Editing X-ray astronomy (section)

==X-ray dark stars==
[[File:The Solar Cycle XRay hi.jpg|thumb|right|A [[solar cycle]]: a montage of ten years' worth of [[Yohkoh]] SXT images, demonstrating the variation in solar activity during a sunspot cycle, from after August 30, 1991, at the peak of [[Solar cycle 22|cycle 22]], to September 6, 2001, at the peak of [[Solar cycle 23|cycle 23]]. Credit: the Yohkoh mission of [[Institute of Space and Astronautical Science]] (ISAS, Japan) and [[NASA]] (US).]]
{{Main|Astrophysical X-ray source}}
{{See also|Supergiant}}
During the solar cycle, as shown in the sequence of images at right, at times the Sun is almost X-ray dark, almost an X-ray variable. [[Betelgeuse]], on the other hand, appears to be always X-ray dark. Hardly any X-rays are emitted by red giants. There is a rather abrupt onset of X-ray emission around spectral type A7-F0, with a large range of luminosities developing across spectral class F. Altair is spectral type A7V and Vega is A0V. Altair's total X-ray luminosity is at least an order of magnitude larger than the X-ray luminosity for Vega. The outer convection zone of early F stars is expected to be very shallow and absent in A-type dwarfs, yet the acoustic flux from the interior reaches a maximum for late A and early F stars provoking investigations of magnetic activity in A-type stars along three principal lines. Chemically peculiar stars of spectral type Bp or Ap are appreciable magnetic radio sources, most Bp/Ap stars remain undetected, and of those reported early on as producing X-rays only few of them can be identified as probably single stars. X-ray observations offer the possibility to detect (X-ray dark) planets as they eclipse part of the corona of their parent star while in transit. "Such methods are particularly promising for low-mass stars as a Jupiter-like planet could eclipse a rather significant coronal area."