The brightness of a stellar object, or any astronomical object in space, is measured by its apparent magnitude from earth. The apparent magnitude of an object is determined by its distance from the earth, inherent luminosity, and any possible interference (mostly interstellar dust) along the line of sight of the star.
The brightness of a star and its magnitude value has an inverse correlation; that is, the more the brightness of a star, the less its apparent magnitude value is. The Sun has an apparent magnitude of -26.74.
Another criterion for measuring stellar brightness is known as absolute magnitude. It measures a celestial object’s luminosity as it would be observed from a fixed distance of 32.6 light-years or 10 parsecs. Below is the list of brightest stars, located relatively close to our planet, based on their apparent magnitude (excluding the Sun).
This false-color WISE infrared image showing Antares in bright white color | Image Courtesy: Judy Schmidt/Flickr
Distance From the Earth: 550 light-years
Apparent Magnitude: 0.6 – 1.6
Current Stage Of Evolution: Red Supergiant
Antares, also known as Alpha Scorpii, is the brightest star in the Scorpius Constellation. The apparent magnitude of Antares varies between +0.6 to +1.6, typical of a slow irregular variable star. It is one of the biggest and brightest stars in the night sky visible to the naked eye.
Antares, despite its single-star appearance, is made up of two stars; Antares and Antares B, possibly forming a binary system. The second, much smaller star, Antares B, is a blue main-sequence star.
According to the most recent estimates, Antares has a mass between 11 to 14.3 M☉ and a radius of 680 R☉ (approximately). If placed at the center of the solar system (in place of the Sun), Antares would likely engulf the orbit of planet Jupiter.
Occultation of Aldebaran by the Moon | Image Courtesy: Christina Irakleous
Distance From the Earth: 65.3 light-years
Apparent Magnitude: +0.86
Current Stage Of Evolution: Red Giant
Aldebaran (alpha Tauri) is the brightest star in the constellation Taurus. It is a red giant of spectral class K and has evolved from the main sequence phase after depleting hydrogen at its core.
According to the current stellar evolutionary model, Aldebaran’s luminosity is about 425 times more than that of the sun (even though it’s only 50 percent more massive than the sun).
Pioneer 10, one of NASA’s oldest and farthest space probes, is heading towards Aldebaran’s direction and should make its nearest approach in approx. two million years.
The four main stars in the Crux constellation. The brightest star is Acrux | Image Courtesy: Wikimedia Commons
Distance From the Earth: 320 light-years
Apparent Magnitude: +0.76
Acrux, or alpha Crucis, is a multiple star system located in the Crux constellation, about 320 light-years away. The Acrux system is comprised of at least five individual stars. The first two components, Acrux A and Acrux B, form a close binary system. These two are accompanied by a distant Acrux C, and together they eventually form a triple star. Both Acrux A and Acrux B themselves are binary stars.
With a combined visual magnitude of 0.76, it is the brightest star in the Crux constellation and the 13th brightest star in the night sky. Acrux is the southernmost first-magnitude star and is not visible above 27 degrees latitude north.
Locations of Altair, Deneb, and Vega
Distance From the Earth: 16.73 light-years
Apparent Magnitude: +0.76
Current Stage Of Evolution: A-type Main sequence star
Altair is the brightest star in the Aquila constellation and is one of the closest stars to the earth visible by the naked eye. The star is about 1.8 times more massive than the sun and 11 times brighter (luminosity).
Along with Vega and Deneb, Altair form the Summer Triangle asterism, an imaginary triangle connecting stars from three different constellations, Aquila, Lyra, and Cygnus, respectively.
The interferometric studies revealed that the star has flattened poles due to the high velocity of its rotation.
11. Beta Centauri
Beta Centauri (the second brightest star on the right) | Image Courtesy: Wikimedia Commons
Distance From the Earth: 390 light-years
Apparent Magnitude: +0.61
Beta Centauri, also known as Hadar, is the eleventh brightest star in the night sky. The Beta Centauri, in fact, is a triple-star system consisting of stars Beta Centauri Aa, Ab, and B. Both Aa and Ab are at least ten times more massive than the Sun.
