Astronomers estimate that Andromeda will collide with the Milky Way in 4 billion years, with the merger concluding 6 billion years from now. By that time, the sun will have swollen into a red giant and swallowed up the terrestrial planets, so Earth will have other things to worry about. Still, the fresh influx of dust should boost star formation in the new "Milkomeda" galaxy, and the Earthless sun may well leave the Milky Way for good.
After a messy phase, where arms project crazily from the combined pair, the two should settle into a smooth elliptical galaxy. Galaxy collisions are a normal part of the universe's evolution. In fact, both Andromeda and the Milky Way bear signs of having already crashed into other galaxies.
Andromeda boasts a large ring of dust in its center, giving it an interesting shape. Astronomers believe this dust may have formed when it swallowed an existing galaxy. In , the Persian astronomer Abd al-Rahman al-Sufi described the galaxy as a "small cloud" in his "Book of Fixed Stars," the first known report of our nearest neighbor. When Charles Messier labeled it M31 in , he incorrectly credited the discovery of what was then called a nebula to the German astronomer, Simon Marius, who provided the first telescopic observation of the object.
The first photographs of Andromeda were taken in , by Isaac Roberts. At the time, astronomers thought the Milky Way composed the whole universe, and the strange patches known as nebulae lay inside of them.
Curtis had spotted various novae in Andromeda, and argued instead that it was a separate galaxy. The discussion wasn't concluded until , when Edwin Hubble identified a special kind of star known as a Cepheid variable — a star whose characteristics allow for precise measurements of distance — within Andromeda.
Because Shapley had previously determined that the Milky Way was only , light-years across, Hubble's calculations revealed that the fuzzy patch was too far away to lie within the Milky Way. Hubble went on to use his measurements of the Doppler shifts of the galaxies to determine that the universe was expanding. The calculated distance to Andromeda doubled in the s when Walter Baade was the first to observe individual stars in the central region of the galaxy, and found two different types of Cepheid variables.
Our understanding of the size of the Andromeda galaxy has grown bigger in recent years. In , observations from the Hubble Space Telescope found that a halo of material surrounding Andromeda is six times larger and 1, times more massive than what was previously measured.
At the time, astronomers said the Milky Way may have a halo as well — and perhaps the two galaxies' halos are already starting to merge.
This follows on from revised size estimates in and , based on observing stars and star motions. In , scientists released the most detailed photo of Andromeda ever using a mosaic of images from the Hubble Space Telescope.
The image included 7, exposures taken over pointings of the telescope. The image revealed more than million stars within the galaxy, as well as dust structures and other features. But while our Galaxy has up to billion stars, Andromeda is thought to contain a trillion. Download a PDF guide to help you take the Andromeda tour. Start your journey into M31 by looking for the dark dust lane that runs along the northwest edge of the core. Look for the faint glow of the spiral arms beyond this lane.
This glow continues toward another dark lane located further out. Notice also how the central region fades inwards towards a star-like core. This mag. Like M31, the core of M32 appears almost stellar in nature, but larger apertures will reveal it as an extended region about arcseconds across. M32 is an elliptical dwarf galaxy with a mass equal to around 3 billion Suns. Like M32, M is another gravitationally bound satellite galaxy of M32 and another elliptical galaxy.
M sits 35 arcminutes northwest of the centre of M31, 1. The surface brightness of this mag. We return to the main galaxy for our next object, NGC , a bright star cloud. When observing the Andromeda Galaxy through a scope, it is obvious that the main galaxy is dominated by its bright core. And it takes patience to see anything other than the core. The dark dust lanes mentioned previously are obvious candidates but there are other parts of the galaxy to see. NGC is truly a part of M NGC is another example.
This lesser-known satellite of M31 is a moderately bright dwarf spheroidal galaxy. A mm scope will reveal it as about 4 arcminutes across, a bit elongated in an east-west direction. NGC , is a tricky object, even with large instruments. This is another dwarf spheroidal galaxy and another M31 satellite. Pete Lawrence is an experienced astronomer and a co-presenter of The Sky at Night. The stars of constellation Andromeda and the magnificent neighboring galaxy M31 rise above the highest peak of Mount Gongga, Sichuan province, China.
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