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About the image & its photographer.

This Milky Way panorama is a mosaic of almost 1,200 separate images taken by French astropho-tographer Serge Brunier and composited by his friend Frederic Tapissier. Brunier shot most of the photos from Chile, but he completed the northern section in the French Alps and on a mountaintop in the Canary Islands, pictured at left.

The detail shown here is limited by the printing process; the original 800-gigapixel mosaic can easily fill a 12-foot-wide poster. You can zoom in to see any piece of it in more detail at sergebrunier. com/gallerie/pleinciel/index-eng.html.

The Spiral Galaxy That We Call Home

The drawing at right was made by Robert Hurt, artist and astronomer for the California Institute of Technology, and published by NASA. It represents the current best information about our Milky Way Galaxy's structure. It's based on recent studies with infrared and radio telescopes, which can penetrate the dust clouds that block most of our galaxy from view in visible light. The dust clouds lining the inner edge of our own Orion Spur form the Great Rift that stretches from Deneb on the left to Alpha Centauri on the right.

These dust clouds are actually composed almost entirely of hydrogen and helium, but those gases are transparent, so we don't see them. The "dust" that blocks the light actually consists of tiny solid particles, more like soot or smoke than what we normally call dust.

Totally unseen is the huge halo of dark matter that envelops the entire galaxy. We know it's there only because of its gravitational effects.

Like most spiral galaxies, the Milky Way is nearly flat, except for a spherical bulge near its center. It's very difficult to discern our galaxy's spiral structure because we see it edge-on, with all the spiral arms superposed. For instance, the cluster M7, roughly 1,000 light-years distant, lies in front of the stars of the galaxy's central bulge, more than 20,000 light-years distant.

Even when we can measure the distances to various features accurately, it is still very difficult to say how they link up into spiral arms. So the precise structure of our galaxy is still an open question.

Galactic Longitude

Galactic longitude is marked out at 30[degrees] intervals by tick marks along the galactic equator. Longitude 0 [degrees] marks the position of the galactic center. When we look in that direction, toward Sagittarius and Scorpius, we see at least three spiral arms (including our own) superposed in front of the central bulge. This area is discussed and charted in more detail in the article "Observing the Milky Way, Part I," on page 24.

When we look toward galactic longitude 180[degrees], between Auriga and Gemini, we're looking directly away from the galactic center. That's why the Milky Way appears so much dimmer and thinner in this direction. Because our Sun is located toward the inside of our own Orion Spur, our view in this direction includes many stars and a few star clusters within the Orion Spur, notably the Alpha Persei Cluster. But most of the clusters and nebulae in this area lie in the Perseus Arm, the next spiral arm out from our own. The brightest of these is the magnificent Double Cluster in Perseus.

Galactic Equator: the plane of our Milky Way Galaxy.

Gould's Belt: Most nearby star clusters and nebulae, both bright and dark, lie near this line.

Stellar associations: large groups of young stars, usually types O and B, that all have a common origin. Most of the associations shown above have been adapted from Glenn LeDrew's detailed Milky Way charts in The Backyard Astronomer's Guide (Firefly Books, 2010).

Dark nebulae: areas where the Milky Way is hidden from view by nearby dust clouds.

Star clouds: rich patches of spiral arms or the central bulge that are visible through windows in dust clouds.

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Title Annotation:Serge Bruinier's shots of the Milky Way
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jul 1, 2013
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