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Doubt cast on collision between the Milky Way and the Andromeda galaxy

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collision Milky Way Andromeda galaxy A three-panel image, two at the top and one stretched across the bottom. At top left, two spiral galaxies are widely separated against the black background of space. At top right, two face-on spiral galaxies are close together. Their spiral arms appear stretched toward each other. At bottom, two spiral galaxies have collided, resulting in a broad X-shaped patch of milky white. Mottled clouds of dark brown dust are superimposed.

Hubble and a new study published in Nature Astronomy cast doubt on the certainty of a collision between the Milky Way and the Andromeda galaxy

Over a decade’s worth of NASA/ESA Hubble Space Telescope data was used to re-examine the long-held prediction that the Milky Way galaxy will collide with the Andromeda galaxy in about 4.5 billion years. The astronomers found that, based on the latest observational data from Hubble as well as the Gaia space telescope, there is only a 50-50 chance of the two galaxies colliding within the next 10 billion years. The study also found that the presence of the Large Magellanic Cloud can affect the trajectory of the Milky Way and make the collision less likely. The researchers emphasize that predicting the long-term future of galaxy interactions is highly uncertain, but the new findings challenge the previous consensus and suggest the fate of the Milky Way remains an open question.

A three-panel image, two at the top and one stretched across the bottom. At top left, two spiral galaxies are widely separated against the black background of space. At top right, two face-on spiral galaxies are close together. Their spiral arms appear stretched toward each other. At bottom, two spiral galaxies have collided, resulting in a broad X-shaped patch of milky white. Mottled clouds of dark brown dust are superimposed.
Hubble and a new study published in Nature Astronomy cast doubt on the certainty of a collision between the Milky Way and the Andromeda galaxy. This selection of images of external galaxies illustrates three encounter scenarios between our Milky Way and the neighboring Andromeda galaxy. In the top left panel, a wide-field DSS image showing galaxies M81 and M82 serves as an example of the Milky Way and Andromeda passing each other at large distances. The top right panel shows NGC 6786, a pair of interacting galaxies displaying the telltale signs of tidal disturbances after a close encounter. The bottom panel shows NGC 520, a cosmic train wreck as two galaxies are actively merging together.
Credit: NASA, ESA, STScI, Till Sawala (University of Helsinki), DSS, J. DePasquale (STScI)

As far back as 1912, astronomers realized that the Andromeda galaxy — then thought to be only a nebula — was headed our way. A century later, astronomers using the NASA/ESA Hubble Space Telescope were able to measure the sideways motion of Andromeda and found it was so negligible that an eventual head-on collision with the Milky Way seemed almost certain.

A smashup between our own galaxy and Andromeda would trigger a firestorm of star birth, supernovae, and maybe toss our Sun into a different orbit. Simulations had suggested it was inevitable.

However, a new study using data from Hubble and ESA’s Gaia suggests this may not necessarily be the case. Researchers combining observations from the two space observatories re-examined the long-held prediction of a Milky Way – Andromeda collision, and found it is far less inevitable than astronomers had previously suspected.

“We have the most comprehensive study of this problem today that actually folds in all the observational uncertainties,” said Till Sawala, astronomer at the University of Helsinki in Finland and lead author of the study, which appears today in the journal Nature Astronomy.

His team includes researchers at Durham University, United Kingdom; the University of Toulouse, France; and the University of Western Australia. They found that there is approximately a 50-50 chance of the two galaxies colliding within the next 10 billion years. They based this conclusion on computer simulations using the latest observational data.

Sawala emphasized that predicting the long-term future of galaxy interactions is highly uncertain, but the new findings challenge the previous consensus and suggest the fate of the Milky Way remains an open question.

“Even using the latest and most precise observational data available, the future of the Local Group of several dozen galaxies is uncertain. Intriguingly, we find an almost equal probability for the widely publicized merger scenario, or, conversely, an alternative one where the Milky Way and Andromeda survive unscathed,” said Sawala.

The collision of the two galaxies had seemed much more likely in 2012, when astronomers Roeland van der Marel and Tony Sohn of the Space Telescope Science Institute in Baltimore, Maryland published a detailed analysis of Hubble observations over a five-to-seven-year period, indicating a direct impact in no more than 5 billion years.

