Scientists have long understood that in the course of cosmic evolution, galaxies become larger by consuming smaller galaxies. The evidence of this can be seen by observing galactic halos, where the stars of cannibalized galaxies still remain. This is certainly true of the Andromeda Galaxy (aka. M31, Earth’s closest neighbor) which has a massive and nearly-invisible halo of stars that is larger than the galaxy itself.
For some time, scientists believed that this halo was the result of hundreds of smaller mergers. But thanks to a new study by a team of researchers at the University of Michigan, it now appears that Andromeda’s halo is the result of it cannibalizing a massive galaxy some two billion years ago. Studying the remains of this galaxy will help astronomers understand how disk galaxies (like the Milky Way) evolve and survive large mergers.
Using computer models, Richard D’Souza and Eric Bell were able to piece together how a once-massive galaxy (named M32p) disrupted and eventually came to merge with Andromeda. From their simulations, they determined that M32p was at least 20 times larger than any galaxy that has merged with the Milky Way over the course of its lifetime.
M32p would have therefore been the third-largest member of the Local Group of galaxies, after the Milky Way and Andromeda galaxies, and was therefore something of a “long-lost sibling”. However, their simulations also indicated that many smaller companion galaxies merged with Andromeda over time. But for the past, Andromeda’s halo is the result of a single massive merger. As D’Souza explained in a recent Michigan News press statement:
“It was a ‘eureka’ moment. We realized we could use this information of Andromeda’s outer stellar halo to infer the properties of the largest of these shredded galaxies. Astronomers have been studying the Local Group—the Milky Way, Andromeda and their companions—for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it.”
This study will not only help astronomers understand how galaxies like the Milky Way and Andromeda grew through mergers, it might also shed light on a long-standing mystery – which is how Andromeda’s satellite galaxy (M32) formed. According to their study, D’Souza and Bell believe that M32 is the surviving center of M32p, which is what remained after its spiral arms were stripped away.
“M32 is a weirdo,” said Bell. “While it looks like a compact example of an old, elliptical galaxy, it actually has lots of young stars. It’s one of the most compact galaxies in the universe. There isn’t another galaxy like it.” According to D’Souza and Bell, this study may also alter the traditional understanding of how galaxies evolve. In astronomy, conventional wisdom says that large interactions would destroy disk galaxies and form elliptical galaxies.
But if Andromeda did indeed survive an impact with a massive galaxy, it would indicate that this is not the case. The timing of the merger may also explain recent research findings which indicated that two billion years ago, the disk of the Andromeda galaxy thickened, leading to a burst in star formation. As Bell explained:
“The Andromeda Galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago. When I was at graduate school, I was told that understanding how the Andromeda Galaxy and its satellite galaxy M32 formed would go a long way towards unraveling the mysteries of galaxy formation.”
In the end, this method could also be used to study other galaxies and determine which were the most massive mergers they underwent. This could allow scientists to better understand the complicated process that drives galaxy growth and how mergers affect galaxies. This knowledge will certainly come in handy when it comes to determining what will happen to our galaxy when it merges with Andromeda in a few billion years.
Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at dwarf elliptical galaxy known as Messier 32. Enjoy!
During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.
One of these objects is the dwarf elliptical galaxy known as Messier 32 (aka. NGC 221). Located about 2.65 million light-years from Earth, in the direction of the Andromeda constellation, this dwarf is actually a satellite galaxy of the massive Andromeda Galaxy (M31). Along with Andromeda, the Milky Way and the Triangulum Galaxy (M33) is a member of the Local Group.
M32 is an elliptical dwarf galaxy which contains about 3 billion solar masses. While it looks small compared to its massive neighbor, this little guy actually stretches across space some 8,000 light years in diameter. Once you pick it up, you’ll notice it’s really quite bright on its own – and with very good reason – its nucleus is almost identical to M31. Both contain about 100 million solar masses in rapid motion around a central supermassive object!
