The stimulus intensity that is detected half the time, on average, is called the threshold.

The answer to this question is: absolute

Absolute threshold is the lowest intensity of a stimulus that can be detected by an organism. This normal value was determined by taking data of the lowest intensity of the populaion, then taking the 50% percentile value of the data. That means the absolute threshold would be right for half of the population. 

0 = 0.40 / 0 = 0

= 0.20 = 9.80 ⁄2 =?

=0 , = = 0.20 =0.50 0 0.40 ⁄

=0−122 =0−12(9.80 ⁄2)(0.50)2 =1.2

We can infer that star y is closer but other factors must be considered including the heat of the star, the mass, and the brightness of the star. even though star x seems dimmer it may just be less bright or contain less heat.