“It’s somewhat ironic that, despite the addition of more precise Hubble data taken in recent years, we are now less certain about the outcome of a potential collision. That’s because of the more complex analysis and because we consider a more complete system. But the only way to get to a new prediction about the eventual fate of the Milky Way will be with even better data,” said Sawala.

Astronomers considered 22 different variables that could affect the potential collision between our galaxy and our neighbor, and ran 100,000 simulations called Monte Carlo simulations stretching to 10 billion years into the future.

“Because there are so many variables that each have their errors, that accumulates to rather large uncertainty about the outcome, leading to the conclusion that the chance of a direct collision is only 50% within the next 10 billion years,” said Sawala.

“The Milky Way and Andromeda alone would remain in the same plane as they orbit each other, but this doesn’t mean they need to crash. They could still go past each other,” said Sawala.

Researchers also considered the effects of the orbits of Andromeda’s large satellite galaxy, M33, and a satellite galaxy of the Milky Way called the Large Magellanic Cloud (LMC).

“The extra mass of Andromeda’s satellite galaxy M33 pulls the Milky Way a little bit more towards it. However, we also show that the LMC pulls the Milky Way off the orbital plane and away from Andromeda. It doesn’t mean that the LMC will save us from that merger, but it makes it a bit less likely,” said Sawala.

In about half of the simulations, the two main galaxies fly past each other separated by around half a million light-years or less (five times the Milky Way’s diameter). They move outward but then come back and eventually merge in the far future. The gradual decay of the orbit is caused by a process called dynamical friction between the vast dark-matter halos that surround each galaxy at the beginning.

In most of the other cases, the galaxies don’t even come close enough for dynamical friction to work effectively. In this case, the two galaxies can continue their orbital waltz for a very long time.

The new result also still leaves a small chance of around 2% for a head-on collision between the galaxies in only 4 to 5 billion years. Considering that the warming Sun makes Earth uninhabitable in roughly 1 billion years, and the Sun itself will likely burn out in 5 billion years, a collision with Andromeda is the least of our cosmic worries.

Bibliographic information:

Sawala, T., Delhomelle, J., Deason, A.J. et al, No certainty of a Milky Way–Andromeda collision. Nat Astron (2025), DOI: https://doi.org/10.1038/s41550-025-02563-1

Press release from ESA Hubble.

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Astronomy

Webb peers into the tendrils of NGC 604

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Webb NGC 604 At the centre of the image is a nebula on the black background of space. The nebula is composed of clumpy, red, filamentary clouds. At the centre-right of the red clouds is a large cavernous bubble, and at the centre of the bubble there is an opaque blue glow with speckles of stars. At the edges of the bubble, the dust is white. There are several other smaller cavernous bubbles at the top of the nebula. There are also some smaller, red stars and a few disc-shaped galaxies scattered about the image.

Two new images from the NASA/ESA/CSA James Webb Space Telescope’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) showcase the star-forming region NGC 604, located in the Triangulum Galaxy (M33), 2.73 million light-years away from Earth. In these images, cavernous bubbles and stretched-out filaments of gas etch a more detailed and complete tapestry of star birth than seen in the past.

Sheltered among NGC 604’s dusty envelopes of gas are more than 200 of the hottest, most massive kinds of stars, all in the early stages of their lives. These types of stars are known as B-types and O-types, the latter of which can be more than 100 times the mass of our own Sun. It’s quite rare to find this concentration of them in the nearby Universe. In fact, there’s no similar region within our own Milky Way galaxy.

This concentration of massive stars, combined with its relatively close distance, means NGC 604 gives astronomers an opportunity to study these objects at a fascinating time early in their life.

At the centre of the image is a nebula on the black background of space. The nebula is composed of clumpy, red, filamentary clouds. At the centre-right of the red clouds is a large cavernous bubble, and at the centre of the bubble there is an opaque blue glow with speckles of stars. At the edges of the bubble, the dust is white. There are several other smaller cavernous bubbles at the top of the nebula. There are also some smaller, red stars and a few disc-shaped galaxies scattered about the image.
This image from the NASA/ESA/CSA James Webb Space Telescope’s NIRCam (Near-Infrared Camera) of star-forming region NGC 604 shows how stellar winds from bright, hot young stars carve out cavities in surrounding gas and dust.
The bright orange streaks in this image signify the presence of carbon-based molecules known as polycyclic aromatic hydrocarbons, or PAHs. As you travel further from the immediate cavities of dust where the star is forming, the deeper red signifies molecular hydrogen. This cooler gas is a prime environment for star formation. Ionised hydrogen from ultraviolet radiation appears as a white and blue ghostly glow.
NGC 604 is located in the Triangulum Galaxy (M33), 2.73 million light-years away from Earth. It provides an opportunity for astronomers to study a high concentration of very young, massive stars in a nearby region.
Credit: NASA, ESA, CSA, STScI

