July - The Milky Way

Find the Milky Way in the Sky

You can observe the Milky Way with the unaided eye, binoculars, or a telescope. Follow these easy observing tips from the July IYA Discovery Guide: The hazy band of the Milky Way passes through the constellations of Cygnus, Aquila, Scorpius, and Crux, the Southern Cross. Use binoculars to look in the direction of one of those constellations and you will see a multitude of stars appearing to be crowded together. You will only be able to see the hazy band itself far from city lights.

  • The Milky Way

    The Milky Way and Sagittarius ConstellationThe Milky Way and Sagittarius Constellation. Credit: Terrence Dickinson.

    Because of increasing light pollution, fewer and fewer people these days are able to see the most spectacular structure in the sky Ė the Milky Way. This hazy ribbon of light is best seen during the summer and winter when it spans horizon to horizon and arches high overhead. Although it looks like a continuous band, the Milky Way is composed of the billions of stars in our galaxy, along with vast clouds of gas and dust.


Galileo Galilei was among the first to look at the Milky Way with the newly invented telescope. He could resolve its contents, like realizing that a smooth sand dune is made of fine particles. He wrote in 1610, "The Milky Way... with the aid of the spyglass, may be observed so well that all the disputes that for so many generations have vexed philosophers are destroyed by visible certainty, and we are liberated from wordy arguments. For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view."

This revelation was staggering because the total number of individual stars that can be seen on the celestial sphere numbers approximately 8,500. Galileo realized there were untold millions more. The universe suddenly got bigger and more complex. This discovery set the stage for future telescope advances right up through the powerful Hubble Space Telescope. Each major step in viewing ability would unveil a bigger and more complicated universe.

As telescopes got bigger, astronomers found that the Milky Way was dotted not only with star clusters, but also with glowing nebulae that revealed the stages of star birth and star death. The Milky Way became sort of the main street of the heavens, laced with opulent celestial sights.

Regions of darkness along the Milky Way, which are conspicuously devoid of stars, were thought by British astronomer William Herschel to be holes in the starry population that surrounded Earth. In the 18th century Herschel constructed a rough 3D map of the galaxy assuming the dark areas were empty tunnels through space. In fact, the inverse is true. What Herschel thought to be empty are really very dense dark clouds of dust that block our view of the more distant stars.

By the early 1900s, models of the Milky Way featured a thin disk of stars that bulged near the center (looking like a convex lens). It was assumed that the Sun was near the middle of this pancake-like structure. But about 90 years ago American astronomer Harlow Shapley studied globular star clusters and determined the true center of the galaxy. These spherical clumps of ancient stars were concentrated in the direction of the constellation Sagittarius. Shapley reasoned that if the globular clusters orbit the center of the galaxy, its location was tens of thousands of light-years from Earth. Today we know the Sun is located about halfway out in the disk of the galaxy.

In the 1920s, Edwin Hubble made observations that conclusively showed that the Milky Way was not the only galaxy. Hubble studied stars in the Andromeda nebula and calculated their distance as being more than 10 times the size of the Milky Way. Hence, it must be a separate "island universe" and should really be called the Andromeda galaxy.

By studying external galaxies, Hubble gained further insight into the nature of the Milky Way. Like many other galaxies, it is spiral-shaped with a central bulge of stars. The location of the center of the galaxy can be seen in the summer sky in the constellation Sagittarius, just off the tip of the constellationís "teapot spout."

Observations of the infrared light from stars, painstakingly collected over the past two decades with powerful telescopes on the ground, have gathered definitive evidence for the presence of a 3-million-solar-mass black hole at the core of the galaxy. The black hole has been "weighed" by measuring the motions of stars caught in the black holeís gravitational pull. In one stunning observation a star whipped past the black hole so fast that it could have traveled the distance between Earth and Pluto in less than a day!

The power of all three of NASAís Great Observatories, the Spitzer Space Telescope, the Chandra X-ray Observatory, and the Hubble Space Telescope, has been brought to bear on the tumultuous galactic core.

X-ray observations from Chandra show massive stars as very bright, point-like X-ray sources. Winds blowing off the massive stars' surfaces collide with winds from orbiting companion stars and produce X-rays. When the most massive stars in these clusters reach the end of their lives, they explode as supernovas. These explosions release vast amounts of energy and heat the gas between the stars to million degree temperatures. The stars near the galactic center also can emit X-rays as stellar corpses -- either in the form of neutron stars or black holes in binary systems. Chandra reveals the intense high energy activity at the core of the Milky Way.

Both Spitzer and Hubble can observe in infrared light and have revealed a firestorm of star birth. Together, these telescopes have produced an infrared panoramic mosaic of an area about 300 light-years around the core including three known clusters of massive stars: the Central cluster, the Arches cluster, and the Quintuplet cluster. While these clusters are easily seen as tight concentrations of bright, massive stars, other lone massive stars may have formed in isolation, or they may have originated in clusters but then were tossed out due to strong gravitational tidal forces. Because these massive stars are rare and live for only a short time, they indicate current and active star birth in the galactic center.

Spitzer looked beyond the galactic core and made landmark observations when it scanned the entire Milky Way plane in infrared light. Note that in visible light we can see only about six percent of the galaxy because the dusty and star-cluttered galactic plane obscures the rest. Using longer infrared wavelengths, Spitzer allows us to see to the far side of the galaxy better than ever before. Spitzerís data allowed astronomers to produce the most detailed map ever made of our galaxy. Our Milky Way galaxy is a barred spiral galaxy with two major spiral arms. These major arms, the Scutum-Centaurus and Perseus arms, have the greatest densities of both young, bright stars, and older red-giant stars. The two minor arms, Sagittarius and Norma, are filled with gas and pockets of young stars. The two major arms seem to connect up with the near and far ends of a central bar of stars in the galaxy.

It is strange to recognize that we can see the structure of many other galaxies much more clearly than we can see our own. The view from the inside of our Milky Way hides many details that an external view would easily reveal. Only after centuries of observation and deduction have we arrived at our modern view. We are the first generation to understand the architecture of our galaxy, as Galileo, Herschel, and Hubble could barely have imagined.

Galileo found the Milky Way.

Galileo found the Milky Way was made of many faint stars, each of them too faint to be seen by the naked eye.

Stars orbiting the supermassive black hole at our Galactic Center.

Stars orbiting the supermassive black hole at our Galactic Center. Credit: COSMUS: Randy Landsberg, Mark SubbaRao, Dinoj Surendran, Daniela Rosner.

Center of our Milky Way

The Chandra X-ray Observatory observes the center of our Milky Way.

Star clusters near the Galactic Center

The Hubble Space Telescope spies giant star clusters near the Galactic Center.

The Galactic center

The Galactic Center as seen in the infrared by the Spitzer Space Telescope.

Milky Way galaxy - artist's concept.

The modern view of the Milky Way galaxy (artistís concept).

Venus transit

Milky Way Galaxy

This artist's rendering shows a view of our own Milky Way Galaxy and its central bar as it might appear if viewed from above. An arrow indicates the location of our Sun.