Red dwarfs

To everyone s surprise, no event lasting less than two weeks has ever been observed. The mean mass inferred is half the solar mass, which rules out brown and red dwarfs but favours white dwarfs. However, too many of these remnants from intermediate-mass stars (1-8 Mq at birth) would contradict the traditional tenets of astrophysics. Indeed, it would imply a frenzied spate of nucleosynthesis during the formation of the galactic halo. The nature of the compact massive objects thus remains a mystery. 

A magnification of the H-R diagram in figure 5.3 can be quite instructive. Figure 5.8 shows the 50 nearest known stars to Earth, with the sizes of the stars indicated by the size of the circles. Looking at the plot, you can see that most of the Sun s nearest neighbors are relatively small stars, the slow-burning red dwarfs. 

Bob washes his faces and hands and returns to the chariot. This solar equilibrium almost seems to be a miracle. If gravity were much stronger or much weaker, this balance between gravitational force and the outward pressure of gas would be disturbed. For example, if the gravity was off by just a little bit, nature would tend to produce more extreme stars—hot blue giants or cool, dim red dwarfs— neither of which seem very conducive to the evolution of life. 2... 

My father had no idea, so I had to wait a few decades until 1 learned the answer. In about a hundred trillion years, the last generations of stars will have been born, and the few remaining red dwarfs will die. Let s digress for a moment and discuss red dwarfs before talking about the end of the Universe. We ve mentioned red dwarfs a few times in this book without giving a detailed description of what they are. 

One of the most fascinating aspects of red dwarfs is their longevity. The Main Sequence lifetime of a red dwarf is about a hundred billion years. Since this is older than the age of the Universe (which is 10-15 billion years), red dwarfs have not reached the white dwarf stage. 

When you look up into the night sky, you never see red dwarfs. They re faint because of their small size and low temperature. Only the nearest red dwarf stars, up to about 90 light-years away, are visible to even the most powerful earth-based telescopes. Proxima Centauri, a member of the Alpha Centauri triple star system, is a red dwarf, and we ve discussed that it is the closest star to the Earth. Despite its proximity, it is a hundred times too faint to see with the naked eye. In contrast, we can see many larger stars using our eyes, even when these stars are thousands of light-years away. 

Astronomers have found about 450 young red dwarf stars within 80 light-years of the Earth. This means that there are more red dwarfs closer to Earth than all the other types of stars combined. Some astronomers suggest that red dwarfs are the most common type of star in the Universe.12 Others suggest that even cooler brown dwarfs are the most common star, but they are so dim they are more difficult to detect (figure 8.7). 

Red dwarfs Small, cool, very faint, Main Sequence star. Surface temperature is less than 4,000 K. Some speculate that these are the most common type of star in the Galaxy. The nearest red dwarf is Proxima Centauri. Red dwarfs are not visible to the naked eye and are about 0.08 to 0.43 solar masses. 

There are also brown dwarfs to consider. Brown dwarfs are astronomical objects somewhat between a planet and a star and have a mass less than 0.08 times the mass of our sun and a surface temperature below 2,500 K. (As comparison, the cool red dwarfs are about 3,000-3,400 K). A large number of brown dwarfs would not change how bright the Galaxy appears in optical observations but would change its total mass quite substantially. 

Like many red dwarfs, Proxima or Alpha Centauri C is a flare star. Flare stars can brighten suddenly to many times their normal luminosity. The cause is thought to be a sudden and intense outburst of radiation on or above the star s surface. An increase in radio emission is often detected simultaneously with the optical outburst. 

Reid N. and Hawley S.L. New Light on Dark Stars Red Dwarfs, Low-Mass Stars, Brown Stars. 2" edition. Springer Verlag Berlin, 2005. 

M Class Star The most common class of stars. These are red dwarf stars and are relatively small and cool. 

This small class of unique stars presents the opportunity to study the structure and behavior of matter in extremely strong magnetic fields. It is well known that these systems eonsist of eompact objects and red dwarfs. The general... 

Duchess had risen above the horizon by the time Colsterworth came into view. The red dwarf sun occupied a portion of the horizon diametrically opposite Duke, the two of them struggling to contaminate the landscape below with their own unique spectrum. 

He lifted his gaze above the flames darting out of the brazier. A vermillion sky was visible through the gaping rents in the broken window. Less than a dozen first magnitude stars twinkled above the wolds, the rest of the universe had been washed out in the red dwarf s sullied glow. The tiny blue-white lights seemed so delicate and pure. 

The red dwarfs are the stars whose diameter is less by 2-3 times, and whose density is higher by 2-3 times than in the Sun, and the luminosity is comparable with the Sun s. 

There may be other ways to arrange a solar system to get sunlit, liquid water. One intriguing option is if a planet like Titan orbits a cooler, bigger type of star called a red dwarf. Red dwarfs are cooler, so the planet would have to be very close to catch enough heat for liquid water. That closeness has its own drawbacks, because gravity could lock the planet in place, making weather even more extreme than Titan s. But the same effect could heat the planet and keep the oceans liquid. We don t know enough to be sure. 

The near infrared range is very sensitive to evolved objects (red dwarfs, late giant stars, spiral galaxies) and is much less affected by interstellar extinction than the optical range. The K luminosity is also an excellent tracer of the mass of galaxies. Infrared wide field observations are thus destined to... 

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