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We are going on a journey of the imagination that will take us to the 10 closest stars to the Earth. As we leave orbit we will swing arond the most important star in the universe to all of us on Earth, the Sun.
At a distance of 93 million miles (150 million km) from the Earth, we find that our Sun is an average type star, with a surface temperature of about 6000°C. It has a spectral classification of - G2V, and is considered a yellow dwarf star. The Sun’s radius is 432,000 miles (695,500 kilometers); it is about 333,000 times more massive than the Earth and is composed mainly of hydrogen (75%) and helium (24%).
Leaving our home star we will dive below the ecliptic plane and head off to our nearest stellar neighbor!
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The Alpha Centauri System
The closest star to our Solar System is actually part of a trinary, or three star system, consisting of Alpha Centauri A, B and Proxima Centauri. As we move into this system we find that Centauri A and B are about 2.1 billion miles (3.4 billion km) apart; about the distance from the Sun to Uranus. Proxima Centauri is the closest star to our Sun at a distance of 4.22 light-years (ly), while its companions are 4.35 ly distant.
Proxima Centauri, a red dwarf - spectral type M, is the dimmest of the three stars. Centauri A is a type G2V star like our Sun, and it is slightly larger than Sol, while Centauri B, slightly smaller than the Sun, is a type K1 V star. This star system can be seen in the southern hemisphere near the constellation of the Southern Cross
From this trinary star system we will head off to the next star on our journey - Barnard’s Star.
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At a distance of 5.9 ly from Earth, we find another red dwarf known as Barnard’s Star, located in the constellation of Ophiuchus (sometimes called the 13th constellation of the zodiac.) The star is named after Edward Barnard, who discovered it in 1916. It is of particular note because the star has the largest proper motion relative to the Sun of any star. This means that as we observe the star over a period of time it moves across the sky, or its angular position changes, and for Barnard’s star this proper motion is 10.3 arcseconds per year.
Being a red dwarf, this star has a surface temperature of about 3000°C and a mass of 0.17 times the mass of the Sun. Red dwarfs have exceptionally long lifetimes, on the order of 8-12 billion years. This is because they burn their fuel at such a low rate, hence the cool surface temperature and red color.
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If we travel about 7.8 ly toward the constellation Leo, we will find yet another red dwarf, Wolf 359. This red dwarf is even cooler than Barnard’s Star with a surface temperature of about 2500°C, and it emits about 0.1% of the energy our Sun does. It has a mass of about .09 solar masses.
If you are a fan of Star Trek: The Next Generation, you might recognize Wolf 359 as the location of the Federation’s last stand against the rampaging Borg, who were on their way to attack Earth in the episode “The Best of Both Worlds, Part II”.
Leaving behind the lore of Star Trek ,we head on to the 7th most distant star from the Earth - Lalande 21185.
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Now off to the Great Bear!
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This time we are headed to the constellation of Ursa Major to visit—yup, you guessed it—another red dwarf!
Lalande 21185 is 8.3 ly from Earth. Even though it is further from us than Wolf 359, Lalande 21185 is brighter with an apparent magnitude of +7 versus +13.5 for Wolf 359. It is a little less than half the size of the Sun with a surface temperature of about 3100°C. Knowing that it is hotter and larger than Wolf 359 it is easy to see why it is brighter, although further away.
Time to move on to something other than red dwarfs—grab your sunglasses and let’s go to Sirius!
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Put your sunglasses on because Sirius is bright! In fact it is the brightest star in the night sky at the magnitude of -1.46. Sirius is a binary star system consisting of the white star Sirius A, spectral type A1V, and Sirius B, a white dwarf, spectral type DA2.
Known as the Dog Star, Sirius lies in the constellation of Canis Major. Sirius A is about twice as big as the Sun in mass and diameter, while Sirius B, the white dwarf, is about as massive as the Sun, but the size of the Earth. Sirius B is the remnant of a large star that burned off its hydrogen and after going through a red giant stage collapsed to the white dwarf we see tugging on Sirius A today. In the above image of the Sirius system we see Sirius B as the brighter x-ray emitter of the pair, which is contrary to what we see in the visible spectrum where it has a magnitude of +8.44. One interesting fact about white dwarfs is that they are extremely dense stars with a typical mass of 1.7 metric tons per cubic centimeter!
Next we go to the constellation of Cetus, the whale.
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We now drop into another binary system, Luyten 726-8, located about 8.7 ly from Earth.
This system is composed of two red dwarfs (guess we can get rid of the sun glasses!), but they are both variable stars and flare stars. Luyten 726-8A is designated as BL Ceti. Luyten 726-8B is designated as UV Ceti, and is the more dynamic of the pair, as it increased in brightness by 75 times in 20 seconds in 1952. Many red dwarfs emit violent flares, which can be as bright as the entire star is by itself.
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Back To Earth
We’ve visited the 11 closest stars to the Earth and it is interesting to note the 6 out of those 11 are red dwarfs. Statistically, red dwarfs make up more than half of the stars in the Milky Way. Our neighborhood seems to bear out that statistic. It is also interesting to see that 7 of the 11 stars exist in a binary or larger system. The majority of the stars in the galaxy may very well be part of multi-star systems.
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Listing of Nearest Stars - http://www.astro.wisc.edu/~dolan/constellations/extra/nearest.html