Eta Carinae

From Exampleproblems

Hubble Space Telescope image showing Eta Carinae and the bipolar Homunculus Nebula which surrounds the star. The Homunculus was created in an eruption of Eta Car whose light reached Earth in 1843. Eta Car itself appears as the white patch near the center of the image, where the 2 lobes of the Homunculus touch.

Eta Carinae (η Car / η Carinae) is a very large (100-150 times as much mass as the Sun) and bright (luminosity about four million times that of the Sun) luminous blue variable (LBV) star located about 7,500 light-years from Earth, in the constellation Carina (right ascension 10 h 45.1 m, declination −59°41m). The star is surrounded by a large, bright nebula, known as the Eta Carinae Nebula, the Keyhole Nebula, or NGC 3372. LBV stars are extraordinarily rare and are thought to approach the theoretical upper limit of stellar mass. As of July 2005, Eta Car is the fifth most luminous star yet discovered, and still the most massive, together with the Pistol Star.

One remarkable aspect of Eta Carinae is its changing brightness. When it was first catalogued in 1677 by Edmond Halley, it was of the 4th magnitude, but by 1730 observers noticed it had brightened considerably, and was now one of the brightest stars in Carina. From there it dimmed again, and by 1782 was back to its former obscurity, but in 1820 it started growing in brightness again. By 1827 it has brightened more than tenfold, and reached its greatest brightness in April 1843, when with a magnitude of −0.8 it was the second brightest star in the sky (after Sirius), despite its enormous distance (7,000–10,000 light-years). After that, it faded away, and between about 1900 and 1940 it was only of the 8th magnitude, and thus not visible to the unaided eye. Currently (2005) it is of magnitude 5–6, having suddenly and unexpectedly doubled its brightness in 1998–1999.

Eta Carinae sometimes has large outbursts, the last one just around its brightness maximum, in 1841. The reason for these outbursts is not yet known. The most likely possibility is believed to be that they are caused by built-up radiation pressure from the star's enormous luminosity.

Very large stars like Eta Carinae use up their fuel very fast as evidenced by their high luminosity, and the star is expected to go supernova or hypernova within about 1 million years of its first appearance (the total lifetime of the Sun is estimated at 10,000 million years which equals 10 billion years, half of which has passed now).

Spectrographic monitoring of Eta Carinae showed that some emission lines faded precisely every 5.52 years, and that this period was stable for decades. The star's radio emission, along with its X-ray brightness, also drop precipitously during these "events" as well. Many astronomers believe that these variations imply that eta Carinae is actually a binary star, in which a hot, lower mass star revolves around eta Car in a 5.52-year, highly eccentric elliptical orbit. However, this is somewhat controversial, and some astronomers believe that the variations could be produced by atmospheric changes in a single star.

Image:Etacar rxte lc.jpg

X-ray brightness variation of Eta Car, 1996-2004, as seen by the Rossi X-ray Timing Explorer. The bottom axis shows the time in Julian Day Number, and the upper axis shows cycle phase. The second X-ray minimum began on June 29, 2003.

A "spectroscopic minimum"/"X-ray eclipse" occurred in the midsummer of 2003. Astronomers organized a large observing campaign, which included every available space and ground-based observatory, including major observations with the Hubble Space Telescope, the Chandra X-ray Observatory, the INTEGRAL Gamma-ray space observatory, and the Very Large Telescope. Primary goals of these observations were to determine if in fact eta Car is a binary star (and if so, to identify the companion star), and to determine the physical mechanism behind the "spectroscopic minima", and to understand their relation (if any) to the large scale eruptions of the 19th century.