Star-Forming Regions

Since the 1980s, the Einstein Observatory has shown that the Young Stellar Objects (YSOs) emit X-rays with luminosities, in the 0.3–8 keV bandpass, up to 103–104 times than the Sun and that the X-ray emission is highly variable. ROSAT has confirmed the pervasiveness of X-ray emission from YSOs and ASCA has provided evidence that the emission of Class I YSOs is largely originating from optical thin plasma at temperature of 1–50 × 106 K. These intrinsic, unexpected properties and the transformational capabilities of the Chandra and XMM-Newton observatories has made X-rays a powerful tool to trace the star formation process up to distance of a few kpc around the Sun. Starting from the early evidences of the 1980s and the intriguing questions they raised, I will summarize the results obtained and how they have influenced our current understanding of physical processes at work, and I will discuss some of the still open issues and some of the likely avenues that next-generation X-ray observatory will open.