Which of these stars has the coolest surface temperature? (a) an A star (b) an F star (c) a K star.
The Correct Answer and Explanation is:
The correct answer is (c) a K star.
The surface temperature of a star depends on its spectral type, which is determined by its color and the characteristics of its emitted light. The spectral classification system ranks stars from hottest to coolest using letters: O, B, A, F, G, K, and M. Stars are grouped into these spectral classes based on their temperature and the types of absorption lines present in their spectra. The classes are divided further into subclasses numbered 0 through 9, where 0 represents the hottest stars and 9 the coolest within each class.
- A stars are relatively hot stars with surface temperatures ranging from about 7,500 to 10,000 K. These stars emit a bluish-white light, and their spectra show strong hydrogen absorption lines.
- F stars have slightly cooler surface temperatures than A stars, ranging from about 6,000 to 7,500 K. They appear yellowish-white and have weaker hydrogen absorption lines than A stars but still show some metallic lines in their spectra.
- K stars are cooler than both A and F stars, with surface temperatures ranging from about 4,500 to 6,000 K. These stars appear orange and have prominent molecular bands, particularly from molecules like titanium oxide (TiO), which are more noticeable at lower temperatures.
Thus, K stars have the coolest surface temperatures compared to the other options provided (A and F stars). Stars with cooler temperatures like K-type stars are typically red-orange in color, as their surface temperature is lower, and they radiate less energy in the blue and violet parts of the spectrum. This cooler temperature also affects the type of absorption lines observed in their spectra, which are dominated by molecules rather than ions or neutral atoms, as seen in hotter stars.