Are We Really Andromeda’s Twin?

While there is not concrete, photographic evidence that Andromeda and the Milky Way are galactic twins, astronomers have found evidence that points them to believe so. This picture shows the twin galaxies surrounded by tens of dwarf galaxies in their local group. This image was uploaded from

Astronomers have declared that our galaxy, the Milky Way is one of the largest two galaxies in our Local Group, rivaled only by its own twin, Andromeda. However, while technology has advanced greatly within the realm of astronomy, we have not yet reached the point of searching beyond the halo of the Milky Way and observing our own galaxy as outsiders. Considering this then begs the questions: How do we know that we are one of the largest galaxies in our Local Group? What evidence points us to the theory that we are Andromeda’s twin?

Let’s first discuss the evidence that tells us that the Milky Way is one of the largest in our local group (so large that other galaxies actually orbit around us!). In the 1920s, it was first believed that the Universe was 300,000 light years across. This was revised shortly after to only 30,000 light years across. However today, astronomers are fairly confident that the Milky Way spans 100,000 to 150,000 light years in diameter. This was discovered by the complex tools of distance measurements that astronomers call “the cosmic distance ladder.” One of the first steps in this ladder is radio waves which are shot out to distances even beyond our solar system so that astronomers can measure the time is takes for the radio waves to come back. The next step in the ladder to uncovering the large size of our galaxy is parallax, which allows scientist to gather distances of close stars within the galaxy. Following this, astronomers use main sequence fitting ( a technique that compares the brightness and color of a far away star to that of a near star) to find the distance to stars that are of even greater lengths from us. Using these techniques, astronomers are able to measure to the ends of the galaxy and get a pretty good idea of how grand our galaxy truly is in comparison to others (Baraniuk).

Yet, this still does not answer how astronomers came to believe that the Milky Way and Andromeda are twins. Previous theories presented Andromeda as three times the size of the Milky Way. Nevertheless, this changed Australian astronomers published an academic article denouncing the previous theory and replacing it with the idea that Andromeda is less than or equal to the size of the Milky Way. Using a measurement technique that measured a the necessary escape velocity of a star leaving the galaxy’s gravity, these astronomers discovered the true size of our neighbor (Parks).

Furthermore, scientist came to discover that the Milky Way is also a spiral galaxy through many clues, the first being the disk of stars that we see in our night sky. This disk of compressed stars can be seen with the naked eye at night and leads scientist to believe that this image is really a look into the disk of our galaxy. Another clue as to the shape of our galaxy is the movement of our stars. Through locating stars’ locations by their rotational velocities, scientist discovered that these stars are located in concentrated spiraling branches, or the “arms” of our galaxy (Peshin). These clues (plus the “duck test” which you can read more on at give way to the theory that, just like Andromeda, our Milky Way is a large spiral galaxy.

Baraniuk, Chris. “Earth – It Took Centuries, but We Now Know the Size of the Universe.” BBC, BBC, 13 June 2016,

Parks, Jake. “Andromeda Is the Same Size as the Milky Way.”, 14 Feb. 2018,

Peshin, Akash, et al. “How Do We Know The Milky Way Is A Spiral Galaxy?” Science ABC, 12 Apr. 2019,

2 thoughts on “Are We Really Andromeda’s Twin?

  1. This is an excellent piece of work on evaluating the process of scientific reasoning. I really appreciated reading this as it outlines the process of discovery so thoughtfully. Well done.

    Liked by 1 person

  2. TA Response:

    Very informative; I had no idea the mass of Andromeda was estimated to be the “same” as the Milky Way.

    A bit of googling revealed that there have been even more recent measurement that suggest Andromeda could be smaller by mass.

    Overall it looks like measuring the dark matter content in either galaxy is still a significantly uncertain task, but it does appear that the mass estimates are close enough that we just don’t know which (if either) is bigger yet!

    Either way we do know they will both come together and make a single galaxy one day.


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