Sirius 2

Greg Bacon, STScI,
ESA, NASA





Large illustration.





An exterrestrial visitor
to the Sirius System could
see the "Summer Triangle"
of bright stars with dim Sol
in the distance (more).

The bright star is the title member of the Sirius stellar moving group (also known as the Sirius Super Cluster or Ursa Major star stream), which include all five stars of the Great Dipper as well as Gemma and are mostly around 490 million years old and all moving towards the galactic center. Athough Ejnar Hertzsprung (1873-1967) claimed that Sirius was a likely member of the Ursa Major moving group as early as 1909, a 2003 study of possible moving group members using HIPPARCOS' parallax data led by Jeremy King was not able to confirm the system's membership (Ken Croswell, Astronomy.com, March 2005), and the Sirius system appears to be too young, only about half the apparent age of the Ursa Major star stream (Liebert et al, 2005; and Ken Croswell, 2005). (See Akira Fujii's color photo of Sirius -- at the top center of photo.)

VLTI, ESO




Larger and jumbo images.




Sirius A is substantially larger
than Sol, although its white
dwarf companion B is smaller
than Jupiter (more from ESO).

AB Binary Star System

After analyzing the motions of Sirius from 1833 to 1844, Friedrich Wilhelm Bessel (1784-1846) concluded that it had an unseen companion. Sirius B itself was not actually observed until January 31, 1862 by Alvan Graham Clark (1832-1897), who was testing a new 18-inch telescope built for Dearborn Observatory by the famous company founded by his father (Alvan Clark) and later run by his brother and himself. Clark was soon honored with the Lalande prize of the French Academy for his discovery. Star B's peculiar high temperature, small size, and great density, however, were not established until 1925 by Walter Sydney Adams (1876-1956).

McDonald Observatory -- larger image



Sirius is a binary star system.

Sirius is composed of a main-sequence star and a white dwarf stellar remnant. They form a close binary, Alpha Canis Majoris A and B, that is separated "on average" by only about 20 times the distance from the Earth to the Sun -- 19.8 astronomical units (AUs) of an orbital semi-major axis -- which is about the same as the distance between Uranus and our Sun ("Sol"). The companion star, is so dim that it cannot be perceived with the naked eye.

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	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Solar)	Diameter (Solar)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
AB Mass Center	0.0	...	...	...	...	...	...	...	...
Sirius A	6.43	50.1	0.592	136.5	2.02-2.14	1.68	...	...	1-7.4
Inner HZ Edge?	2.7	3.0-3.1	0	136.5	...	...	...	...	...
Outer HZ Edge?	5.8	9.5-9.8	0	136.5	...	...	...	...	...
Sirius B	13.4	50.1	0.592	136.5	1.03	0.0084	...	...	1-7.4

Sirius A

Alpha Canis Majoris A, the Dog Star, is the brightest star in the night sky as well as the brightest star in its constellation. Unlike Sol, it is a slightly bluish, white main sequence dwarf star of spectral and luminosity type A0-1 Vm. This relatively large star has 2.02 (+/- 0.03, Liebert et al, 2005) to 2.14 times Sol's mass, but RECONS estimates a mass of 2.0 Solar based on luminosity, while the NASA Star and Exoplanet Database calculated 2.4 Solar-masses based on David F. Gray). It probably has around 1.68 to 1.71 (+/- 0.013) times Sol's diameter (Kervalla et al, 2003; and Gatewood and Gatewood, 1978), although the NASA Star and Exoplanet Database derived 1.80 +/-0.05 Solar-masses based on luminosity, using Kenneth R. Lang, 1980). According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 2491, diameter estimates vary from 0.00560 to 0.00589". Compared to Sol, Sirius A is hotter and more than 21 times brighter (with 23.8 times Sol's bolometric luminosity (NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980). It may be only about 225 to 250 million years old (Liebert et al, 2005; and Ken Croswell, 2005), but being so much bigger and hotter than Sol, the star will exhaust its core hydrogen within only a billion years and turn into a red giant or Cepheid variable before puffing away its outer layers to reveal a remnant core as a white dwarf.

NASA Observatorium




See a discussion of
the "main sequence"
as part of stellar
evolution and death.

Dust has been detected in this binary system (Backman et al, 1986; and Kuchner and Brown, 2000 -- in postscript). Sirius A is rich in elements heavier than hydrogen ("metals rich"), as it has about one to 7.4 times the iron abundance of Sol (Cayrel de Strobel et al, 1991, pages 285-286). It was probably enriched by its companion star, which was once bigger and hotter than Sirius A and so evolved and "burnt out" even faster. Sirius B manufactured lots of heavier elements which it puffed out into space and onto Sirius A before becoming a white dwarf.

