1999JAVSO..27....1P

The Hipparcos astrometric and photometric database has many applications for undergraduate research projects. As one example, we have investigated the onset of variability in red giants by noting the excess scatter (above and beyond the instrumental scatter) in the Hipparcos photometric measurements of 7600 giants with +1.35 < (V-I) < +1.70. Microvariability (0.00 < ΔV < 0.01) sets in gradually for (V-I) > +1.46 ± 0.01 and variability (ΔV > 0.01) increases rapidly for (V-I)>+1.55. These are, however, reddened colors, but the small contamination by distant reddened supergiants may be offset by the contamination by reddened earlier-type giants. According to three recent effective temperature scales, these two (V-I) colors correspond to 4085 K and 3905 K, respectively, with an internal uncertaintly of ± 10 K.