Tuesday, 21 March 2017

Hubble's variable nebula

Ask ten astronomers about their favourite winter objects and maybe only one of them will mention NGC2261. This is really astonishing because this little nebula's quite bright, peculiar in shape and offers some remarkable detail at high magnifications. Yet, it roams in the shadows of much more famous objects such as the Orion, Rosette or Horsehead nebulae. It never ceases to amaze me that many astronomers prefer to peer for hours trying to spot the almost invisible Cone Nebula rather than having a look at its small but o so beautiful neighbour. And a real neighbour it is because NGC2261's an outlying part of the large Cone Nebula complex, which lies 2.500 lightyears away from us. In turn, this nebula complex is but a part of the gigantic Orion molecular cloud.

When Sir William Herschel discovered NGC2261 in 1783, he mistook the nebula for a comet and I'm sure that you can see why. The bright tip envelops the young double star R Monocerotis, which is thought to be only 300.000 years old. The system's surrounded by a thick disk of gas and debris, much like the one in which the planets of our solar system were born 4.5 billion years ago. As some of this debris falls into the stars, much of it gets ejected again. The dust around the stars' equators blocks this outflow, but at the poles the way out's mostly clear. The result is that gas and dust blows out in two opposed jets and one of these happens to be directed at the bright nebula. Just like the smoke from an oversized industrial chimney, the expelled matter forms dense clouds that drift in front of the nebula and sometimes block some of its light. In 1916 Edwin Hubble noted that the nebula changes brightness quite rapidly indeed, up to two full magnitudes in a matter of months. Comparing recent photographs to older ones shows that the nebula's also changed shape considerably over time and will continue to do so at a very fast rate until the newly formed stars stabilise and enter their adult life. Then the nebula will gradually expand and dissipate until it will have disappeared completely. 

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