Do you know what is star magnitude/apparent brightness?
It is a measure of how bright objects look from earth – very subjective, also nearer stars look brighter but are not necessarily actually brighter than those farther away.
Number 6 or less means that the object should be visible to the naked eye. Note: Sun is -26.8 magnitude and full moon -13.
Astronomers measure brightnesses of stars using a refined version of the magnitude system introduced by the Greek astronomer Hipparchus about 130 BC. Hipparchus ranked the visible stars from first to sixth magnitude, with first being brightest. The magnitude system was elaborated and made quantitative after it became possible to measure accurate relative brightnesses of stars with telescopes and auxiliary instruments. The system is logarithmic because biological systems like the eye respond in proportion to the logarithm of the stimulus.
Image: this image is made by me, original picture here: http://astropixels.com/stars/Aldebaran-01.html and here: http://spot.pcc.edu/~aodman/physics%20122/lecture1/lecture1.htm