Alpha Canis Majoris-시리우스

Alpha Canis Majoris - Sirius

At apparent magnitude -1.46, Sirius is the brightest star in our sky. It is the fifth nearest star system to our own, and contains the first white dwarf star discovered, Sirius B.

History and Mythology

The name "Sirius" comes from the Greek word for "searing" or "scorching". Sirius is also known as the "Dog Star", belonging to the constellation Canis Major, the Great Dog. In the northern hemisphere, Sirius first peeks above the eastern horizon on late summer mornings just before sunrise. For this reason, the long, hot days of summer are often called "dog days".

In many cultures, Sirius was used as a time marker for harvesting and celebration. It was an important marker in the ancient Egyptian calendar. The Egyptians knew that when Sirius rose just before dawn, the Nile was about to begin its yearly flood. (This is no longer true, however, because precession - the 26,000-year wobble of the Earth's axis - has shifted the start of the seasons relative to the positions of the "fixed" stars.)

Today, Sirius and the bright star Procyon in Canis Minor are visible on clear winter nights, separated by the band of light that is the Milky Way. An old Arabic legend explains their origin: the two stars were young sisters who decided one day to follow their older brother into the field. The sisters soon became lost. When they neared a wide river in the sky, the older sister, Sirius, jumped into the water and swam to the other side. Procyon, afraid of the water, stayed behind, and so she was forever parted from her sister. The two separated sisters now look at each other across the celestial river of the Milky Way. Procyon's tears are said to add to the waters that cause the yearly flooding of the Nile.

Physical Characteristics

At a distance of 8.7 light years, Sirius is the fifth nearest star system. Its brilliance in our sky is due primarily to its close proximity; its actual luminosity of about 23 suns is not extraordinary, as naked-eye stars go. Sirius is a typical, hydrogen-fusing main-sequence star of spectral class A1 V, with a surface temperature of 9,900 K. It has a diameter 1.8 times the Sun's, and a mass of 2.35 times the Sun's. With a minimum equatorial rotation speed of 16 km/sec, Sirius turns in under 5.5 days, again typical for a star of its class. Sirius is metal rich, with an iron content triple the Sun's, indicating that it is relatively young.

Sirius compared in size to the Sun (bottom left) and other well-known bright stars.
Being nearby, Sirius appears to move relatively quickly, about 1.3" per year. This "proper motion" was discovered by the 17th century astronomer Edmund Halley. By comparing its modern position against the position recorded by the ancient Greek astronomer Ptolemy, Halley noted that Sirius had moved southward by 30' (about the diameter of the moon) over the intervening 1800 years.

In 1868, Sirius also became the first star to have its radial velocity measured, by observing the Doppler shift in its spectrum. Spectroscopy shows that Sirius is moving toward the solar system at about 7.6 km/sec. Sirius shares the space motion of the widely-scattered Ursa Major Moving Group, which includes the five central stars of the Big Dipper, as well as α Oph, β Aur, α CrB, and δ Leo.

Sirius B

Sirius has a famous 8th-magnitude companion star, Sirius B, which is sometimes referred to as the "Pup". In 1844, Sirius was observed to have a periodic wobble in its motion, indicating that an unseen companion was affecting its motion. In 1862, the famous telescope maker Alvan Clark detected the faint companion while testing a new 18-inch telescope.

The apparent orbit of Sirius B relative to A. (NASA)
Sirius B is a challenging object to observe in small telescopes because of the great magnitude difference and close separation (2.5" to 11.3") between it and Sirius A. The pair may be resolved for a few seconds, but air currents keep them combined into an elongated blur. Their separation was closest in the year 1995, at 3.1", but widened again to 4.6" by the start of the 21st century. Sirius B's average distance from Sirius A is about 20 AU, with an orbital period of approximately 50 years.

Sirius B was the first white dwarf star discovered. Its surface is actually hotter than Sirius A's - a scorching blue-white 24,800 K - yet its total luminosity is only 1/360th of our Sun's. The only way Sirius B can be both hot and dim is to be small - only 92% the size of Earth. From its observed orbital motion and Kepler's laws, Sirius B must have a mass nearly equal to the Sun's - yet its diameter is 50 times smaller. This means that Sirius B has an incredibly high density: about 25 tons per cubic inch, compared to the Sun's density of about 0.5 ounce per cubic inch.

White dwarf stars represent the final stage in the evolution of sunlike stars, and are the remnants of stellar cores that have run out of fuel. Without the radiation pressure released by nuclear fusion to counteract the pull of gravity, they collapse to the point where, essentially, the empty space inside their atoms has been squeezed out. They are composed of "degenerate" carbon and oxygen atoms, supported against further collapse only by quantum mechanics, slowly cooling as they radiate their remaining heat into space until the end of time.

Evolution and Controversy

To have reached this state before Sirius A, Sirius B must once have been the more massive and luminous of the pair, showing that stars lose considerable mass as they age. Given the system's 250 million year age, Sirius B may have once been a hot class B3 star containing as much as 6 or 7 solar masses. Over its lifetime, it has lost over 80% of its matter into interstellar space through stellar winds, particularly during its red giant phase. Within about a billion years, Sirius A will also exhaust its core hydrogen, turn into a red giant, puff away its outer layers into space, and leave its remaining core behind as a second white dwarf.

The positions of the closest star systems, including Sirius, relative to our Sun. (ESO)
Some mystery still surrounds our understanding of the Sirius system. Many ancient records, including those of the Greek astronomer Ptolemy, describe the star as being reddish in color, like the red giant stars Betelgeuse and Antares. If these records are not mistaken, there is a discrepancy, since Sirius appears blue-white today. It is doubtful that stellar evolution could be responsible for this change, because the timescale of thousands of years is too short, and there is no sign of the nebulosity that would be expected in the system. (There is some faint nebulosity, NGC 2327, appearing near Sirius, but it merely lies along the same line of sight and is not physically related.)

Another proposed explanation involves interaction with a third red-dwarf component - but a search for faint companions by the Hubble Space Telescope found no evidence for any Jupiter- or brown-dwarf sized objects in the system. A third explanation involves an intervening dust cloud, dimming and reddening the system's light, which has since moved out of our line of sight due to its (and the system's) proper motion. But no direct evidence for such a cloud exists either, and so the "Red Sirius" controversy remains unexplained.