The massive stars in the universe are either red hypergiants or red supergiants; in extreme cases, they are more than 1,500 times bigger than the Sun. But what about the smallest stars? Are there stars smaller than our Sun? Well, actually there are.
The smallest stars in the universe are generally the tiny (astronomically speaking) red dwarfs. These red dwarfs are the most abundant and among the least luminous stars in the universe. One popular example of a red dwarf is the Proxima Centauri located 4.2 light years away from the Earth. It has only 12% of the Sun’s mass and is about 14% of its total size. The radius of Proxima is about 100,000 km.
That’s pretty small right, but it’s not the smallest star discovered till date. If you’re curious, here’s the exciting list of some of the smallest stars in the universe. We haven’t included any white dwarf, neutron star or stellar-mass black hole, because they all are dead stars.
12. CM Draconis A
Radius: 0.2534 R☉
Mass: 0.2310 M☉
About 47 light years away in the constellation of Draco lies two almost identical red dwarfs in an eclipsing binary system known as CM Draconis. Both of the red dwarfs are among the lightest stars observed till date. The binary system enables astronomers to test current stellar theories on very-low mass stars. The bigger of the two stars in this binary is CM Draconis A.
11. Ross 154
Image Courtesy: Alchetron
Radius: 0.24 R☉
Mass: 0.17 M☉
Ross 154 is one of the closest neighbors of the Sun and it’s located in the constellation of Sagittarius. It is a red dwarf, means it’s producing energy by burning hydrogen in its core. Compared to the Sun, Ross 154 has only 17% of its mass and about 24% of its radius.
Due to its relatively higher rotational speed, astronomers believe that it might be younger that the Sun. In about 157,000 years or so, this star will make its closest approach to the Sun at 6.39 light years. Currently, it is located at a distance of 9.6 light years.
10. CM Draconis B
Radius: 0.2396 R☉
Mass: 0.2141 M☉
CM Draconis B is the second red dwarf in the CM Draconis star system mentioned above. It is slightly less massive and have a smaller radius than CM Draconis A. It is also cooler than its companion. According to the Combined General Catalog of Variable Stars, at-least one of them is a variable star and another is a flare star.
9. Barnard’s Star
Position of Bernard’s Star (relative to the Sun)
Radius: 0.196 R☉
Mass: 0.144 M☉
As you can see above, the Bernard’s star is the fourth nearest star to our Sun after the three stars in the Alpha Centauri system. It is a very low mass red dwarf with a spectral classification of M4. The star has been a subject of extensive studies over the years, mostly due to its favorable position (in respect to the Earth) and close proximity.
Even though the star is relatively close to Earth, its ultra dim apparent magnitude of +9.5 makes the it completely invisible to the naked eye. Only telescopes operating at infrared wavelength can detect this star more clearly. Scientists are certain that Bernard’s star is much older than our Sun and might just be one of the oldest in the entire Milky Way.
In the early 1960s and 1970s, Bernard’s star was a subject of major controversy. Few astronomers like Peter van de Kamp repeatedly argued that the star is orbited by at least one gas giant. However, researchers were not able to find any gas giant orbiting the star.
8. Ross 248
Radius: 0.16 R☉
Mass: 0.136 M☉
Ross 248 also known by the names of Gliese 905 or HH Andromedae, is a low mass, red dwarf star located about 10.3 light years away from the Sun. It has about 16% of the solar radius and 12% of its mass. With just 0.2% of Sun’s luminosity, Ross 248 is too dim be observed with naked eyes and home-telescopes.
Recent researches have shown that it is a flare star, which is characterized by a sudden burst in their luminosity. The star was first cataloged by astronomer and physicist Frank Elmore Ross back in 1926.
7. Wolf 359
Wolf 359 is the orange colored star located just above the center of the image
Radius: 0.16 R☉
Mass: 0.09 M☉
Wolf 359 is a red dwarf located about 7.9 light years away from the Earth in the constellation of Leo. It is among the faintest and least massive stars discovered till date. Wolf 359 is barely above the lowest limit at which any star can sustain hydrogen fusion in its core.
Its radius is estimated to be about 16% of that of the Sun or in other words somewhere about 110,000 km. To understand it better, we can compare Wolf 359 with the planet Jupiter, which is slightly smaller with an equatorial radius of about 71,492 km.
Furthermore, Wolf 359 is also characterized as a flare star due to the sudden bursts in its luminosity. This is due to elevated magnetic activity on its surface. Researchers believe the star is relatively younger with age less than 1 billion years.
