Luminosity
Luminosity is the total brightness of a star or galaxy.
Absolute and apparent magnitudes
| Rank | Star | Absolute Magnitude | Apparent Magnitude | Distance from Earth (light-years) |
|---|---|---|---|---|
| . | The Sun | +4.8 | -26.72 | . |
| 1 | Sirius (in Canis Major) | +1.4 | -1.46 | 8.6 |
| 2 | Canopus (in Carina) | -2.5 | -0.72 | 74 |
| 3 | Rigel Kentaurus (Alpha Centauri) (in Centaurus) |
+4.4 | -0.27 | 4.3 |
| 4 | Arcturus (in Boötes) | +0.2 | -0.04 | 34 |
| 5 | Vega (in Lyra) | +0.6 | 0.03 | 25 |
| 6 | Capella (in Auriga) | +0.4 | +0.08 | 41 |
| 7 | Rigel (in Orion) | -8.1 | +0.12 | 900 |
| 8 | Procyon (in Canis Minor) | 2.8 | +0.38 | 11 |
| 9 | Archenar (in Eridanus) | -1.3 | +0.46 | 75 |
| 10 | Betelgeuse (in Orion) | -7.2 | +0.50 | 1,500 |
| 11 | Hadar (in Centaurus) | -4.3 | +0.61 | 300 |
| 12 | Altair (in Aquila) | +2.3 | +0.77 | 17 |
| 13 | Acrux (in Crux) | -3.8 | +0.79 | 270 |
| 14 | Aldebaran (in Taurus) | -0.2 | +0.85 | 65 |
| 15 | Antares (in Scorpius) | -4.5 | +0.96 | 400 |
| … | … | … | … | … |
| 20 | Deneb (in Cygnus) | -7.2 | +1.25 | 1,500 |
| -- | Proxima Centauri (in Centaurus) | +15.5 | +11.05 (var.) | 4.3 |
Apparent magnitude is a measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness. The full moon has an apparent magnitude of -12.6; the sun’s is -26.8. We can see objects up to 6th magnitude without a telescope. Apparent magnitude is abbreviated m. This system of rating the brightness of celestial objects was developed by the Greek astronomer Hipparchus in 120 B.C.
Absolute magnitude is a measure of the inherent brightness of a celestial object. This scale is defined as the apparent magnitude a star would have if it were seen from a distance of 32.6 light-years (10 parsecs). The lower the number, the brighter the object. Negative numbers indicate extreme brightness.
Some bright stars that can be seen from Earth
Antares
(Alpha Scorpii) Antares (meaning “Rival of Mars”) is the brightest star in Scorpius, one of the constellation in the zodiac. Antares is a M1.5Iab variable red supergiant star that is about 520 light-years from Earth and is about 230 times as big as the Sun. This incredibly massive, old, low-temperature (3500 K) star is the 15th brightest star in the sky; it has a visual (apparent) magnitude of +0.96 (var.) and an absolute magnitude of -5.2.
Arcturus
(Alpha Boötis) Arcturus is the brightest star in the constellation Boötes (the herdsman). It is a red giant (spectral type K1.5IIIp) that is the fourth brightest star in the sky. Arcturus is 34 light-years from Earth. It has an apparent magnitude of -0.04 and an absolute magnitude of 0.2.
Betelgeuse
(pronounced “beetle juice”) Betelgeuse (alpha Orionis) is the second-brightest star in the constellation Orion and one of the brightest stars in the sky. It is a supergiant star, reddish in color, and over 600 million miles in diameter (almost 1,000 times bigger than the Sun but cooler than the Sun). If Betelgeuse were at the center of our Solar System, it would extend beyond the orbit of Jupiter. It is 520 light-years from Earth. It is a variable star, varying in magnitude from 0.3 to 1.2 over a period of about 7 years, averaging about 0.70. It is the only star (other than our sun) for which we have surface images.
Deneb
Deneb (which means “tail” in Arabic) is the brightest star in the constellation Cygnus (the swan); Deneb is also referred to as alpha Cygni, and is the tail of the swan. This young, bright, white supergiant star is about 1,500 light years away. Deneb is about 60,000 times more luminous than the sun!
Polaris
Polaris (alpha UMi) is the current pole star for the Northern Hemisphere; it is 1 degree from the exact Northern celestial pole. In 1780, Sir William Herschel discovered that Polaris was a double star with a faint companion star. Polaris is a blue-green Cepheid variable star (its size brightness changes periodically, with period of 3.969778 days; it varyies between mag 1.92 and 2.07). Polaris has a relatively dim companion star (9th magnitude). Polaris’ distance from Earth has been estimated to be from 360 to 820 light years. At its brightest, Polairs is about 6,000 to 10,000 times brighter than our Sun. It is the larger star at the end of the handle of the Little Dipper (Ursa Minor). Polaris is also called the Lodestar or the Cynosure.
Orion
Orion, also known as “The Hunter,” is a constellation in the zodiac. The brightest stars in Orion are Rigel, Betelgeuse, and Bellatrix. The Horsehead Nebula and the nebulae M42 and M43 are also in this constellation.
Rigel
Rigel (beta Orionis) is the brightest star in the constellation Orion and one of the brightest stars in the sky. It is a blue (very hot) supergiant, over 60 million miles in diameter (almost 100 times bigger than the sun). It is more than 50,000 times more luminous than the Sun. It has an absolute magnitude of -7.1 and an apparent magnitude of +0.12. It is over 900 light-years from Earth.
Sirius
Sirius (meaning “scorching” in Greek), also known as the dog star, is the brightest star in the sky (except for the sun). It is in the constellation Canis Major (The Great Dog). Sirius is a main sequence star that is about 70 times more luminous than the sun. It is about 8.6 light-years from Earth. It has an apparent magnitude of -1.46 and an absolute magnitude of +1.4. Sirius has a companion star (called the Pup), which is a white dwarf.