ASTRONOMY FUNDAMENTALS - Chapter 4, Exercise 1 Solution
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Classifying a Star by Temperature

PROBLEM
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A star has a measured surface temperature of 4,500 K. Determine its
spectral class using the chapter's own OBAFGKM temperature table, and
name one real example star from that same class.

SOLUTION
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Checking the real temperature ranges from the chapter's table:

    O: >= 33,000 K
    B: 10,000-33,000 K
    A: 7,300-10,000 K
    F: 6,000-7,300 K
    G: 5,300-6,000 K
    K: 3,900-5,300 K      <- 4,500 K falls here
    M: 2,300-3,900 K

4,500 K falls within the K class range (3,900-5,300 K).

ANSWER: This star is a K-class star. Real example stars from this
class, per the chapter, include Sigma Draconis and Epsilon Eridani.

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WHY THIS WORKS AS AN ANSWER

Spectral classification is fundamentally a temperature-ordering system
- the OBAFGKM sequence runs from hottest to coolest, and each letter
covers a specific, non-overlapping real temperature band. Placing 4,500
K correctly requires checking it against each band in order until
finding the one range it actually falls inside: it's too cool to be G
class (which requires at least 5,300 K) but too hot to be M class
(which tops out at 3,900 K), leaving K class as the only real range
that fits. This is exactly how astronomers use a star's own measured
temperature - typically determined via the spectroscopy techniques
covered in Chapter 1 - to assign it a spectral class in practice.