At the Heart of the Milky Way

Our galaxy, the Milky Way, is a ridiculously huge collection of gas, dust, and stars … something like 200 billion stars, give or take.

That’s a lot.

In our neck of the woods stars are pretty far apart, from 20 trillion to 50 trillion kilometers on average. The nearest star known to the Sun, Proxima Centauri, is about 40 trillion kilometers away. But in the center of the galaxy—downtown—they’re packed a lot more closely together. A cube just big enough to fit the Sun on one side and Proxima on the other would hold 1 million stars if it were in the galactic core. A million.

What does something like that look like? Why, it looks like this:

That is a Hubble observation, using the Wide Field Camera 3. The colors you see aren’t what you'd actually see; the shot is a combination of three colors in the near infrared (just outside what the human eye can detect) displayed in blue, red, and green to mimic colors we can see.

Despite the small section of sky displayed (about one-fifththe apparent width of the full Moon on the sky), more than a half-million stars are visible! They’re packed together so tightly they overlap, creating an overall background glow.

The dark ribbons and patches you see are made of dust; silicate and complex carbon molecules created when stars are born and when they die. Dust is opaque to visible light, though infrared does a better job getting through it. So the places you see dark patches really are dense and thickly strewn clouds of dust. Note that the stars you see in those patches look redder; that’s because we only see longer wavelength light able to make its way through.

This image is stunning. It’s also a key to understanding galaxies. It’s part of an ongoing project to map the precise positions of the stars in the galactic core. Over time the stars move, orbiting the center of the galaxy. From 26,000 or so light-years away that motion is incredibly small, but Hubble’s eyesight is incredibly sharp. After a few years those tiny motions are measurable, and the stars’ velocities can be mapped. It’s been known for some time that the speeds of stars in the centers of galaxies is related in some way to the total mass of the galaxy and how the galaxy itself formed, but that relationship is difficult to quantify. These observations will help nail it down. Then we can use that to extrapolate to other galaxies, and understand them better.

Sometimes, to understand the Universe, you have to look into your own heart. Poetry? Perhaps. But it’s true nonetheless, and sometimes metaphor is a lovely way to understand science.