BETA DOR (Beta Doradus). Among the most important of all stars are the Cepheid variables, named after the prototype, Delta Cephei. Many of them dot the naked- eye starry sky, their number including Eta Aquilae, Zeta Geminorum (Mekbuda), even Polaris, the brightest of them (though Polaris's variations are too small to be witnessed by eye). Here is another bright one, fourth magnitude (averaging 3.76) Beta Doradus, the second brightest star (after Alpha) in the modern southern constellation Dorado, the Swordfish. At its peak, Beta Dor actually reaches into third magnitude, 3.41, and then falls to 4.08, oscillating between the two over a slightly variable 9.842... day period. Cepheids are evolved supergiants, and this one is no
The top graph shows the visual (V) light curve of Beta Doradus over two cycles, where "phase" is the relative period, which begins with maximum light at 0 and ends with maximum light at 1. The middle graph gives the relative change in "color," the blue magnitude (B) minus the usual visual magnitude (V). Color correlates with temperature and spectral class, wherein the more positive values indicate cooler temperature. (These are not true B-V colors, but show only relative change.) The bottom graph shows the "radial velocity" of the star, that is, how fast the star appears to be moving along the line of sight as determined from the Doppler shift (positive values indicating recession). The variation is coming from the expansion and contraction of the stellar surface during the pulsation cycle. The star is brightest not when it is biggest or smallest, but when it expanding the fastest, and dimmest when it is contracting the fastest. From an article by D. Bersier in the Astrophysical Journal Supplement Series, vol. 140, p. 465, 2002.
exception, changing during its variation cycle from class F6 to about G5, which shows that the temperature changes as well (the average about 6000 Kelvin). From the distance of 1040 light years, the star shines with a mean luminosity of 3000 Suns, which implies a radius of 50 times solar and a mass 6.5 solar. Direct measure of angular diameter through interferometry, however, suggests a larger star, one closer to 65 times bigger than our Sun. Cepheids pulsate, alternately growing larger then smaller, hotter then cooler (which may explain part of the difference), as a deep layer within them traps and releases heat. In an unstable state, they expand, shoot past equilibrium, fall back under the force of gravity, then over-compress and expand again. The effect involves only the outer layers -- the core has no idea of what is going on. Though we know they long ago gave up hydrogen fusion in their cores, they can be in any of a variety of conditions. Beta Dor may still have a contracting helium core, or it could be fusing hydrogen into helium; it could even have given up core helium fusion. No matter which, the star is only about 60 million years old: higher mass stars age fast. Cepheids follow a strict "period-luminosity" relation. The more luminous they are -- in total power output -- the longer the pulsation period (which makes sense, since brighter stars are the larger, and they just take longer to go through the cycle). The pulsation period of any random Cepheid thus gives its absolute brightness, while comparison with apparent brightness gives the distance. Find a Cepheid in another galaxy, and you know that galaxy's distance. They are the key to finding the structure of the Universe and the nature of its expansion. As expected, Beta Dor fits the relation almost perfectly.
Written by Jim Kaler. Return to STARS.