Andromeda Galaxy

The Andromeda Galaxy, a spiral galaxy which also has alternate scientific names of M31 and NGC224, is the closest full-sized galaxy to the Milky Way Galaxy, home of the Earth. There are smaller galaxies, such as the Large and Small Magellanic Clouds which orbit the Milky Way, is the closest large galaxy at a distance of about 2.5 million light-years from Earth. It gets its name from the fact that it is located in the night sky in the constellation Andromeda.

The Andromeda Galaxy is thought to be somewhat larger than the Milky Way Galaxy, although not by quite as much as previously thought – we will go into more detail on this a little later. It is also easily visible from the night skies on moonless nights with the naked eye, despite being about 2.5 million light-years away.

In this section I will go over some facts about the Andromeda Galaxy, some of which we may explore in later sections. The Andromeda Galaxy is also known by various scientific designations such as M31, NGC 224, UGC454, and PGC 2557.

The Andromeda Galaxy can be seen in the constellation of Andromeda at the coordinates of right ascension 00 hours 42 minutes and 44.3 seconds, and declination of +41 degrees 16 minutes and 9 seconds.

The redshift value for Andromeda is -.001001; the minus sign indicates that this value is actually a blueshift which means that the Andromeda Galaxy is moving towards Earth and the Milky Way Galaxy.

The radial velocity of Andromeda is about 187 miles(301 kilometers) per second and its distance has been determined to be approximately 2.54 million light-years away from Earth.

The apparent magnitude of Andromeda is about 3.44 which means it is easily visible in moonless night skies with low light pollution, while it’s apparent magnitude is -21.5.

It is scientifically classified as a type SA(s)b galaxy – a spiral galaxy with spiral arms and medium sized nucleus. It has a diameter of about 220000 light-years with a mass of around 1.5 trillion solar masses and is thought to contain approximately 1 trillion stars.

Observations of the Andromeda Galaxy go back much further in time than one might think. The first recorded observation of the Andromeda Galaxy appears to be around the year 964 when a Persian astronomer named Abd al-Rahman al-Sufi described it in his Book of Stars as a ‘nebulous smear’.

Later on in 1612 a German astronomer named Simon Marius described it based on some telescopic observations he had. The French mathematician and philosopher Pierre Louis Maupertuis observed Andromeda in 1745 as a blurry spot, conjecturing that it was some sort of island universe.

Charles Messier was a French astronomer famous for his astronomical catalogue of nebulae and star clusters known as Messier objects, and he cataloged Andromeda as M31 in his book. William Herschel was a German born British astronomer who made observations of Andromeda and claimed he saw a reddish color in it’s core in 1785. Making an assessment of its color and magnitude, he came to a conclusion that the Andromeda Galaxy was around 2000 times the distance away as the star Sirius, which is about 8 light-years from Earth. This would have put Andromeda at a distance of around 16000 light-years from Earth, which as we now know, is grossly incorrect.

In 1850 William Parsons, an Anglo-Irish astronomer who had built a 72 inch telescope known as the Leviathan of Parsonstown, made the first known drawing of the spiral structure of the Andromeda Galaxy.

In 1864 William Huggins, an English astronomer who pioneered in astronomical spectroscopy, observed that the spectra of Andromeda displayed various frequencies which were superimposed with dark absorption lines corresponding to various elements that were more indicative of stars than of nebulae, leading him to believe that Andromeda was composed of stars.

Later in 1885, a supernova was observed in the Andromeda Galaxy, known as S Andromedae – this was the only supernova(essentially an explosion of a massive star) that has ever been observed in Andromeda, even to this day. At the time it was first discovered it was believed to be only a nova(the transient bright appearance of a new star), since it was thought at that time that Andromeda was much closer to Earth than it actually was, thus grossly underestimating it’s luminosity.

Isaac Roberts was a Welsh engineer who was also an amateur who is believed to have taken the first photographs of Andromeda in 1887. At that time it was widely believed that Andromeda was only a nebula in our own Milky Way Galaxy, and so Isaac Roberts thought that Andromeda was an early forming solar system in our galaxy.

Vesto Melvin Slipher was an American astronomer who made the first measurements of the radial velocities of galaxies, also noting their redshift values which helped establish the fact that the Universe is expanding. In 1912 he used spectroscopy to measure the radial velocity of Andromeda with respect to our Solar System, which came out to a value of 190 miles per second, the largest velocity that had ever been recorded up to that time.

In 1917 Heber Curtis, an American astronomer, discovered 11 novae in Andromeda and observed that they were about 10 magnitudes fainter than those occurring in other areas of the night sky. Because of this he estimated that the distance of the Andromeda Galaxy from Earth was about 500000 light-years, still far less than its actual distance of 2.5 million light-years that we know today, but definitely a big improvement over previous estimates. He thus came to the conclusion that Andromeda was an independent galaxy far outside our own Milky Way.

