho-Ophiuchi cloud is located at (119 +/- 6) pc and the Lupus complex is located at (155 +/- 8) pc; in addition, we are able to put constraints on the thickness of the clouds and on their orientation on the sky (both these effects are not included in the error estimate quoted above). For Ophiuchus, we find some evidence that the streamers are closer to us than the core.

    The method applied in this paper is currently limited to nearby molecular clouds, but it will find many natural applications in the GAIA-era, when it will be possible to pin down the distance and three-dimensional structure of virtually every molecular cloud in the Galaxy.

    Comments: A&A in press – Corrected typo (Lupus distance) in the electronic abstract

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.3346v2 [astro-ph]

    Submission history

    From: Marco Lombardi [view email]

    [v1] Tue, 22 Jan 2008 11:39:44 GMT (319kb)

    [v2] Thu, 24 Jan 2008 12:53:19 GMT (319kb)


  • ljk June 20, 2008, 16:44

    What will Gaia tell us about the Galactic disk?

    Authors: C.A.L. Bailer-Jones

    (Submitted on 16 Jun 2008)

    Abstract: Gaia will provide parallaxes and proper motions with accuracy ranging from 10 to 1000 microarcsecond on up to one billion stars. Most of these will be disk stars: for an unreddened K giant at 6 kpc, it will measure the distance accurate to 2% and the transverse velocity to an accuracy of about 1 km/s.

    Gaia will observe tracers of Galactic structure across the whole HR diagram, including Cepheids, RR Lyrae, white dwarfs, F dwarfs and HB stars. Onboard low resolution spectrophotometry will permit — in addition to a Teff estimate — dwarf/giant discrimination, metallicity measurement and extinction determination.

    For the first time, then, Gaia will provide us with a 3D spatial/properties map and at least a 2D velocity map of these tracers (RVs will be obtained too for brighter stars.) This will be a goldmine of information from which to learn about the origin and evolution of the Galactic disk.

    I briefly review the Gaia mission, and then show how the expected astrometric accuracies translate into distance and velocity accuracies and statistics. I examine the impact Gaia should have on a few scientific areas relevant to the Galactic disk.

    I discuss how a better determination of the spiral arm locations and pattern speed, plus a better reconstruction of the Sun’s orbit over the past billion years (from integration through the Gaia-measured gravitational potential) will allow us to assess the possible role of spiral arm crossings in ice ages and mass extinctions on the Earth.

    Comments: Proceedings of IAU 254 “The Galaxy disk in a cosmological context”, Copenhagen, June 2008, invited talk, 8 pages

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.2575v1 [astro-ph]

    Submission history

    From: Coryn Bailer-Jones [view email]

    [v1] Mon, 16 Jun 2008 13:25:55 GMT (35kb)