M31 (M32, M101) – the Andromeda Galaxy and its satellite galaxies
| Polish version is here |
The Andromeda Galaxy (M31 in the Messier catalog) is located in the constellation Andromeda, named after the mythological princess, the daughter of King Cepheus of Ethiopia. Its distance from Earth is approximately 2.52 million light-years.
It is the largest and brightest external galaxy visible in the northern sky. Under favorable conditions, far from the light pollution of urban areas, its brightest central regions can often be seen with the naked eye as a small, slightly fuzzy star or a faint patch of light. However, to fully appreciate its beauty, appropriate optical equipment is required.
The Andromeda Galaxy is classified as an Sb galaxy in Hubble’s classification or SA(s)b in the revised system, meaning it is a spiral galaxy without a central bar, featuring a medium-sized core and well-developed spiral arms. However, scientific reports from 2006 suggest that M31 may have a small bar, which would classify it as SAB(s)b. Additionally, Andromeda is categorized as a Low-Ionization Nuclear Emission-line Region (LINER), placing it in the most common group of galaxies with active nuclei.
The Andromeda Galaxy has a dense double core containing at least one black hole, at least two spiral arms accompanied by a ring of cosmic dust which may originate from the smaller galaxy M32, and more than 450 globular clusters some of which are among the densest known.
Interestingly, 14 dwarf satellite galaxies have been identified orbiting Andromeda, the most well-known being M32 and M110.
Observations
August 17, 2018 – Jaworzno, Poland
urban conditions, high level of light pollution
The peak period for observing meteors from the Perseid meteor shower is slowly coming to an end, but that doesn’t mean the night sky above our heads has become any less interesting. On the contrary, late summer and early autumn provide excellent opportunities to observe many fascinating deep-sky objects. One of them is the Andromeda Galaxy.
The photo below were taken without the use of a telescope. I used an older Canon EOS 300D DSLR with a 250mm telephoto lens. The final image is a stack of 10 exposures, each with a 120-second exposure time, resulting in a total exposure of approximately 20 minutes (of course, for stacking purposes, the required number of dark and flat frames was also captured). This was my first photograph of this object, and unfortunately, it is somewhat blurry due to an accidental misfocus while adjusting the camera. Nonetheless, I have great sentimental value attached to this photo.
The photograph shows the Andromeda Galaxy, a beautiful example of a spiral galaxy. Its structural elements are clearly visible such as the bright central bulge and the galactic disk. Dark bands within the disk marked with arrows are also noticeable. Additionally, two other extragalactic star systems namely the dwarf galaxies M32 and M110 are distinctly visible. Both are satellites of the Andromeda Galaxy orbiting around it. Slightly below is the star ν Andromedae (Nu Andromedae), a spectroscopic binary star whose components orbit their common center of mass with a period of just over four Earth days.
September 5, 2019 – Jaworzno, Poland
urban conditions, high level of light pollution
Just over a year after my previous attempt at photographing M31, and having upgraded to a newer camera (Canon EOS 60D), I planned another session.
As seen, the image sharpness is significantly better, revealing more details of the galaxy’s structure and the dark lanes within its spiral arms. The longer exposure time helped highlight a greater number of details. The above photograph is cropped; the original image is presented below:
The view of this distant stellar system, suspended in an incomprehensibly vast cosmic void, never fails to make me ponder the mysteries of the universe.
September 11, 2025 – Katowice, Poland
urban conditions, very high level of light pollution
The Andromeda Galaxy continues to fascinate and captivate. That’s why, even after some time, I returned to it with great pleasure. Although the photograph below is in black and white, it offers a much more detailed view of this beautiful object than the previous one (Photo 4). Achieving such results, despite challenging conditions caused by bright moonlight, was possible thanks to the use of more advanced optical instruments and an astronomical camera.
