Science

Hubble data predicts Milky Way galactic collision

Hubble data predicts Milky Way...
3.75 billion years from now - the nighttime sky showing the Andromeda galaxy (M31) early in its collision with our Milky Way galaxy (Image: NASA / STScI)
3.75 billion years from now - the nighttime sky showing the Andromeda galaxy (M31) early in its collision with our Milky Way galaxy (Image: NASA / STScI)
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Four billion years from now, after its first close pass through the Milky Way, a view of Earth's nighttime sky shows that M31 is tidally stretched out and the Milky Way is also warped (Image: NASA / STScI)
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Four billion years from now, after its first close pass through the Milky Way, a view of Earth's nighttime sky shows that M31 is tidally stretched out and the Milky Way is also warped (Image: NASA / STScI)
An image of the present day line-up for the collision between our Milky Way galaxy and the Andromeda galaxy. The galaxies are moving toward each other, and being accelerated by their mutual gravity (Image: NASA / STScI)
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An image of the present day line-up for the collision between our Milky Way galaxy and the Andromeda galaxy. The galaxies are moving toward each other, and being accelerated by their mutual gravity (Image: NASA / STScI)
Before-and-after comparison of the size of our Milky Way galaxy at present, and after it merges with M31. The merged galaxies will form a large elliptical galaxy. Our Sun now orbits in the Milky Way's disk, but after the merger will be tossed into a highly elliptical orbit (Image: NASA / STScI)
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Before-and-after comparison of the size of our Milky Way galaxy at present, and after it merges with M31. The merged galaxies will form a large elliptical galaxy. Our Sun now orbits in the Milky Way's disk, but after the merger will be tossed into a highly elliptical orbit (Image: NASA / STScI)
This image describes how the transverse motion of M31 was measured by Hubble. As the galaxy drifts through space, its stars will move uniformly against the (essentially) fixed background galaxies (Image: NASA / STScI)
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This image describes how the transverse motion of M31 was measured by Hubble. As the galaxy drifts through space, its stars will move uniformly against the (essentially) fixed background galaxies (Image: NASA / STScI)
Astrophotograph of M31, also known as the Andromeda nebula, taken using an 85mm telescope with a hydrogen-alpha filter to enhance nebulosity (Photo: Adam Evans)
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Astrophotograph of M31, also known as the Andromeda nebula, taken using an 85mm telescope with a hydrogen-alpha filter to enhance nebulosity (Photo: Adam Evans)
3.75 billion years from now - the nighttime sky showing the Andromeda galaxy (M31) early in its collision with our Milky Way galaxy (Image: NASA / STScI)
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3.75 billion years from now - the nighttime sky showing the Andromeda galaxy (M31) early in its collision with our Milky Way galaxy (Image: NASA / STScI)
Mosaic image of Earth's nighttime sky as the collision of M31 and the Milky Way unfolds:1. (Upper left): Present Day - the bright belt of our Milky Way stretches across the sky, while the Andromeda galaxy looks like a faint spindle, several times the diameter of the full Moon.2. (Upper right): 2 Billion Years - M31's approaching disk is noticeably larger.3. (2nd row left): 3.75 Billion Years - M31 fills the field of view. The Milky Way begins to show distortion due to tidal pull from Andromeda.4. (2nd row right)-5(3rd row left): 3.85-3.9 Billion Years - during the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters.6. (3rd row right): 4 Billion Years - After its first close pass, Andromeda is tidally stretched out. The Milky Way, too, becomes warped.7. (Final row left): 5.1 Billion Years - During the second close passage, the two galactic cores maintain their separate identity. The level of star formation is much smaller because interstellar gas and dust has been reduced during earlier stages of the collision.8. (Final row right): 7 Billion Years - The merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.
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Mosaic image of Earth's nighttime sky as the collision of M31 and the Milky Way unfolds:1. (Upper left): Present Day - the bright belt of our Milky Way stretches across the sky, while the Andromeda galaxy looks like a faint spindle, several times the diameter of the full Moon.2. (Upper right): 2 Billion Years - M31's approaching disk is noticeably larger.3. (2nd row left): 3.75 Billion Years - M31 fills the field of view. The Milky Way begins to show distortion due to tidal pull from Andromeda.4. (2nd row right)-5(3rd row left): 3.85-3.9 Billion Years - during the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters.6. (3rd row right): 4 Billion Years - After its first close pass, Andromeda is tidally stretched out. The Milky Way, too, becomes warped.7. (Final row left): 5.1 Billion Years - During the second close passage, the two galactic cores maintain their separate identity. The level of star formation is much smaller because interstellar gas and dust has been reduced during earlier stages of the collision.8. (Final row right): 7 Billion Years - The merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.
This composite image shows a region in the halo in the neighboring Andromeda galaxy that astronomers used to precisely measure the galaxy's sideways motion on the sky (Image: NASA / STScI)
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This composite image shows a region in the halo in the neighboring Andromeda galaxy that astronomers used to precisely measure the galaxy's sideways motion on the sky (Image: NASA / STScI)
Two billion years from now, the disk of the approaching Andromeda galaxy is noticeably larger (Image: NASA / STScI)
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Two billion years from now, the disk of the approaching Andromeda galaxy is noticeably larger (Image: NASA / STScI)
During the first close approach just prior to four billion years in the future, the sky is ablaze with new star formation, which is evident in vast numbers of emission nebulae and open young star clusters (Image: NASA / STScI)
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During the first close approach just prior to four billion years in the future, the sky is ablaze with new star formation, which is evident in vast numbers of emission nebulae and open young star clusters (Image: NASA / STScI)
At 3.75 billion years, Andromeda fills the field of view, and the Milky Way begins to show distortion due to tidal pull (Image: NASA / STScI)
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At 3.75 billion years, Andromeda fills the field of view, and the Milky Way begins to show distortion due to tidal pull (Image: NASA / STScI)
During the second close passage at 5.1 billion years, the cores of the Milky Way and M31 galaxies appear as a pair of bright lobes, while bright nebulae are sparse because interstellar gas and dust were largely depleted by the earlier collision (Image: NASA / STScI)
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During the second close passage at 5.1 billion years, the cores of the Milky Way and M31 galaxies appear as a pair of bright lobes, while bright nebulae are sparse because interstellar gas and dust were largely depleted by the earlier collision (Image: NASA / STScI)
After seven billion years, the merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky (Image: NASA / STScI)
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After seven billion years, the merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky (Image: NASA / STScI)
This is a nighttime image of our Milky Way galaxy, which includes a view of M31, that lies 2.5 million light-years distant and looks like a faint spindle, several times the diameter of the full Moon (Image: NASA / STScI)
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This is a nighttime image of our Milky Way galaxy, which includes a view of M31, that lies 2.5 million light-years distant and looks like a faint spindle, several times the diameter of the full Moon (Image: NASA / STScI)
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When Galaxies Collide! It sounds like an early science fiction novel. However, analysis of Hubble measurements shows that our own Milky Way galaxy is moving toward a head-on collision with our nearest neighbor, the Andromeda galaxy (also known as M31). The collision will start in about four billion years, and over the following three billion years the two spiral galaxies will coalesce into a large elliptical galaxy. Based on this data, NASA has produced a video of the upcoming collision.

