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Saturday, May 1, 2010

Red Dwarf Stars

“It may be that the old astrologers had the truth exactly reversed, when they believed that the stars controlled the destinies of men. The time may come when men control the destinies of stars.”

Arthur C. Clarke (First on the Moon, 1970)

As you get to know your stellar neighborhood, you will find that more than two-thirds of the nearby stars are red dwarfs. These stars are much smaller, cooler and dimmer than Sol (our Sun). Because they are so dim, even those nearest to us cannot be seen without a telescope. We might have a slightly better chance of seeing them if we could see in the infrared part of the spectrum, where they emit most of their energy.

Since they are so plentiful, red dwarfs (not dwarves!) and their planetary systems might seem to be good places to look for extraterrestrial life. They might also be promising for colonization. These are the subjects of a lot of debate recently, and there are good arguments to be made on both sides.

Red dwarfs may be small and dim, but they are hardly boring. For one thing, most of them are prone to violent convulsions. These are usually called flares, but they are much bigger, compared to the size of the star, than solar flares. A red dwarf flare can increase its power output fivefold or more, for a period of a few seconds or minutes. This would make life very exciting and dangerous for space travelers who venture too close.

Cool stars, like red dwarfs, burn their nuclear fuel so slowly that they last a very long time – the smaller, the longer. The smaller ones have lifetimes measured in trillions of years. Provided the Universe last that long! They cool gradually as they age, the flare activity settles down, and when they finally run out of fuel, they will turn into black dwarfs. (Sol will go through a red giant stage, then a white dwarf stage, and after many billions of years, it will also end its life as a black dwarf.) There are no black dwarfs yet, though. According to theory, at least. The Universe is not old enough yet to have produced any. Just wait.

Red dwarfs are so common in our immediate vicinity that, assuming our stellar neighborhood is not abnormal, it seems reasonable to extrapolate to the galaxy as a whole. Astronomers think that at least 80% of the stars in our Milky Way galaxy are red dwarfs. Even with powerful telescopes, we can usually only detect red dwarfs within a few hundred light years. Some of the nearest ones were only discovered in the mid 20th century, and there are undoubtedly many more, very near, but very dim, lurking in our stellar neighborhood. The number of star systems known to be within 33 light years has increased by 18% since 2000. That’s not because of newly discovered stars. Most of these stars were known before, but we didn’t know how close they were.

Whether there may also be an abundance of brown dwarfs – smaller bodies that have less than the 0.08 solar mass needed for sustained nuclear reaction – is more uncertain. They are not actually stars, but they do emit a small amount of infrared radiation. If they exist, that is. But it wouldn’t surprise anybody if it turned out that there were a few dozen (or more!) brown dwarfs closer to Sol than any of the stars. They would be practically invisible, and we would only know about them by their gravitational effects.

It’s pretty safe to say that the closest star to Sol, at a distance of 4.2 light years, is Proxima Centauri, a red dwarf. It is small and dim, even by red dwarf standards! With diameter approximately 200,000 km, it is somewhat bigger than Jupiter (diameter 143,000 km); it is 110 times as massive, and 36 times as dense. At 51 grams per cubic centimeter, it is 4.5 times as dense as lead. Its temperature is around 2,800°C, which is pretty cool for a star.

Like all red dwarfs, Proxima is composed almost entirely of hydrogen. In fact, since many red dwarfs are thought to have been born in the early Universe, when elements heavier than hydrogen were rare, it is somewhat of a mystery why there aren’t more red dwarfs with even lower amounts of heavier elements.

Red dwarfs convey heat from the core, where hydrogen is being converted to helium in a fusion reaction, to the surface by convection, rather than radiation. They are fully convective, like a pot of boiling liquid. They are not massive enough to generate the pressure at the core required to fuse helium into heavier elements, as the larger stars do.

When you start making a small planet, as you put in more mass, the diameter gets larger, and the pressure in the interior increases. When the pressure becomes high enough, the chemical bonds are overcome, and the material compresses. Adding more mass after this actually makes the planet smaller. So there is a maximum diameter, which is about 180,000 km, for a pure hydrogen planet. That’s about 0.13 times the diameter of Sol, or 1.3 times the diameter of Jupiter. So if you keep adding mass, the planet shrinks and heats up, until finally it ignites in a thermonuclear reaction, and starts converting hydrogen into helium. Now it is a star, and its size can increase, because it gets hotter, and because it keeps collecting matter that falls into it.

The next closest stars are Alpha and Beta Centauri, which, along with Proxima, form the Centauri system. Alpha is a Sol-like star, just a little bigger and brighter than Sol, and Beta is a reddish-orange dwarf, a little smaller and dimmer than Sol. They form a close binary pair, and Proxima orbits this pair at such a great distance that no one is really sure whether it is gravitationally bound to the system. In other words, Proxima may continue on a very large elliptical orbit around Alpha and Beta, with a period of roughly half a million years, or it may continue off on its own. At its current speed of 31 km/sec, it is just a bit faster than Earth, at 29.3 – 30.3 km/sec, relative to Sol.

