Thursday, 5 October 2017

NGC7538: Super-massive stars in an exciting nebula

Last week I showed you the famous Bubble Nebula. Now let's move the telescope slightly northwest and you're going to bump into this exceptional beauty. NGC7538's not nearly as popular as it's larger neighbour and this is a disgrace I'd like to rectify with this post. 

"Dreyer's Object", named after the great Danish-Irish astronomer John Dreyer who compiled the New General Catalogue (NGC) in the 19th century, is a very active star forming region that lies approximately 9,100 light-years away from us, in the same spiral arm as the Bubble. Many of the stars within it are still very young, "only" 1 to 4 million years old, or are still in the process of forming. What makes this nebula truly special is that it's home to the most massive protostar known to day. On my sketch, it's the lower one of the pair in the nebula's centre, denominated NGC7538S. This star under construction is currently 300 times the size of our Solar System (!), and has a contracting core of anything between 85 and 115 solar masses! Mass accretion is still on-going at an astonishing rate of 1/1,000th of a solar mass per year! The star's surrounded by an enormous disk of gas and dust in which perhaps one day planets will be formed. Unfortunately, the bigger the star, the shorter its lifespan and this extraordinary giant will probably not live longer than a few million years.

The upper star of the pair, MM1, is also one of the largest known giants, although with its "merely" 20 to 30 solar masses it's dwarfed by its incredible sister. This star is slightly more ahead in its formation process and exhibits powerful jets. At least 8 more protostars are currently being formed in MM1's vicinity, in an area only 1 light-year across. This is fairly odd because molecular density and temperature don't seem to be sufficient in that region to induce star formation. Scientists believe that in addition to ordinary mass collapse these stars are being created by shock waves and/or strong magnetic fields.   


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