The Castor-and-Pollux Gemini Dioscuri system contains several red dwarfs classified as type 'dM1e' stars.

I believe the d is dwarf, the M is M-type (redundantly designating a red dwarf?).

What do the '1' and 'e', specifically, stand for?

  • $\begingroup$ What do you mean by "The Castor-and-Pollux Gemini Dioscuri system"? Castor and Pollux look close to each other as seen from Earth, which is why they are named after the twins in Greek mythology. But they are to entirely separate star systems. Castor is about 50 light years from Earth and contains six stars, while Pollux i a single star 34 light years from Earth. $\endgroup$ Mar 25, 2023 at 18:15

1 Answer 1


The 9 main spectral types (the classical OBAFGKM ones plus the more recent L and T ones for brown dwarfs) indicate the general features seen in the spectra and are in descending of temperature (O is hottest, T is coolest). Each spectral type is subdivided from 0 to 9 e.g. M0, M1, M2... M9, again in decreasing temperature order.

Normally there is then a luminosity class which is a Roman numeral from I (supergiants) to V (dwarfs) except in this case, they have indicated the dwarf nature with a 'd' prefix instead. (Since you could have a M giant e.g. a M1III with similar surface temperatures and therefore spectral appearance, the M-type doesn't redundantly designate a dwarf star).

Finally there is a series of suffix letters for unusual features in the spectra. In this case this is 'e' for "emission lines present" - there is a table of these, along with brief summaries of the other parts of the spectral type classification at this site.


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