ANTARES (Alpha Scorpii). A brilliant jewel set within the Milky Way, Antares guides us to one of the great constellations of the sky, the Zodiac's Scorpius (or Scorpio), the celestial scorpion, one of the few constellations that actually looks like what it represents.
Antares, a class M (M1.5) red supergiant gleaming redly at the scorpion's heart, has a color similar to Mars. Since it is found within the Zodiac, which contains the apparent path of the Sun and planets, it is commonly mistaken for the red planet, a fact shown by its name, Antares, or "Ant-Ares," which means "like Mars," "Ares" being the Greek name for the god of war. This magnificent first magnitude (typically 0.96) star, shining opposite Betelgeuse, its counterpart in Orion, is ranked the 15th brightest in the sky. It is, however, a semi-regular variable that can change by several tenths of magnitude over a period of years. Its great distance of 600 light years reveals that it is truly luminous, to the eye over 10,000 times brighter than the Sun.
Because it is cool, only about 3600 degrees Kelvin at its surface, it radiates a considerable amount of its light in the invisible infrared. When that is taken into account, the star becomes some 60,000 times brighter than the Sun. A low temperature coupled with high luminosity tells us that the star must be huge, luminosity and temperature giving a radius of about 3 Astronomical Units. It is so big that astronomers can easily detect and measure the size of its apparent disk, which gives an even bigger radius of 3.8 AU, three-fourths the size of the orbit of Jupiter The difference is caused by uncertainties in distance, temperature, and the actual location of the mass-losing surface, as the star is slowly evaporating under a fierce wind that has encased it in a gas cloud, or nebula, that shines by light scattered from the ultraluminous star within. Buried within the wind is a fifth magnitude (5.5) hot class B (B2.5) companion star (only 3 seconds of arc away) that hides within Antares' bright glare.
The two are separated by roughly 550 AU and take perhaps 2500 years to orbit each other. The companion hollows out a small ionized region within the wind, and although blue-white, has the reputation of appearing green as a result of a contrast effect with its brilliant reddish mate. Antares, with a mass of 15 to 18 solar masses, probably does not have much time left to it. It is massive enough someday to develop an iron core and eventually to explode as a brilliant supernova. The event may be a million years off, an astronomical blink of an eye; or it may occur tonight, so keep a watch on one of the great stars of the nighttime sky. The companion, however, at around 7 to 8 solar masses, seems to be just below the supernova limit and will probably die as a massive white dwarf. See the Moon visit Antares.
Antares, a class M (M1.5) red supergiant gleaming redly at the scorpion's heart, has a color similar to Mars. Since it is found within the Zodiac, which contains the apparent path of the Sun and planets, it is commonly mistaken for the red planet, a fact shown by its name, Antares, or "Ant-Ares," which means "like Mars," "Ares" being the Greek name for the god of war. This magnificent first magnitude (typically 0.96) star, shining opposite Betelgeuse, its counterpart in Orion, is ranked the 15th brightest in the sky. It is, however, a semi-regular variable that can change by several tenths of magnitude over a period of years. Its great distance of 600 light years reveals that it is truly luminous, to the eye over 10,000 times brighter than the Sun.
Because it is cool, only about 3600 degrees Kelvin at its surface, it radiates a considerable amount of its light in the invisible infrared. When that is taken into account, the star becomes some 60,000 times brighter than the Sun. A low temperature coupled with high luminosity tells us that the star must be huge, luminosity and temperature giving a radius of about 3 Astronomical Units. It is so big that astronomers can easily detect and measure the size of its apparent disk, which gives an even bigger radius of 3.8 AU, three-fourths the size of the orbit of Jupiter The difference is caused by uncertainties in distance, temperature, and the actual location of the mass-losing surface, as the star is slowly evaporating under a fierce wind that has encased it in a gas cloud, or nebula, that shines by light scattered from the ultraluminous star within. Buried within the wind is a fifth magnitude (5.5) hot class B (B2.5) companion star (only 3 seconds of arc away) that hides within Antares' bright glare.
The two are separated by roughly 550 AU and take perhaps 2500 years to orbit each other. The companion hollows out a small ionized region within the wind, and although blue-white, has the reputation of appearing green as a result of a contrast effect with its brilliant reddish mate. Antares, with a mass of 15 to 18 solar masses, probably does not have much time left to it. It is massive enough someday to develop an iron core and eventually to explode as a brilliant supernova. The event may be a million years off, an astronomical blink of an eye; or it may occur tonight, so keep a watch on one of the great stars of the nighttime sky. The companion, however, at around 7 to 8 solar masses, seems to be just below the supernova limit and will probably die as a massive white dwarf. See the Moon visit Antares.
Antares in ancient cultures
Antares' name derives from the Greek Αντάρης, meaning "(holds) against Ares (Mars)", due to the similarity of its reddish hue to the appearance of the planet Mars. Its distinctive coloration has made the star an object of interest to many societies throughout history. According to ancient Arab tradition, Antares is the warrior-poet Antar's star. Many of the old Egyptian temples are oriented so that the light of Antares plays a role in the ceremonies performed there. Some writers[citation needed] claim that it is the "lance star" referred to in the Biblical book of Job. Antares was also known as Satevis in ancient Persia and was one of the four "royal stars" of the Persians around 3000 BC. It was also known as Jyeshtha in ancient India. In the religion of Stregheria, Antares is a fallen angel and quarter guardian of the western gate. In astrology Antares is one of the Behenian fixed stars and has the symbol .
Properties
Comparison between the red supergiant Antares and the Sun. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for size comparison.
