What Will Happen When The Milky Way And Andromeda Collide

According to astronomers, our Milky Way galaxy and the Andromeda galaxy will collide in four billion years.

When the Milky Way and Andromeda galaxies collide, it will be a significant event in the history of our universe. Here’s what is expected to happen based on current scientific understanding:

According to astronomers, our Milky Way galaxy and the Andromeda galaxy will collide in four billion years. As galaxies collide, new stars are formed as gasses combine, both galaxies lose their shape, and the two galaxies create a new supergalaxy that is elliptical. And that’s going to happen someday! The Andromeda galaxy is currently racing toward our Milky Way at a speed of about 70 miles (110 km) per second.

Ultimately, the two galaxies will collide and merge. The merging of galaxies will radically affect their shape. For example, two spiral galaxies can merge and form an elliptical galaxy. Sometimes even more than two galaxies can collide with each other. Although galaxies have a lot of stars, it is very unlikely that stars from both galaxies actually collide.

  1. Collision and Merger: Over the course of billions of years, the gravitational interaction between the Milky Way and Andromeda will cause them to approach each other. Eventually, the galaxies will collide and merge to form a single, larger galaxy. The collision is estimated to occur in about 4 billion years.
  2. Distorted Spiral Structures: The collision will distort the spiral structures of both galaxies. The stars within the galaxies will be influenced by the gravitational forces, leading to a rearrangement of their orbits. However, the vast majority of stars are unlikely to collide with each other because of the vast distances between them.
  3. Stellar Halo Formation: As the galaxies merge, their gravitational forces will disrupt the smaller satellite galaxies orbiting them. Some of the stars from these smaller galaxies will be captured by the combined gravitational field, resulting in the formation of a stellar halo around the newly formed galaxy.
  4. Starburst Activity: The collision will trigger intense star formation activity. As gas and dust clouds collide, they will collapse under gravity, leading to the formation of new stars. This phenomenon is known as a starburst. The increased rate of star formation will likely result in the formation of numerous massive, bright blue stars.
  5. Black Hole Interaction: Both the Milky Way and Andromeda host supermassive black holes[1] at their centers. When the galaxies merge, their black holes will also interact. The exact dynamics of this interaction are not yet fully understood, but it is expected that the black holes will eventually merge into a single supermassive black hole.

At 2.5 million light-years from Earth, the Andromeda Galaxy is the most distant object visible to the naked eye. With the naked eye, Andromeda will be extremely faint. But if you have a pair of binoculars, look through them and you’ll see what looks like a cloud.

Blueshift is a phenomenon in which the light emitted from an object appears shifted towards shorter wavelengths, or towards the “blue” end of the electromagnetic spectrum. It occurs when an object is moving toward an observer. In the context of the Milky Way and Andromeda galaxies, blueshift provides evidence for their collision and eventual merger. Here’s how:

  1. Line-of-sight Velocity: By observing the light emitted from distant galaxies, astronomers can measure their line-of-sight velocity using spectroscopy. Spectroscopy allows scientists to analyze the wavelengths of light emitted by objects and identify any shifts.
  2. Blueshifted Spectra: In the case of the Milky Way and Andromeda galaxies, their spectra exhibit a blueshift. This means that the wavelengths of light emitted by these galaxies appear slightly shifted towards the blue end of the spectrum when observed from Earth.
  3. Doppler Effect: The blueshift observed in the spectra of the Milky Way and Andromeda galaxies is a result of the Doppler effect. The Doppler effect occurs when there is relative motion between the source of the waves (in this case, the galaxies) and the observer (Earth). When the source is moving towards the observer, the waves get compressed, causing a blueshift.
  4. Radial Velocity: The magnitude of the blueshift provides information about the radial velocity of the galaxies. By analyzing the amount of blueshift, astronomers can determine the speed at which the Milky Way and Andromeda galaxies are moving towards each other.
  5. Indication of Collision: The blueshift observed in the spectra of the Milky Way and Andromeda galaxies indicates that they are moving towards each other. This supports the idea that these galaxies are on a collision course and will eventually collide and merge to form a larger galaxy.

