The letters represent the initials of the proper names of different stars, while the numbers are the distance to the stars in light years, e.g. RbO860 = Rigel β Orionis at 860 ly. The missing star is Sirius α Canis Majoris at 8.6 ly.
Congratulations 'ylj'........ nicely explained solve and your 8th Challenge solve I believe. Your 'Honorary Gold Medal' is on the way ..... congratulations again.
The picture of 'Atlas' was a clue as 'Atlas' was given the job of holding the 'Heavens' on his shoulders as can be seen by the band of the Zodiac around the globe.
Yes, the picture made me think about Greek gods and then about constellations, helped by your clue. It was really fun solving this ...... thanks for another great challenge!!