Final answer:
The T₁ scale from brightest to darkest for stars' luminosity classes is Hyperintense, Hypointense. Starlight reflected by dust is reddened, a star behind interstellar gas has absorption lines, and an emission nebula shows bright emission lines in its spectrum.
Step-by-step explanation:
The question seems to be related to luminosity classes of stars, involved in the study of astrophysics, which makes it a Physics-related subject. The luminosity class of a star indicates its brightness relative to others. The sequence provided (Ia: Brightest supergiants, Ib: Less luminous supergiants, II: Bright giants, III: Giants, IV: Subgiants, V: Main-sequence stars) describes these classes from the brightest (Ia) to less bright stars (V). Therefore, the answer to the question 'T₁ scale from brightest to darkest' would be Hyperintense, Hypointense (A).
The spectrum associated with different astronomical objects can provide information about their composition and condition. For example, starlight reflected by dust, typically shows a spectrum that is similar to the star but reddened due to scattering. A star behind invisible interstellar gas likely has absorption lines in its spectrum, characteristic of the gas it's passing through. An emission nebula will have a spectrum with bright emission lines corresponding to the gas composition of the nebula.