David Cao/Photography Intern

UC Irvine astronomers recently unraveled one of the great mysteries of the Milky Way: how its renowned spiral arms were formed. Credit for the astounding discovery goes to UCI astronomers James Bullock and Chris Purcell.

The Milky Way is a stream of stars in the sky  that is comprised of our galaxy. The spiral arms include Norma Arm, Scutum-Crux, Sagittarius, Orion, Perseus, and Cygnus.  The arms encompass the brightest stars found in the galaxy.

Relying on a combination of telescope observation and data simulation programs, Professors Bullock and Purcell discovered that when the dwarf galaxy, Sagittarius, collides with our galaxy, the impact causes stars to shoot out from both. Maneuvered by the Milky Way’s far more powerful gravity, the stars eventually form these spiral arms, giving rise to the famous shape of the Milky Way. The streams of stars grow and are then  pulled into the spiral arm shape.

The professors also determined that the dark matter that occupies  Sagittarius is what causes the galaxy to uncontrollably propel in the direction of the Milky Way. The collision between the two galaxies resulted in the spiral shape. The Sagittarius dwarf galaxy has had two collisions in the past 2 billion years.

According to Professor Purcell, the initial collision  broke off a chunk of the Milky Way. Due to the impact of this first collision, “instabilities were amplified.”

The professor also established that the next impact is projected to occur within the next 10 million years.

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