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Spectroscopic Atlas for Amateur Astronomers 90

19 Spectral Class M

19.1 Overview

Some of the orange-red shining stars of the M-class are well-known names such as Betel-
geuse, Antares, Mirach (β And), Scheat (β Peg), Ras Algheti (α Her), Menkar (α Cet), Tejat
Posterior (μ Gem). Some late types of this class are long period variables such as Mira
(M7llle).

19.2 Parameters of the Early to Late M-Class Stars

The following table shows the data exclusively for the Main Sequence Stars of the M-class,
compared to the Sun ( ) and according to [701].

Mass Stay on main Temperature Radius Luminosity L/L
M/M sequence [y] photosphere [K] R/R

0.5 – 0.08 >100bn 3,850 – 2,600 0.63 – 0.17 0.08 – 0.001

Striking is here the huge percental increase of the covered mass range compared to the
earlier classes A, F, G, K. The "late half” of this class already touches the range limit of the
Brown Dwarfs, which are separately classified with L, T and Y (not be treated here). As the
K- also the M-Dwarfs stay longer on the Main Sequence than the estimated age of the uni-
verse of some 13.7 billion years. This means that in the entire universe, not a single star of
the M-class is migrated to the Giant Branch in the HRD! All with the naked eye visible M-
type stars are former class B to G Main Sequence Stars which spend here only a relatively
short time until the end of their Giant Stage. Their parameters are therefore far outside of
the table values above.

With the naked eye, no real M-type Main Sequence Stars are visible, although they provide
76% of the Sun’s near neighbours! The most famous and apparently “brightest” representa-
tive is in the southern sky Proxima Centauri with the spectral class M5 Ve. With a distance
of 4.22 ly it’s the very nearest neighbour to the Sun, but reaching an apparent magnitude of
only 11m! Therefore, it was discovered very late in 1915.
The graphic (Wikipedia) shows the proportion of Proxima
Centauri compared to the Sun. Its diameter was determined
with the HST to about 200,000 km. His life expectancy on
the Main Sequence is estimated to some
4 trillion years! Whether our universe then still exists – in
whatever form – must be answered by cosmological models.

19.3 Spectral Characteristics of the M-Class

The Fraunhofer H + K lines remain striking in the entire M-class. In addition to the Hα line
neutral calcium Ca l at λ 4227 and the sodium double line at about λλ 5890/95, are the
dominant discrete absorptions, which are still visible at this resolution. The once prominent
G-band breaks now up in at least three discrete lines (BSA). The main features here are un-
doubtedly the huge absorption bands of titanium oxide TiO. Their intensity rises signifi-
cantly towards the late subclasses, overprinting now thousands of neutral, atomic absorp-
tion lines which would otherwise visible here [1]. Although they show up already in the late
K-classes, they form here the unmistakable "Brand" of the M-category. Much less frequent
and with smaller intensity we find here a few absorption bands of CaH (calcium hydride)
and MgH molecules (magnesium hydride). In the late M-subclasses the lower temperature