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The Milky Way galaxy
oldest parts c. 13 billion years old; disk built over billions of years· Event· Universe & Earth
Mostly agreedGood evidence; some details, dates or numbers uncertain.
Our home galaxy: a spiral disk of perhaps 100–400 billion stars, about 100,000 light-years across.
See it on the History map →What happened
A light-year is about 6 trillion miles. The Milky Way's oldest stars are about 13 billion years old. It grew by pulling in gas and swallowing smaller galaxies; one big merger, with a galaxy called Gaia-Enceladus, happened about 10 billion years ago. At the center sits a black hole about four million times the Sun's mass. The Sun lies about 26,000 light-years out, circling once every 230 million years or so.
Why it matters
It is the neighborhood where the Sun and Earth formed. A quiet spot partway out in the disk gave the solar system a relatively calm setting for billions of years.
How we know
Galileo saw in 1610 that it is made of stars; the Gaia satellite (2013–2025) mapped over a billion stars; the central black hole was imaged in 2022.
Still debated
The galaxy's exact mass, shape and star count are uncertain, as is the nature of the dark matter surrounding it.
Connections (2)
▸First stars and galaxies merged over time to build The Milky Way galaxyMostly agreed
The connection
Small early galaxies, each a swarm of stars and gas, were drawn together by gravity and merged into larger ones. The Milky Way grew this way over billions of years, absorbing many dwarf galaxies. It is still at it, slowly tearing apart the Sagittarius dwarf galaxy today.
When and how
In our corner of the universe, from about 13 billion years ago; the major Gaia-Enceladus merger happened about 10 billion years ago.
How sure are we?
The Gaia satellite traced streams of stars moving together, the remains of swallowed galaxies; their chemistry and ages match.
Debates and open questions
How many galaxies the Milky Way absorbed, and when its disk took shape, are still being worked out.
▸The Milky Way galaxy was the setting for Birth of the Sun and EarthWell documented
The connection
The solar system formed in the Milky Way's disk, from a cloud enriched by billions of years of star births and deaths. Its location, roughly halfway out from the center, offered enough heavy elements for rocky planets and fewer nearby blasts than the crowded core.
When and how
About 26,000 light-years from the galaxy's center, 4.6 billion years ago, probably in a cluster of sibling stars that later scattered.
How we know
The Sun's chemistry fits the galaxy's pattern for its age and position; astronomers have identified possible sibling stars with nearly identical chemistry.
Debates and open questions
Where the Sun was actually born is uncertain; it may have drifted outward from a birthplace nearer the center.