Astronomical Collisions

It’s the ultimate merging when lightyear-size galaxies attract each other and collide. The process takes billions of years, and although we won’t be able to record a whole collision in our lifetime, the Hubble Space Telescope has given us glimpses of various galaxies around the universe at different stages of their collision.

The Antennae galaxies. composite image from the Chandra X-ray Observatory (blue), the Hubble Space Telescope (gold and brown), and the Spitzer Space Telescope (red)

The Antennae galaxies. Composite image from the Chandra X-ray Observatory (blue), the Hubble Space Telescope (gold and brown), and the Spitzer Space Telescope (red). Zooming through the nighttime sky into the Antennae galaxies!¬†Visit image’s feature.

What Happens When Galaxies Collide?

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 starts, it is very unlikely that starts from both galaxies actually collide. This is because of all the space in between the stars. As the galaxies merge the start would just pass by each other.

Merging galaxies can also trigger the creation of new stars. Galaxies are also composed of gas and dust. The gravitational pull of both colliding stars can interact with this materials, creating friction and shock waves that can ignite the formation of new stars.

The Discovery of Other Galaxies

Edwin Powel Hubble, from which the Hubble Space Telescope is named after, was the American astronomer who discovered that many space objects, previously classified as nebulae laying very close or within the Milky Way Galaxy, were actually galaxies beyond our galaxy.
Around the early 1900s, it was believed that the universe consisted simply of the Milky Way Galaxy. Hubble, through observations at the Mount Wilson Observatory, between 1922 to 1923, was able to prove that some that stars were part of more distant nebulae. Many of them being part of the Andromeda Galaxy.

Through his findings, Hubble was able to classify all the galaxies the discovered based on the photographic appearance, this classification is now known as the Hubble Sequence.
Hubble's Sequence, galaxies classification.

The Collision Simulation

Collaborations between the Advanced Visualization Laboratory (AVL) at the National Center for Supercomputing Applications (NCSA), NASA and, Drs. Brant Robertson (Caltech) and Lars Hernquist (Harvard Univ.) brought to life the data of a scientific theoretical model of two colliding galaxies into a computerized simulation.
The visualization of the two colliding galaxies interacting with each other and finally merging takes a period spanning about two billion years. This computer simulation is the only way to give us a quick view of the whole process. Even so, shapes, dimensions, and speeds of different galaxies can create different outcomes but this simulation is the closest, yet, we can get to observe such an amazing dance between giants.

This is the simulation video by NCSA/NASA/B. Robertson (Caltech) and L. Hernquist (Harvard Univ.).

This is another summation video showing how it resembles different pictures taken by Hubble Space Telescope.


Milky Way Collisions

The Milky Way and Andromeda

It’s predicted that in about four billion years these two galaxies will collide. The Andromeda galaxy has a diameter of 220,00 light years, making it much bigger than our Milky Way at 100,000 light years in diameter. Andromeda is 2.537 million light years from us and getting closer.¬† The merging will change completely the structure of both galaxies and in the end, it will be a completely new galaxy.

Representation of the Andromeda galaxy inEarth's night sky in 3.75 billion years.

In this image, representing Earth’s night sky in 3.75 billion years, Andromeda (left) fills the field of view and begins to distort the Milky Way with tidal pull. (Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger)

The Milky Way Colliding with Dwarf Galaxies

In 2003 a team of International astronomers discovered an even closer galaxy, the Canis Major dwarf galaxy. It lays at about 42.000 light years from the Milky Way’s center and 25,000 light years from Earth. It’s been pulled apart as it orbits the Milky Way. Now it’s stream of gas and dust surrounding the Milky Way.

Canis Major Dwarf

Canis Major Dwarf. Illustration Credit & Copyright: R. Ibata (Strasbourg Observatory, ULP) et al., 2MASS, NASA


Another dwarf galaxy surrounding the Milky Way is the¬†Sagittarius Dwarf. It was discovered in 1994 and just like the Canis Major dwarf galaxy, the massive pull of the Milky Way’s gravity has been tearing it apart over billions of years. It’s now seen an elongated tidal stream around our galaxy and sits at about 70,000 light years away from the Sun.

Sagittarius Dwarf Tidal Stream, artist representation

The Sagittarius Dwarf is small galaxy discovery in 1994 by R. Ibata, G. Gilmore and M. Irwin (RGO). Drawing Credit & Copyright: David Martinez-Delgado (MPIA) & Gabriel Perez (IAC)


Other Galaxies:

  • M51 is a spiral galaxy, about 30 million light-years away, that’s in the process of merging with a smaller galaxy.


  • Galaxy NGC 3921 is an interacting pair of disk galaxies in the late stages of its merger. Observations show that the galaxies collided about 700 million years ago. The tails and loops are characteristic of a post-merger.


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