“M32 is the prototype for the relatively rare class of galaxies referred to as compact ellipticals. It has been suggested that M32 may be a tidally disturbed r1/4 elliptical galaxy or the remnant bulge of a disk-stripped early-type spiral galaxy reveals that the surface bightness profile, the velocity dispersion measurements, and the estimated supermassive black hole mass in M32 are inconsistent with the galaxy having, and probably ever having had, an r1/4 light profile. Instead, the radial surface brightness distribution of M32 resembles an almost perfect (bulge+exponential disk) profile; this is accompanied by a marked increase in the ellipticity profile and an associated change in the position angle profile where the “disk” starts to dominate. Compelling evidence that this bulge/disk interpretation is accurate comes from the best-fitting r1/n bulge model, which has a Sersic index of n=1.5, in agreement with the recently discovered relation between a bulge’s Sersic index and the mass of a bulge’s supermassive black hole.”
By probing deeply into Messier 32, we’ve learned this little galaxy is home to mainly mature red and yellow stars. And they’re good housekeepers, too… because there’s practically no dust or gas to be found. While this seems neat and tidy, it also means there isn’t any new star formation going on either, but there are signs of some lively doings in the not too distant past.
Because M32 has shared “space” with neighboring massive M31, the strong tidal field of the larger galaxy may have ripped away what once could have been spiral arms – leaving only its central bulge and triggering starburst in the core. As Kenji Bekki (et al) wrote in their 2001 study:
“The origin of M32, the closest compact elliptical galaxy (cE), is a long-standing puzzle of galaxy formation in the Local Group. Our N-body/smoothed particle hydrodynamics simulations suggest a new scenario in which the strong tidal field of M31 can transform a spiral galaxy into a compact elliptical galaxy. As a low-luminosity spiral galaxy plunges into the central region of M31, most of the outer stellar and gaseous components of its disk are dramatically stripped as a result of M31’s tidal field. The central bulge component on the other hand, is just weakly influenced by the tidal field, owing to its compact configuration, and retains its morphology. M31’s strong tidal field also induces rapid gas transfer to the central region, triggers a nuclear starburst, and consequently forms the central high-density and more metal-rich stellar populations with relatively young ages. Thus, in this scenario, M32 was previously the bulge of a spiral galaxy tidally interacting with M31 several gigayears ago. Furthermore, we suggest that cE’s like M32 are rare, the result of both the rather narrow parameter space for tidal interactions that morphologically transform spiral galaxies into cE’s and the very short timescale (less than a few times 109 yr) for cE’s to be swallowed by their giant host galaxies (via dynamical friction) after their formation.”
History of Observation:
M32 was discovered by Guillaume Le Gentil on October 29th, 1749 and became the first elliptical galaxy ever observed. Although it wasn’t cataloged by Charles Messier until August 3rd, 1764, he had also seen it some seven years earlier while studying at the Paris Observatory, but his notes had been suppressed. But no matter, for he made sure to include it in his notes with a drawing! As he wrote of the object:
“I have examined in the same night [August 3 to 4, 1764], and with the same instruments, the small nebula which is below and at some [arc] minutes from that in the girdle of Andromeda. M. le Gentil discovered it on October 29, 1749. I saw it for the first time in 1757. When I examined the former, I did not know previously of the discovery which had been made by M. Le Gentil, although he had published it in the second volume of the Memoires de Savans erangers, page 137. Here is what I found written in my journal of 1764. That small nebula is round and may have a diameter of 2 minutes of arc: between that small nebula and that in the girdle of Andromeda one sees two small telescopic stars. In 1757, I made a drawing of that nebula, together with the old one, and I have not found and change at each time I have reviewed it: One sees with difficulty that nebula with an ordinary refractor of three feet and a half; its light is fainter than that of the old one, and it doesn’t contain any star. At the passage of that new nebula through the Meridian, comparing it with the star Gamma Andromedae, I have determined its position in right ascension as 7d 27′ 32″, and its declination as 38d 45′ 34″ north.”
Later, Messier 32 would be examined again, this time by Admiral Symth who said:
“An overpowering nebula, with a companion about 25′ in the south vertical M32 … The companion of M31 was discovered in November, 1749, by Le Gentil, and was described by him as being about an eighth of the size of the principal one. The light is certainly more feeble than here assigned. Messier – whose No. 32 it is – observed it closely in 1764, and remarked, that no change had taken place since the time of its being first recorded. In form it is nearly circular. The powerful telescope of Lord Rosse is a reflector of three feet in diameter, of performance hitherto unequalled. It was executed by the Earl of Rosse, under a rare union of skill, assiduity, perseverance, and muniference. The years of application required to accomplish this, have not worn his Lordship’s zeal and spirit; like a giant refreshed, he has returned to his task, and is now occupied upon a metallic disc of no less than six feet in diameter. Should the figure of this prove as perfect as the present one, we may soon over-lap what many absurdly look upon as the boundaries of the creation.”