In 1920 there was a Great Debate between Heber Curtis and another American astronomer Harlow Shapley concerning the nature of spiral nebulae and the size of the Universe. Shapley believed they were smaller nebulae lying within the outer regions of our own galaxy, while Curtis thought that they were independent galaxies which were quite large and at a very great distance from Earth. Of course, Heber Curtis turned out to be correct and was therefore the winner of the Great Debate.

Ernst Opik was an Estonian astronomer, who in 1922 used the measured velocities of stars in the Andromeda Galaxy to calculate its distance from Earth, which he estimated at 1.5 million light-years, yet another significant improvement. Then in 1925 Edwin Hubble, a famous American astronomer, definitively proved that the Andromeda Galaxy was a large spiral galaxy, far away from Earth by observing the extragalactic Cepheid variable stars(a type of pulsating star) in Andromeda for the first time which confirmed the great distance of Andromeda from the Milky Way.

In 1943 Walter Baade, a German astronomer working in the United States at the time, became the first person to actually resolve stars in the galactic core of Andromeda. He classified two different types of stars; type 1 which were young high velocity stars, and type 2 which were older red colored stars. He also discovered two types of Cepheid variable stars which caused him to come to the conclusion that the Andromeda Galaxy was almost twice as far away as the previous estimate of 1.5 million light-years, much closer to the distance of 2.5 million light-years that we know today.

Radio emissions from the Andromeda Galaxy were first detected in 1950 by Hanbury Brown, a British astronomer, and Cyril Hazard with radio telescopes located at the Jodrell Bank Observatory in Manchester, in the United Kingdom. Later in the 1950’s the British astronomer John Baldwin and his associates made the first radio maps of the Andromeda Galaxy at the Cambridge Radio Astronomy Group at the University of Cambridge.

In 2009 the very first planet is believed to have been discovered in the Andromeda Galaxy using a type of gravitational microlensing technique – detecting deflected light from the gravitational effects of a larger object – to seperate the mass of the planet from its parent star.

It is now thought that the Andromeda Galaxy formed about 10 billion years ago from colliding and then merging with a number of smaller protogalaxies – these are essentially clouds of interstellar gas which are in the early stage of galaxy formation. These violent collisions formed the extended disk and galactic halo of Andromeda. During this formation epoch there would have been a very high rate of star formation in the Andromeda Galaxy causing it to become a luminous infrared galaxy for around another 100 million years.

What is the history of the Andromeda Galaxy after this initial period of formation, roughly 100 million years? For billions of years gravitational forces, along with some possible mergers with smaller galaxies and star clusters, have been the main factor in the evolution of Andromeda.

It is currently thought that somewhere around 85 percent of all the matter in the Universe is dark matter, with only the remaining 15 percent ordinary, or visible, matter. So the gravitational forces of all this matter, both ordinary matter and dark matter, was crucial for Andromeda to evolve to the galaxy that we observe today. I say ‘today’, but since Andromeda is about 2.5 million light-years away we are really seeing it 2.5 million years in the past!

It is very interesting to mention that the Andromeda Galaxy and the Triangulum Galaxy had a very close encounter, or passage, with each other around 2 to 4 billion years ago. The gravitational forces from this event is thought to have resulted in very high rates of star formation across the disk of Andromeda, even creating some globular star clusters.

During the past 2 billion years new star formation in Andromeda has greatly decreased, almost to the point of inactivity. There also seems to have been interactions with a number of satellite galaxies with Andromeda which is thought to have resulted in Andromeda’s Giant Stellar Stream – a stream of stars orbiting Andromeda which may have been smaller satellite galaxies and globular star clusters which were torn apart by intense gravitational tidal forces.

The Andromeda Galaxy is roughly 2.54 million light-years from Earth according our best estimates at the present time. Methods of estimating this distance include variance in light distribution from the luminosity of stars in the galaxy and their fluctuations, cepheid variable stars and their luminosity fluctuations, eclipses of binary stars in the galaxy with the comparison of apparent and absolute magnitudes, and measuring the luminosity of the brightest red-giant branch stars in the galaxy(TRGB method). Using these methods and averaging the results gives us the 2.54 million light-year figure for the distance of the Andromeda Galaxy.

Estimates for the size of the Andromeda Galaxy are still uncertain. In 2006 the spheroid of the Andromeda Galaxy was determined to have a higher stellar density than the Milky Way Galaxy and the stellar disk of Andromeda was estimated at about twice the diameter of the Milky Way – Andromeda was thought to contain over 1 trillion stars compared to around 400 billion stars in the Milky Way.

This gave a total stellar mass estimate about twice that of the Milky Way at about 1.5 trillion solar masses with about 30 percent of this mass in the galactic core, 56 percent in the galactic disk, and the remaining 14 percent in the galactic halo.