Our Milky Way galaxy is around 100,000 light-years across, but that’s fairly average for a spiral galaxy. In comparison, the largest known galaxy, called IC 1101, is 50 times larger and about 2,000 times more massive than our galactic home.

Starburst activity refers to a period of intense star formation that occurs in galaxies. It is characterized by a rapid and enhanced rate of star formation compared to the average star formation rate in a galaxy. Starburst galaxies exhibit high levels of activity, producing a large number of new stars within a relatively short period of time. Here’s a brief explanation of starburst activity:

  1. Triggering Mechanisms: Starburst activity can be triggered by various mechanisms, including galaxy interactions, mergers, gravitational disturbances, and the inflow of gas and dust into the galaxy’s central regions. These processes can compress gas clouds, increase their density, and trigger the collapse of these clouds, leading to the formation of new stars.
  2. High Star Formation Rate: During a starburst, the rate of star formation can be hundreds or even thousands of times higher than the average rate in a typical galaxy. This results in the formation of massive star clusters and the generation of a significant amount of stellar mass.
  3. Massive, Young Stars: Starburst regions are characterized by the formation of massive, young stars. These stars are often more massive than the Sun and have short lifetimes. They emit large amounts of ultraviolet radiation and have strong stellar winds, which can influence the surrounding interstellar medium and trigger further star formation.
  4. Energetic Processes: The high rate of star formation in a starburst galaxy leads to various energetic processes. Supernovae explosions[2], stellar winds, and radiation from massive stars inject energy and momentum into the surrounding gas and can drive turbulence and outflows, affecting the future evolution of the galaxy.
  5. Duration and Evolution: Starburst activity is typically a short-lived phase in the life of a galaxy, lasting around tens of millions to a few hundred million years. After the starburst subsides, the galaxy transitions into a more quiescent phase with a lower star formation rate.

It is likely the sun will be flung into a new region of our galaxy, but our Earth and solar system are in no danger of being destroyed.

When the Milky Way and Andromeda galaxies collide, the supermassive black holes at their centers will also interact. Here’s what is currently understood about black hole interactions during galaxy collisions:

  1. Central Black Holes: Both the Milky Way and Andromeda host supermassive black holes at their galactic centers. These black holes have masses millions or even billions of times greater than that of our Sun.
  2. Black Hole Dynamics: During the galaxy collision, the gravitational forces between the Milky Way and Andromeda will cause their central black holes to approach each other. As the galaxies merge, the black holes will come into close proximity and eventually interact.
  3. Three Possible Outcomes: The exact nature of the black hole interaction depends on various factors, including their masses, relative velocities, and orientations. Generally, there are three possible outcomes:
    • a. Binary Black Hole: The two black holes can form a binary system, orbiting around each other due to their mutual gravitational attraction. This binary pair will gradually lose energy through the emission of gravitational waves, leading to their eventual merger into a single black hole.
    • b. Ejection: In some cases, one of the black holes may acquire enough energy during the interaction to be flung out of the merged galaxy. This ejection can occur if the two black holes do not merge but pass close enough to each other to transfer energy.
    • c. Core Scavenging: If the relative velocities and orientations are not favorable for a binary formation or ejection, the black holes can undergo a process called core scavenging. In this scenario, one black hole dominates and absorbs stars and gas from the vicinity, growing more massive and becoming the central black hole of the merged galaxy.
  4. Gravitational Waves: The process of black hole interaction and eventual merger can produce powerful gravitational waves, which are ripples in spacetime. These waves carry energy away from the system and were first directly detected in 2015.
  5. Observational Signatures: The interaction of black holes during a galaxy collision can leave observable signatures. These include disturbances in the surrounding gas and stars, changes in the distribution of matter, and the emission of electromagnetic radiation across various wavelengths.

The studies also suggest that M33, the Triangulum Galaxy—the third-largest and third-brightest galaxy of the Local Group—will participate in the collision event, too. Its most likely fate is to end up orbiting the merger remnant of the Milky Way and Andromeda galaxies and finally merge with it in an even more distant future. However, a collision with the Milky Way, before it collides with the Andromeda Galaxy or an ejection from the Local Group cannot be ruled out.