Locating Messier 32:
Locating M32 is as easy as locating the Andromeda Galaxy, but it will require large binoculars or at least a small telescope to see. Even under moderately light polluted skies the Great Andromeda Galaxy can be easily be found with the unaided eye – if you know where to look. Seasoned amateur astronomers can literally point to the sky and show you the location of M31, but perhaps you have never tried to find it.
Believe it or not, this is an easy galaxy to spot even under the moonlight. Simply identify the large diamond-shaped pattern of stars that is the Great Square of Pegasus. The northernmost star is Alpha, and it is here we will begin our hop. Stay with the northern chain of stars and look four finger widths away from Alpha for an easily seen star.
The next along the chain is about three more finger widths away… And we’re almost there. Two more finger widths to the north and you will see a dimmer star that looks like it has something smudgy nearby. Point your binoculars there, because that’s no cloud – it’s the Andromeda Galaxy!
Now aim your binoculars or telescope its way… Perhaps one of the most outstanding of all galaxies to the novice observer, M31 spans so much sky that it takes up several fields of view in a larger telescope, and even contains its own clusters and nebulae with New General Catalog designations. If you have larger binoculars or a telescope, you will be able to pick up M31’s two companions – M32 and M110. Messier 32 is the elliptical galaxy to the south.
Why not stretch your own boundaries? Go observing! Halton Arp included Messier 32 as No. 168 in his Catalogue of Peculiar Galaxies. It’s bright, easy and fun! And here are the quick facts on this Messier Object to help you get started:
Object Name: Messier 32 Alternative Designations: M32, NGC 221 Object Type: Type E2, Elliptical Galaxy Constellation: Andromeda Right Ascension: 00 : 42.7 (h:m) Declination: +40 : 52 (deg:m) Distance: 2900 (kly) Visual Brightness: 8.1 (mag) Apparent Dimension: 8×6 (arc min)
In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come. Thanks to the development of modern telescopes and astronomy, this list was amended by the early 20th century to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.
Of these, Andromeda is one of the oldest and most widely recognized. Located north of the celestial equator, this constellation is part of the family of Perseus, Cassiopeia, and Cepheus. Like many constellation that have come down to us from classical antiquity, the Andromeda constellation has deep roots, which may go all the way back to ancient Babylonian astronomy.
What is up with these dwarf galaxies? A survey of thousands of galaxies using the Sloan Digital Sky Survey reveals something interesting, which was first revealed by looking at the massive Andromeda Galaxy nearby Earth: dwarf galaxies orbiting larger ones are often in disc-shaped orbits and not distributed randomly, as astronomers expected.
The finding follows on from research in 2013 that showed that 50% of Andromeda’s dwarf galaxies are in a single plane about a million light-years in diameter, but only 300,000 light-years thick. Now with the larger discovery, scientists suspect that perhaps there is a yet-to-be found process that is controlling gas flow in the cosmos.
“We were surprised to find that a large proportion of pairs of satellite galaxies have oppositely directed velocities if they are situated on opposite sides of their giant galaxy hosts,” stated lead author Neil Ibata of Lycée International in France.
“Everywhere we looked, we saw this strangely coherent coordinated motion of dwarf galaxies,” added Geraint Lewis, a University of Sydney physicist. “From this we can extrapolate that these circular planes of dancing dwarfs are universal, seen in about 50 percent of galaxies. This is a big problem that contradicts our standard cosmological models. It challenges our understanding of how the universe works, including the nature of dark matter.”
The astronomers also speculated this could show something unexpected in the laws of physics, such as motion and gravity, but added it would take far more investigation to figure that out.
