Tukukkk

During the rituals for a Navagraha puja, I saw that the avahana was done for the stars (nakshatra) of the yajamaana and his family. I didn't pay much attention until I heard that the devatha for the star "Magam" (or Makhaa) being the Pitrs! Now I am curious to know where from these devathas for each each Nakshatra are assigned. Same question goes for the Navagrahas.

For Navagrahas one such scriptural source is the Matsya Purana.

A Vedi(platform) is to be prepared with white rice grains with Surya
Deva the Center, Mangal in the South, Brihaspati in the North, North
East with Budha, East as Shukra, South East as Chandra, West as Shani,
South West as Rahu, and North West as Ketu. The Adhi Devatas to each
of the Planets are as follows: Shiva for Surya, Parvati for Chandra,
Skanda for Mangal, Vishnu for Budha, Brahma for Brihaspati, Indra for
Shukra, Yama for Shanaischara, Kaala Devata forRahu and Chitragupta
for Ketu. The Pratyabhidevatas respectively are Agni, Varuna, Prithvi,
Vishnu, Indra, Aindri, Prajapati, Sarpa and Brahma. Besides, Vinayaka,
Durga, Vayu, Aakasha, and Ashvini Kumars and these are also to be
invoked by individual names.

For the Nakshtras, the scriptural source is Krishna Yajurveda.

The Taittiriya Samhita 4.4.10 gives the following list of the usual 27 Nakshatras and their ruling deities:

Krittika (Agni),
Rohini (Prajapati),
Mrigashirsham (Soma),
Ardra
(Rudra),
Punarvasu (Aditi),
Tishya (Brihaspati),
Ashresha (Sarpa),

Magha (Pitrs),
Phalguni (Aryama),
Phalguni (Bharga),
Hasta ( Savita),

Chitra (Indra),
Svasti (Vayu),
Vishakha (Indragni),
Anuradha (Mitra),

Rohini (Indra),
Vichrtau (Pitarau),
Ashada-Purva (Apaha),

Ashadha-Uttara (Viswadeva),
Shrona (Vishnu),
Shravishtha (Vasava),

Shatabhisha (Indra),
Proshtapada (Ekapada),
Proshtapada (Ahirbudhna),

Revati (Pusham),
Ashvayujau (Ashvinau),
Apabharani (Yama).

Note that in the above list, all the 27 Nakshatras, that we know in Jyotisha, are mentioned. But few are having unfamiliar names. For example, we are familiar with Shravana but the list mentions it as Shrona. Similarly, we know about the Bhadrapadas but here they are mentioned as Proshtapadas.