There's a slight misunderstanding in the question. Stars are generally classified by their spectral type, which is determined by their surface temperature. Group 2 and 3 stars are not a recognized classification in stellar astronomy.

Here's a breakdown of how stellar classification works:

* Spectral Types: Stars are categorized based on their spectral lines, which are unique patterns of light emitted due to different elements present in their atmospheres. The primary spectral types are:

* O: The hottest and bluest stars.

* B: Hot, blue-white stars.

* A: White stars.

* F: Yellow-white stars.

* G: Yellow stars (like our Sun).

* K: Orange stars.

* M: Red, cool stars.

* Main Sequence: The majority of stars spend most of their lives on the main sequence, a diagonal band on the Hertzsprung-Russell diagram. Stars on the main sequence are fusing hydrogen into helium in their cores, and their luminosity and surface temperature are directly related to their mass.

Why are there no "Group 2 and 3" stars?

There's no established system for grouping stars based on the numbers 2 and 3 in stellar astronomy. The classification system is based on spectral type and other characteristics. It's possible you might be thinking of something else, or the information you encountered is inaccurate.

Let me know if you have any more questions or would like to explore specific aspects of stellar classification further.