DX Cancri

Also known as DX Cnc, GJ 1111, G 51-015,

LTT 18058, LHS 248, GCTP 2016.01, and USNO 311

     DX Cancri, at 11.8 ly distance the 18th closest star system to our own sun, is unfortunately the first star on our list, as it is also the hardest to find. Being a red dwarf of only 9% of the mass of our Sun and 0.00132% of its visual luminosity (and 8% of total luminosity), DX Cancri weighs in at a measly magnitude 14.78, or more than a full magnitude dimmer than what I can reasonably spot with my 5-inch (132 mm) refractor on a good night in light-polluted Howard County.

     DX Cancri is also the first of many flare stars that we will encounter throughout our year-long adventure.  Since it can increase in brightness by as much as 500% over an extremely short interval, one might just be lucky enough to catch the star during one of its intermittent and unpredictable flares, when it would shine at magnitude 13.1. Now that I’d might be able to see (just barely), without having to borrow somebody else’s gear.

     Since several of the objects we’re going to encounter in our exploration of the solar neighborhood will be, like DX Cancri, a red dwarf flare star, now is as good a time as any to explain just what such a star is like. (Brace yourself. Here comes the “science” part of this posting.)

     Red Dwarfs are the most common star type in the universe. More than half the stars in the Milky Way are red dwarfs, as are 19 out of the 33 closest stars to the Sun. None are visible to the naked eye from the surface of the Earth (although one comes close). 

     Red dwarfs are main-sequence stars of Spectral type M, which means that its surface temperature is below 3,700º Kelvin (our own Sun, for comparison, has a surface temperature of 5778º Kelvin).  They rarely have more than about one third the mass of the Sun, and are generally much less massive than that (CX Cancri being a case in point, with only 9% of the Sun’s mass). As with all main sequence stars, a red dwarf’s outward pressure from the fusion processes within its core is perfectly balanced by the gravitational force of the star’s total mass, a state referred to as hydrostatic equilibrium. 

     As in every star, the energy being generated in the core needs to somehow get out. In our own Sun, this energy escapes by two means: convection and radiation. But a red dwarf manages to get by on convection alone. Radiation is not an option, since material within a red dwarf is too dense for its temperature to avoid being opaque to energy from the core. This has huge implications for the star’s life cycle. Being fully convective, there is a thorough and continual remixing of 100% of the stellar material extending from very center all the way to the very surface. This prevents a buildup of helium (which for our purposes can be considered as a nuclear “ash” left over from hydrogen fusion) within the core, resulting in a far more efficient usage of the star’s available hydrogen for energy production. This greater efficiency, combined with a far slower rate of consumption of hydrogen fuel (compared to more massive and hotter stars), results in a red dwarf existing in an essentially changeless state, with practically no long-term alteration in either temperature or luminosity for many thousands of billions of years. (Theoretically, of course. Since the universe is less than 14 billion years old, such enormous life spans must be inferred from modeling.) So keep in mind when observing a star of this type that you are looking at an object which will likely outlast the universe (or at least the galaxy)! Long after the Sun, the Earth, and the rest of our solar system are but a distant memory in the long history of the Milky Way, the typical red dwarf star will still be in its infancy, with its perhaps 10 trillion year long maturity still to look forward to.

     But however awe-inspiring such longevity might be, one must admit that it probably doesn’t add much to the visual experience of actually observing a red dwarf. Far more exciting for the casual stargazer might be the fact that so many of them are flare stars, (also known as UV Ceti variables – we’ll get to the star UV Ceti in a later posting). Flare stars are red dwarfs that experience a dramatic increase in luminosity (by as much as 5 magnitudes!) across the spectrum within a few minutes or hours. In the case of DX Cancri, its apparent magnitude can spike from 14.78 to 13.1 in the time it might take you to read the remainder of this posting.

     What makes a red dwarf a flare star? It turns out that there is nothing particularly exotic about what makes them tick. Our own Sun has solar flares all the time, and they are very similar to the flares which cause the dramatic luminosity increases for flare stars. The big difference is mainly the much greater intrinsic luminosity of the Sun compared to a red dwarf. Even the largest solar flares add only an insignificant amount of light to the total luminosity of the Sun, whereas the same flare would be quite a noticeable addition to the far dimmer normal output of a red dwarf, which might have as little as one ten-thousandth solar luminosity. Add to this the fact that, for uncertain reasons, solar flares on a red dwarf tend to be far more massive in comparison with the stars on which they occur than is the case with the Sun, and you can see why the relative increase in brightness is so much greater. But that is not all. Compounding the tendency of red dwarfs toward extremes in luminosity is that they can frequently experience truly gargantuan starspots (what we call “sunspots” on our own star), that can at times cover as much as 40% of their surface and thus contribute significantly toward dimming episodes.

     As exciting as all this can be for the visual observer hoping for something to make that tiny reddish dot in his field of view at least a little more interesting, it is terrible news for anyone hoping to find potential “New Earths” out there. The potential habitable zone (the distance from a star at which an Earthlike planet could support life) around a typical red dwarf is less than 0.04 astronomical units (about 3,720,000 miles) from the star. Such a close orbit would subject any theoretical planet to frequent, lethal dosages of radiation during flare episodes, not to mention the erratic luminosity associated with them. Plus, such a tight orbit (a “year” for said planet would be only 10 Earth days long!) would most likely result in a situation analogous to the Earth-Moon system, where the orbiting body is tidally locked to the parent one, keeping one hemisphere forever facing the star, and the other in perpetual darkness. And what light that does reach the planet’s surface would be skewed (from our perspective) toward the infrared end of the spectrum, making photosynthesis problematical, should that turn out to be a necessary requirement for life.

As it is, no planets have as yet been observed around DX Cancri.