THE MILKY WAY

Our Home Galaxy

If you’ve ever been outside at a good dark observing place, you might have seen the Milky Way. It almost looks like a cloud, and in a way it is. It’s a star cloud, and it’s also how our galaxy looks from the inside. If you could somehow travel outside our galaxy and look back at the Milky Way, it would resemble a giant pinwheel of light with spiral arms winding around a bright core that has a bar of light extending across the center. Our galaxy has been around for about 11 billion years, forming from smaller clumps of stars into the gigantic stellar city we inhabit today.

Milky Way Facts

1. There are more than 400 billion stars in our home galaxy.
2. The Milky Way measures about 120,000 light-years across.
3. Our galaxy rotates once every 220 million years.

A Galactic Tour

The central region of the Milky Way is densely populated with stars and a black hole called Sagittarius A*. You can’t see the core of our galaxy because it’s hidden behind clouds of gas and dust, but it’s a very busy place. The innermost part of the core has some of the oldest stars in the galaxy packed into a roughly spherical area about 10,000 light-years across. Extending out from the core is a bar of gas and stars that connects to the two major spiral arms of the galaxy called the Scutum-Centaurus Arm and the Perseus Arm. Other, smaller arms extend out from the center:

• The Sagittarius Arm
• The Near 3KPC Arm (short for “three kiloparsecs”)
• The Far 3 KPC Arm
• The Outer Arm

The Sun is located about 26,000 light-years out from the center, in a “sub arm” called the Orion Spur. The spiral arms form the galactic disk, which is the flat plane of the galaxy. It’s made mostly of stars and clouds of gas and dust that are the sites of ongoing star formation.

Surrounding the disk is the galactic halo, which encloses a region of space about 200,000 light-years across. It contains very old stars and many globular clusters, which are gravitationally bound collections of old stars. The globulars orbit the core of the galaxy and are thought to have formed about when the galaxy did. Beyond the visible halo is a spherical concentration of unknown material called dark matter. Recent studies of our galaxy using x-ray data show that the galaxy travels the universe embedded in a bubble of very hot gases.

Forming the Milky Way Galaxy

The formation history of galaxies is a story that astronomers are really just starting to understand. Here’s how the story goes: A few hundred thousand years after the Big Bang, the universe was a distributed mass of matter that was denser in some areas than in others. The first stars began to form inside these “overdensities,” and these stars became the seeds of our own galaxy and of the globular clusters that swarm around its core. Billions of years later, the spherical-shaped early Milky Way had a critical mass of stars and other material, and it began to spin. This motion caused it to collapse into the disk shape we know today.

The Milky Way continued to grow and change through galaxy mergers. Over time, these collisions helped develop the spiral arms. In fact, our galaxy is still assimilating stars from a merger with small dwarf spheroidals and is pulling material from the Large and Small Magellanic Clouds, two satellite galaxies. The process isn’t through yet. The Milky Way is moving through space at an estimated velocity of 630 kilometers per second, and it’s headed for a merger with the nearby Andromeda Galaxy. That will take place in about 4 to 5 billion years, and the two galaxies will ultimately mingle stars, gas, dust, and possibly their central black holes. The process will radically change the shape of the resulting merged object, triggering great bursts of star formation and creating the so-called Milkdromeda Galaxy, a giant elliptical that will ultimately combine with the other galaxies in the Local Group.