This image is a composite of photographs taken with two different telescopes: a refractor, which captured the central region with the galaxy, and a Newtonian reflector, used to complete the outer areas filled with surrounding stars. I must admit, this is one of my favorite photos.
While previous observations have focused on the Andromeda Galaxy and its satellite systems, this time the attention turns to another intriguing object that also appears in the frame.
Variable stars have long intrigued astronomers. Their brightness does not remain constant but fluctuates, sometimes within seconds, sometimes over the course of years. The mechanisms behind these variations are as diverse as the stars themselves. One particularly fascinating group includes binary systems, where two gravitationally bound stars orbit a shared center of mass and influence each other’s evolution. Within this group, symbiotic stars are especially notable. These systems consist of two strikingly different components: a cool yet still active red giant and a hot, dense companion, most often a white dwarf. A remarkable example of such a system is HIP 3494, also known as EG Andromedae, located about 2,200 light years from Earth. Its brightness ranges from 11.5m to 12.3m. This is an eclipsing binary, which means that from our perspective one star periodically passes in front of the other, causing noticeable dips in brightness. However, the variability of HIP 3494 involves more than just eclipses.
The cooler and larger component of this system is a red giant nearing the final stages of its life. Its estimated mass falls between 1.2 and 2.4 M⊙. At this point in its evolution, the star has already left the main sequence, undergone significant expansion, and cooled enough to display its characteristic reddish color. Its companion is a hot and compact white dwarf, a typical feature of symbiotic systems. This stellar remnant, with a mass of about 0.4 M⊙, has ended its nuclear fusion and contracted to a size similar to that of Earth.
The geometry and orbital dynamics of HIP 3494 provide important insights into how mass is transferred between the stars in the system. The two components revolve around a shared center of mass with a period of 483.5 days. This relatively long orbital period indicates a significant distance between them, estimated at approximately 1.3 AU. The orbit is nearly circular, with an eccentricity of just 0.07, which means the distance between the stars changes very little over time. This detail is particularly important because it excludes Roche lobe overflow as a likely mechanism for mass transfer. Since the red giant does not fill its Roche lobe, matter must be exchanged through another process.
In this case, mass is transferred through the red giant’s stellar wind. At this advanced stage of its evolution, an M-type star produces strong streams of gas and dust that are expelled into surrounding space. The white dwarf, with its powerful gravitational field, captures some of this outflow. Instead of falling directly onto the white dwarf’s surface, the captured material begins to orbit it, forming an accretion disk. This process, common in symbiotic systems, results in the release of substantial energy.
As the gas in the disk gradually spirals inward, it heats up to extremely high temperatures and emits strong radiation across various regions of the electromagnetic spectrum. Observations of HIP 3494 in both ultraviolet and X-ray wavelengths confirm the occurrence of these energetic processes.
Photo 1 Parameters:
- Total exposure time: 20 minutes (stack of 10 RAW frames at 120s each, using an appropriate number of dark, bias, and flat frames)
- Camera: Canon EOS 60D
- ISO: 2500
- Fake colors
- Lens: zoom type (used at fmax = 250mm)
- Aperture: f/4 (the smallest possible for the lens used)
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system
Photos 2 and 3 Parameters:
- Total exposure time: 36 minutes (stack of 36 RAW frames at 60s each, using an appropriate number of dark, bias, and flat frames)
- Camera: Canon EOS 60D
- ISO: 800
- Lens: zoom type (used at fmax = 250mm)
- Aperture: f/4 (the smallest possible for the lens used)
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system
Photo 4 Parameters:
- Total exposure time: 260 minutes (stack of 130 RAW frames at 120s each, using appropriate dark, bias, and flat frames)
- Camera: ZWO ASI 662MM
- Telescope 1: Achromatic refractor Messier AR-152S (152/760), prime focus (galaxies)
- Telescope 2: Newtonian telescope (150/750), prime focus (surrounding stars)
- A light pollution and atmospheric glow reduction filter was used
- Mount: Equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system
Marek Ples