Star Facts: Mimosa

Beta Crucis
Image Credit: John Glossop

Mimosa (Beta Crucis) is the second most luminous star in the Southern Cross asterism that defines the constellation Crux, and the 20th most luminous star in the entire night sky. It is also a spectroscopic binary system consisting of the stars Beta Crucis A and Beta Crucis B, in which the primary component (A) is thought to be the hottest first magnitude star ever discovered with a temperature of 27,000 kelvins.

Quick Facts

• Constellation: Crux
• Coordinates: RA: 12h 47m 44s | Dec: -59° 41′ 19″
• Distance: 280 light years
• Star Type: (A) B0.5 III / (B) B2V
• Mass: (A) 16 sol/ (B) 10 sol
• Radius: (A) 8.4 sol
• Apparent Magnitude: +1.25 (Combined)
• Luminosity: (A) 34,000 sol
• Surface Temperature: (A) 27,000K
• Rotational Velocity: (A) 35 km/sec
• Age: 8 to 11 million years
• Other Designations: Becrux, ß Crucis, HR 4853, CPD-59°4451, HD 111123, FK5 481, SAO 240259, HIP 62434

Visibility

Mimosa is located in the southern constellation of Crux, which is the smallest of the 88 recognized constellations, but also one of its brightest and most beautiful members. This cross-shaped constellation be observed from between +20° and -90° of latitude, where it occupies just 68 square degrees west of Centaurus. However, due to precession, Crux was at one time visible to ancient Greek and Roman observers as far north as latitude 40° N or higher, with its stars once considered a part of the constellation Centaurus.

Physical Properties

Size, Mass and Luminosity

The principal component in the binary system, Beta Crucis A, is a blue-white giant (B0.5 III) located 280 light years from Earth that is about 8.4 times bigger than the Sun, and 16 times more massive. It is around 3,000 times brighter than our sun in terms of visible light, but more than 34,000 more energetic once radiation frequencies invisible to the naked eye are considered.

Beta Crucis A has a projected rotational velocity of around 35 km/sec, but since the orbital plane of the system is inclined by only about 10 degrees with respect to our line of sight, it is expected that the star’s actual azimuthal rotational velocity will be substantially higher. With a projected value of about 120 km/sec, this means that Mimosa will rotate once on its axis roughly every 3.6 days.

Mimosa is also a confirmed Beta Cephei variable star, but with an effective surface temperature of around 27,000K, it falls into the upper limit of the instability strip, which is a position on the Hertzsprung–Russell diagram where several types of variable stars are found. In the case of Mimosa, three distinctly different periods of pulsation, ranging between 4.03 – 4.59 hours, have been identified, none of which is related to the star’s high rotational velocity.

Evolutionary Stage

Being a B0.5 III-class star, Mimosas’ blue-white colour is characteristic of giant stars that have exhausted their supply of hydrogen fuel. In its present evolutionary phase, the star is emitting a strong stellar wind that is blowing off material at a rate that is about equivalent to one solar mass every one hundred million years or so. As a point of interest, the solar wind leaves the star at a speed of about 2,000 km/sec.

Binary System

Beta Crucis B, the secondary star in the system, is thought to be a blue main sequence dwarf star (B2V), with the two A and B components having an orbital period of about 5 years, in a highly eccentric orbit that brings the stars to within 5.4 AU and as far as 12 AU from each other. A recently discovered third companion star is thought to be a relatively light pre-main sequence star, while two more stars, located 44 and 370 arcseconds away, have been proposed as companion stars to the primary star, but recent observations seem to suggest that these stars are more likely to be line-of-sight companions than true members of the system. Like Acrux, the Beta Crucis system is suspected to be a member of the Lower Centaurus-Crux sub-group of the Scorpius-Centaurus Association.

History

Along with the other stars that make up the Southern Cross, Beta Crucis (Mimosa) was well known to Aboriginal people and islanders of the Southern Hemisphere. These days, it appears on the flags of Australia, New Zealand, Samoa and Papua New Guinea, as well as on the flag of Brazil, where it represents the State of Rio de Janeiro. Chinese observers place Beta Crucis in a four-star asterism known as the “Cross”, in which Mimosa is known as Shí Zì Jià san, meaning “the third star of Cross”.

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