ज्योतिरायुषः स्तोमः

Operational rule vidhi

ष् is substituted for the स् of स्तोम after ज्योतिस् and आयुस् in a compound.

Word by word (padaccheda)

Left context ज्योतिर्- आयुषः pañcamī · ekavacanaSubstitute / term स्तोमः prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

ज्योतिर्-आयुषः स्तोमः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

ष् is substituted for the स् of स्तोम after ज्योतिस् and आयुस् in a compound.
As ज्योतिष्टोमः, आयुष्टोमः, but ज्योतिः स्तोमं दर्शयति where there is no compounding.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.