Alpha Canis Minoris - Procyon

Procyon is a yellow-white star of magnitude 0.34. It is the brightest star in Canis Minor, and the 8th brightest in the sky. The reason for its brightness is not its intrinsic luminosity but its closeness to the Sun; at a distance of 11.4 light years, Procyon is the 14th-closest star system to our own. Its closest neighbor in space is Luyten's star, 1.11 light years away.

Name and Mythology

The name Procyon comes from Greek, and means "before the dog", referring to the fact that Procyon rises slightly before the "Dog Star", Sirius, in Canis Major. Procyon is the eastern anchor of the "Winter Triangle" with Betelgeuse and Sirius; the Milky Way separates Procyon from Sirius. According to an Arab myth, the two stars were young sisters, who decided one day to follow their older brother into the fields, and became lost. When they neared a wide river in the sky (the Milky Way) the older sister, Sirius, jumped into the water and swam to the other side. Procyon, afraid of the water, stayed behind and so was forever parted from her sister. Procyon's tears are said to add to the waters that cause the yearly flooding of the Nile.

Properties

Procyon is actually a binary star system, consisting of a white main sequence star of spectral type F5 IV-V (Procyon A), and a faint white dwarf companion of spectral type DA (Procyon B). Procyon B's existence was inferred from irregularities in the proper motion of Procyon A, which were first detected in 1840 - long before Procyon B was observed directly. Though B's orbit was computed as far back as 1861, it was not visually confirmed until 1896, with the 36-inch refractor at Lick Observatory.

Procyon B is even more difficult to observe than Sirius B, due to a greater apparent magnitude difference (at least 15,000 times fainter than Procyon A) and smaller angular separation (4.3"). The average separation of the two components is 15 AU - a little less than the distance between Uranus and the Sun - though their eccentric orbit (e=0.407) carries them as close as 9 AU and as far as 21 AU over its 40.82-year period.

Procyon A is 1.5 times the mass, twice the diameter, and 7.5 times more luminous than the Sun. With an equatorial rotation speed of at least 3.2 km/sec, Procyon's rotational period could be as long as 33 days. It is surrounded by a hot solar-type corona heated to 1.6 million K.

Procyon A appears to be a relatively young star, around 1.7 billion years old. It is bright for its spectral class, suggesting that it is a subgiant that has completely fused its core hydrogen into helium, and begun to expand as fusion moves outside the core. As it continues to expand, the star will eventually swell to about 80 to 150 times its current diameter, and become a red giant before expelling its outer layers and much of its mass into interstellar space. This will probably happen within 10 to 100 million years.

Procyon A is also a BY Draconis-type variable star. In 2004, Canada's orbital MOST satellite telescope carried out a 32-day survey of Procyon A, intended to confirm solar-like oscillations in its brightness. No oscillations were detected, and models of stellar oscillations for measuring stellar oscillations are being reexamined.

Procyon B

Procyon A's companion is a white dwarf, like Sirius B, with only 1/1700th the luminosity of the Sun. At 0.62 solar masses, Procyon B is considerably less massive than Sirius B. However, the peculiarities of degenerate matter ensure that it is larger, with an estimated radius of ~8600 km, versus ~5800 km for Sirius B. This is only about 30% larger than the Earth; with a stellar mass squeezed into a planetary volume, Procyon B's average density is over two tons per cubic inch.

At a surface temperature of 7740 K, Procyon B is also much cooler than Sirius B; this is a testament to its lesser mass and greater age. Procyon B was once bigger and hotter than Procyon A, perhaps originally around 2.1 solar masses, and so has evolved faster. It manufactured lots of heavier elements which it puffed out into space and onto Procyon A before becoming a white dwarf. Indeed, Procyon A is rich in "metals" (elements heavier than hydrogen), with about 1.4 times the Sun's iron abundance.

A recent search for faint companions using the Hubble Space Telescope found no evidence for any large Jupiter- or brown dwarf- sized objects. However, dust has been detected in the system.