The story begins and ends with Hubble. The beginning is Edwin Hubble, who established in 1924 that some nebulae in the skies were far enough away that they could not be part of the Milky Way galaxy. Hubble accomplished this by using the new 100-inch Mount Wilson telescope to discover Cepheid variable stars in M31. Cepheid variables have a fixed relationship between brightness and their period of variability. As such, their apparent brightness allows their distance to be easily estimated.

Astrophotograph of M31, also known as the Andromeda nebula, taken using an 85mm telescope with a hydrogen-alpha filter to enhance nebulosity (Photo: Adam Evans)
Astrophotograph of M31, also known as the Andromeda nebula, taken using an 85mm telescope with a hydrogen-alpha filter to enhance nebulosity (Photo: Adam Evans)

Hubble also demonstrated that the redshift of galaxies (first discovered in 1912 by Vesto Silpher) was proportional to their distance from our galaxy. Redshift of the spectral lines in the light from a galaxy reflects its velocity relative to our galaxy. The cosmological redshift is caused by the expansion of the Universe. Some nearby galaxies, however, such as M31, show a blue shift rather than a red shift, showing that they have a physical motion toward us that is larger then the cosmological redshift.

Some 90 years later, we know that M31 is about 2.5 million light-years away, and is approaching our galaxy at about 109 km/sec. However, until now we have not been able to measure the perpendicular speed of M31, so could not tell if M31 would collide with us, brush by, or miss.

The end of the story also owes much to a different Hubble - the Hubble Space Telescope. The only accurate approach to measure the tangential speed of M31 is to observe proper motion of M31's stars against a background of distant galaxies. This is a difficult problem, as to make an accurate go-nogo collision prediction we must measure proper motions with a resolution of 20-30 microarcseconds. For comparison, Earth's Moon has an angular diameter of about two billion microarcseconds, and the required resolution is that required to resolve objects separated by two inches on the Moon's surface.

This image describes how the transverse motion of M31 was measured by Hubble. As the galaxy drifts through space, its stars will move uniformly against the (essentially) fixed background galaxies (Image: NASA / STScI)
This image describes how the transverse motion of M31 was measured by Hubble. As the galaxy drifts through space, its stars will move uniformly against the (essentially) fixed background galaxies (Image: NASA / STScI)

Only the Hubble is capable of such precise measurement, and then only of objects which appear in the same deep view image. Such an image is about a thousand light-years wide at the distance of M31, which does not allow measurement of sufficiently small transverse motions. It is therefore necessary to perform sophisticated averaging procedures over images of stars bright enough that their image is spread over a number of pixels on the Hubble CCD detectors. When this was carried out, the measured transverse motion was statistically equal to zero - meaning a head-on collision with M31 is in our remote future.

