4. Our Galaxy: The Milky Way

A galaxy is a massive system of stars, gas, dust, and dark matter held together by gravity. Our home galaxy is called the Milky Way.

Structure of the Milky Way:

• Shape: a barred spiral galaxy — a flat disk with spiral arms and a central bar-shaped bulge.
• Size: about 100,000 light-years in diameter (a light-year is the distance light travels in one year — about 9.46 × 10¹² km).
• Stars: estimated 200–400 billion stars.
• Our location: the Sun sits in a minor spiral arm, the Orion Arm, about 26,000 light-years from the galactic center.

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What we see as the "Milky Way" in the night sky:
The faint band of light stretching across the night sky is our edge-on view of the galactic disk — billions of distant stars blending into a glowing stripe. Because we are inside the disk, we cannot see the full spiral structure from Earth.

The galactic center:
At the very center of the Milky Way sits a supermassive black hole called Sagittarius A*, with a mass about 4 million times that of the Sun. Its intense gravity influences the motion of stars near the galactic center.

The halo:
Surrounding the disk is a roughly spherical halo containing old stars (in globular clusters) and most of the galaxy's dark matter — invisible matter detected only by its gravitational effects.