The joke is a pun.  'Sirius' is the name of the brightest star visible in the Earth's night sky.  'Sirius' is pronounced the same as 'serious' which would mean 'important' or 'not a joking matter'.  The claim about a star wasn't serious, and it also may not have been about Sirius.
The astronomer made a claim about a well - known star , but it wasn ' t sirius .