On a clear autumn night, the Andromeda galaxy, also known as Messier 31, M31, or NGC 224 is barely visible to the naked eye. It was described as early as the 10th century by the Persian astronomer Abd al-Rahman al-Sufi and observed by the German astronomer Simon Marius in 1612, shortly after the invention of the telescope. Andromeda is the closest neighbor to our Milky Way galaxy – a colossal spiral of hundreds of billions of stars, 2.5 million light years away.
Only in 1924 scientists have learned that the Andromeda galaxy lies far beyond the Milky Way galaxy. This was discovered by the American astronomer Edwin Hubble when he observed individual Cepheid variables in the nebula. To determine their distance he used the 2.5 m Hooker telescope, at the Mount Wilson Observatory. Cepheid variable is a type of star that pulsates radially. It varies in both diameter and temperature and produces changes in brightness with a well-defined stable period and amplitude.
A brief history of Andromeda galaxy exploration
The Andromeda galaxy was undoubtedly known in ancient times. The Persian astronomer al-Sufi described it in the 10th century. Abd al-Rahman al-Sufi was a Muslim astronomer at the court of Emir Adud ad-Daula in Persian Isfahan, some 350 kilometers south of the current Iranian capital Tehran. After the invention of the telescope, more than 400 years ago, the nebula continued to be in the spotlight. Irish nobleman William Parsons (also known as Earl of Rosse) discovered in the mid-1800s that some galaxies, including Andromeda, exhibit a beautiful, symmetrical spiral structure.
In 1885, a new star flared up in the Andromeda Nebula. But not exactly in the center. Was it a so-called nova explosion? In that case, the nebula would simply be part of our own Milky Way galaxy, and it might be a complete star cluster in the making. But what about the much fainter ‘new stars’ that were also found later? If those were nova eruptions, the nebula had to be much farther away, and the 1885 explosion was much more energetic.
Measuring cosmic distances
Calculating distance is a notorious problem in astronomy. The distance of the stars that are relatively close to the sun is known quite well. They show a small annual fluctuation in the sky – an apparent change in position caused by the earth revolving around the sun. The size of that parallax is a direct measure of the distance from the star, but unfortunately the effect is always so small that it can only be used to properly measure the closest stars. At greater distances, you need other methods, and in the early 1900s, no one had any idea how to determine the distance of a hazy nebula.
Hubble’s contribution to the study of Andromeda
In the case of the Andromeda galaxy, this changed in the early 1920s. Using the giant 2.5-meter Hooker telescope on Mount Wilson, near Los Angeles, American astronomer Edwin Hubble was able to observe individual stars in the Andromeda galaxy. It didn’t take long before everyone was convinced that this was a separate galaxy, comparable to our Milky Way galaxy. Today, astronomers prefer to speak of the Andromeda Galaxy instead of the Andromeda Nebula. They also learned that the 1885 explosion was no ordinary nova, but an extremely bright supernova.
When Hubble discovered a so-called Cepheid variable in Andromeda – a star that changes brightness in a very characteristic and regular way – he was even able to determine its distance from the galaxy. Henrietta Leavitt had discovered more than ten years earlier that there is a relationship between the speed of the brightness variations and the actual luminosity of these stars. Hubble measured the phase of Cepheid variables — the time it takes for the star to complete one cycle of brightness. Using Leavitt’s Law, he was then able to determine the star’s true luminosity. Then by comparing that to the perceived brightness, it was not difficult to calculate the distance.
We now know that the Andromeda Galaxy is 2.5 million light years away. Although it is roughly the same size as the Milky Way galaxy, it has more stars, estimated at one trillion. Like the Milky Way, Andromeda contains twinkling star clusters, glowing gas nebulae, dark molecular clouds, active star forming regions, ancient globular clusters, planetary nebulae, supernova remnants and spinning neutron stars.
Characteristics of Andromeda Galaxy
From Earth, we can see the Andromeda Galaxy at a fairly narrow angle. Hence it looks like an elongated ellipse in simple binoculars. Since we can only observe it from the side, a lot of starlight is absorbed by dust clouds located in the flat disk of the galaxy. If we had seen Andromeda directly from above it would have been a spectacular sight in the night skies. A bright, nebula-like spiral galaxy amidst the twinkling foreground stars of the Milky Way.
Structure of the galaxy
If we had a better view of the galaxy, astronomers would have discovered much earlier that the galaxy contains a gigantic ring of bright nebulae and young stars. With an ordinary ‘optical’ telescope it cannot be visible easily.
