Today's subject comes from a question asked during the "Reach for the Stars" program I presented last Saturday at Cerrillos Hills State Park. How do we know that the Milky Way is a spiral galaxy?

To uncover the true shape of our galaxy, astronomers combined observations from visible light and other parts of the electromagnetic spectrum. With telescopes, we see distant galaxies and can recognize their spiral structure. So, we know that spirals are out there, but just how did we figure out the shape of the Milky Way?

To find the answer, we first have to look at observations made by William Herschel in the 1780s. Herschel conducted a survey by counting stars. By plotting his findings, he created a map that showed a discrete band of stars embedded in a more general distribution. Under moderately dark and clear conditions, you can see this same swath of light spanning the sky. This band of stars reveals a primary characteristic of our galaxy — a flattened disk shape.

A mass that starts out bulky will flatten as it begins to rotate. So, we had reason to believe that the Milky Way rotates. But to understand more about the gross structure our galaxy, we had to determine its center of rotation. Many believed the center was located in the direction of the constellation Sagittarius because it is the brightest part of the Milky Way.

In 1918, Harlow Shapely mapped the locations and estimated distances for 69 globular star clusters. Assuming a spherical distribution, he calculated their average center of motion. Shapely's results confirmed earlier suspicions that the galactic center is in the direction of Sagittarius.

Astronomers had predicted the Milky Way was a spiral based upon observations of hot, young blue stars and their mother nebulae in distant galaxies. These blue stars and star-forming nebulae populate the spiral arms. Ultraviolet radiation from the hot, blue stars cause the nebulae to glow with the distinct wavelength of ionized hydrogen gas. If similar structure could be found in our galactic neighborhood, we would be one step closer to confirming the Milky Way spiral.

Direct evidence of the Milky Way's spiral arms was obtained in 1951. With help from graduate students Stewart Sharpless and Donald Osterbrock, astronomer William W. Morgan photographically mapped regions of ionized hydrogen gas in our galaxy. This process allowed Morgan to identify two of our galaxy's spiral arms.

The Morgan-Sharpless-Osterbrock model of our galaxy became one of the most important discoveries of the last century, but not all of the galaxy's structure exists as light emitting nebulae and stars. Cold, dark clouds of gas and dust block light making it impossible to see regions beyond. To go further, astronomers had to leave behind the limitations of visible light and use other methods to explore galactic structure. Observations made with radio and infrared telescopes allowed us to create a more complete picture.

Radio telescopes can be tuned to detect specific bands of radiation. In the 1950's Hendrik van de Hulst and Jan Hendrik Oort conducted a radio survey of hydrogen gas clouds. The rotation of our galaxy shifts the wavelength of a gas cloud's radio signature revealing which clouds are receding or coming towards us. This effect is because of the Doppler shift. An common example of this phenomenon is the increased pitch in sound from an approaching ambulance siren and the decrease in pitch as the vehicle races on by.

Continued radio surveys have increased the accuracy of mapping Milky Way spiral arms, but every improvement in sensitivity brings new questions. Evidence suggested that gas in the core doesn't just rotate, it also moves outward. This expanding gas led some to suggest that our galaxy has a central bar. To learn more, astronomers turned to infrared detectors. Infrared emissions pass unimpeded through thick clouds of dark nebulae and haze giving us the ability to see deep inside the galactic center.

Working with a toolbox of observations ranging from visible light to radio and infrared emissions, astronomers have mapped our galaxy from the inside out.

Our understanding of the Milky Way is that it is a barred spiral approximately 120,000 light years in diameter. Our solar system is 26,000 light years from the center speeding along at 140 miles per second. Something I'll try to keep in mind the next time I'm stuck at a red light feeling like I'm going nowhere.

Peter Lipscomb shares the wonder of the night sky as lead guide for Astronomy Adventures and works to promote sensible and energy efficient lighting practices. He keeps a blog at www.eyesfullofstars.com Contact him at pslipscomb@gmail.com.