Caption: The Greek alphabet of 24 Greek letters: alpha α, beta β, gamma γ, ...

alpha, beta, ... --- you get it?

1. α, β, γ, and δ turn up in Bayer designation system for bright stars and in designating the hydrogen Balmer lines.

2. θ is the conventional symbol for angle.

3. λ is the conventional symbol for wavelength.

4. μ is the metric prefix meaning 10**(-6).

5. π the letter is the symbol for the irrational number Pi: π = 3.14159265358979 ... (see One Million Digits of Pi).

6. And others as need arises.

7. Actually all small Greek letters are used in the aforesaid Bayer designation system to label stars in a given constellation in order of their apparent magnitude (approximately their apparent V magnitude): the brightest one was called α, the 2nd brightest one was called β, etc. The ordering by brightness is NOT always correct. Johann Bayer (1572--1625) and other early implementers of the Bayer designation system did NOT work at modern accuracy/precision. When these implementers ran out of Greek letters, they used Roman letters and in later times numbers were used too. So omicron ο is used astronomy even though it looks almost like o.