6. Proxima Centauri
Radius: 0.14 R☉
Mass: 0.1221 M☉
Located at a distance of 4.25 light years, Proxima Centauri is perhaps the closest star to our solar system. It is the third and the smallest component in the Alpha Centuari Triple star system. Due to its apparent magnitude of 11.05, Proxima is not visible to the naked eye. However, it can register a sudden increase in brightness due to surge in magnetic activities.
Compared to the Sun, Proxima Centauri has just 12% of its mass and 33 times its density. As a low-mass M type star, it is one of the smallest stars observed till date, whose angular diameter can be measured directly.
Read: 15 Biggest Stars In The Universe | With Fascinating Details
5. Luyten 726-8 A and B
Radius: 0.14 R☉
Mass: 0.102 M☉, 0.100 M☉
Gliese 65 or Luyten 726-8 is a binary star system located about 8.7 light years away from the Sun in the constellation of Cetus. Both the stars in this system have similar radius, however Luyten 726-8 A is slightly more massive than its sibling Luyten 726-8 B. They also exhibit similar brightness when seen from the Earth.
Both the stars, Luyten-726-8 A and B are variable star (a star with fluctuating brightness) and flare star (sudden outburst of powerful flares) simultaneously. However, Luyten-726-8 B appears to be more active and violent than its companion.
4. OGLE-TR-122 B
Comparison between OGLE-TR-122 B, Sun and Jupiter Image Courtesy: SOHO/ESA, Cassini/NASA/JPL/University of Arizona/ESA)
Radius: 0.120 R☉
Mass: 0.092 M☉
OGLE-TR-122 is a binary star system that contains one of the smallest known hydrogen-fusing-main sequence star known as OGLE-TR-122 B. In comparison, the star is about 20% bigger than the planet Jupiter and has 100 times more mass, which is less than 0.1 solar mass. It has the lowest possible mass of a hydrogen-fusing star. The primary, OGLE-TR-122A is generally considered a Sun like star.
Radius: 0.121 R☉
Mass: 0.089 M☉
Right now, TRAPPIST-1 might be the most popular star among astronomers and space fanatics. And the reason is quite obvious; the potential habitability of the planets that are orbiting around the star. But let’s concentrate on the star here. TRAPPIST-1 is an ultra cool red dwarf star with just 11% the Sun’s radius and 8% of its mass.
If by any chance the star were smaller than its current state, it wouldn’t have the needed mass to burn hydrogen and would be a brown dwarf. Various scientific researches have concluded that TRAPPIST-1 is about 7.6 billion years old with an error of 2.2 billion years. Because to its low luminosity, TRAPPIST-1 can live as long as 12 trillion years, which is much longer than the Sun.
Read: 15 Brightest Stars In The Sky
2. 2MASS J0523-1403
Image Courtesy: Martin Silvertant
Radius: 0.086 R☉
Mass: Less than 0.08 M☉
In 2003, a jointly funded astronomical survey project known as the Two Micron All-Survey Program or 2MASS, discovered an intriguing low mass red dwarf in the constellation of Lepus about 40 light years away from the Earth. After initial studies, it turns out that the star was indeed fascinating.
With a mass less than 0.08 M☉ , luminosity of 0.000126 L☉ (solar luminosity) and an effective temperature of 2074 K, the 2MASS J0523-1403 lies on the edge between main sequence stars and brown dwarfs.
1. EBLM JO555-57Ab
An artist’s impression of star EBLM J0555-57Ab compared to Saturn, Jupiter and TRAPPIST-1 Image Courtesy: Armanda Smith
Radius: 0.84 jup
Mass: 85.2 M jup
Perhaps the smallest red dwarf discovered till date is star EBLM J0555-57Ab, located about 600 light years away from the Earth in the constellation Pictor. It is a part of a triple star system which appears to be orbiting its primary star in every 7.8 days.
It was first observed by a group of researchers working on the EBLM (Eclipsing Binary Low Mass) project to find transiting exoplanets. Upon discovery, researchers implemented the Doppler spectroscopy method to derive important information about the red dwarf.
Read: Will We Ever Make It to Stars Outside Our Solar System?
EBLM J0555-57Ab’s radius is comparable to that of Saturn’s and it has a mass of about 0.081 M☉ or 85.2 M jup (Jupiter mass). It is currently the smallest star known to have enough mass to support the hydrogen fusion process.
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