The galaxy product of the collision has been nicknamed Milkomeda or Milkdromeda. According to simulations, this object is likely to be a giant elliptical galaxy, but with a center showing less stellar density than current elliptical galaxies. It is, however, possible the resulting object will be a large lenticular or super spiral galaxy, depending on the amount of remaining gas in the Milky Way and Andromeda. Over the course of the next 150 billion years, the remaining galaxies of the Local Group will coalesce into this object, effectively completing its evolution.

Footnotes
  1. Supermassive black holes are extremely massive celestial objects that reside at the centers of most galaxies, including the Milky Way. They have masses ranging from millions to billions of times that of the Sun. These black holes are characterized by their immense gravitational pull, which can significantly influence the dynamics and evolution of their host galaxies. They are thought to form through a combination of processes, including the accretion of surrounding matter and the merging of smaller black holes. Supermassive black holes play a crucial role in galactic evolution, regulating star formation, and driving energetic phenomena such as active galactic nuclei and quasars. Studying these enigmatic objects is an active field of research, aiming to better understand their formation, growth mechanisms, and their impact on the surrounding galaxies. [Back]
  2. Supernovae explosions are powerful events that occur at the end of a massive star’s life cycle. When a star exhausts its nuclear fuel, it undergoes gravitational collapse, leading to a rapid release of energy. This explosive event results in the ejection of the outer layers of the star and the creation of a shockwave that propagates through the interstellar medium. Supernovae are classified into different types based on their characteristics, such as the presence of hydrogen, helium, or other elements in their spectra. These stellar explosions play a crucial role in various astrophysical phenomena, including the dispersal of heavy elements into space, the triggering of star formation, and the release of energy that can impact the surrounding galaxies and interstellar medium. [Back]
Further Reading
Sources
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  • “Andromeda–Milky Way collision” ( 13:34, February 20, 2023, Last Update) https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision
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  • ESA/Hubble. (2019). When Galaxies Collide! [Website]. Retrieved from: https://www.spacetelescope.org/news/heic1917/
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  • Kormendy, J., & Ho, L. C. (2013). Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies. Annual Review of Astronomy and Astrophysics, 51, 511-653.
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  • Ferrarese, L., & Ford, H. (2005). Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research. Space Science Reviews, 116(3-4), 523-624.
  • Scharf, C. (2018). What Happens When Galaxies Collide? [Article]. Scientific American. Retrieved from: https://www.scientificamerican.com/article/what-happens-when-galaxies-collide/
  • NASA. (2019). Blueshift and Redshift. [Website]. Retrieved from: https://imagine.gsfc.nasa.gov/features/cosmic/blueshift_info.html
  • NASA. (2012). Andromeda’s Parallax and Speed. [Website]. Retrieved from: https://imagine.gsfc.nasa.gov/features/cosmic/andromeda_info.html
  • Choi, C. (2012). Milky Way and Andromeda Galaxy Heading for Catastrophic Collision. [Article]. Space.com. Retrieved from: https://www.space.com/13325-milky-andromeda-galaxy-collision.html
  • Kennicutt, R. C., Jr. (1998). Star Formation in Galaxies Along the Hubble Sequence. Annual Review of Astronomy and Astrophysics, 36, 189-231.
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  • Blecha, L., et al. (2013). Ejection of Supermassive Black Holes from Galaxy Cores: Implications for Gravitational Wave Signatures. The Astrophysical Journal, 775(1), 7.
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  • Filippenko, A. V. (1997). Optical Spectra of Supernovae. Annual Review of Astronomy and Astrophysics, 35, 309-355.
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Author: Doyle

I was born in Atlanta, moved to Alpharetta at 4, lived there for 53 years and moved to Decatur in 2016. I've worked at such places as Richway, North Fulton Medical Center, Management Science America (Computer Tech/Project Manager) and Stacy's Compounding Pharmacy (Pharmacy Tech).

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