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I have need to say the spectral type of a star for a poster presentation I'm making, which includes the luminosity class. However, I've been unable to determine the standard way of doing so, and searching the internet has been fruitless. The spectral type is M4 V. I don't know whether I should say Em four vee or Em four five. I thought the first sounded better, but then I thought, "What if it were M4 III instead?" After all, Em four eye eye eye is a bit clunkier than Em four three, and consistency is important. Is there a standard way to say these aloud, and if so, what is it?

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The luminosity classes are indicated by Roman numbers. So you pronounce them as numbers if you don't spell out the actual name of the luminosity class you are referring to. Pronouncing them as letters would sound wrong and - at least to me - incomprehensible. The preferred way is to actually use the luminosity class name like 'dwarf' for V etc.

More generally, it helps comprehension often A LOT when you talk in actual words or names of the quantities and not - as often done when referring to physical formula - in terms of the symbols commonly used for describing the quantities, like saying "luminosity equals temperature to the fourth power times stefan-boltzmann constant times area" instead of "L equals sigma times A times T to the power of 4". For one it's easier to follow as no transfer is needed from 'vee' to 'luminosity class: dwarf', on the other hand it reduces ambiguity. That's especially helpful when a person had their thoughts wander for a second, and as every symbol is overloaded with many common meanings, making them helps to not immediately loose track.

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Usually when spoken aloud, you say the actual description of the class, e.g. "main sequence", "giant", "sub-giant", and so on. In this case you would say "Em four main sequence."

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    $\begingroup$ Usually I just say M four V but I guess your answer is in layman's terms. +1 $\endgroup$ Oct 25 at 12:40

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