Closer to the center of the galaxy there is a smaller ring of dust, and the thin disk of the Andromeda Galaxy is slightly domed. All of these structures are almost certainly the result of colliding a few billion years ago with another, smaller galaxy – presumably one of Andromeda’s neighboring elliptical galaxies.
Andromeda satellite galaxies
The Andromeda galaxy has satellite galaxies just like the Milky Way. The Andromeda galaxy has at least 13 dwarf satellite galaxies. These are tiny, faint galaxies that orbit the great mother system at a considerable distance, caught in the grip of its gravity. A few dozen of these small satellite galaxies have been discovered in the Milky Way galaxy. The actual number of Andromeda’s satellites is undoubtedly much higher. At a distance of 2.5 million light years, they are simply very difficult to observe.
Because it is difficult to determine the distance of all those small satellite galaxies very accurately, their three-dimensional distribution in space was not well known for a long time. But more than a decade ago, new measurements showed that the dwarf galaxies are not randomly distributed around the large Andromeda galaxy. Instead, they are more or less in one plane, which is roughly perpendicular to the central plane of the great spiral galaxy. The dwarf satellites of the Milky Way and of the Centaurus A galaxy also seem to have such a preferential distribution. The cause of this phenomenon is not well understood yet.
How big is Andromeda compared to the Milky Way?
Until recently, astronomers have always assumed that Andromeda is about twice as massive as the Milky Way, but that turns out to be wrong. In 2018 Australian astronomers have ‘weighed’ the Andromeda galaxy using a new technique for measuring the mass of galaxies. Their study indicates that the Andromeda galaxy is 800 billion times heavier than the sun, which is roughly the same weight as the Milky Way. According to the researchers it is possible that earlier studies overestimated the amount of dark matter in the Andromeda galaxy, making it appear much more massive than it really is.
Black holes and colliding galaxies
At the galactic center of the Andromeda Galaxy is a large black hole, comparable to the black hole in the core of the Milky Way Galaxy. However, Andromeda’s black hole is much more massive. The black hole in the center of the Milky Way, Sagittarius A* (Sagittarius A-Star) is about four million times the mass of the sun, but the black hole at the center of the Andromeda galaxy weighs perhaps a hundred million solar masses. Remarkably, a second bright ‘core’ has been discovered just five light years away from that black hole – possibly a large collection of gas and stars held in orbit by the black hole’s gravity.
Hubble space telescope data
The Hubble Space Telescope, which has been taking unprecedentedly sharp images of the Universe since 1990, can easily distinguish individual stars in the Andromeda Galaxy, provided they are not very much fainter than our own sun. The space telescope is named after Edwin Hubble, the astronomer who first determined the distance to Andromeda. This instrument, however, has only a small field of view. Therefore, the Andromeda galaxy never fits in one Hubble image. But in recent years, more than 400 individual images have been taken, from which a gigantic mosaic could be assembled. By comparing the razor-sharp images to images from many years ago, Hubble was able to measure even the minimal displacement of the stars in the sky.
These measurements are important to determine the spatial movement of the galaxy. Research from the Andromeda star indicates that the galaxy is approaching our Milky Way galaxy at a speed of 110 kilometers per second. However, the sideways movement was not known until recently. Now that it is outdated, astronomers are sure that the two great galaxies will collide in the distant future. In four to five billion years, Andromeda will merge with the Milky Way into one giant elliptical galaxy already known as Milkomeda.
What about the mass of the Andromeda galaxy? That remains a remarkable story indeed. You would think that a galaxy that contains more stars should automatically be heavier. But the mass of galaxies is largely determined by the amount of dark matter contained in the galaxy. And you simply cannot see it. Dark matter does not consist of ordinary atoms and molecules, but mysterious elementary particles that have never been observed until now.
The fact that galaxies contain large amounts of dark matter was also demonstrated for the first time in the Andromeda galaxy. The more mass a galaxy contains, the faster the stars will move around the center – also and especially in the outer parts of the galaxy. If you can measure the rotation speed of a galaxy at different distances from the center, you can calculate its total mass quite accurately. And if it turns out to be much higher than the mass of all visible stars, star clusters, gas nebulae and dust clouds put together, then there must be large amounts of dark matter.
Rotation speed of the Andromeda Galaxy
In 1970, Vera Rubin and Kent Ford in the United States were the first to fairly accurately determine the rotation speed of the Andromeda Galaxy this way, up to some 22,000 light-years out of the center. However, the real breakthrough did not come until eight years later, with the work of the Dutch radio astronomer Albert Bosma. Bosma used the Westerbork Synthesis Radio Telescope (WSRT) to measure the rotational speeds of clouds of hydrogen gas in the outer parts of no fewer than twenty-five galaxies. Those hydrogen clouds are much greater distances from the center than the stars measured by Rubin and Ford, making the conclusions much more inevitable.
