Eta Carinae
Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around 7,500 light-years (2,300 parsecs) distant in the constellation Carina. Previously a 4th-magnitude star, it brightened in 1837 to become brighter than Rigel, marking the start of its so-called "Great Eruption". It became the second-brightest star in the sky between 11 and 14 March 1843 before fading well below naked-eye visibility after 1856. In a smaller eruption, it reached 6th magnitude in 1892 before fading again. It has brightened consistently since about 1940, becoming brighter than magnitude 4.5 by 2014.
The Homunculus Nebula, surrounding Eta Carinae, imaged by WFPC2 at red and near-ultraviolet wavelengths Credit: Jon Morse (University of Colorado) & NASA Hubble Space Telescope | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Carina |
| Right ascension | 10h 45m 03.591s |
| Declination | −59° 41′ 04.26″ |
| Apparent magnitude (V) | −1.0 to ~7.6 4.8 (2011) 4.6 (2013) 4.3 (2018) |
| Characteristics | |
| Evolutionary stage | Luminous blue variable |
| Spectral type | variable (LBV) + O (WR?) |
| Apparent magnitude (U) | 6.37 |
| Apparent magnitude (B) | 6.82 |
| Apparent magnitude (R) | 4.90 |
| Apparent magnitude (J) | 3.39 |
| Apparent magnitude (H) | 2.51 |
| Apparent magnitude (K) | 0.94 |
| U−B colour index | −0.45 |
| B−V colour index | +0.61 |
| Variable type | LBV & binary |
| Astrometry | |
| Radial velocity (Rv) | −125.0 km/s |
| Proper motion (μ) | RA: −17.6 mas/yr Dec.: 1.0 mas/yr |
| Distance | 7,500 ly (2,300 pc) |
| Absolute magnitude (MV) | −8.6 (2012) |
| Orbit | |
| Primary | η Car A |
| Companion | η Car B |
| Period (P) | 2,022.7±1.3 days (5.54 yr) |
| Semi-major axis (a) | 15.4 AU |
| Eccentricity (e) | 0.9 |
| Inclination (i) | 130–145° |
| Periastron epoch (T) | 2009.03 |
| Details | |
| η Car A | |
| Mass | ~100 M☉ |
| Radius | ~240 (60 – 881) R☉ |
| Luminosity | 4.6 million (2.96 million – 4.1 million) L☉ |
| Temperature | 9,400–35,200 K |
| Age | <3 Myr |
| η Car B | |
| Mass | 30–80 M☉ |
| Radius | 14.3–23.6 R☉ |
| Luminosity | <1 million L☉ |
| Temperature | 37,200 K |
| Age | <3 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
At declination −59° 41′ 04.26″, Eta Carinae is circumpolar from locations on Earth south of latitude 30°S (for reference, the latitude of Johannesburg is 26°12′S), and is not visible north of about latitude 30°N, just south of Cairo, which is at a latitude of 30°2′N.
The two main stars of the Eta Carinae system have an eccentric orbit with a period of 5.54 years. The primary is an extremely unusual star, similar to a luminous blue variable (LBV). It was initially 150–250 M☉, of which it has already lost at least 30 M☉, and it is expected to explode as a supernova in the astronomically near future. This is the only star known to produce ultraviolet laser emission. The secondary star is hot and also highly luminous, probably of spectral class O, around 30–80 times as massive as the Sun. The system is heavily obscured by the Homunculus Nebula, which consists of material ejected from the primary during the Great Eruption. It is a member of the Trumpler 16 open cluster within the much larger Carina Nebula.
Although unrelated to the star and nebula, the weak Eta Carinids meteor shower has a radiant very close to Eta Carinae.