M31 and M33 are two of the nearest spiral galaxies, and can form the basis for determining distances to more remote spiral galaxies and constraining the expansion rate of the Universe (the Hubble constant). Hence the relevance and importance of several new studies that employed near-infrared data to establish solid distances for M31 (Andromeda) and M33 (Triangulum) (e.g., Gieren et al. 2013), and aimed to reduce existing uncertainties tied to the fundamental parameters for those galaxies. Indeed, reliable distances for M31 and M33 are particularly important in light of the new Hubble constant estimate from the Planck satellite, which is offset relative to certain other results, and that difference hinders efforts to ascertain the nature of dark energy (the mysterious force theorized as causing the Universe’s accelerated expansion).
Gieren et al. remarked that, “a number of new distance determinations to M33 … span a surprisingly large interval … which is a cause of serious concern. As the second-nearest spiral galaxy, an accurate determination of [M33’s] distance is a crucial step in the process of building the cosmic distance ladder.” Concerning M31, Riess et al. 2012 likewise remarked that “M31, the nearest analogue of the Milky Way Galaxy, has long provided important clues to understanding the scale of the Universe.“
The new Gieren and Riess et al. distances are based on near-infrared observations, which are pertinent because radiation from that part of the electromagnetic spectrum is less sensitive than optical data to absorption by dust located along our sight-line (see the figure below). Properly accounting for the impact of dust is a principal problem in cosmic distance scale work, since it causes targets to appear dimmer. “different assumptions about [dust obscuration] are a prime source for the discrepancies among the various distance determinations for M33.” noted Gieren et al., and the same is true for the distance to M31 (see Riess et al.).
Gieren et al. observed 26 Cepheids in M33 and established a distance of ~2,740,000 lightyears. The team added that, “As the first modern near-infraredCepheid study [of] M33 since … some 30 years … we consider this work as long overdue …” Astronomers often cite distances to objects in lightyears, which defines the time required for light emitted from the source to reach the observer. Despite the (finite) speed of light being 300,000,000 m/s, the rays must traverse “astronomical” distances. Gazing into space affords one the unique opportunity to peer back in time.
The distances to M33 shown below convey seminal points in the evolution of humanity’s knowledge. The scatter near the 1920s stems partly from a debate concerning whether the Milky Way and the Universe are synonymous. In other words, do galaxies exist beyond the Milky Way? The topic is immortalized in the famed great debate (1920) featuring H. Shapley and H. Curtis (the latter argued for an extragalactic scale). The offset between the pre-1930 and post-1980 data result in part from a nearly two-fold increase in the cosmic distance scale recognized circa 1950 (see also Feast 2000). Also evident is the scatter associated with the post-1980 distances, which merely reinforces the importance of the new high-precision distance estimates.
Score another point for the National Science Foundation’s Green Bank Telescope (GBT) at the National Radio Astronomy Observatory (NRAO) in Green Bank. They have opened our eyes – and ears – to previously undetected region of hydrogen gas clouds located in the area between the massive Andromeda and Triangulum galaxies. If researchers are correct, these dwarf galaxy-sized sectors of isolated gases may have originated from a huge store of heated, ionized gas… Gas which may be associated with elusive and invisible dark matter.
“We have known for some time that many seemingly empty stretches of the Universe contain vast but diffuse patches of hot, ionized hydrogen,” said Spencer Wolfe of West Virginia University in Morgantown. “Earlier observations of the area between M31 and M33 suggested the presence of colder, neutral hydrogen, but we couldn’t see any details to determine if it had a definitive structure or represented a new type of cosmic feature. Now, with high-resolution images from the GBT, we were able to detect discrete concentrations of neutral hydrogen emerging out of what was thought to be a mainly featureless field of gas.”
So how did astronomers detect the extremely faint signal which clued them to the presence of the gas pockets? Fortunately, our terrestrial radio telescopes are able to decipher the representative radio wavelength signals emitted by neutral atomic hydrogen. Even though it is commonplace in the Universe, it is still frail and not easy to observe. Researchers knew more than 10 years ago that these repositories of hydrogen might possibly exist in the empty space between M33 and M32, but the evidence was so slim that they couldn’t draw certain conclusions. They couldn’t “see” fine grained structure, nor could they positively identify where it came from and exactly what these accumulations meant. At best, their guess was it came from an interaction between the two galaxies and that gravitational pull formed a weak “bridge” between the two large galaxies.
The animation demonstrates the difference in resolution from the original Westerbork Radio Telescope data (Braun & Thilker, 2004) and the finer resolution imaging of GBT, which revealed the hydrogen clouds between M31 and M33. Bill Saxton, NRAO/AUI/NSF Credit: Bill Saxton, NRAO/AUI/NSF.
