Mimosa (star)

Mimosa

Template:Location mark Location of Mimosa (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Crux
Pronunciation	Template:IPAc-en
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	1.25 (1.23 - 1.31)
Characteristics
Spectral type	B0.5 III / B2V
U−B Template:Engvar	−0.98
B−V Template:Engvar	−0.23
Variable type	β Cep
Astrometry
Radial velocity (Rv)	+15.6 km/s
Proper motion (μ)	RA: −42.97 mas/yr Dec.: −16.18 mas/yr
Parallax (π)	11.71±0.98 mas
Distance	Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)	−3.92
Details
A
Mass	Template:Val Template:Solar mass
Radius	Template:Val Template:Solar radius
Luminosity	Template:Val Template:Solar luminosity
Surface gravity (log g)	Template:Val cgs
Temperature	28,840 K
Metallicity [Fe/H]	−0.08 dex
Rotation	3.6 days
Rotational velocity (v sin i)	35 km/s
Age	Template:Val Myr
B
Mass	10 Template:Solar mass
Surface gravity (log g)	4.0 cgs
Temperature	23,000 K
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
Template:Odlist
Database references
SIMBAD	data

Mimosa is the second-brightest object in the southern constellation of Crux (after Acrux), and the 20th-brightest star in the night sky. It has the Bayer designation β Crucis, which is Latinised to Beta Crucis and abbreviated Beta Cru or β Cru. Mimosa forms part of the prominent asterism called the Southern Cross. It is a binary star or a possible triple star system.

Nomenclature

β Crucis (Latinised to Beta Crucis) is the system's Bayer designation. Although Mimosa is at roughly −60° declination, and therefore not visible north of 30° latitude, in the time of the ancient Greeks and Romans it was visible north of 40° due to the precession of equinoxes, and these civilizations regarded it as part of the constellation of Centaurus.

It bore the traditional names Mimosa and the historical name Becrux Template:IPAc-en. Mimosa, which is derived from the Latin for 'actor', may come from the flower of the same name. Becrux is a modern contraction of the Bayer designation. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Mimosa for this star.

In Chinese, (), meaning Cross, refers to an asterism consisting of Acrux, Mimosa, Gacrux, and δ Crucis. Consequently, Mimosa itself is known as (, Template:Langx).

The Mursi people of Ethiopia call this star Thaadoi; it forms an asterism with δ Crucis (Imai), β Centauri (Waar), and α Centauri (Sholbi).

Stellar system

File:Deep Crux wide field with fog.jpg

The constellation Crux

Based on parallax measurements, Mimosa is located at a distance of Template:Convert from the Earth. In 1957, German astronomer Wulff-Dieter Heintz discovered that it is a spectroscopic binary with components that are too close together to resolve with a telescope. The pair orbit each other every 5 years with an estimated separation that varies from 5.4 to 12.0 Astronomical Units. The system is only 8 to 11 million years old.

The primary, β Crucis A, has a stellar classification of B0.5 III, classifying it as a blue giant that exausted the hydrogen supply at its core. Asteroseismic observations have measured its mass to be 15 times the Sun's mass, its radius to be between 7.3 and 8.9 times the Sun's radius and its age to be 11 million yearsTemplate:Emdashit is the most massive star with an age derived by this method. Mimosa has sufficient mass to explode as a supernova, which might occur in roughly 6 million years. The projected rotational velocity of this star is about 35 km s⁻¹. However, the orbital plane of the pair is only about 10°, which probably means the inclination of the star's pole is also likely to be low. This suggests that the azimuthal rotational velocity is quite high, at about 120 km s⁻¹. This star has an orbital period of only about 3.6 days.

File:BetaCruLightCurve.png

A light curve for Beta Crucis, plotted from TESS data

β Crucis A is a known β Cephei variable, although with an effective temperature of about 27,000 K it is at the hot edge of the instability strip where such stars are found. It has three different pulsation modes, none of which are radial. The periods of all three modes are in the range of 4.03–4.59 hours. The high temperature of the star's outer envelope is what gives the star the blue-white hue that is characteristic of B-type stars. It is generating a strong stellar wind and is losing about Template:Solar mass per year, or the equivalent of the mass of the Sun every 100 million years. The wind is leaving the system with a velocity of 2,000 km s⁻¹ or more.

The secondary, β Crucis B, may be a main sequence star with a stellar class of B2. In 2007, a third companion was announced, which may be a low mass, pre-main sequence star. The X-ray emission from this star was detected using the Chandra X-ray Observatory. Two other stars, located at angular separations of 44 and 370 arcseconds, are likely optical companions that are not physically associated with the system. The β Crucis system may be a member of the Lower Centaurus–Crux sub-group of the Scorpius–Centaurus association. This is a stellar association of stars that share a common origin.

In culture

Mimosa is represented in the flags of Australia, New Zealand, Samoa and Papua New Guinea as one of five stars making up the Southern Cross. It is also featured in the flag of Brazil, along with 26 other stars, each of which represents a state. Mimosa represents the State of Rio de Janeiro.

A vessel named MV Becrux is used to export live cattle from Australia to customers in Asia. An episode dedicated to the vessel features in the television documentary series Mighty Ships.

External links

• http://jumk.de/astronomie/big-stars/becrux.shtml