page_title: Homunculus Nebula
section_title: Distance
hierarchical_section_title: Homunculus Nebula / Distance
caption_attribution_description: English: At the turn of the 19th century, the binary star system Eta Carinae was faint and undistinguished. In the first decades of the century, it became brighter and brighter, until, by April 1843, it was the second brightest star in the sky, outshone only by Sirius (which is almost a thousand times closer to Earth). In the years that followed, it gradually dimmed again and by the 20th century was totally invisible to the naked eye. The star has continued to vary in brightness ever since, and while it is once again visible to the naked eye on a dark night, it has never again come close to its peak of 1843. The larger of the two stars in the Eta Carinae system is a huge and unstable star that is nearing the end of its life, and the event that the 19th century astronomers observed was a stellar near-death experience. Scientists call these outbursts supernova impostor events, because they appear similar to supernovae but stop just short of destroying their star. Although 19th century astronomers did not have telescopes powerful enough to see the 1843 outburst in detail, its effects can be studied today. The huge clouds of matter thrown out a century and a half ago, known as the Homunculus Nebula, have been a regular target for Hubble since its launch in 1990. This image, taken with the Advanced Camera for Surveys High Resolution Channel is the most detailed yet, and shows how the material from the star was not thrown out in a uniform manner, but forms a huge dumbbell shape. Eta Carinae is not only interesting because of its past, but also because of its future. It is one of the closest stars to Earth that is likely to explode in a supernova in the relatively near future (though in astronomical timescales the “near future” could still be a million years away). When it does, expect an impressive view from Earth, far brighter still than its last outburst: SN 2006gy, the brightest supernova ever observed, came from a star of the same type. This image consists of ultraviolet and visible light images from the High Resolution Channel of Hubble’s Advanced Camera for Surveys. The field of view is approximately 30 arcseconds across.
context_page_description: The Homunculus Nebula is a bipolar emission and reflection nebula surrounding the massive star system Eta Carinae, about 7,500 light-years from Earth. The nebula is embedded within the much larger Carina Nebula, a large star-forming H II region. From the Latin homunculus meaning Little Man, the nebula consists of gas which was ejected from Eta Carinae in 1841. It also contains dust which absorbs much of the light from the extremely luminous central stellar system and re-radiates it as infra-red. It is the brightest object in the sky at mid-IR wavelengths.
  Within the Homunculus is a smaller Little Homunculus, and within that a shell of shocked material from stellar winds that has been called Baby Homunculus.
context_section_description: The Homunculus and Eta Carinae are assumed to be at approximately the same distance as Trumpler 16 and the Carina Nebula, but the distances to these objects are not known with any great accuracy.  Instead, the distance of the Homunculus Nebula itself can be calculated using measurements of its expansion. The velocity of particular locations within the thin shell of the Homunculus lobes can be measured using the doppler shift of the spectral lines at that point, assuming the lobes are symmetrical.  There are two different doppler shifts visible in the spectrum: one for direct emission lines; and one for reflected lines from Eta Carinae A.  The direct line doppler shift gives the expansion velocity of the shell project onto the line of sight.  Assuming an origin in Eta Carinae's Great Eruption and a constant expansion velocity, this gives the linear distance of the shell from the central star projected along the line of sight.  The difference between the reflected line doppler shift velocity and the direct line velocity gives the distance of the shell from the central star, again assuming expansion at constant velocity since the Great Eruption.
  Observations of the spectrum of the Homunculus at a particular angular distance from the central star has shown the actual linear distance of that point from the central star, which defines the distance.  Values obtained using this method are around 2.3 kpc with a margin of error around 100 pc.
  The same calculations also return the inclination of the axis of the Homunculus relative to the line of sight.  This turns out to be 41°, or 49° relative to the plane of the sky, which means it is slightly more "end on" than "side on".
caption: Homunculus Nebula around Eta Carinae (HST UV and visual image)
exif: {"Image ImageDescription": "At the turn of the 19th century, the binary star system Eta Carinae was faint and undistinguished. In the first decades of the century, it became brighter and brighter, until, by April 1843, it was the second brightest star in the sky, outshone only by Sirius (which is almost a thousand times closer to Earth). In the years that followed, it gradually dimmed again and by the 20th century was totally invisible to the naked eye.The star has continued to vary in brightness ever since, and while it is once again visible to the naked eye on a dark night, it has never again come close to its peak of 1843.The larger of the two stars in the Eta Carinae system is a huge and unstable star that is nearing the end of its life, and the event that the 19th century astronomers observed was a stellar near-death experience. Scientists call these outbursts supernova impostor events, because they appear similar to supernovae but stop just short of destroying their star.Although 19th century astronomers did not have telescopes powerful enough to see the 1843 outburst in detail, its effects can be studied today. The huge clouds of matter thrown out a century and a half ago, known as the Homunculus Nebula, have been a regular target for Hubble since its launch in 1990. This image, taken with the Advanced Camera for Surveys High Resolution Channel is the most detailed yet, and shows how the material from the star was not thrown out in a uniform manner, but forms a huge dumbbell shape.Eta Carinae is not only interesting because of its past, but also because of its future. It is one of the closest stars to Earth that is likely to explode in a supernova in the relatively near future (though in astronomical timescales the \u201cnear future\u201d could still be a million years away). When it does, expect an impressive view from Earth, far brighter still than its last outburst: SN 2006gy, the brightest supernova ever observed, came from a star of the same type.This image consists of ultraviolet and visible light images from the High Resolution Channel of Hubble\u2019s Advanced Camera for Surveys. The field of view is approximately 30 arcseconds across. Links  Previous images of Eta Carinae from the Hubble Space Telescope:       http://www.spacetelescope.org/images/opo9623a/ http://www.spacetelescope.org/images/opo9409a/ http://www.spacetelescope.org/images/opo9110a/", "Image Artist": "", "Image Copyright": ""}