Sirius (or Alpha Canis Majoris) is the brightest star in the night sky, with an apparent magnitude of -1.47, nearly twice as bright as Canopus, the second brightest star.
Facts About the Sirius Constellation
The name Sirius is derived from the ancient Greek word Σείριος. According to the Bayer designation, its name is α Canis Majoris (α CMa, or Alpha Canis Majoris). To the naked eye, it appears as a single star, but it is actually a binary star system, consisting of a main-sequence white star of spectral type A1V, named Sirius A, and a faint white dwarf companion, known as Sirius B, which has a spectral type of DA2.
Location of Sirius
Sirius shines brightly in the sky because it is an intrinsically luminous star and relatively close to Earth. With a distance of 2.6 parsecs (8.6 light years), the Sirius system is one of our closest neighbors. Sirius A has a mass approximately twice that of the Sun and an absolute magnitude of 1.42.
It is twice as bright as the Sun but still much dimmer compared to other luminous stars such as Canopus and Rigel. This system is about 200 to 300 million years old. Initially, it consisted of two blue stars. About 120 million years ago, the more massive star, Sirius B, exhausted its nuclear fuel and became a red giant, subsequently shedding its outer layers and collapsing into a white dwarf as it is today.
Commonly, Sirius is also known as the “Dog Star,” because it is located in the Canis Major constellation. Beyond the Sun, it is more frequently mentioned in the folklore and customs of various cultures than other stars. The appearance of Sirius in the night sky marks the flooding season of the Nile in ancient Egypt and is known as the “Dog Days” in summer in ancient Greece; in the Polynesian islands, it signifies winter and is an important day for maritime navigation in the Pacific.
Discovery of the Companion
American telescope maker and astronomer Alvan Graham Clark was the first to observe this faint companion, now known as Sirius B or “the Little Dog.” The visible star is referred to as Sirius A. Since 1894, there have been observations of perturbations in the apparent orbit of the Sirius system, suggesting the presence of a very small third companion, which has yet to be detected. The best-fit data indicates it should orbit Sirius A with a period of six years and have a mass of only 0.06 solar masses. This star would have a luminosity ten times dimmer than the white dwarf Sirius B, making it very difficult to observe. Recent observations have consistently failed to confirm the existence of this third object, but it does not rule out the possibility of its existence at a distance close enough to the Sirius system to be observable. In the 1920s, some observations suggested that this “third star” could be one of the background celestial objects.
Simulation of Sirius A and B using Celestia software
In 1915, Walter Sydney Adams, using a 1.5-meter reflector telescope at Mount Wilson Observatory, observed the spectrum of Sirius B and identified it as a faint white star. This led astronomers to conclude it was a white dwarf. The diameter of Sirius A was first measured by Robert Hanbury Brown and Richard Q. Twiss in 1959 at Jodrell Bank using an intensity interferometer. In 2005, astronomers using the Hubble Space Telescope determined that Sirius B has a diameter close to that of Earth, measuring 12,000 km, with a mass of 98% that of the Sun.
Observation
The brightness and glare of Sirius are due to the effects of optical instruments. With an apparent magnitude of -1.47, Sirius is the brightest star in the night sky, nearly twice as bright as the second brightest star, Canopus. However, it cannot outshine the Moon, Venus, Jupiter, Mercury, or Mars, which are significantly brighter than Sirius. Sirius is observable from most places on Earth; however, those located north of latitude 73 degrees will not be able to see it. It does not rise high in the sky for northern cities, only reaching an altitude of 13 degrees in Saint Petersburg. Together with Procyon and Betelgeuse, Sirius forms the three points of the Winter Triangle for observers in the northern hemisphere. With an approximate altitude of -17 degrees, Sirius is a circumpolar star for those at 73 degrees South latitude. In the southern celestial sphere at the beginning of June, Sirius may have nights when it sets after the Sun and nights when it rises before the Sun.
Sirius A and B captured by the Hubble Space Telescope. Sirius B is the smaller star below.
Sirius can be observed even during the daytime with the naked eye under suitable conditions. Ideally, the sky should be exceptionally clear, the observer should be at a high location, and the star must be overhead while the Sun is near the horizon.
The orbit of the Sirius system brings them as close as 3 arc seconds and as far apart as 11 arc seconds. When they come close, it is an opportunity to observe the faint white dwarf alongside its bright companion, requiring a telescope of at least 300 mm and exceptionally good conditions. The last close approach occurred in 1994, and the two stars are currently moving apart, which was an ideal time for observation through a telescope.
