M Stars: Definition

The defining characteristic of an M type spectrum is the presence of absorption bands of titanium oxide molecules. The M dwarfs has a temperature range of 2,500-3,500K, cover the mass range from 0.08 to 0.5 solar masses and have radii between 0.1 and 0.6 times that of the Sun.

M Stars: Description

The hottest M type dwarfs have a surface temperature of 3,500K; define the lower end of the main sequence. According to theory, stars with masses less than 0.08 solar mass do not ignite hydrogen burning nuclear reactions in their interiors; they simply contract and cool as Brown Dwarfs. As a result, the main sequence terminates at its low end with stars having surface temperatures of about 2,500KAt their most massive they have a luminosity of nearly one tenth that of the Sun, but this diminished rapidly at the lower masses, to become less than one thousandth of the Sun’s luminosity at the bottom of the main sequence. As these very low-mass stars are also very cool, most of their radiation is emitted in the infrared and their visual luminosity decreases even more precipitously; from one-twenty-fifth of the Sun for the hottest M dwarfs to one-ten-thousandths for the coolest.

The lifetime of the M stars on the main sequence are very long, even greater than the age of the Galaxy. As a result all of the M stars formed during the life of our galaxy are still with us. This, and the fact that the process of star formation makes far more low-mass than high-mass stars, means that they are the most common of the dwarf stars. Because of their limited individual luminosities, they don’t contribute greatly to the total radiation of the galaxy. However, they are so numerous that almost all the luminous material in the Galaxy is in the M dwarfs.

All of the M dwarfs (with the exception of the small number in short-period binary systems) are slow rotators; their periods range from a few days to a few months. From computations of the structure of low-mass dwarfs it is found that convection in the envelop extends deeper at lower masses, until at 0.3 solar mass it reaches the center of the star. Consequently, M dwarfs cooler than about 3,500K have their interiors completely mixed.

At the low temperatures of M star atmospheres molecules can for and remain intact. In the infrared spectra of M stars, molecular bands of CO and OH are strong and at temperatures below 3,500K bands of H2O become strong. Certain other bands such as CaH (calcium hydride), have strengths that are sensitive to the pressure in the atmosphere in the star and therefor are useful as indicators of the luminosity of the star. The high luminosity and low surface gravity of M giants and super-giants causes a wind of gas in which mass loss can be as high as one solar mass in 100,000 years. In the relatively cool environment surrounding the M stars, molecules and solid particles quickly condense from the expanding gas shells. At the highest rates of mass loss the stars themselves are hidden from view by the resulting dust shells, but can be detected by their far infrared radiation or OH microwave emission (OH/IR stars). At lower rates the M star is visible, but the spectra bands of the dust particles, which are mostly composed of silicates, appear in the infrared. The shells themselves, whose temperatures are only a few hundred Kelvin, are detectable from their far infrared radiation. They also can be observed from the light they scatter: the shell around Betelgeuse is in this way observed to have a diameter of three minutes of arc. At the low velocities of expansion of the shells (typically 6m/s to 10km/s) material at the outer edge of this shell left Betelgeuse 10,000 years ago.

As with the K giants, the M giant stars can at some stages in their evolution dredge up nuclear-processed material from their interiors. This material is rich in carbon and heavy elements and when sufficient has mixed into the interior of the M star its spectrum and hence classification changes. In this way, many M giants pass through the N-type or S type spectral classes.

For all stars that reach the M giant or M supergiant phase, these represent their final period of life as an extended star, at the end of which their envelopes are ejected, leaving the hot core to settle down as a degenerate star. The process can be a spectacular supernova explosion for the massive supergiants, or a quieter event producing a planetary nebula in the lower mass stars and so Wolf-Rayet stars.

M stars appear red to the eye. There are no M dwarfs visible to the naked eye. Bright examples are Betelgeuse M2Iab, Gamma Crux M3II, Antares M1Iab, Alpha Ceti M2III, Mu Chephei M2Ia.

Once they have exhausted their fuel, they will simply cool to large planet like cinders. No such bodies have been found.
There has not been enough time since the universe began for them to form.*Bodies smaller than M8 stars cannot
become stars. They do not develop enough pressure and temperature to sustain fusion. They are classified as
Brown Dwarfs and Gas Giant Planets.

M Stars: Variability/Peculiarity

M-type giants and super-giants are all evolved stars with masses much larger than those of the M dwarfs. There surface temperatures are typically a few hundred degrees cooler than dwarfs of the same type. The most luminous M super-giants rival the energy radiated by the luminous hot stars (up to one million times the luminosity of the Sun). As the temperatures of M super-giants are so much lower it follows that they must have enormous surface areas. The largest have radii several thousand times that of the Sun – as large as the orbits of Saturn and even Uranus around the Sun. Even the coolest M stars have radii as large as that of the Earth’s orbit. Such distended stars are unstable so the M super-giants and most of the M giants are irregular or semi-regular variable stars. For example the bright star Betelgeuse (M2Iab) varies with a range of about half a magnitude on a timescale of years. Many M stars vary in brightness more regularly and are known as long-period variables, or Mira variables, with periods ranging from 200 to 1,000 days.

Variability of a different kind is also common among the M dwarf stars. The surface activity visible in the Sun – spots and flairs – associated with magnetic fields is common among all low mass stars, but the very low luminosities of the M dwarfs make such activity more readily observable. Most low-mass stars are flare-stars (UV Ceti) or spotted stars (BY Dra variables) with dMe spectral types. The emission lines of ionized calcium, which on the Sun are associated with regions of magnetic activity, are strong in the dMe stars and observations made over many years show that intensities vary regularly, enabling both the rotational periods of the stars to be measured and also longer periods, probably analogous to the sunspot cycle in the Sun.