Milky Way
The Milky Way is the name of our galaxy— that gigantic collection of stars, gas, dust, and dark matter that contains the Sun, Earth, and everything we can see with the naked eye in the sky. It is a barred spiral galaxy containing between 200 and 400 billion stars, of which our Sun is just one among many.
Why this name?
The name Milky Way comes from Greek mythology: the milky band that crosses the night sky is said to have been formed from the milk of the goddess Hera. It was not until 1610 that Galileo, pointing his telescope at this luminous band, discovered that it was composed of a multitude ofstars. This lateral projection in the sky corresponds to the plane of the galactic disk as seen from within.
Structure of Our Galaxy
The Milky Way is composed of several distinct regions. The central bulge, dense with old stars, is home to Sagittarius A*, the central supermassive black hole with a mass of 4 million solar masses. The disk, which spans about 100,000 light-years, contains the spiral arms where new stars are actively forming. The halo, which is spherical and extends over several hundred thousandlight-years, contains very old stars, globular clusters, and most of the dark matter.
Our Place in the Galaxy
The Sun is located in the Orion Arm, a minor spiral arm, about 27,000 light-years from the galactic center. It takes about 230 million years to complete one full orbit around the center of the galaxy—this period is called a galactic year. Since the Sun’s formation 4.6 billion years ago, it has completed approximately 20 galactic revolutions.
A Galaxy in Motion
The Milky Way is part of the Local Group, a cluster of about fifty galaxies, the two largest of which are the Milky Way and Andromeda. These two giants are approaching each other at a speed of about 110 km/s and will collide in about 4.5 billion years, forming a new elliptical galaxy sometimes nicknamed Milkomeda.
Did you know?
The Milky Way is currently absorbing several dwarf satellite galaxies. The stellar streams of Sagittarius and the Big Dipper are evidence of ongoing galactic mergers. The Sagittarius Dwarf Galaxy is currently being torn apart by the Milky Way’s tidal forces.
Frequently Asked Questions
Can you see the Milky Way with the naked eye?
Yes, but only far away from any light pollution. Under a dark sky, the band of the Milky Way appears as a luminous veil stretching across the sky. More than a third of humanity can no longer see it from where they live due to light pollution.
How many stars are there in the Milky Way?
Estimates range from 200 to 400 billion stars. This uncertainty stems from the difficulty of counting the dim red dwarfs, which make up the majority of the stellar population but are hard to detect individually.
Is there life beyond Earth in the Milky Way?
There is currently no evidence, but the conditions seem favorable: billions of stars have planets, including billions in the habitable zone. The SETI project and the James Webb Space Telescope’s analysis of exoplanet atmospheres are actively searching for signs of life elsewhere in the galaxy.