Observations of M91

These are the observations available for M91. If you have any of your own that you'd like to submit we'd love to put them on the website.

• Messier 91 (NGC 4548) and Starlink Satellites in Coma Berenices

  Messier 91 was discovered by Charles Messier on March 18 1781, and at magnitude 10.2, it is the faintest object in his catalogue. However, he made a mistake recording the position of M91 and other observers could not find any object at its recorded location. William Herschel observed the same galaxy on April 8, 1784.

  M 91 became one of the "missing Messier objects", and it wasn't until 1969 that amateur astronomer, W C Williams, reconstructed Messier's mistake in recording the position. There are few bright stars in this area of the sky to provide fixed points, so Messier had recorded the location of M91 relative to another galaxy. Messier had recorded the position relative to M89, but Williams wondered if he should have written M58, rather than M 89. He tested this hypothesis and landed on NGC 4548 to better than one minute of arc. So M91 is now accepted as NGC 4548.

  M91 is classified as an anaemic galaxy, that is a spiral galaxy with low star formation and gas content compared with other galaxies of its type. The core and bar are relatively bright, but the outer regions and spiral arms are very faint with a surface brightness around mag 22/arc-seconds-sq, providing a tough observational challenge.

  

  On the evening of April 25, my data gathering was interrupted by the passage of a stream of Starlink satellites. One of my red subs captured the passage of three of them through Coma Berenices around 20:12 to 20:22 UT. The camera shutter closed as the last one crossed the field of view. I've used this sub as a luminance layer on the finished image to give an impression of their appearance.

  

  Image Details

  Data were gathered between April 20 and 26, and test subs confirmed the faint nature of the outer regions of the galaxy. The image, therefore, comprises 50 x five-minute luminance and 15 x 10-minutes, each of RGB, binned 2 x 2.

  I processed the image in Pixinsight and used the photometric colour calibration tool to check the star colours. The galaxy has a B-V value of 0.8, that is the orange side of white, so I think the presented colour is more or less correct.

  • Telescope: 8" Ritchey-Chretien.
  • Camera: QSI 683 with Astrodon filters.
  • Mount: Skywatcher EQ8.

  David Davies - (13 May 2020).