by Sergio Dieterich, Carnegie Institution for Science
When members of our research group go observing at Cerro Tololo Inter-American Observatory in the Chilean Andes we spend most of our time in a cozy heated control room. Modern astronomical observing is done mostly by monitoring computer screens and entering commands to tell the telescope where to point next. If we have to put on our winter jackets and climb the flight of stairs to where the telescope is—under the open dome—it is because something went wrong and we are frantically trying to fix the problem and minimize the loss of precious telescope time. There is one exception. Our group’s tradition dictates that when we are training a new student, and the season and time of night is just right, we will go up to the dome and have our new colleague look through the telescope’s eyepiece. Photography does not do justice to the sight that emerges: a bright ruby red speck of light floats seemingly in front of a vast ocean of fainter and whiter stars. That red speck is Proxima Centauri, the closest star to us other than the Sun, the subject of the Pale Red Dot project, and a typical low mass star. Stars like Proxima Centauri, or just Proxima for short, are amongst the smallest but also the most common types of stars in the Galaxy. Let’s take a few minutes to understand our smallest and closest stellar neighbors a little better.
Imagine for a moment that we drop a large ceramic dinner plate on a hard kitchen floor. The plate shatters into many, many, pieces, of all different sizes. We then look down and examine the results of our carelessness. Our attention is first drawn to the handful of large fragments. After a more careful look we see that for every one of those large ceramic fragments there are dozens, if not hundreds, of much smaller pieces. Further, we soon realize that if we have any hope of reconstructing the original plate or figuring out what happened we cannot simply ignore those smaller pieces and sweep them under the rug. This unfortunate kitchen accident is a rough analogy to the stellar formation process, and it sheds some light on how the Milky Way Galaxy ended up with the stellar population we observe today. Stars are formed when clouds of interstellar gas and dust, called giant molecular clouds, are somehow perturbed—causing the cloud to start collapsing under its own gravitational pull. Several points in the collapsing cloud achieve higher and higher density, and therefore exert an even greater gravitational force. Over the course of hundreds of thousands of years these high density regions consume enough gas and become compact enough to form stellar embryos, or protostars. When the protostar’s core becomes hot enough to ignite and sustain nuclear fusion, a star is born. In a manner similar to what happens with our shattering plate, but for different physical reasons, the result of this cloud collapse mechanism heavily favors the production of stars whose masses are anywhere from about 60% to about only 8% of our Sun’s mass. When fully formed and contracted these are tiny stars, with the majority having radii between 20% and only 10% our Sun’s radius. The smallest are very close in size (but not in mass or density!) to the planet Jupiter. What these small stars lack in terms of size they make up for in their sheer numbers. Indeed, out of the 366 stars whose accurately measured distances place them within 32.6 light-years (or 10 parsecs, in astronomical lingo) of our Solar System, 275 belong to this type. These objects are commonly known as red dwarfs, or M dwarfs, in the stellar classification system used by professional astronomers. Using the fair assumption that our solar neighborhood is typical of much of the Milky Way Galaxy, that means that about 75% of the stars in our galaxy are M dwarfs. The M dwarf class is sometimes subdivided, with stars having about 20% or less the mass of our Sun being called Very Low Mass, or VLM stars. Proxima is in the upper mass range of the VLM stars.
What are red dwarfs like as stars, and how does their energy output compare to our Sun’s? These stars are incredibly faint, and not even Proxima can be seen with the naked eye despite its proximity of only 4.25 light-years. To put this distance in context, the best estimates for the diameter of the Milky Way Galaxy place it at anywhere between 100,000 to 180,000 light-years; if our galaxy were a city 10 km across Proxima would be so close to us as to be knocking on our front door! And yet stars that are intrinsically more luminous can be seen with the naked eye from distances almost one fifth of the way across the galaxy. If a representative sample of red dwarfs were all placed at the same distance to us as the Sun the brightest ones would shine only about 7 percent as bright as the Sun. Recent research by our group indicates that the faintest of the VLM stars would shine with only about 0.016 percent, or about 1/6,000th , the brightness of our Sun. Proxima has a total energy output about 0.2% that of our Sun.