In 2018 study of radio frequency emissions from Andromeda seemed to indicate a mass more or less equal to that of the Milky Way in the range of .8 trillion solar masses – this contradicts the earlier studies but is by no means certain. Much ongoing research is still being done to determine how big the Andromeda Galaxy is.

The Andromeda Galaxy appears to have a significantly older population of stars than the Milky Way Galaxy with many having ages greater than 7 billion years. However the estimated luminosity of Andromeda is about 26 billion units of radiant flux, which is about 25 percent greater than that of the Milky Way. This may be in large part because Andromeda is believed to have over twice the number of stars than in the Milky Way, around a trillion, despite having a predominately older star population.

The absolute magnitude of Andromeda is believed to be about -21.5, although this is only an estimate, and some believe that the Andromeda Galaxy is the second brightest galaxy within a radius of about 32.6 million light-years of Earth, behind the Sombrero Galaxy, a lenticular galaxy visible in the constellation of Virgo about 31.1 million light-years from Earth.

The Andromeda Galaxy is currently classified as a SA(s)b galaxy – a spiral disk galaxy with tightly bound spiral arms and a medium size nucleus. Andromeda may have to be reclassified as a barred galaxy since the Two Micron All-Sky Survey(2MASS) seems to indicate that it has a bar structure spanning its long axis.

In past years Andromeda was thought to have a diameter of 70000 to 120000 light-years across, comparable to the Milky Way. However, starting in 2005 with observations from the two telescopes at the Keck Observatory near the top of Mauna Kea in the state of Hawaii in the USA(the primary mirrors are both 394 inches in diameter making them the 2nd largest astronomical telescope behind the Gran Telescopio Canarias on the island of La Palma in the Canaries, Spain which has a primary mirror 410 inches in diameter) discoveries have been made indicating that the diameter of the disk of Andromeda is much greater than previously thought, perhaps as much as three times as great.

Observations of the Keck telescopes show that stars extending outward from Andromeda are actually part of the disk itself giving strong evidence of a much larger and expansive stellar as much as 220000 light-years across. Andromeda is inclined at an angle of 77 degrees relative to our galaxy(90 degrees would be edge on), the Milky Way, which enables a marginal view of some of the cross section structure, which seems to show a significant S-shaped warp rather than only a flat disk. This may be due in part to the gravitational influences of some of Andromeda’s closer satellite galaxies.

Spectroscopic studies of Andromeda have enabled us to estimate rotational velocities inside the galaxy as a function of the distance from the galactic core. At a distance of 1300 light-years from the galactic core the rotational velocity is around 140 miles per second, and when moving out to about 7000 light-years from the core it drops to about 31 miles per second. From this point rotational velocities will keep rising until they reach a peak of about 160 miles per second at a radius of 33000 light-years from the galactic core.

Going outward from the 33000 light-year radius velocities will slowly decline until they drop to 120 miles per second at a radius from the core of about 80000 light-years. From these radial velocity measurements it is possible to make an estimate that there is a concentrated mass equivalent to about 6 billion solar masses in the nucleus region of the Andromeda Galaxy.

The Andromeda Galaxy is viewed close to edge on with respect to its orientation from Earth, 77 degrees as opposed to 90 degrees for a complete edge on view, so its spiral structure is somewhat difficult to discern. Given this fact, it still seems that the Andromeda Galaxy is pretty much an ordinary spiral galaxy with 2 spiral arms beginning at about 1600 light-years from the core and extending outward, separated from each other by at least 13000 light-years. The spiral arms are somewhat indistinct, possibly due to interaction with the satellite galaxies M32 and M110 and the resultant gravitational influence.

In 1998 images from the European Space Agency’s Infrared Space Observatory indicated the presence of several overlapping rings of gas and dust in Andromeda, with an especially prominent one, called the ring of fire by some astronomers, at a distance of 32000 light-years from the galactic core. This is mostly cold gas and dust which cannot be seen in the visible wavelengths of light. The presence of these rings of gas and dust have led some scientists to believe that Andromeda could be evolving into a ring type of galaxy in the far distant future, although this is by no means certain at this time.

Close examinations of the inner structure of Andromeda seem to suggest that a collision with the smaller satellite galaxy M32 some 200 million years ago may have been responsible for some of the ring structures. Computer simulations show that this smaller galaxy, M32, may have passed through the disk of Andromeda along its polar axis, stripping away about half of the mass of M32 and creating the ring structures in Andromeda.

The galactic halo of Andromeda, an extended halo of stars surrounding the galactic disk, is thought to have followed a similar evolutionary path to the galactic halo of the Milky Way, resulting from the assimilation of up to one hundred smaller galaxies over the course of about 12 billion years.

