(1) Sapt means seven, and Rishi means very learned Braahman, so Sapt Rishi or Saptarshi seven very learned Braahman.

(2) But there are Sapt Rishi who are seven stars and stay with Dhruv Taara (Polar Star) also. In English the group of these seven stars is called Ursa Major. In fact in every Manvantar, these Sapt Rishi are different (see their list in different Manvantar and even listed by different sources here).

The Sapt Rishi are mentioned as early as in Rig Ved as under: Rig Ved, Book 10, Hymn 130.
The verse from 5 to 7 run as indicating the reference to 7 sages, but they were told as known but never named. The 7th verse means - 7 They who were versed in ritual and meter, in hymns and rules, were the Seven Godlike Rishi. Viewing the path of those old sages have taken up the reins like chariot-drivers.

The earliest list of the Sapt Rishi is given in Jaimineeya Braahman 2.218-221:
(1) Vashishth,  (2) Bharadwaaj,  (3) Jamadagni,  (4) Gautam,  (5) Atri,  (6) Vishwaamitra, and  (7) Agastya.

This list is followed by Shatapath Braahman and Brihadaaranyak Upanishad, 2.2.6 with a single different list -
(1) Vashishth,  (2) Bharadwaaj,  (3) Jamadagni,   (4) Gautama,  (5) Atri.   (6) Vishwaamitra,  and  (7) Kashyap,

Krishn Yajur Ved in the Sandhyaa Vandan Mantra lists the seven names likes this --
(1) Angiras,  (2) Atri,  (3) Bhrigu,  (4) Gautam,  (5) Kashyap,  (6) Kuts,  and  (7) Vashishth

Mahaabhaarat gives the seven Rishi names like this --
Agastya, Atri, Kuts, Vashishth

The following 7 names of Sapt Rishi appear in Rig Ved under creation. Hence the Manu of every Yug is deemed to have had 7 Rishi as safe guard for Ved and hence different Rishi mentioned as the protector for each Manu and his Manvantar. Vaivaswat Manvantar, the current Manvantar, is
supposed to have Brihat Sanhitaa gives the seven Rishi names like this--
Mareechi - Aachaarya of Punishment code,
Vashishth - Guide to Puraan, Aachaarya of Yog Vidyaa, Guru of Raam
Angiras - BHakti Darshan and writer of Smriti
Atri - lived in South, wrote Atri Sanhitaa
Pulastya - his son was Vishravaa, his grandsons are Kuber and Raavan etc, told Vaaman Puraan to Naarad,
Pulah - famous Shiv Bhakt, built Pulaheshwar Shiv Temple in Kaashee
Kratu - divided Ved in Varaah Kalp

The late Gopath Braahman, 1.2.8 has the following Sapt Rishi -
(1) Vashishth,  (2) Vishwaamitra,  (3) Jamadagni,  (4) Gautam,  (5) Bharadwaaj,  (6) Agastya, and  (7) Kashyap.

In post-Vaidik texts, different lists appear; some of these Rishi were recognized as the 'mind-born sons' (Sanskrit: Maanas Putra) of Brahmaa, the Supreme Being as Creator. Since these seven Rishi were also among the primary eight Rishi, who were considered to be the ancestors of the Gotra of Braahman, the birth of these Rishi was mythicized.

This difference in the names is because they were the Sapt Rishi at different points of time. Every Manvantar has its own set of Sapt Rishi