MONTHLY OBSERVER’S CHALLENGE

Compiled by:

Roger Ivester, North Carolina

&

Sue French, New York

October 2021

Report #153

Click on the following link, for the complete report:

october-2021-observers-challenge-_ngc-6857-1

This month’s target:

William Herschel discovered NGC 6857 on 6 September 1784. His handwritten journal for that date reads: A patch containing some nebulosity…irregularly long.

Heinrich d’Arrest writes of this object and his observation of it in his 1867 Siderum Nebulosorum Observationes Havnienses. My very loosely paraphrased English for the Latin text: Minute, faint; it is most probably a cluster. A 12th-magnitude star precedes it. – Rechecked shortly after: it was not so small; not all of the nebula is resolved, there is at least some cloudiness. I’m not surprised that this was missed by Rosse.

NGC 6857 is the brightest part of the larger, star-forming emission region Sharpless 2-100, which is a much more difficult visual target than NGC 6857. 

A 2010 paper by Manash Samal and colleagues in the Astrophysical Journal indicates that the main ionizing source at the center of NGC 6857 is the bright, massive star at its heart. This compact nebula is estimated to be approximately 28 thousand light-years away from us, and the star is thought to have a spectral type of about OIII. The most likely age of the nebula is in the vicinity of 1 to 2 million years. (Intro and object information by Sue French)