star chart; astronomical chart
A map of the night sky showing stars and constellations, used for astronomy or navigation.
I looked for stars in the night sky while consulting a star chart.
天体図 is a broader term for a celestial chart, including planets and other objects, whereas 星座図 focuses specifically on constellations.
Compound of 星座 (seiza, 'constellation') and 図 (zu, 'chart, diagram').