We are here

We live in the Milky Way galaxy, a fairly typical open spiral galaxy galaxy (Type S, for you naming geeks out there). Those spirally bits spread out from the galactic core, and the whole thing is slowly (quickly) rotating, making one turn every 120 million years or so.

Well, we once thought there were four arms to the Milky Way; now we think there are only two. Regardez la photo:

We’re on a mini arm called the Orion Spur. Here’s a honking big image – you might be able to see better there.

From the MSNBC site that brought me the story (there’s more to read, so click through and take a look):

For decades, astronomers have pictured our galaxy as sporting four major, spiral arms, however new images effectively sever two appendages, revealing the Milky Way has just two major arms.

“We’re not proposing that they change the positions of the arms,” said Robert Benjamin of the University of Wisconsin, Whitewater. “What we’re proposing is a change in the emphasis of the arms.” Benjamin will present his team’s results today here at a meeting of the American Astronomical Society (AAS).

The Milky Way debuted as a spiral celebrity in 1951 when astronomical morphologist William Morgan of the Yerkes Observatory presented his results showing the galaxy’s three arms of hot stars, which he were then named Perseus, Orion and Sagittarius.

“Those were the first three arms of the spiral galaxy,” Benjamin told SPACE.com. “Actually, he got a standing ovation at the AAS meeting, which is something I’ve never seen.”

This is what 19th century astronomy William Herschel thought we looked like:

BTW – it’s -20C on Mars today. That’s a high temperature.

The galactic plane

Our galaxy, the Via Lactae, is round and flat. Like a frisbee, only with a bulge in the core. It’s called, oddly enough, the galactic core, and there’s very likely a black hole in that core. (But that’s another story.)

So yeh, we live on the rim of a flattened galaxy. When we view the Milky way from Earth, we’re looking through the edge of it, sort of like when we look at the edge of the rings of Saturn. Here’s a photograph from Japan’s Akari satellite taken in the infrared wavelength (ummm, infrared? huh?).

Remember, we’re taking this picture from within the Milky Way… we’re in the outer reaches!

Notice that the galactic disc is thin. Ish. The Milky Way is about 1 000 light years thick, but over 100 000 light years in diameter.

Now, to the point of this posting. Our solar system is on the edge of the galaxy. But guess what? we’re not standing still. No, we’re slowly rotating around the galactic core (like a satellite around a planet, or a planet around a star). We make one orbit in about 150 million years or so, and this is known as the galactic year.

But our “orbit” also wobbles a bit, up and down with respect to the galactic plane. You can imagine that when we cross through the middle of it, where it’s thicker, that there’s a lot of dust a debris and stuff.

Well, there is. And every 35 to 40 million years we cross the dusty area, and our chances of colliding with a comet increase by ten times. It’s what scientists think wiped out the dinosaurs 65 million years ago. Here’s the article from Cardiff University (it’s in Wales, in the UK):

Scientists at the Cardiff Centre for Astrobiology built a computer model of our solar system’s movement and found that it “bounces” up and down through the plane of the galaxy. As we pass through the densest part of the plane, gravitational forces from the surrounding giant gas and dust clouds dislodge comets from their paths. The comets plunge into the solar system, some of them colliding with the earth.

The Cardiff team found that we pass through the galactic plane every 35 to 40 million years, increasing the chances of a comet collision tenfold. Evidence from craters on Earth also suggests we suffer more collisions approximately 36 million years. Professor William Napier, of the Cardiff Centre for Astrobiology, said: “It’s a beautiful match between what we see on the ground and what is expected from the galactic record.”

Milky Way

The Astronomy Picture of the Day for Saturday was this:  the Milky Way

Clicking on the image will get it large.  It’s highly recommended.

It was taken early in the morning in northern Iran.  The Milky Way curves more than it normally would because of the distortion in the lens (it’s more flat than curved), but you can clearly see the bulge of the galactic core to the right.

If you look carefully, you can also see the Big Dipper on the far left, and the planets Jupiter and Saturdn to the right.  They’re the really bright ones.