Next in the list of nearest stars, at 6.0 light years, is Barnard’s Star, another red dwarf. It appears to be an old star, perhaps 11 – 12 billion years, more than twice the age of Sol. A little bigger and hotter than Proxima, it will probably only be another 40 billion years or so before its nuclear reaction stops, and it becomes a black dwarf. Although relatively old and quiet, it had a small flare event in 1998, so it has been designated as a flare, or variable star. With a surface temperature of about 3,100°C, it is an average red dwarf.

Barnard’s Star is moving at 142 km/sec, almost 5 times as fast as Proxima, and it will pass within 3.8 light years from Sol in about 10,000 years. Because of its high speed and nearness, Barnard’s Star has the largest proper motion of any star. In other words, its angular position, as seen from our solar system, is changing faster than any other star: 10.3 arcseconds, or 0.00286 degrees, per year. In 200 years, it will have crossed more than the width of the full moon, in angular position.

The next closest star, at 7.78 light years, is Wolf 359, another red dwarf. This one is probably about the same size as Proxima Centauri. It’s named after its discoverer, Max Wolf – not because of any reference to wolves! It is also known as CN Leonis. According to currently used models of stellar evolution, Wolf 359 appears to be a vey young star, somewhere between 100 and 350 million years. On the other hand, it has a slow rotation rate, which we can detect because rotation blurs the spectrum, through the Doppler Effect. This would seem to indicate an older star, since it takes a long time for magnetic effects to slow the rotation. Its radius is 1.6 times Jupiter’s radius, so its volume is about 4 times Jupiter’s volume. But its mass is 90 times Jupiter’s mass, making Wolf 359 about 22.5 times as dense as Jupiter, almost 30 times as dense as liquid water. Its rotation period is at least 67 hours, compared with 10 hours for Jupiter. Coincidentally, it is very close to the ecliptic, which is the plane containing the Earth’s orbit around Sol.

Next in the list, at 8.26 light years, is Lalande 21185. This is a larger red dwarf, with 0.46 times the mass of Sol, and also 0.46 times the radius of Sol. That makes it 10.3 times as dense. That’s much less dense than the smaller red dwarfs. The large size is supported by higher temperature, as indicated by the surface temperature of 3,400°C. Most of the other nearby stars are moving more or less with Sol, in their orbits around the galactic center. They were born in the “thin disk” of the galaxy, and will remain there, along with Sol, for billions of years. But Lalande 21185 is moving in a direction perpendicular to the disk, so it is just passing through our stellar neighborhood.

The brightest star in the sky is Sirius, only 8.6 light years distant. Sirius is actually two stars: Sirius A, which is about twice the mass of Sol, with 25 times the luminosity, and Sirius B, a small white dwarf companion. A whole book could be written on Sirius, but in this article, I want to talk about red dwarfs, so I just mention Sirius in passing.

Next, at 8.73 light years, is a binary system, Luyten 726-8. The two stars in the system are both red dwarfs, both about 0.1 times the mass of Sol, and both about 0.14 times the radius. Both have very low luminosity. They orbit their common center of mass in a highly eccentric orbit, coming as close as 2.1 AU and as far apart as 8.8 AU, with a period of 26.5 years. Luyten 726-8A is not much different than the other red dwarfs we have seen so far, besides being a bit smaller and dimmer. But its companion, Luyten 726-8B, also known as UV Ceti, is a remarkable flare star, exhibiting very violent outbursts of ultraviolet radiation. In 1952, within a period of 20 seconds, its luminosity increased by a factor of 75.

I’ve been talking about distances to stars, and their diameters, speeds, temperatures, etc; as if these are things we can measure. In practice, what we can actually measure very precisely is the angular position, as viewed from Earth. That takes 2 variables to describe – think of latitude and longitude. By making observations over a period of time, we can also determine the rate of change of a star’s angular position, or proper motion, as mentioned above. That’s 2 more variables. Now we have 4 variables that can be determined with very high precision. We can also measure the intensity of light reaching us from a star, in any band of wavelength. Putting this information together gives us the electromagnetic spectrum of all the light coming to us from the star. The spectrum can be determined very precisely, but it can change over time.

Astrometry, which involves determining the positions and motions of astronomical objects, is one of the most important parts of astronomy. As a general rule, distances are more difficult to determine than angles measured from Earth.

