Answer
See below.
Work Step by Step
Figure 30.7 shows the radiation curve of an incandescent source. It spans a broad band of frequencies. A star that you see as violet-hot must have its peak frequency in the ultraviolet, with a lot of emitted light in the higher-frequency (blue-violet) part of the visible spectrum. If the star were any cooler, with its peak frequency in the violet range, it would emit significant light in all parts of the visible spectrum, and appear to be white, not violet.
A star with peak frequency in the ultraviolet emits enough light in the higher-frequency part of the visible spectrum to appear “violet-hot.” As in the previous exercise, if it were cooler, all frequencies would be more balanced in intensity that would make it look whiter.
