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| Title: Eta Carinae
Source: Hubble Space Telescope
Information: An "almost true color" image of the material surrounding the star Eta Carinne. The picture was obtained with the second generation Wide-Field and Planetary Camera-II, designed and built at the Jet Propulsion Laboratory in Pasadena, California. The new camera was installed during the Hubble Space Telescope servicing mission, STS-61, in December 1993. WFPC-II incorporates optics that correct for the aberration of the telescope's primary mirror, restoring the optical quality of images obtained with the telescope to the level that the telescope was originally designed to provide. Eta Carinae is one of the best studied and most fascinating objects in the sky. The star has a mass of approximately 150 times that of the sun, and is about 4 million times brighter than our local star, making it one of the most massive and most luminous stars known. Unlike the benevolent and quiescent center of our solar system, Eta Carinae is highly unstable and prone to violent outbursts. The last of these occurred in 1841, when despite its distance (more than 10,000 light years away), Eta Carinne briefly became the second brightest star in the sky. Since that time the star has grown over 600 times fainter in visible light, so that today, Eta Carinae is only barely visible to the naked eye. The rapidly expanding shell of material ejected during the last century's outburst (named the "homunculus" or the "little man" in 1950 by the Italian astronomer Gaviola) was the target of pre-servicing mission Hubble Space Telescope observations taken with JPL's original Wide-Field and Planetary Camera-I. This observation demonstrated the potential for discovery which has always been one of the strongest motivations for a mission such as HST. However, the WFPC-I image of Eta Carinae suffered from the effects of HST's spherical aberration. In particular, the structure of the material very near Eta Carinae itself -- a question of great scientific interest -- was totally obscured in the original images by the spherical aberration "skirt" around the bright star. How the clear view of Eta Carinae provided by WFPC-II dramatically demonstrates the ability of HST to reliably study faint structure near bright objects -- a demonstration of the capability that will allow the HST to carry out many of the high priority scientific programs (e.g. imaging of disk systems surrounding stars) which were most hampered by spherical aberration. On the other hand, the observations of Eta Carinae also demonstrates how pre-servicing mission HST science complements work to be done with the restored capabilities of the telescope. By comparing the WFPC-I and WFPC-II images, astronomers are watching the nebula grow and change with time. The picture shown is actually a combination of three different images taken in red, green, and blue light. The ghostly red outer glow surrounding the star is composed of the very fastest moving of the material which was ejected during the last century's outburst. This material, much of which is moving in excess of 2 million miles per hour, is largely composed of nitrogen and other elements formed in the interior of the massive star, and subsequently ejected into interstellar space. Massive stars convert the hydrogen and helium which were present in the early universe into heavier elements, then disperse this enriched material into space, where it can be incorporated into other stars and solar systems (and eventually people). Thus, by looking at Eta Carinae, we are looking at one way that the universe conspired to make our own existence possible. The bright blue-white nebulosity closer in to the star also consists of ejected stellar material. Unlike the outer nebulosity, this material is very dusty and is seen in reflected starlight. The new data show that this structure consists of two lobes of material, one of which (lower left) is moving toward us and the other of which (upper right) is moving away. This is called a "bipolar flow." The knots of ejected material have sizes comparable to that of our solar system. Astronomers study bipolar flows in a number of contexts: the principal feature of most models of bipolar flows is a dense disk surrounding the star which funnels the ejected material out of the poles of the system. Such disks are used to explain almost all directional outflows from stars, and are also thought to be linked to the formation of solar systems. In Eta Carinae, however, high velocity material is seen to be spraying out in the same plane as the disk which is supposed to be channeling the flow. This is quite unexpected: bullets don't normally shoot out of the sides of a gun. How can it be that the same disk that keeps material from the star flowing into the two lobes also lets other material through, actually concentrating it in the very direction which should be the hardest for it to go? Does the disk exist at all, or is there something fundamentally wrong with our understanding of how bipolar flows are formed. As with all good scientific experiments, the WFPC-II observations of Eta Carinae raise as many questions as they answer.