In Webb’s near-infrared NIRCam image, the most noticeable features are tendrils and clumps of emission that appear bright red, extending out from areas that look like clearings, or large bubbles in the nebula. Stellar winds from the brightest and hottest young stars have carved out these cavities, while ultraviolet radiation ionises the surrounding gas. This ionised hydrogen appears as a white and blue ghostly glow.

The bright orange streaks in the Webb near-infrared image signify the presence of carbon-based molecules known as polycyclic aromatic hydrocarbons, or PAHs. This material plays an important role in the interstellar medium and the formation of stars and planets, but its origin is a mystery. As you travel further from the immediate clearings of dust, the deeper red signifies molecular hydrogen. This cooler gas is a prime environment for star formation.

Webb’s exquisite resolution also provides insights into features that previously appeared unrelated to the main cloud. For example, in Webb’s image, there are two bright, young stars carving out holes in dust above the central nebula, connected through diffuse red gas. In visible-light imaging from the NASA/ESA Hubble Space Telescope, these appeared as separate splotches.

At the centre of the image is a nebula on the black background of space. The nebula is composed of wispy filaments of light blue clouds. At the centre-right of the blue clouds is a large cavernous bubble. The bottom left edge of this cavernous bubble is filled with hues of pink and white gas. Hundreds of dim stars fill the area surrounding the nebula.
This image from the NASA/ESA/CSA James Webb Space Telescope’s MIRI (Mid-Infrared Instrument) of star-forming region NGC 604 shows how large clouds of cooler gas and dust glow at mid-infrared wavelengths. This region is a hotbed of star formation and home to more than 200 of the hottest, most massive kinds of stars, all in the early stages of their lives.
In the MIRI view of NGC 604, there are noticeably fewer stars than Webb’s NIRCam image. This is because hot stars emit much less light at these wavelengths. Some of the stars seen in this image are red supergiants — stars that are cool but very large, hundreds of times the diameter of our Sun. The blue tendrils of material signify the presence of polycyclic aromatic hydrocarbons, or PAHs.
Credit: NASA, ESA, CSA, STScI

Webb’s view in mid-infrared wavelengths also illustrates a new perspective on the diverse and dynamic activity of this region. In the MIRI view of NGC 604, there are noticeably fewer stars. This is because hot stars emit much less light at these wavelengths, while the larger clouds of cooler gas and dust glow. Some of the stars seen in this image from the surrounding galaxy are red supergiants — stars that are cool but very large, hundreds of times the diameter of our Sun. Additionally, some of the background galaxies that appeared in the NIRCam image also fade. In the MIRI image, the blue tendrils of material signify the presence of PAHs.

NGC 604 is estimated to be around 3.5 million years old. The cloud of glowing gases extends to some 1300 light-years across.

At the centre of the image is a nebula on the black background of space. The nebula is composed of clumpy, red, filamentary clouds. At the centre-right of the red clouds is a large cavernous bubble, and at the centre of the bubble there is an opaque blue glow with speckles of stars. At the edges of the bubble, the dust is white. There are several other smaller cavernous bubbles at the top of the nebula. There are also some smaller, red stars and a few disc-shaped galaxies scattered about the image.
This image from the NASA/ESA/CSA James Webb Space Telescope’s NIRCam (Near-Infrared Camera) of star-forming region NGC 604 shows how stellar winds from bright, hot young stars carve out cavities in surrounding gas and dust.
The bright orange streaks in this image signify the presence of carbon-based molecules known as polycyclic aromatic hydrocarbons, or PAHs. As you travel further from the immediate cavities of dust where the star is forming, the deeper red signifies molecular hydrogen. This cooler gas is a prime environment for star formation. Ionised hydrogen from ultraviolet radiation appears as a white and blue ghostly glow.
NGC 604 is located in the Triangulum Galaxy (M33), 2.73 million light-years away from Earth. It provides an opportunity for astronomers to study a high concentration of very young, massive stars in a nearby region.
Credit: NASA, ESA, CSA, STScI

Press release from ESA Webb.

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