Antares is a class M supergiant star, with a diameter of approximately 700 times solar. I.e., if Antares were placed in the centre of our solar system, its outer surface would lie between the orbits of Mars and Jupiter. Antares is approximately 600 light years from our solar system. Its visual luminosity is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the bolometric luminosity equals roughly 65,000 times that of the Sun. The mass of the star is calculated to be 15 to 18 solar masses.[2] Its large size and relatively small mass give Antares a very low average density.
The best time to view Antares is on or around May 31 of each year, when the star is at "opposition" to the Sun. At this time, Antares rises at dusk and sets at dawn, and is thus in view all night (depending on your position on Earth). For approximately two to three weeks on either side of November 30, Antares is not visible at all, being lost in the Sun's glare; this period of invisibility is longer in the Northern Hemisphere than in the Southern Hemisphere, since the star's declination is significantly south of the celestial equator.
Companion star
Antares has a hot blue companion star, Antares B, of spectral type B2.5 at a separation of about 2.9 arcseconds, or 550 AUs at Antares' estimated distance.[2] At magnitude 5.5, it is only 1/370th as bright visually as Antares A, although it shines with 170 times the Sun's luminosity. It is normally difficult to see in small telescopes due to Antares' glare, but becomes easy in apertures over 150 mm (6 in.).[3] The companion is often described as green, but this is probably a contrast effect.[2] Antares B can be observed with a small telescope for a few seconds during lunar occultations while Antares itself is hidden by the Moon; it was discovered during one such occultation on April 13 1819.
The orbit is poorly known, with an estimated period of 878 years.
Position on the ecliptic
Antares near the Sun, event which occurs on December 2 every year. Image from SOHO.
Antares is one of the 4 first magnitude stars which lie within 5° of the ecliptic and therefore can be occulted by the Moon and rarely by the planets. On 17 Nov 2400 Antares will be occulted by Venus.
Of the 21 first-magnitude stars, Antares now lies farthest in angular distance from any other first-magnitude star; i.e. it is possible to draw a larger circle centered around Antares without including any other first-magnitude star inside that circle, than around any other first-magnitude star. The nearest first-magnitude star to Antares is Alpha Centauri, lying approximately 39°6.75′ away. The high proper motion of Alpha Centauri is gradually increasing this angle. Before about March 2000, Achernar and Fomalhaut held this distinction of being the most isolated from other first-magnitude stars.
Antares' name derives from the Greek Αντάρης, meaning "(holds) against Ares (Mars)", due to the similarity of its reddish hue to the appearance of the planet Mars. Its distinctive coloration has made the star an object of interest to many societies throughout history. According to ancient Arab tradition, Antares is the warrior-poet Antar's star. Many of the old Egyptian temples are oriented so that the light of Antares plays a role in the ceremonies performed there. Some writers[citation needed] claim that it is the "lance star" referred to in the Biblical book of Job. Antares was also known as Satevis in ancient Persia and was one of the four "royal stars" of the Persians around 3000 BC. It was also known as Jyeshtha in ancient India. In the religion of Stregheria, Antares is a fallen angel and quarter guardian of the western gate. In astrology Antares is one of the Behenian fixed stars and has the symbol .
Properties
Comparison between the red supergiant Antares and the Sun. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for size comparison.
Antares is a class M supergiant star, with a diameter of approximately 700 times solar. I.e., if Antares were placed in the centre of our solar system, its outer surface would lie between the orbits of Mars and Jupiter. Antares is approximately 600 light years from our solar system. Its visual luminosity is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the bolometric luminosity equals roughly 65,000 times that of the Sun. The mass of the star is calculated to be 15 to 18 solar masses.[2] Its large size and relatively small mass give Antares a very low average density.
The best time to view Antares is on or around May 31 of each year, when the star is at "opposition" to the Sun. At this time, Antares rises at dusk and sets at dawn, and is thus in view all night (depending on your position on Earth). For approximately two to three weeks on either side of November 30, Antares is not visible at all, being lost in the Sun's glare; this period of invisibility is longer in the Northern Hemisphere than in the Southern Hemisphere, since the star's declination is significantly south of the celestial equator.
Companion star
Antares has a hot blue companion star, Antares B, of spectral type B2.5 at a separation of about 2.9 arcseconds, or 550 AUs at Antares' estimated distance.[2] At magnitude 5.5, it is only 1/370th as bright visually as Antares A, although it shines with 170 times the Sun's luminosity. It is normally difficult to see in small telescopes due to Antares' glare, but becomes easy in apertures over 150 mm (6 in.).[3] The companion is often described as green, but this is probably a contrast effect.[2] Antares B can be observed with a small telescope for a few seconds during lunar occultations while Antares itself is hidden by the Moon; it was discovered during one such occultation on April 13 1819.
The orbit is poorly known, with an estimated period of 878 years.
Position on the ecliptic
Antares near the Sun, event which occurs on December 2 every year. Image from SOHO.
Antares is one of the 4 first magnitude stars which lie within 5° of the ecliptic and therefore can be occulted by the Moon and rarely by the planets. On 17 Nov 2400 Antares will be occulted by Venus.
Of the 21 first-magnitude stars, Antares now lies farthest in angular distance from any other first-magnitude star; i.e. it is possible to draw a larger circle centered around Antares without including any other first-magnitude star inside that circle, than around any other first-magnitude star. The nearest first-magnitude star to Antares is Alpha Centauri, lying approximately 39°6.75′ away. The high proper motion of Alpha Centauri is gradually increasing this angle. Before about March 2000, Achernar and Fomalhaut held this distinction of being the most isolated from other first-magnitude stars.