An image of the present day line-up for the collision between our Milky Way galaxy and the Andromeda galaxy. The galaxies are moving toward each other, and being accelerated by their mutual gravity (Image: NASA / STScI)
An image of the present day line-up for the collision between our Milky Way galaxy and the Andromeda galaxy. The galaxies are moving toward each other, and being accelerated by their mutual gravity (Image: NASA / STScI)

Computer simulations derived from Hubble's 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. The dynamics of the collision are shown in the following video, which traces the evolution of the Milky Way and Andromeda galaxies over the next 8.2 billion years at a rate of 105 million years per second.

Attracted by their mutual gravity, the two galaxies encounter each other in a head-on collision about four billion years from now. The thin disk shapes of these spiral galaxies are strongly distorted and irrevocably transformed by the encounter. After an additional two billion years, the two galaxies will merge to form a single elliptical galaxy.

The mosaic image below shows what the predicted galactic collision will look like in Earth's night sky over the next seven billion years.

Mosaic image of Earth's nighttime sky as the collision of M31 and the Milky Way unfolds:1. (Upper left): Present Day - the bright belt of our Milky Way stretches across the sky, while the Andromeda galaxy looks like a faint spindle, several times the diameter of the full Moon.2. (Upper right): 2 Billion Years - M31's approaching disk is noticeably larger.3. (2nd row left): 3.75 Billion Years - M31 fills the field of view. The Milky Way begins to show distortion due to tidal pull from Andromeda.4. (2nd row right)-5(3rd row left): 3.85-3.9 Billion Years - during the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters.6. (3rd row right): 4 Billion Years - After its first close pass, Andromeda is tidally stretched out. The Milky Way, too, becomes warped.7. (Final row left): 5.1 Billion Years - During the second close passage, the two galactic cores maintain their separate identity. The level of star formation is much smaller because interstellar gas and dust has been reduced during earlier stages of the collision.8. (Final row right): 7 Billion Years - The merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.
Mosaic image of Earth's nighttime sky as the collision of M31 and the Milky Way unfolds:1. (Upper left): Present Day - the bright belt of our Milky Way stretches across the sky, while the Andromeda galaxy looks like a faint spindle, several times the diameter of the full Moon.2. (Upper right): 2 Billion Years - M31's approaching disk is noticeably larger.3. (2nd row left): 3.75 Billion Years - M31 fills the field of view. The Milky Way begins to show distortion due to tidal pull from Andromeda.4. (2nd row right)-5(3rd row left): 3.85-3.9 Billion Years - during the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters.6. (3rd row right): 4 Billion Years - After its first close pass, Andromeda is tidally stretched out. The Milky Way, too, becomes warped.7. (Final row left): 5.1 Billion Years - During the second close passage, the two galactic cores maintain their separate identity. The level of star formation is much smaller because interstellar gas and dust has been reduced during earlier stages of the collision.8. (Final row right): 7 Billion Years - The merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.

Although the galaxies will plow into each other, 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.

Before-and-after comparison of the size of our Milky Way galaxy at present, and after it merges with M31. The merged galaxies will form a large elliptical galaxy. Our Sun now orbits in the Milky Way's disk, but after the merger will be tossed into a highly elliptical orbit (Image: NASA / STScI)
Before-and-after comparison of the size of our Milky Way galaxy at present, and after it merges with M31. The merged galaxies will form a large elliptical galaxy. Our Sun now orbits in the Milky Way's disk, but after the merger will be tossed into a highly elliptical orbit (Image: NASA / STScI)

What could be worse for the Earth than a galactic collision? Well ... just about anything, it seems. The closest average approach of stars in the spiral arms during such a collision will be about a tenth of a light year. Such distant encounters produce very little terror for individual worlds.

In addition, the Earth faces much more significant problems over this time scale. The Sun is approaching its transition to a red giant star. This will cause the oceans to evaporate in about one billion years. All water will be lost from the planet after three billion years, so any remnants of our present biosphere will be destroyed by that time, if not sooner. And two billion years later the Sun will expand into a red giant large enough to engulf the Earth. In view of such a interesting future history, a galactic collision seems a very small concern indeed.

Source: Space Telescope Science Institute (STScI)

View gallery - 14 images
3 comments
Myron J. Poltroonian
So, "the Earth faces much more significant problems over this time scale. The Sun is approaching its transition to a red giant star. This will cause the oceans to evaporate in about one billion years. All water will be lost from the planet after three billion years, so any remnants of our present biosphere will be destroyed by that time, if not sooner. And two billion years later the Sun will expand into a red giant large enough to engulf the Earth.". Hmm, so much for the significance of Man's impact on "Climate Change". Looks like ol' Sol's got us beat by a country mile, 'n then some. Maybe we'd better get some invention' done in the next million years or so because "We gotta get out'a This Place, if it's the last thing we ever do". (Hey, I'm just saying'. Y'know?)
donwine
Why 4 Billion? The number: 3,673,980,201 sounds more accurate. Now I need to know the month and day so I can start planing now.
kellory
" Now I need to know the month and day so I can start planing now." Morning, or afternoon? What does that matter? Because the plumber is coming in the morning, followed by the cable installer.