Using observations made from widely separated points in Earth’s orbit, the angular position of a star changes with the position of the observation. In other words, we can imagine a very acute triangle with the star at one vertex and the two observing points at the others. If we can work out the geometry of this triangle, then we know the distance to the star. It’s a pretty easy trigonometry problem. However, most stars are moving faster than the Earth moves, relative to Sol. Hence a star may move significantly between observations. The triangle gets a little bent out of shape, and you have to take this into account. This is called the method of parallax. It allows us to determine the distances to nearby stars to within a few percent, or better, for the nearest stars. It only works for objects within a few hundred light years, at most. We have Proxima Centauri pinned down to 4.243 ± 0.002 light years. For more distant objects, other methods must be used. In recent years, parallax measurements made by the satellite Hipparcos have greatly increased the accuracy of our stellar geography. Several ground-based and space-based efforts are under way or planned for the coming decade, which will greatly increase the accuracy of our stellar geography. Astronomers will go to great lengths to determine distances!

The surface temperature of a star can be determined pretty accurately (we think) by its spectrum. Hot stars, as hot as 10,000°C or more, radiate most of their energy at high frequencies, so they look blue. Yellow stars, like Sol, are medium on the temperature scale, around 5,500°C. Red dwarfs are on the cool side, at less than 3,500°C, which is why they appear red. Stars are usually much hotter in their interiors, due to nuclear reactions, but the surface, or photosphere, is where the light that reaches us comes from.

Given the distance and the intensity of all the light coming from a star, we can calculate how much total energy it is putting out in the form of light. That’s called the bolometric luminosity, L. We use a model (a mathematical equation) relating L with the surface temperature and radius. Basically, the model says that L is proportional to the square of the radius and the 4th power of the temperature. Of course, this is only a model, and it could be way off. We don’t have a really good way to test it. But it’s the best we have, so we use it to calculate the radius of a star, given its surface temperature and L. The results are good to within 10% at best.

We also have a model that says L is proportional to the 4th power (technically, it’s the 3.9th power) of its mass. So now we can compute the mass of the star. Again, the model could be way off. However, recent discoveries of planets orbiting red dwarfs give an independent way to measure the mass, using the radius and period of the orbit, so the model can be refined, if needed.

Several of the recently discovered exoplanets (planets orbiting stars other than Sol) are orbiting red dwarfs. That is partly because red dwarfs are so numerous, but also because their nearness to us and their low mass both cause the angular motion of the star, due to the planet’s orbit, as seen from Earth, to be detectable. The motion of the star due to its planet(s) provides the most common means of detecting exoplanets.

Imagine a red dwarf star in our Solar System, in place of Jupiter. It would have a dominating effect on the orbits of the other planets and asteroids, much more than Jupiter does now. Everything would either be ejected from the Solar System, or crash into something, or get captured by the Dwarf and become its moon, or else its orbit would evolve into a resonant one, in which the period is some low order rational multiple of the orbital period of the Dwarf. Because of its size, the Dwarf would be a little brighter than Jupiter, in terms of visible light. It would appear very red. Flare events would be very noticeable from Earth, but not dangerous, because of the distance. The solar wind from Sol would still dominate the electromagnetic environment in most of the Solar System, but the Dwarf would have its own domain, much larger than Jupiter’s. It would be interesting to have two solar winds, with different flavors.

Red dwarfs may not be the best candidates for colonization. They may not be the best places to look for life, for the same reasons. Since they put out so little energy for their mass, a spaceship, or planet, would have to get pretty close, in order to get enough heat. For a planet to have liquid water, it would have to orbit at 0.02 – 0.05 AU, or 1/50th to 1/20th of the average distance from Sol to the Earth. At this distance, the planet’s rotation would be tidally locked to its orbit, so that it would always keep one side facing its dwarf sun, just as the Moon keeps one side facing the Earth. One side would be a lot hotter than the other. There has been some argument lately as to whether this would make conditions unsuitable for life. Also, photosynthesis (as we know it) doesn’t work very well with red or infrared light. But there is a much bigger problem with orbiting so close to a red dwarf: the flares! Even red dwarfs that seem old and tired, like Barnard’s Star, can surprise you. A flare can appear in seconds, scorching any spaceship or colony that had been at a nice, comfortable distance before the flare.

On the other hand, many people are arguing that red dwarfs (actually, planets near red dwarfs) would be the most likely places to look for life. A planet with a strong magnetic field might have enough of a magnetosphere to protect its atmosphere from being blown away by flares. The atmosphere might just be able hold onto oxygen, making it possible for liquid water to exist on the surface.

Red dwarfs are like the glowing embers of the galaxy. They are invisible until you get close. Yet they comprise about 25% of the galactic mass. They might make nice places to visit.

Red dwarfs have made convenient venues for science fiction. The list is too long to include here, so I refer the reader to the Wikipedia article, Stars and Planetary Systems in Fiction.
A lot of good information about individual stars can be found at

Wikipedia has an excellent list of nearby stars. They also have an article on each individual star in the list, and they are pretty good about keeping up to date.

The RECONS (Research Consortium on Nearby Stars) website is also a great resource. They tend to focus on the kind of information scientists need, or can measure directly (angular position, proper motion, parallax, apparent magnitude) than on the kind of things an amateur would want to know (how big is it, how massive, how hot, how far away, what does it look like).

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