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About red dwarf stars

Red dwarfs are possibly the smallest of all stars. The list of “negative” records they hold is really impressive. They are the smallest, the least bright and massive, and the coldest of stars. Nevertheless, they also possess “positive” records that counterbalance the previous ones. To wit, they are nothing less that the most long-living and (by far) numerous of stars. They actually outnumber all other kinds of stars added together.

Let’s take a closer inspection to the features of these celestial bodies. Their name, to begin with: what does “red dwarf” mean? Of course everybody knows what is the meaning of such words like “red” or “dwarf”, but these are scientific terms that require a more detailed explanation.

Fig. 1: A modern H-R Diagram. It can be seen that most stars lie on the main sequence plus the giant branch. (From Wikipedia, credit: Richard Powell)

The story beging more or less one century ago. In the years around 1910, two astronomers, Ejnar Hertzsrpung and Henry Norris Russell, independently from one another, were wondering about the possible relations existing between the luminosities and colors of the stars. They knew that color were in their turn related to surface temperatures, as epitomized by the black body laws (that state that the colder/hotter a star is, the redder/bluer its color is expected to be). So the question was: are hotter stars brighter or dimmer than colder ones? To address this issue, the two astronomers built up a diagram that was later called, after their names, the Hertsprung-Russell Diagram or simply H-R Diagram. In it, the abscissa represents the surface temperature and the ordinate the luminosity. Please notice that, unlike what usually happens, the abscissa increases towards the left, i.e bluer stars are on the left, and redder stars on the right side of the diagram. Moreover, notice that, according to the black body laws, the more a star is shifted upwards and to the right, the larger is its radius.

The H-R Diagram appears like the one depicted in Fig. 1. The vast majority of the stars line up in a sort of diagonal that spans form the top left corner to the bottom right one. This is called the Main Sequence, and some 85 to 90% of all stars lie in it. Apart from the main sequence, the most populated region is the branch that from the middle of the diagram moves to the right and slightly upwards and enters the reddest stars domain. Hertzsprung himself suggested to call dwarfs the main sequence stars, and giants the ones in this quasi-horizontal branch (later to be split in the giants and supergiants branches, the latter spannig horizontally in the topmost part of the diagram). Clearly enough, these names are due to the fact that giants are larger than main sequence stars.

Let’s now focus our attention on the main sequence. Not all the so-called dwarf stars are indeed dwarf. The blue and brighter ones can be actually quite large: their radius can attain values around ten times the Sun’s. The term dwarf, in this case, should rather be intended as meaning “smaller than the giants”, whose radius can reach values around 100 times solar, or even 1000 and more in the case of supergiants. The red dwarfs, on the other hand, are actually small. Their radius and their mass are half the Sun’s, or less, and their brightness is merely 1/10 to 1/1000, or even less, as compared to our parent star.

Fig. 2: Comparison of the dimensions of Main Sequence stars of different colors. (From Wikipedia)

Red Dwarfs are also called “M Dwarfs”, the “M” being the letter that is attributed to the coldest stars in the Spectral Classification that was pursued at the Harvard University by Miss Annie J. Cannon, more or less in those same years Hertzsrung and Russell were mulling over the diagram. According to Miss Cannon’s scheme, stars are to be classified from hotter to colder in seven classes labelled by the cap letters O, B, A, F, G, K and M. This is called “spectral classification” because the spectral features (absorpton lines or bands) are in their turn mainly determined by surface temperature of the stars and are hence tied to their color.

But how is a red dwarf made like? Like any main sequence star, it has a hydrogen-fusing core that sustains the stellar brightness, but unlike the other stars, it is thought to be fully or mainly convective, like a boiling pot. Convection is believed to trigger a complex magnetic pattern that in its turn brings about flaring phenomena.

Fig. 3: A picture of Proxima Centauri taken with an infrared (left) and red (right) filter. As usual for cold stars, Proxima is brighter in infrared rather than visible. (Credit: ESO)

In fact, several red dwarfs are flare stars, i.e. stars that undergo unpredictable and quick brightness increases. The prototype of this kind of stars is UV Ceti, the minor component of a closeby binary star constituted by two red dwarfs named Luyten 726-8. In 1952, this star increased its brightness by 75 times in a mere 20 seconds lapse of time.

One further remark, in the end: what about the visibility of red dwarfs in the sky? What should you do to see one with your own eyes? With the naked eye, you can’t: albeit they are by large the most common stars, red dwarfs are so faint they can’t be seen without a telescope.

The closest star to the Sun, Proxima Centauri, is a typical red dwarfs that and it is a mere 4,2 light years away, but it is much dimmer than the dimmest stars visible to the naked eye! Just compare it to such stars like Deneb or Mu Cephei, that are visible althoug their distance is some 2 to 3 thousand light years from us, because they exceed the Sun’s brightness by a factor 1 to 2 * 105!

(Author: Davide Cenadelli)

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