Universe Today
Glossary · Galaxies & Cosmology
Galactic merger
Two galaxies colliding, which takes a billion years and almost never breaks a star.
Galaxies are mostly empty, so a collision is a gravitational event rather than a physical one: the stars pass through, but the gas clouds do not, and the resulting compression triggers enormous bursts of star formation. Tidal forces draw out the long tails visible in the famous images. The Milky Way has eaten several small galaxies already and will merge with Andromeda in about four billion years, by which time the Sun will be a red giant and the question will be academic. The gas is where the drama is. Two discs meeting funnel enormous quantities of it into their centres, which both lights a starburst and feeds whatever black holes are there — so a merger is usually the moment a quiet galaxy becomes an active one. The final black holes are expected to spiral together as well, and that signal is what the pulsar timing arrays are listening for.
Where it has come up lately
Related terms
Fast radio burst
A millisecond flash of radio energy from another galaxy, cause still unsettled.
Redshift
The stretching of light from distant objects, and the universe's odometer.
Hubble tension
Two good measurements of the universe's expansion rate that stubbornly disagree.
Dark energy
The name for whatever is making the expansion of the universe speed up.
Questions
What is Galactic merger?
Two galaxies colliding, which takes a billion years and almost never breaks a star.
Why does it matter?
Two discs meeting funnel enormous quantities of it into their centres, which both lights a starburst and feeds whatever black holes are there — so a merger is usually the moment a quiet galaxy becomes an active one. The final black holes are expected to spiral together as well, and that signal is what the pulsar timing arrays are listening for.