Its combined visual magnitude of +0.61 makes Beta Centauri the second brightest star in the Centaurus constellation only after our neighbor Alpha Centauri.
Moreover, Beta Centauri exhibits rapid changes in its brightness and is thus classified as a Beta Cephei variable. These changes in brightness, however, are minute and cannot be observed by the naked eye.
Betelgeuse (the reddish star) at its usual brightness on the left compared to an unusual minimum on the right. The bright star at the bottom is Rigel. Image Courtesy: Wikimedia Commons/H. Raab.
Distance From the Earth: 727 light-years
Apparent Magnitude: +0.50
Current Stage Of Evolution: Red Supergiant
Betelgeuse, also known as Alpha Orionis, is the second-brightest star in the Orion constellation. It is a semiregular variable star whose magnitude fluctuates between 0.3 to +1.8, the largest known of any first-magnitude star.
The variations in Betelgeuse’s brightness were first reported by Sir John Herschel between 1836 and 1840. During that period, Herschel observed drastic changes in the star’s magnitude when it outshone, usually brighter, Rigel, on multiple occasions.
Between 1927 and 1941, based on the American Association of Variable State Observers data, the minimum observed magnitude of Betelgeuse was at 1.2.
According to the current estimates, Betelgeuse’s mass could be anywhere between 10 to slightly over 20 times that of the Sun. It’s one of the most massive stars that can be observed with an unaided eye.
Image Courtesy: Wikimedia Commons
Distance From the Earth: 139 light-years
Apparent Magnitude: +0.46
Achernar, designated as Alpha Eridani, is a binary star system located in the Eridanus constellation. The two components of the star system are Alpha Eridani A and B. The brighter of the two, Eridani A is classified as a B-type main-sequence star, one of the most luminous of all known star types.
The star is about 3,150 times more luminous than the Sun and seven times more massive. Achernar is best observed from the southern hemisphere at a 33-degree latitude south, while it becomes invisible above 33-degree latitude north.
Procyon star | Image Courtesy: Yunji Kitahara
Distance From the Earth: 11.46 light-years
Apparent Magnitude: +0.34
Procyon, also known as Alpha Canis Minoris, is the brightest star in the constellation Canis Minoris and the eighth brightest in the night sky. Procyon is, in fact, a binary star comprised of a main-sequence star (Procyon A) and a white dwarf (Procyon B).
The atmospheric temperature of Procyon A is estimated to be around 6,530 K and luminosity about seven times that of the Sun. Together with Sirius and Betelgeuse, Procyon forms the Winter Triangle asterism.
7. Rigel A
Rigel | Image Courtesy: Fred Espenak/ astropixels.com
Distance From the Earth: 860 light-years
Apparent Magnitude: 0.13
Current Evolutionary Stage: Blue Supergiant
Rigel is the brightest star in the Orion Constellation. Though it appears as a single star, Rigel is a system of at least four stars. The most prominent star of the group, Rigel A, is as much as 120,000 times more luminous than the Sun and 21 times more massive (both values vary depending on the method). Other components include Rigel Ba and Bb (spectroscopic binary) and a distant Rigel C.
Rigel is classified as an Alpha Cygni variable, a group of variable stars that simultaneously exhibit contraction on one portion and expansion on the other part of the stellar surface. Its brightness (apparent magnitude) varies between 0.05 to 0.18.
Though Rigel is usually the brightest star in the Orion Constellation, it is outshone by the red supergiant Betelgeuse on different occasions.
6. Capella Aa/Ab
The components of Capella in comparison to the Sun (SOL)
Distance From the Earth: 42.9 light-years
Apparent Magnitude: +0.08
Current Evolutionary Stage: Red giant, main sequence
After Arcturus and Vega, Capella is the third brightest star in the northern celestial hemisphere and the brightest in the constellation Auriga. Capella is a multiple star system, rather than a single star, comprised of four stars in (binary) pairs of two.
