The Andromeda Galaxy, also known as Messier 31, and originally named the Andromeda Nebula, is a barred spiral galaxy approximately 2.5 million light-years (770 kiloparsecs) from Earth and the nearest large galaxy to the Milky Way. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the Ethiopian (or Phoenician) princess who was the wife of Perseus in Greek mythology.

The virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at 1 trillion solar masses. The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%. This has been called into question by a 2018 study that cited a lower estimate on the mass of the Andromeda Galaxy, combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way. The Andromeda Galaxy has a diameter of about 220,000 ly (67 kpc), making it the largest member of the Local Group in terms of extension.

The number of stars contained in the Andromeda Galaxy is estimated at one trillion, or roughly twice the number estimated for the Milky Way.

The Milky Way and Andromeda galaxies are expected to collide in around 4-5 billion years, merging to form a giant elliptical galaxy or a large lenticular galaxy. With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects, making it visible to the naked eye from Earth on moonless nights, even when viewed from areas with moderate light pollution. 

Acquisition and processing details

This data was taken over the course of 2 nights, August 31st and September 2nd 2021 during a waning crescent Moon (about 30%). I used the QHY168C OSC on my TS Photoline Triplet 130/910 with the reducer/flattener (x 0.79) giving an f-ratio of 5.5. The field of view is about 1°54"x1°15".  In total 146 usable frames from 180 seconds each were stacked in Astro Pixel Processor. The post-processing was done in PixInsight with a special attention to the "HDRMultiscaleTransformation" to take care of the core. 

Total exposure 7.3 hrs at f5.5.  

Acquisition and processing details

This data was taken during the same period as above on September 6th 2021. No filter used in bortle 5 area. I used the QHY168C OSC on my TS61-EDPH-II. The field of view in this case is about 4°58"x3°18".  In total 114 usable frames from 180 seconds each were stacked in Astro Pixel Processor. The post-processing was done in PixInsight with a special attention to the "HDRMultiscaleTransformation" and "LocalHistrogram" to take care of the core. 

Total exposure 5.7 hrs at f4.5.  

The image on the left is a new capture of M31 with my TS61-EDPH-II telescope after it got a new lens cel. The previous version had a lot of chromatic aberration. The version on the right is the same data, but this time processed with the newly releases PixInsight plugin "BlurXTerminator". It gives obviously more details in the dust clouds.