Given the dependence of magnetic activity on rotation and the dependence of rotation on age, one expects a dependence of activity on age. Already more than 30 years ago, prior to the advent of stellar X-ray astronomy, the so-called "Skumanich law" has been formulated, describing the decay of chromospheric Call emission in the form ~t-1/2 .
Individual stellar ages are rather hard to come by, and thus X-ray studies of open clusters, where the age is known from main-sequence fitting, are the main source of information on the coronal age-activity relations. Already the Einstein Observatory observations of the Hyades cluster (d = 45 pc, age = 600 Myr), Pleiades cluster (130 pc, 70 Myr) and Ursa Major stream (40 - 100 pc, 400 Myr) showed the dramatic increase of X-ray luminosity for young stars compared to the Sun. In particular, they demonstrated that solar-like stars at an age of the Hyades emit X-rays at levels two orders of magnitude above solar levels. With ROSAT the numbers and especially the age range of the observed clusters has been tremendously expanded.
The very young clusters observed with ROSAT, XMM-Newton, and Chandra include IC 2391 (160 pc, 40 Myr), IC2602 (160 pc, 30 Myr), a Per (160 pc, 50 Myr) and IC 4665 (350 pc, 40Myrs), NGC2516 (380 pc, 60 Myr), and the intermediate age clusters Coma (80pc, 500Myr), Praesepe (160pc, 700Myrs), NGC6475 (240 pc, 130Myrs), and IC4756 (480 pc, 600Myrs). Old clusters containing stars with ages comparable to that of the Sun such as M67 (800 pc, 4500 Myrs) and NGC752 (380pc, 2500 Myr) were also studied, but because of their large distances, solar-like X-ray emission levels are not detectable even with XMM-Newton or Chandra. The stellar X-ray sources detected in such old clusters are mostly active binaries (see section on Close Binaries) with X-ray luminosities of up to LX - 1031 erg s-1.
Figure 10.11 shows a plot of the median of the X-ray luminosity of G dwarfs derived for a number of star forming regions and clusters, the Sun, and field stars. During the first 100 million years in a star's life, its activity changes only little, afterwards the activity decays by a factor of 10 in a little less than a decade in logarithmic age. Figure 10.11 strongly suggests that the early evolution of the solar system occurred under conditions very different from the present conditions. The exploration of the consequences of the enhanced UV, X-ray, and particles fluxes in the early solar system has become vital for an understanding of planet formation .
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