Andromeda appears through Earth-bound telescopes to have a singular central bulge in it’s center, but in 1991 the Hubble Space Telescope was used to image the center of the Andromeda Galaxy, and what resulted was rather surprising. The nucleus of Andromeda really consists of two concentrations of stars, separated by about 4.9 light-years. The brighter concentration, called P1, is offset from the center of Andromeda, while the dimmer concentration of stars, called P2, is in the true center of the Andromeda Galaxy.

In the true center of Andromeda, in the P2 concentration, there is a black hole which is estimated to be in the range of 110 million to 230 million solar masses. The radial velocity of the stars and other materials dispersed around it is estimated to be about 99.4 miles per second, or 160 kilometers per second. At the current time, there is not thought to be any black hole in the center of the P1 concentration, due to the characteristics of the distribution of stars and other materials around it.

The types of objects which the Andromeda Galaxy contains are probably somewhat similar to that of our own Milky Way Galaxy, such as planets in orbit around stars, various types of stars in different stages of evolution, black holes, rogue planets drifting in the void of interstellar space, interstellar gas and dust, and so on. One difference is that Andromeda is more massive and contains over twice the number of stars as the Milky Way.

At the current time it is thought that there are about 460 globular clusters in the Andromeda Galaxy. The most massive of these, called Globular One, is the most luminous globular cluster in the Local Group of galaxies, which includes Andromeda, the Milky Way, and Triangulum as the largest galaxies. It has several large stellar populations with a total of several million stars and is about twice as luminous as Omega Centauri, the most luminous globular cluster in the Milky Way Galaxy. It seems to be much too massive to be an ordinary globular cluster – it is now thought that Globular One might be the remnant core of a dwarf galaxy that was consumed by Andromeda in the distant past.

G76 is the globular cluster with the greatest apparent brightness and is located in the eastern half of the southwest spiral arm of Andromeda. In 2006 another massive globular cluster was discovered which seems to have similar properties to G1, Globular One, and has a reddish color because of the concentration of interstellar gas.

The globular clusters in the Andromeda Galaxy have a much larger range of ages than those in the Milky Way Galaxy, with some as old as Andromeda itself, around 12 billion years, and other much younger ones in the range of from several hundred million years to five billion years old.

In 2005 a completely new type of star cluster was discovered in Andromeda – they contained hundreds of thousands of stars like regular globular clusters. However they were much different in that they were far larger across, hundreds of millions of light-years, and so hundreds of times less dense, making the distance between the stars in these new types of clusters far greater than in ordinary clusters.

In 2012 a microquasar(a burst of radio emissions from a small black hole), which is a smaller version of a quasar, was discovered in the Andromeda Galaxy. The black hole which is believed to be causing this microquasar is only about 10 solar masses large and is located near the galactic center of Andromeda. This was the first microquasar observed outside of the Milky Way Galaxy.

The Andromeda Galaxy is like the Milky Way in that it has a number of smaller satellite galaxies, 14 of them are classified as dwarf galaxies. M32 and M110 are the best known and most easily observable of these. It is thought that M32 used to be a larger galaxy but had its stellar disk removed by M31 in the distant past. M110 may have a younger star population and also appears to be interacting with Andromeda, contributing to the galactic halo of Andromeda.

In 2006 it was discovered that nine of these satellite galaxies lie in the same plane which intersects the galactic core of Andromeda, instead of being randomly distributed – this may very well be because of a common gravitational tidal force effect upon these galaxies.

It is now know that the Andromeda Galaxy is slowly moving towards the Milky Way Galaxy at a rate of around 68 miles per second(110 kilometers per second), giving it a blueshift value of about .001001. The approaching velocity of the Andromeda Galaxy is much greater than its tangential, or sideways, velocity – this means that there is likely to be a direct collision between Andromeda and the Milky Way in about 4 billion years.

One possible outcome of this is that the two galaxies will merge to form a giant elliptical galaxy or disc galaxy, which is a frequent occurance among galaxies in the Universe. The fate of the Earth and its Solar System would, of course, be unknown in such an occurance. We should not be concerned though since it is highly unlikely that humans and their civilization will be around in 4 billion years.

The Andromeda Galaxy has an apparent magnitude of about 3.44, which makes it bright enough to be easily visible in moonless night skies with low levels of light pollution. It is probably best to view Andromeda during autumn nights in the Northern Hemisphere – here in the mid latitudes Andromeda reaches its zenith, the highest point, around midnight and so is visible for almost the whole night. In the Southern Hemisphere Andromeda is visible in the same months, which is spring there, but it stays close to the horizon and is much more difficult to observe unless you are close to the equator.

Andromeda is easily visible through binoculars which can reveal some of its larger structure along with its two brightest satellite galaxies, M32 and M110. With a decent size amateur telescope Andromeda’s disk, some of the brightest globular clusters, dark dust lanes, and the large star cloud designated as NGC 206 are visible.