The theory that almost all galaxies in the universe contain large amounts of dark matter is nowadays almost no longer doubted. But the measurements of rotational speeds do not yield very accurate values for the mass of a galaxy. Until 2018, mass estimates of the Andromeda galaxy have ranged from about 1.5 trillion solar masses to 2.5 trillion solar masses, compared to about 1.5 trillion solar masses for the Milky Way. This contradicted earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass. In 2018, the equality of mass was re-established by radio results as approximately 1.2 trillion solar masses.
New mass estimate
Measurements of the motion velocities of planetary nebulae in the far outer reaches of the Andromeda galaxy have now yielded a new mass estimate. A planetary nebula is created when a sun-like star comes to the end of its life, blowing its outer layers into space in an expanding gas shell. As many as more than 2,500 have been discovered in the Andromeda galaxy, and velocity measurements of those nebulae provide a fairly accurate value for Andromeda’s escape velocity – the speed an object must have to escape the galaxy’s gravity.
Australian astronomers calculated that the escape speed is 470 kilometers per second. And that is lower than the escape speed of our Milky Way Galaxy (about 550 kilometers per second). The conclusion must therefore be that the total mass of Andromeda is less than that of the Milky Way: about 800 billion solar masses. Because there are still quite a few uncertainties at play, the astronomers conclude that the two galaxies have approximately the same mass. That Andromeda is twice as massive as the Milky Way seems out of the question.
Andromeda contains less dark matter than the Milky Way
Considering the size of the galaxy, and the number of stars it contains, there is really only one way to explain this result. It is likely that the Andromeda Galaxy contains about three times less dark matter than the Milky Way. It is unclear why (if only because the true nature of dark matter is still a mystery), but it does mean that previous computer simulations of the future collision of the two galaxies must be adjusted. These models were based on the assumption that Andromeda is twice as heavy as the Milky Way.
That our own galaxy will be engulfed by the much heavier Andromeda galaxy in the distant future, therefore does not seem to be true. Instead, there will be a much more equal collision. By the way, we can safely assume that the last word on the mass of the Andromeda Galaxy has not yet been spoken. In the past, mass estimates have also been continually adjusted and revised; it is simply not easy to ‘weigh’ a galaxy.
So it turns out that the cosmos still holds many unsolved riddles, and that surprising things can still be discovered about our closest neighbors. That fuzzy smear of light in the night sky, just 2.5 million light-years away in the constellation Andromeda, keeps telling us again and again that we are just beginning to understand the universe.
Andromeda galaxy collided with many dwarf galaxies
An analysis of data from the Pan-Andromeda Archaeological Survey (PAndAS) has revealed two major accretion episodes, probably separated by billions of years, in which the Andromeda galaxy, a neighbor of our galaxy, has engulfed several small galaxies.
Large galaxies, like our own galaxy, grow because they devour smaller dwarf galaxies. Gas and dwarf galaxies in space follow the force of gravity caused by filaments of dark matter. As a result, they slowly move towards this dark matter and merge into larger galaxies. Dwarf galaxies attracted by gravity are simultaneously being pulled apart. They leave long traces of stars and groups of stars behind.
Astronomer Dougal Makcey and his colleagues from the Australian National University have found evidence of two major past migration events of the Andromeda galaxy. The last migration took place several billion years ago and the oldest about 10 billion years ago.
The evidence was obtained through using galactic archeology methods. Astronomers have used the motion and properties of stars and groups of stars to reconstruct the formation and evolution of galaxies.
The future collision of Andromeda and Milky Way galaxies
Macky says our galaxy is on a collision course with the Andromeda galaxy. The two galaxies will collide in about 4 billion years. Now that astronomers know what they are dealing with, they can try to determine the fate of our galaxies.
Dr. Mack and his colleagues studied the motions and properties of 77 globular clusters and several larger stellar substructures in the halo of the Andromeda galaxy. In this way they were able to reconstruct the way the galaxy consumed the smaller galaxies.
The discovery also opens up new riddles because the both streams come from completely different directions. This can be seen in the way the two populations turn globular clusters.
According to Macky and his colleagues, these currents may tell something about the cosmic web in which the Andromeda system and our own galaxy are located. Perhaps the information will provide more insight into the origins of our galactic neighbor.