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA
(Sirius B is a young white dwarf, a remnant stellar core, which enriched Sirius A with elements
heavier than hydrogen when it cast off its outer gas layers, like planetary nebula NGC 2440)

Sirius A is considered to be a variable star because its companion can boost its brightness when located in front of the star as observed from Earth. The distance separating Sirius A from its companion varies between 8.1 and 31.5 AUs as the two swing around in a highly eccentric orbit (e= 0.59) that takes 50.1 years to complete (Willem Henrik van den Bos, 1960; in the new Sixth Catalog of Visual Orbits of Binary Stars). (See a larger, interactive animation of the orbits of Stars A and B, and a possibly disrupted habitable zone around Star A.) Useful star catalogue numbers and designations include: Alp or Alf CMa, 9 CMa, HR 2491, Gl 244 A, Hip 32349, HD 48915, BD-16 1591, SAO 151881, FK5 257, LHS 219, and ADS 5423 A.

Lick Observatory (Lick 3-m Telescope)

Short exposure on left reveals the faint white dwarf Sirius B, then located about 25 AUs
below Sirius A, which is absent in the overexposed, longer exposure on right

Disrupted Habitable Zone around Star A?

According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Sirius A's habitable zone is located relatively far from the star at around 2.7 AUs from the star, while the outer edge lies even farther out at around 5.8 AUs. The distance from Sirius A where an Earth-type planet would be comfortable with liquid water is about 4.25 AUs (at about the distance of Jupiter from the Sun in the Solar System), where its orbital period would be about 6.0 years long. The orbit of any protoplanet forming around Sirius A at that distance could have been disrupted by the close orbit of Sirius B, and also probably during B's mass loss when it became a nova then a white dwarf. Even if it is possible for an Earth-type planet to orbit youthful Sirius A and develop life, it is likely to be primitive single-cell, anaerobic bacteria under constant bombardment by meteorites and comets as Earth was for the first billion years. Since there is unlikely to be free oxygen in the atmosphere of such a planet, it probably would not have an ozone layer (O₃) although Sirius A puts out a lot more hard radiation (especially ultraviolet) than Sol. (For an illustrated discussion, see Christoph Kulmann's web page on the unlikely habitable zone around Sirius A.)

Justin Cantrell,
Todd J. Henry,
RECONS




Larger illustration.




In general, brighter stars
like A-spectral-type Sirius A
have wider habitable zones
than dimmer ones, if for much
shorter lifetimes (more).

Sirius B

Nicknamed "the Pup" as the much smaller companion to the Dog Star, this much dimmer object is a white dwarf (DA2-5 or A2-5 VII). It is more than 8,200 times fainter than Sirius A and 360 times fainter than Sol. Compared with our Sun, it appears to have the same mass (97.8 to 103.4 percent, Hubble press release; Barstow et al, 2005; Liebert et al, 2005; and Holberg et al, 1998) but less than one percent (0.0864 +/- 0.012) of its diameter (Barstow et al, 2005; and Holberg et al, 1998). In fact, Sirius B's diameter of about 11,700 km (about 7,300 miles) is about 92 percent of Earth's. Its mass and diameter are consistent with the theoretical size for a carbon-core white dwarf, one that may have evolved from a 5.05 +0.374/-0.276 Solar-mass, B-type main-sequence star about 124 +/- 5 million years ago, after 101 to 126 million years as a giant star (Liebert et al, 2005; and Ken Croswell, 2005). Hence, Sirius B was once brighter than Regulus A, currently a B7 main-sequence star. While now tiny compared to main sequence stars, white dwarf stars are actually intensely hot, but without the internal heat of fusion to keep them burning, they gradually cool and fade away.

NASA, CXC, and SAO

(larger x-ray image)

Sirius B is a much brighter
source of x-rays than Sirius A.
For more information, see the
Chandra X-Ray Observatory's
photo album and an
Astronomy Picture of the Day.

Sirius B was the first White Dwarf to be discovered, but it took almost a century from its discovery to the wide acceptance of its nature as a stellar remnant. The star's calculated average density is 92,000 times that of Sol's, so that one cubic inch of its material would weigh about 15 tons at the surface of the Earth. Useful catalogue numbers include Gl 244 B and ADS 5423 B.

NASA Observatorium





See a discussion of
"stellar remnants"
as part of stellar
evolution and death.

Hunt for Sirius C and Substellar Companions

According to Duchner and Brown (2000 preprint -- in postscript), three analyses of the proper motion of Sirius found a perturbation in the orbit of Sirius B with a period around six years (Ch. Volet, 1932; Walbaum and Duvent, 1983; and Benest and Duvent, 1995). The analyses did not resolve whether the perturbing body orbits Sirius A or B, although dynamical simulations suggest that stable orbits exist around both stars at circumstellar distances up to more than half the binary system's closest separation of 8.1 AUs (Daniel Benest, 1989). Because ancient astronomers believed that Sirius was red in color as late as 2,000 years ago, some investigators wonder if the system may have a third stellar component, Sirius C, with about five percent of Sol's mass that implies a spectral type M5-9 in a six-year elliptical orbit around Sirius A (Benest and Duvent, 1995). A search for faint companions using the Hubble Space Telescope found no supporting evidence for a large Jupiter or brown dwarf sized object, although the observed positions of Sirius AB -- Gl 244 AB -- differed from published orbital elements (Schroeder et al, 2000). In 2008, a search using high-contrast, infrared imaging for a planet below 10 Jupiter-masses within 25 AUs of the binary stars was negative (Bonnet-Bideaud and Pantin, 2008).

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