At a distance of 2.6 parsecs or 8.6 light-years, Sirius is among the six closest star systems to the Solar System and is the fifth closest system to us. This proximity is the primary reason for its brilliant brightness, similar to the nearby star system Alpha Centauri, in contrast to the distant supergiants like Canopus, Rigel, or Betelgeuse, which have extreme luminosity but are far away. Nevertheless, it is still about 25 times brighter than the Sun. The closest bright star to Sirius is Procyon, located 1.61 parsecs or 5.24 light years away. Voyager 2, launched in 1977 to study the four giant gas planets in the Solar System, is expected to reach a distance of 4.3 light years from Sirius after 296,000 years of travel.
System Composition
Sirius is a binary star system consisting of two white stars orbiting each other at a distance of about 20 astronomical units (approximately the distance between the Sun and Uranus) with an orbital period of only 50 years. The brighter star, known as Sirius A, is a main-sequence star of spectral type A1V, with a surface temperature of about 9,940 K. Its companion, Sirius B, has left the main sequence and entered the white dwarf phase. It is 10,000 times dimmer than its counterpart but has a greater mass. The age of this system is approximately 230 million years. In its early phases, they may have been two blue-white stars orbiting each other with a period of 9.1 years. This system emits more infrared radiation than predicted, as measured by the ground-based IRAS observatory. This effect may be due to the presence of dust in the system, which is considered an anomaly in a binary system.
Sirius A
Sirius A has a mass about 2.1 times that of the Sun. Its diameter, measured by an astronomical interferometer, reveals an angular diameter of approximately 5.936±0.016 arc minutes. Its relatively low rotational speed of 16 km/s prevents it from bulging at the equator. This distinguishes it from Vega, which has a similar size but rotates much faster at 274 km/s, leading to a significant equatorial bulge.
Stellar models suggest it formed from the collapse of a molecular cloud, and after 10 million years, energy was generated from nuclear fusion. The core has transitioned to using the CNO cycle (carbon-nitrogen-oxygen cycle, a type of cycle that occurs in the nuclear fusion reactions of stars) for energy production. It is estimated that Sirius A will exhaust its hydrogen fuel in the core after one billion years. At that point, it will transition to the red giant phase and eventually collapse into a white dwarf.
The spectrum of Sirius A shows a significant amount of heavy elements beyond helium, such as iron. Compared to the Sun, the iron-to-hydrogen ratio in the atmosphere of Sirius A is Fe/H=0.5, indicating it has an iron abundance of 316% relative to the iron found in the Sun’s atmosphere. Such a large quantity of metals should not exist on this star; however, it is possible that they are suspended in a thin convective layer near the surface.
Illustration of the Sirius system. Sirius A is the larger star compared to Sirius B.
Sirius B
Sirius B is one of the heaviest known white dwarfs. Despite its large mass, it is only about the size of Earth. Its current surface temperature is 25,200 K. However, Sirius B is gradually cooling and will eventually extinguish in about 2 billion years.
A white dwarf star is formed from a main-sequence star that has gone through the red giant phase. This occurs when Sirius B was about half its current age, approximately 120 million years ago. The original star weighed five times that of the Sun and was a type B star while it was still on the main sequence. As it transitioned through the red giant phase, Sirius B likely transferred a significant amount of metals to its companion star.
This star primarily consists of a carbon-oxygen mixture produced by helium fusion in its early stages. It is covered by a layer of lighter elements, with a completely different mass composition due to the strong gravitational pull at the surface. As a result, the atmosphere of Sirius now consists solely of hydrogen—the lightest element—and no other elements have been detected in the spectrum of this star.
The Sirius Supercluster
In 1909, Ejnar Hertzsprung became the first to suggest that Sirius is a member of the Ursa Major Moving Group, based on his observations of the system’s motion across the sky. The Ursa Major Moving Group is a collection of 220 stars with similar spatial movements, which were once a open star cluster but have since been dispersed due to gravitational influences. However, measurement results from 2003 and 2005 cast doubt on Sirius’s membership; the Ursa Major Group has an age estimated around 500±100 million years, while Sirius, with a metallicity similar to that of the Sun, can only be about half that age, making it too young to belong to this group. Nevertheless, it may be a member of the Sirius Supercluster, which includes stars such as Beta Aurigae, Alpha Coronae Borealis, Beta Crateris, Beta Eridani, and Beta Serpentis. This is one of three major star clusters within a radius of 500 light-years from the Sun. The other two clusters are the Hyades and Pleiades, containing hundreds of stars.
