Sorry. Like Chris pointed out, Hipparcos didn't collect any B-V measurements, but another European satellite, Tycho, did. I think of the Tycho measurements as if they were made by Hipparcos, because my software, Guide, mentions the Hipparcos distance parallax and the Tycho color measurement in more or less the same go, as if they were two peas in a pod.neufer wrote:Please explain the difference between the (ground based) Johnson B-V and the (space based) Hipparcos B-V.Ann wrote:
The Johnson B-V of Achernar is (a lot) more negative than -0.2, but the Hipparcos B-V is well below (that is, a lot less negative than) -0.2. The difference between the Johnson and the Hipparcos color measurements is remarkable. And the most likely reason for it that I can think of is the oblateness of Achernar and the temperature variations that come with it.
(Ground based systems can't observe UV and 20,000K peaks in the UV.)
I do know that Tycho didn't measure exactly the same wavelengths as groundbased Johnson, so you would expect Tycho and Johnson to show slightly different colors for the same star. You will have to take me at my word when I say that I have used my software to compare the Johnson and the Hipparcos - that is, the Tycho - B-V color of hundreds if not thousands of blue stars. The color indexes very often differ a little, but usually just a little. The difference between the Johnson and the Tycho B-V color for Achernar is remarkable. My guess, for what it is worth, is that Achernar was in a relatively pole-on position when the Johnson measurement was made, and in a far more equator-on position when Tycho made its measurement.
Either that, or else the Johnson measurement was partly guessed at from the spectral class of Achernar.