The most prominent of the four, Capella Aa, is a red giant with a mass 2.5 times that of the Sun, while almost 79 times more luminous. Its binary companion, Capella Ab (subgiant), is slightly smaller and less luminous. The second pair, Capella H and L, are much smaller and fainter red dwarfs.
Capella is the closest first magnitude star to the north celestial pole. Due to its location, Capella is visible throughout the year above 44-degree latitude north. In contrast, it is invisible below 44-degree latitude south.
Astrophoto of Vega | Stephen Rahn/Flickr
Distance From the Earth: 25.4 light-years
Apparent Magnitude: +0.03
Current Evolutionary Stage: Main sequence
Vega, also known as Alpha Lyrae, is one of the most extensively studied stars in the immediate vicinity of the Sun. It was among the first stars whose distance was estimated using the stellar parallax displacement. The star also has uses in astrophotography (to calibrate photometric brightness).
Vega is the brightest star in the constellation Lyra and the second brightest in the northern celestial hemisphere after Arcturus. The star is estimated to be 2.1 times more massive and 40 times more luminous than the Sun. However, both its age and estimated lifetime are much shorter than our star.
In about 11,707 years from now (the year 13,727), Vega will become a north pole star. Its current declination is +38° 47′
Optical image of Arcturus
Distance From the Earth: 36.7 light-years
Apparent Magnitude: -0.05
Arcturus is the brightest star in the Bootis constellation and the northern celestial hemisphere. It is outshone only by three stars in the night sky. A red giant, Arcturus, has burnt through the hydrogen at its core. Though the star is only 0.8 times more massive than the Sun, it is 25 times bigger and 170 times more luminous.
In 1635, Arcturus became the first star (other than the Sun and supernovae) to be observed in the daytime with a telescope by French astronomer Jean-Baptiste Morin.
3. Alpha Centauri A
A wide-field image of Alpha Centauri A created by DSS2 | Image Courtesy: ESO/DSS 2
Distance From the Earth: 4.37 light-years
Apparent Magnitude: -0.27
Alpha Centauri is a multiple star system consisting of two close binary, Alpha Centauri A and Alpha Centauri B, and a relatively distant Alpha Centauri C or Proxima Centauri. Although Proxima Centauri is the nearest star to the solar system, it is much fainter than Alpha Centauri AB.
The distance between Proxima Centauri and Alpha Centauri AB is estimated to be around 0.21 light-years (towards the solar system).
The most prominent of the three stars, Alpha Centauri A, also known as Rigil Kentaurus, is slightly more massive and 1.519 times more luminous than the Sun. However, its binary companion is slightly less massive and half as luminous as the star in our solar system.
An Image of Canopus taken from the International Space Station
Distance for the Earth: 310 light-years
Apparent Magnitude: -0.74
Canopus, also known as Alpha Carinae, is the brightest star in the constellation of Carina and the second brightest in the night sky. Under normal conditions, this A-type bright giant star is visible throughout the year in the southern hemisphere, especially during the summer.
Canopus’s luminosity is believed to be as much as 10,700 times that of the Sun, while it is about eight times more massive. Before the launch of the Hipparcos satellite in 1989, the estimated distance between the Sun and Canopus varied between 90 to 1200 light-years.
HST image of Sirius A and B | Image Courtesy: NASA, ESA, H. Bond (STScI)
Distance From the Earth: 8.6 light-years
Apparent magnitude: -1.47
Sirius, or Canis Majoris, is the brightest star in the night sky and the second brightest overall after the Sun (it can also be observed in daylight). It is a binary star consisting of a main-sequence star (Sirius A) and a white dwarf (Sirius B).
Sirius A is more than twice as massive as the Sun, while it has 25 times more luminosity. Its absolute magnitude is +1.42. Its companion, Sirius B, is significantly less massive and luminous.
Though Sirius is remarkably less luminous than Canopus and even Rigel, it appears much brighter due to its distance from the earth (intrinsic luminosity).
Sirius has considerable mythological significance. Ancient Greeks feared Sirius and believed that it brings hot summer times as a punishment for humanity. On the contrary, Egyptians worshiped Sirius as the goddess of fertility.