Answer:
Recall that stars are classified with types from the spectra sequence OBAFGKM. In general, an ordinary (hydrogen-fusing, or main-sequence) star of spectral type A is hotter than a star of spectral type K.
Explanation:
The Harvard classification system (OBAFGKM) is a sequence of temperature, in which type O correspond to a star with an effective temperature around 50000K and type M to a star with an effective temperature of 3000K. This classification is made by an analysis of the spectrum¹ of the star and by searching for spectral lines which are dependent to temperature.
Each spectral type will have different lines that are stronger as a consequence of the temperature of the type. For example, the Hydrogen lines are more intense in the type A than in others spectral types (OBFGKM).
It is important to know that the spectral lines in a star's spectrum are created when the light coming from the inner parts of the star interacts with the elements that are in its atmosphere.
Since the letters to the left represents higher temperatures that the ones to right in the Harvard classification system (OBAFGKM) it is correct to say that a star of spectral type A is hotter than a star of spectral type K.
Key terms:
¹Spectrum: Decomposition of light in its characteristic colors (wavelengths).
