About 1000 X-ray emitting young pre-main sequence ( PMS ) stars distributed in mass from \sim 0.05 M _ { \odot } brown dwarfs to a \sim 50 M _ { \odot } O star 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 .
Individual X-ray luminosities in the Orion Nebula Cluster range from the sensitivity limit of < 2 \times 10 ^ { 28 } erg s ^ { -1 } to \sim 10 ^ { 32 } erg s ^ { -1 } .
ACIS sources include 85 - 90 % of V < 20 stars , plus a lower but substantial fraction of deeply embedded stars with extinctions as high as A _ { V } \simeq 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 .
In a subsample of 17 unabsorbed stars with mass \simeq 1 M _ { \odot } , X-ray luminosities are constant at a high level around L _ { x } \simeq 2 \times 10 ^ { 30 } erg s ^ { -1 } for the first \simeq 2 My while descending the convective Hayashi track , but diverge during the 2 - 10 My phase with X-ray emission plummeting in some stars but remaining high in others .
This behavior is consistent with the distribution of X-ray luminosities on the zero-age main sequence and with current theories of their rotational history and magnetic dynamos .

The sources in the Becklin-Neugebauer/Kleinman-Low ( BN/KL ) region of massive star formation are discussed in detail .
They include both unabsorbed and embedded low-mass members of the Orion Nebula Cluster , the luminous infrared Source n , and a class of sources without optical or infrared counterparts that may be new magnetically active embedded PMS stars .
Several X-ray sources are also variable radio emitters , an association often seen in magnetically active PMS stars .
Faint X-ray emission is seen close to , but apparently not coincident with the Becklin-Neugebauer object .
Its nature is not clear .