  About 1000 X-ray emitting young pre-main sequence (PMS) stars distributed in
mass from 0.05 to 50 solar masses are detected in an image of the Orion Nebula
obtained with the Advanced CCD Imaging Spectrometer on board the Chandra X-ray
Observatory. This is the richest field of sources ever obtained in X-ray
astronomy. ACIS sources include 85-90% of V<20 stars, plus a substantial
fraction of deeply embedded stars with extinctions as high as A_V~60.
  The relationships between X-ray and other PMS stellar properties suggest that
X-ray luminosity of lower-mass PMS stars depends more on mass, and possibly
stellar rotation, than on bolometric luminosity as widely reported. A subsample
of unabsorbed solar-mass stars show high X-ray luminosities around L_x ~ 2 x
10^30 erg/s for the first ~2 My but diverge during 2-10 My with X-ray emission
plummeting in some stars but remaining high in others. This behavior is
consistent with theories of stellar rotational history and magnetic dynamos.
  The Becklin-Neugebauer/Kleinman-Low (BN/KL) region of massive star formation
shows both unabsorbed and embedded low-mass members of the Orion Nebula
Cluster, the luminous infrared Source n, and a class of sources without
photospheric counterparts that may be new magnetically active embedded PMS
stars. Several X-ray sources are also variable radio emitters. Faint X-ray
emission is seen close to, but apparently not coincident with the BN object.
Its nature is not clear.
