The Sun

The Sun is close and has been studied intensively. It radiates strongly from radio- to X-ray frequencies and, because of solar-terrestrial effects, has been monitored by an armada of spacecraft for 50 years. There were the OSO spacecraft (which also observed other cosmic sources) (1962–1978), Skylab (1973), Solar Max (1980–1989), Yohkoh (1991–2001), SOHO (1995–), TRACE (1998), and Hinode (2006). Solar X-ray emission is now continuously measured by a series of GOES spacecraft, and current data are available online almost instantaneously (NOAA/SWPC, 2009a). In this section we show only a few observations which illustrate things to keep in mind when considering the emission of other stars. The data are spectacular, and we regret not having room to include more. For a more thorough overview of solar observations and theory, there is an excellent book by Golub and Pasachoff (1997). Movies of EUV and X-ray images of the Sun can be viewed on several websites (e.g. TRACE, 2009; XRT, 2009).

An historical puzzle

Why should there be detectable X-rays from the Sun at all? Certainly not on the basis of its everyday visible appearance. The optical spectrum of the Sun can be represented quite well by a simple blackbody at a temperature of about 6000 K. Such an object should produce no detectable X-ray flux, whereas the amount actually seen implies the presence of material at a temperature of at least 1 million degrees!