The upcoming merger between our Galaxy and the Sagittarius Galaxy

Context of the Collision

Discovery and dimensions of the Sagittarius Galaxy

The Sagittarius Galaxy was discovered in 1994 by a team of astronomers (Rodrigo Ibata, Michael Irwin, and Gerard Gilmore). The Sagittarius dwarf galaxy has a diameter of 10,000 light-years and contains about 200 million stars. The Milky Way has a diameter of 100,000 light-years and contains about 200 billion stars. The Sagittarius Galaxy is located about 70,000 light-years from Earth, traveling on a nearly polar orbit (an orbit passing above the galactic poles of the Milky Way) at a distance of about 50,000 light-years from the center of the Milky Way.

Orbit of Sagittarius

A nearly polar and elliptical trajectory

The orbit of the Sagittarius Galaxy is highly inclined, with an angle of 60 to 90 degrees relative to the galactic plane. This means it moves not only above and below the galactic plane, but also makes passages through regions at a high inclination relative to that plane. The trajectory of this galaxy around the nucleus of the Milky Way is highly elliptical, so that it passes near the galactic center at certain times and then moves away, ending up farther out in the periphery.

A continuous gravitational interaction

This polar orbit causes the Sagittarius Galaxy to pass through the galactic halo, where it is influenced by the gravity of the Milky Way, which contributes to its accretion process and the possible partial dissolution of the Sagittarius Galaxy. Because of this trajectory, Sagittarius is in constant gravitational interaction with the Milky Way, leading to effects such as the stripping of its stars and gas by gravitational tides, thus contributing to its slow destruction.

A merger process already begun

The Sagittarius Galaxy began merging with our galaxy about 4 to 5 billion years ago. This merger process is a major cosmic event that unfolds over billions of years. The collision between the Milky Way and the Sagittarius Galaxy is an example of galactic cannibalism, in which a larger galaxy slowly absorbs a smaller galaxy.

The stars of the Sagittarius Galaxy are gradually integrated into the Milky Way, enriching our galaxy with new stars and interstellar matter.

Impact on the Milky Way

Star formation and perturbation of orbits

The collision with the Sagittarius Galaxy will have several impacts on the Milky Way. In addition to stimulating the formation of new stars in our galaxy, this merger could disturb the orbits of existing stars, modifying the structure of the Milky Way.

However, it is important to note that stars are so far apart that direct collisions between stars are extremely rare. Most stars from the Sagittarius Galaxy will simply be integrated into the Milky Way without colliding with other stars.

Density waves and spiral arms

Waves of matter, called "density waves" or "tidal waves", will propagate through our Galaxy and contribute to the formation of spiral arms. These waves are disturbances in the distribution of matter (stars, gas, and dust) that propagate through the galaxy in response to the gravitational forces exerted by the merging galaxy.

Consequences for the Solar System

No direct impact

Although the merger of the Sagittarius Galaxy with the Milky Way is a major event, it will probably have no direct impact on our solar system. The Sun and the planets will continue their orbit without being significantly disturbed by this collision.

The Local Bubble, a protective cocoon

Our solar system is currently in a region of the Milky Way called the "Local Bubble". This bubble is a cavity relatively empty of interstellar matter; it is composed of hot (about one million degrees Kelvin) and rarefied gas. This region contrasts with the denser and colder molecular clouds that exist elsewhere in the galaxy. The Local Bubble was created by supernova explosions about 10 to 20 million years ago, which swept away interstellar gas and dust, leaving behind a low-density region. Our solar system is located near the edge of the Local Bubble. Observations suggest that we entered this bubble about 5 to 10 million years ago and that we could remain sheltered in this region for several million more years.

Formation of Galaxies

Hierarchical mergers

Galaxies form from the aggregation of matter in the primitive universe. Over time, small galaxies merge to form larger galaxies. Most large galaxies, including our own Milky Way, are the result of mergers of smaller galaxies during their history. Galactic mergers play a crucial role in the evolution of galaxies and are a common process in the universe.

Comparative histories of the Milky Way and Andromeda

The Milky Way has experienced a period of relative stability over the last few billion years. This means it has not undergone major mergers or significant disturbances that would have radically altered its overall structure. The stability of the Milky Way has allowed relatively constant star formation and a well-defined galactic structure, with spiral arms and a central bulge.

The Andromeda Galaxy has a more turbulent history. It has experienced major mergers with other galaxies, which led to a more complex structure and intense episodes of star formation. Andromeda probably merged with a significant-sized galaxy about 2 to 3 billion years ago, which disturbed its structure and led to the formation of new stars and galactic structures. Because of these mergers, Andromeda displays distinctive characteristics, such as a more disturbed disk and more active star-forming regions.

Observations and Studies

A laboratory for galactic merger

The Sagittarius Galaxy offers a unique opportunity to observe an ongoing galactic merger, making it possible to test theoretical models and better understand galactic dynamics.

Hubble and Gaia telescopes

Observations of the Sagittarius Galaxy are carried out using ground-based and space telescopes, such as the Hubble Space Telescope and the Gaia telescope. These observations make it possible to map the distribution of stars and interstellar matter, as well as to track the evolution of the merger.