Eta Carinae - Brightness Variations

Brightness Variations

One remarkable aspect of Eta Carinae is its changing brightness. It is currently classified as a luminous blue variable (LBV) binary star due to peculiarities in its pattern of brightening and dimming.

When Eta Carinae 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, at that point, one of the brightest stars in Carina. Subsequently it dimmed again, and by 1782 it appeared to have reverted to its former obscurity. In 1820, it was observed to be growing in brightness again. By 1827, it had brightened more than tenfold and reached its greatest apparent brightness in April 1843. With a magnitude of −0.8, it was the second brightest star in the night-time sky (after Sirius at 8.6 light years away), despite its enormous distance. (To put the relationship in perspective, the relative brightness would be like comparing a candle (representing Sirius) at 14.5 meters (48 feet) to another light source (Eta Carinae) about 10 kilometers (6.2 mi) away, which would appear almost as bright as the candle.)

Eta Carinae sometimes has large outbursts, the last of which appeared in 1841, at around the time of its maximum brightness. The reason for these outbursts is not known. The most likely explanation is that they are caused by a build-up of radiation pressure from the star's enormous luminosity. After 1843, Eta Carinae faded and between about 1900 and 1940 it was only 8th magnitude, invisible to the naked eye. A sudden and unexpected doubling of brightness was observed in 1998–1999. In 2007, at magnitude 5, Eta Carinae could be seen with the naked eye.

In 2008, the formerly clockwork regularity of the dimming was upset. Following its 5.52-year cycle, the star would normally have started its next dimming in January 2009, but the pattern was noticed starting early in July 2008 by the southern Gemini Observatory near La Serena, Chile. Spectrographic measurements showed an increase in blue light from superheated helium, which was formerly assumed to occur with the wind shock. However, if the cause is a binary star, it would be located too far away at this point in time for the wind to interact in so significant a fashion. There is some debate about the cause of the recent event.

In 2010, astronomers Duane Hamacher and David Frew from Macquarie University in Sydney showed that the Boorong Aboriginal people of northwestern Victoria, Australia, witnessed the outburst of Eta Carinae in the 1840s and incorporated it into their oral traditions as "Collowgulloric War", the wife of "War" (Canopus, the Crow — pronounced "Waah"). This is the only definitive indigenous record of Eta Carinae's outburst identified in the literature to date.

In 2011, light echoes from the 19th century Great Eruption of η Carinae were detected using the U.S. National Optical Astronomy Observatory's Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. Analysis of the reflected spectra indicated the light was emitted when η Carinae was a 5000 K G2-to-G5 supergiant, some 2000 K cooler than other supernova imposter events.