Just last year, the GBT observed the tell-tale fingerprint of hydrogen gas. It might be thin, but it is plentiful and it’s spread out between the galaxies. However, the observations didn’t stop there. More information was gathered and revealed the gas wasn’t just ethereal ribbons – but solid clumps. More than half of the gas was so conspicuously aggregated that they could even have passed themselves off as dwarf galaxies had they a population of stars. What’s more, the GBT also studied the proper motion of these gas pockets and found they were moving through space at roughly the same speed as the Andromeda and Triangulum galaxies.
“These observations suggest that they are independent entities and not the far-flung suburbs of either galaxy,” said Felix J. Lockman, an astronomer at the NRAO in Green Bank. “Their clustered orientation is equally compelling and may be the result of a filament of dark matter. The speculation is that a dark-matter filament, if it exists, could provide the gravitational scaffolding upon which clouds could condense from a surrounding field of hot gas.”
And where there is neutral hydrogen gas, there is fuel for new stars. Astronomers also recognize these new formations could eventually be drawn into M31 and M33, eliciting stellar creation. To add even more interest, these cold, dark regions which exist between galaxies contain a large amount of “unaccounted-for normal matter” – perhaps a clue to dark matter riddle and the reason behind the amount of hydrogen yet to revealed in universal structure.
“The region we have studied is only a fraction of the area around M31 reported to have diffuse hydrogen gas,” said D.J. Pisano of West Virginia University. “The clouds observed here may be just the tip of a larger population out there waiting to be discovered.”
More of our readers had success in capturing the awesomeness of seeing Comet PANSTARRS encounter the Andromeda Galaxy (M31) in the night sky. Göran Strand sent us this absolutely gorgeous image, taken from 70 km north of Östersund, Sweden — a really dark site with no light pollution. “This photo is a 30 minute exposure through my 300mm/f2.8 lens using my full format Nikon D3s camera,” Göran said. “Besides seeing the comet and the galaxy, I also got to see 4 elks, 2 meteors, 1 bolide and 1 aurora. So all in all, it was a good night!”
That’s for sure!
See more images below of this great meet-up in the skies, and see our earlier post of our readers’ images here.
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Cold rings of dust are illuminated in this image taken by Herschel’s Spectral and Photometric Imaging Receiver (SPIRE) instrument. Credit: ESA/NASA/JPL-Caltech/B. Schulz (NHSC)
Looking wispy and delicate from 2.5 million light-years away, cold rings of dust are seen swirling around the Andromeda galaxy in this new image from the Herschel Space Observatory, giving us yet another fascinating view of our galaxy’s largest neighbor.
The colors in the image correspond to increasingly warmer temperatures and concentrations of dust — blue rings are warmer, while pinks and reds are colder lanes of dust only slightly above absolute zero. Dark at shorter wavelengths, these dust rings are revealed by Herschel’s amazing sensitivity to the coldest regions of the Universe.
The image above shows data only from Herschel’s SPIRE (Spectral and Photometric Imaging Receiver) instrument; below is a mosaic made from SPIRE as well as the Photodetecting Array Camera and Spectrometer (PACS) instrument:
“Cool Andromeda” Credit: ESA/Herschel/PACS & SPIRE Consortium, O. Krause, HSC, H. Linz
Estimated to be 200,000 light-years across — almost double the width of the Milky Way — Andromeda (M31) is home to nearly a trillion stars, compared to the 200–400 billion that are in our galaxy. And within these cold, dark rings of dust even more stars are being born… Andromeda’s star-making days are far from over.
Herschel’s mission will soon be coming to an end as the telescope runs out of the liquid helium coolant required to keep its temperatures low enough to detect such distant heat signatures. This is expected to occur sometime in February or March.
Herschel is a European Space Agency cornerstone mission with science instruments provided by consortia of European institutes, and with important participation by NASA. Launched May 14, 2009, the telescope orbits the second Lagrange point of the Earth-Sun system (L2), located 1.5 million km (932,000 miles) from Earth. Read more from the Herschel mission here.
An ancient passing between two nearby galaxies appears to have left the participants connected by a tenuous “bridge” of hydrogen gas, according to findings reported Monday, June 11 by astronomers with the National Radio Astronomy Observatory (NRAO).