Red dwarfs are not only faint, but the little light they do emit is also very different from the warm sunlight we enjoy on a Caribbean beach on Earth. The surface of our Sun shines at a temperature of approximately 5,500 degrees Celsius (10,000 F). At that temperature most of the light is emitted in the yellow-green region of the visible light spectrum. It therefore makes sense that the human eye has evolved to be the most sensitive to the yellow-green light that most strongly bathes our planet. Low mass stars have significantly cooler surface temperatures: about 3,500 C (6,400 F) for the hottest red dwarfs and approximately 1,800 C (3,300 F) for the smallest and faintest VLM stars. At these temperatures not only does the star emit considerably less light overall, but the light emitted is also shifted to longer wavelengths, which we perceive as redder colors. The color spectrum of the hottest red dwarfs has its peak at a deep red color that is just at the limit of the detection range of the human eye. For the faintest VLM stars the color spectrum peaks in the near infrared range of the electromagnetic spectrum, well beyond the detection capabilities of the human eye. In both cases the human eye’s enhanced sensitivity to yellow-green light will shift the perceived colors to shorter wavelengths than the peak color emission. A future interstellar voyager who sees a hot red dwarf up close will likely perceive a distinctive orange hue, whereas one of the cooler red dwarfs may appear to be a lively red (Figure 2ab). To make these faintest of faint stars even more unusual, there is evidence to suggest that they have strong surface magnetic fields. These magnetic fields would cause dark spots analogous to sunspots, but they may be more numerous and larger—perhaps covering a substantial portion of the star’s surface.
Astronomers currently think that as many as 1/3 of red dwarfs may harbor rocky planets with compositions similar to Earth’s. Could life evolve on these planets, and what would life around a red dwarf be like? The idea of life evolving on planets around red dwarfs is extremely exciting. If for no other reason, their sheer numbers means that the question of red dwarf habitability has tremendous implications in determining whether we live in a Universe teeming with life or whether life is a sparse occurrence. Despite this huge potential, the notion of life on low mass star systems is not without its challenges.
Because of their lower mass and consequentially weaker gravitational pull, red dwarfs take a very long time to settle into their fully contracted configuration, once they stop accreting material from their parent star forming cloud. Similarly, the comparatively slow rate of nuclear reactions in a low mass star’s core causes these stars to have extremely long lives when compared to more massive stars. Their slow evolution and long lives are both a blessing and a curse for the possibility of life. Once fully formed and contracted, red dwarfs change very little for hundreds of billions of years. The oldest red dwarfs may therefore have provided a stable environment for life for as long as they have existed, roughly 10 billion years based on current estimates for the age of the Galaxy. Compare that with only 4.1 billion years of biological evolution on Earth. Even if evolution around a planet hosting red dwarf happened slower and hit a few dead ends, the final result might still mean a complex and diverse ecosystem. However, the prospect of a prolonged period of stability suitable for biological evolution is only exciting if we assume that the right conditions for life were present to begin with, and that is where a red dwarf’s life in the slow lane becomes a problem. Liquid water is essential for life as we know it on Earth, and liquid water can only exist if the temperature on a planet’s surface allows it. A planet’s temperature is governed primarily by the planet’s orbital distance from its parent star and the star’s intrinsic luminosity. Astronomers call the range of orbital radii allowing the existence of liquid water the ‘habitable zone’ around a star. All stars are significantly brighter during their initial contraction phase, when most of the star’s energy comes from its gravitational collapse and not from nuclear fusion. For red dwarfs this initial period of increased luminosity may last up to 3 billion years, which is well beyond the formation time for planets. Any planet that forms in what will eventually become the star’s habitable zone will be subject to scorching heat during its early life. Calculations suggest that this fiery youth may cause all water to evaporate away, thus effectively sterilizing the planet. A possible way out of this scenario involves the retention of water in minerals called chondrites. If chondrites are present in sufficient amounts in the rocky material that coalesces to form planets, the fully formed planets could have substantial water reserves in their interiors. The water could then be released from the planet’s interior by volcanic activity at later times when the surface temperature is right for liquid water. Whether or not this scenario is likely is an area of active research.
Another interesting aspect of the idea of life in planets orbiting red dwarfs has to do with the extreme proximity of the star’s habitable zone to the star itself. These stars are so faint that planets in their habitable zones would have orbits smaller than the orbit of Mercury in our Solar System. At such small distances the slight difference in the star’s gravitational pull from the planet’s side facing the star to the planet’s far side causes a phenomenon called tidal locking. In a tidally locked planet the same side of the planet always faces the star, causing it to be much hotter than the side that is perpetually facing away from the star. The Earth-Moon system is a good example of a tidally locked satellite. The habitable conditions in a tidally locked planet may be confined to a narrow ring shaped region where the illuminated side meets the dark side of the planet. This habitable region would be in perpetual twilight, with the star shining low in the horizon. Such low illumination conditions may seem rather depressing to us humans, but low light levels peaking at redder wavelengths are the norm around red dwarfs, and it is quite possible that any existing life form in these otherworldly environments may have evolved to use infrared light in much the same way we utilize the bright yellow-green light of our parent star. Perhaps venturing too close to the planet’s illuminated side would cause these creatures to get a “star burn” from red light in much the same way we get can get a sunburn from the small portion of our Sun’s energy that is emitted as ultraviolet light.
