How far are the Pleiades?

ToSeek · #1 (**permalink**) 02-June-2004, 05:11 PM

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The mystery began in 1997, when the European Space Agency's satellite Hipparcos measured the distance to the Pleiades and found it is 10 percent closer to Earth than traditional estimates, which were based on comparing the Pleiades to nearby stars. If the Hipparcos measurements were correct, then the stars in the Pleiades are peculiar because they are fainter than Sun-like stars would be at that distance. This finding, if substantiated, would challenge our basic understanding of the structure of stars.

But measurements made by the Hubble telescope's Fine Guidance Sensors show that the distance to the Pleiades is about 440 light-years from Earth, essentially the same as past distance estimates and differing from the Hipparcos results by more than 40 light-years. The Hubble results will be presented June 1 at the American Astronomical Society meeting in Denver, Colo.

The new results agree with recent measurements made by astronomers at the California Institute of Technology and NASA's Jet Propulsion Laboratory, both in Pasadena, Calif. Those astronomers used interferometer measurements from Mt. Wilson and Palomar observatories in California, reporting that the star cluster is between 434 and 446 light-years from Earth.

Normandy6644 · #2 (**permalink**) 02-June-2004, 06:39 PM

Whew, now I can sleep much easier at night. :wink:

daver · #3 (**permalink**) 02-June-2004, 06:53 PM

Any word yet on why Hipparcos was so far off? When this first cropped up they suggested a systematic error in Hipparcos, which would seem to throw a number of its results into question.

George · #4 (**permalink**) 02-June-2004, 11:30 PM

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Originally Posted by ToSeek

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This finding, if substantiated, would challenge our basic understanding of the structure of stars.

So now it's the challenge to our basic understanding of the structure of Hipparcos.

tracer · #5 (**permalink**) 03-June-2004, 12:45 AM

Quote:

Originally Posted by daver

Any word yet on why Hipparcos was so far off? When this first cropped up they suggested a systematic error in Hipparcos, which would seem to throw a number of its results into question.

When I started adding HIPPARCOS measurements to the Internet Stellar Database, I quickly realized that I had to completely discard some of them, because they resulted in ridiculous values (e.g. 150+ parsecs away for some of the main sequence stars listed in the Gliese catalog of nearby stars). Despite the wildness of these measurements, they were listed as having a standard error of 1 or 2 milli-arcsec at most (!) .

Kullat Nunu · #6 (**permalink**) 03-June-2004, 07:21 AM

Quote:

Originally Posted by daver

Any word yet on why Hipparcos was so far off? When this first cropped up they suggested a systematic error in Hipparcos, which would seem to throw a number of its results into question.

Could the wrong orbit of Hipparcos be the culprit?
It was meant to work in a geosynchronic orbit, but because of apogee boost motor failure it ended up in a highly eccentric orbit.

Anyway, if GAIA gets launched and works properly we'll have much better distance measurements of stars than ever.

AstroSmurf · #7 (**permalink**) 03-June-2004, 08:38 AM

The Pleiades are kind of interesting in that the cluster is stretched out over a rather long distance. If we weren't seeing the cluster end-on, it'd be much less spectacular.

(I noticed this while playing with Celestia, btw)

eburacum45 · #8 (**permalink**) 03-June-2004, 08:51 AM

Yes; as much as I like Celestia, the distances given for binary stars are unreliable; this is because the Hipparchos measured distance by using parallax.
if the stars are moving around each other they make the parallax readings wrong. Celestia either ignores double stars or has their distances quite badly out.

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