Using the National Science Foundation’s Green Bank Telescope in West Virginia — the world’s largest fully-steerable radio telescope — astronomers have confirmed the existence of a vast bridge of hydrogen gas streaming between the Andromeda galaxy (M31) and the Triangulum galaxy (M33), indicating that they likely passed very closely billions of years ago.
The faint bridge structure had first been identified in 2004 with the 14-dish Westerbork Synthesis Radio Telescope in the Netherlands but there was some scientific dispute over the findings. Observations with the GBT confirmed the bridge’s existence as well as revealed the presence of six large clumps of material within the stream.
Since the clumps are moving at the same velocity as the two galaxies relative to us, it seems to indicate the bridge of hydrogen gas is connecting them together.
“We think it’s very likely that the hydrogen gas we see between M31 and M33 is the remnant of a tidal tail that originated during a close encounter, probably billions of years ago,” said Spencer Wolfe of West Virginia University. “The encounter had to be long ago, because neither galaxy shows evidence of disruption today.”
Astronomers have known for years that our Milky Way and its closest neighbor, the Andromeda galaxy, (a.k.a M31) are being pulled together in a gravitational dance, but no one was sure whether the galaxies would collide head-on or glide past one another. Precise measurements from the Hubble Space Telescope have now confirmed that the two galaxies are indeed on a collision course, headed straight for a colossal cosmic collision.
No need to panic for the moment, as this is not going to happen for another four billion years. And while astronomers say it is likely the Sun will be flung into a different region of our galaxy, Earth and the solar system will probably just go along for the ride and are in no danger of being destroyed.
“In the ‘worst-case-scenario’ simulation, M31 slams into the Milky Way head-on and the stars are all scattered into different orbits,” said team member Gurtina Besla of Columbia University in New York, N.Y. “The stellar populations of both galaxies are jostled, and the Milky Way loses its flattened pancake shape with most of the stars on nearly circular orbits. The galaxies’ cores merge, and the stars settle into randomized orbits to create an elliptical-shaped galaxy.”
The simulations Besla was talking about came from precise measurements by Hubble, painstakingly determining the motion of Andromeda, looking particularly at the sideways motion of M31, which until now has not been able to be done.
“This was accomplished by repeatedly observing select regions of the galaxy over a five- to seven-year period,” said Jay Anderson of STScI.
Right now, M31 is 2.5 million light-years away, but it is inexorably falling toward the Milky Way under the mutual pull of gravity between the two galaxies and the invisible dark matter that surrounds them both.
Of course, the collision is not like a head-on between two cars that takes place in an instant. Hubble data show that it will take an additional two billion years after the encounter for the interacting galaxies to completely merge under the tug of gravity and reshape into a single elliptical galaxy similar to the kind commonly seen in the local universe.
Astronomers said the stars inside each galaxy are so far apart that they will not collide with other stars during the encounter. However, the stars will be thrown into different orbits around the new galactic center. Simulations show that our solar system will probably be tossed much farther from the galactic core than it is today.
There’s also the complication of M31’s small companion, the Triangulum galaxy, M33. This galaxy will join in the collision and perhaps later merge with the M31/Milky Way pair. There is a small chance that M33 will hit the Milky Way first.
The astronomers working on this project said that they were able to make the precise measurements because of the upgraded cameras on Hubble, installed during the final servicing mission. This gave astronomers a long enough time baseline to make the critical measurements needed to nail down M31’s motion.
The Hubble observations and the consequences of the merger are reported in three papers that will appear in an upcoming issue of the Astrophysical Journal.
First Row, Left: Present day.
First Row, Right: In 2 billion years the disk of the approaching Andromeda galaxy is noticeably larger.
Second Row, Left: In 3.75 billion years Andromeda fills the field of view.
Second Row, Right: In 3.85 billion years the sky is ablaze with new star formation.
Third Row, Left: In 3.9 billion years, star formation continues.
Third Row, Right: In 4 billion years Andromeda is tidally stretched and the Milky Way becomes warped.
Fourth Row, Left: In 5.1 billion years the cores of the Milky Way and Andromeda appear as a pair of bright lobes.
Fourth Row, Right: In 7 billion years the merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.