Finally, a treatment of low mass stars would not be complete without making a connection to their lower mass cousins, the substellar brown dwarfs. Looking back to our shattered plate analogy of star formation, the cloud collapse process that produces stars with a wide range of different masses can also produce objects whose mass is too small to create the conditions necessary for sustainable core nuclear fusion. These objects are called brown dwarfs. Brown dwarfs look much like their VLM star counterparts in their youth because during that phase gravitational contraction releases a large amount of energy for both stars and brown dwarfs. However, once brown dwarfs are fully contracted they keep cooling down over the course of billions of years. For much of the red dwarf range of temperatures and colors it is difficult to tell whether a given object is a young brown dwarf or a VLM star of any age. Recent research by my collaborators and I indicates that the stellar sequence comes to an end when we reach objects with surface temperatures of about 1,800 C (3,300 F) and luminosities of roughly 1/6,000th that of our Sun (interested in the technical details? read the paper here). We came to this conclusion by performing the observations necessary to estimate the radius of a sample of 63 objects thought to lie close to the end of the stellar sequence. We then noted that for temperatures higher than 1,800 C the objects cover a wide range of radii, including the radii expected for old and fully contracted stars. At cooler temperatures we encountered larger radii that can only be explained if the objects in question are young brown dwarfs that are not yet fully contracted (Figure 3).
The temperature we obtained for the end of the stellar sequence is substantially higher than that predicted by theoretical models, and we are now trying to pinpoint the root causes of this discrepancy. As a part of this research we have found what we believe to be the smallest known star to date and also a representative of the smallest possible stars. This star is called 2MASS J0523-1403, and shines faintly in the constellation Lepus the hare, under the feet of Orion the hunter. (Figure 4). 2MASS J0523-1403 has a radius of only percent the radius of our Sun. That radius makes 2MASS J0523-1403 about 15 percent smaller than the planet Jupiter. Indeed, perhaps coincidentally, the size we calculate for 2MASS J05234-1403 is within 1 percent of the size of the planet Saturn. Therefore while we can say that VLM stars in general have sizes comparable to Jupiter, we can go one step further and say that the smallest stars are Saturn sized. In making these comparisons we must be careful not to confuse volume and mass. While these stars have the volume of giant planets their mass is theoretically predicted to be anywhere from 70 to 80 times the mass of Jupiter, making them incredibly dense. In fact, it is the quantum mechanical limit on the allowed upper density that causes brown dwarfs to stop contracting before nuclear fusion ignites.
Over the last few decades our knowledge of red dwarfs has gone from simply knowing that they exist, to realizing just how numerous they are, and finally to being able to characterize them and assess their suitability as hosts for habitable planets. This progress is in part due to advances in observational astronomy, such as the substitution of blue sensitive photographic film to red sensitive digital CCD detectors and infrared detectors. Those advances in sensitivity and data management were then utilized to conduct large all-sky surveys that revealed a multitude of new red dwarfs and gave astronomers the unprecedented ability to study them not only as individual objects but also as a population. We now have a good understanding of how red dwarfs contribute to the overall stellar population of the Galaxy and are gaining greater understanding of their promises and challenges as hosts of livable planets. The history of astronomy has taught us that we cannot predict what the next discovery will be and how it will change our understanding of things. It could well be that after thorough study we may realize that the roughly 75 percent of the stars in the Galaxy that we call red dwarfs are not suitable as hosts of living planets. That alone would let us know that life in the Universe might be a bit more special than previously thought and how fortunate we are to have a home on planet Earth. On the opposing view, we know from our experience on Earth that evolution usually finds a way to make life flourish in the most extreme and odd environments. If life forming mechanisms are able to overcome the challenges we discussed here, plus many others that we have not yet even imagined, it is quite possible that our solar neighborhood abounds with beings of unimaginable forms thriving under the soft red twilight of their tiny parent star.
About the author. Sergio (Serge) Dieterich is an observational astronomer who studies the properties of the smallest stars in the solar neighborhood, and the differences and similarities between these stars and their lower mass substellar cousins, the brown dwarfs. He is particularly interested in how stellar structure, and evolution processes happening deep within the core of a star or brown dwarf, are related to the colors and spectroscopic features of the surface of the star, which is the only part probed by telescope observations. Serge also specializes in the technique of astrometry, which measures minute changes in the relative position of a star in the sky to determine the star’s distance from Earth, as well as any orbital motion the star may have about an unseen companion. Serge was born in Porto Alegre, Brazil, and moved to Miami, Florida, just before starting high school. He has a B.A. in physics from Johns Hopkins University, an M.S. in physics from Georgia State University, and recently obtained his Ph.D. in astronomy also from Georgia State. After college and before starting graduate school Serge taught high school physics and middle school physical science for two years. He currently holds a National Science Foundation postdoctoral fellowship at the Department of Terrestrial Magnetism from the Carnegie Institution for Science. In addition to cutting edge research, he is also developing contents for high school level students about astronomy and stars.