The Earth orbits the Sun in the Solar System, and the Solar System is embedded within a vast galaxy of stars. Just one in hundreds of billions of galaxies in the Universe. Ours is called the Milky Way because the disk of the galaxy runs across the sky as a band of glowing light, like spilled milk.

Astronomers had suspected the Milky Way was made up of stars, but it wasn’t proven until 1610, when Galileo Galilei turned his rudimentary telescope towards the heavens and resolved individual stars in the band across the sky. With the help of telescopes, astronomers realized that there were many many more stars in the sky, and all of the stars that we can see are a part of the Milky Way.

If you could travel outside the galaxy and look down on it from above, you’d see that the Milky Way is a barred spiral galaxy measuring about 120,000 light-years across and about 1,000 light-years thick. For the longest time, the Milky Way was thought to have 4 spiral arms, but newer surveys have determined that it actually seems to just have 2 spiral arms: they are called Scutum–Centaurus and Carina–Sagittarius.

A research team has developed the first complete map of the Milky Way galaxy's spiral arms. The map shows the inner part of the Milky Way has two prominent, symmetric spiral arms that extend into the outer galaxy where they branch into four spiral arms.

"For the first time these arms are mapped over the entire Milky Way," said Martin Pohl, an Iowa State associate professor of physics and astronomy. "The branching of two of the arms may explain why previous studies - using mainly the inner or mainly the outer galaxy - have found conflicting numbers of spiral arms."

Pohl, Peter Englmaier of the University of Zurich in Switzerland, and Nicolai Bissantz of Ruhr-University in Bochum, Germany, developed the new map.

As the Sun and other stars revolve around the center of the Milky Way, researchers cannot see the spiral arms directly, but have to rely on indirect evidence to find them. In the visible light, the Milky Way appears as an irregular, densely populated strip of stars. Dark clouds of dust obscure the galaxy's central region, so it cannot be observed in visible light.

The NASA's Cosmic Background Explorer satellite mapped the Milky Way in infrared light using an instrument called the Diffuse IR Background Experiment. The infrared light makes the dust clouds almost fully transparent.

Englmaier and Bissantz used the infrared data from the satellite to develop a kinematic model of gas flow in the inner galaxy. Pohl used the model to reconstruct the distribution of molecular gas in the galaxy. And that led to the researchers' map of the galaxy's spiral arms.

Studies of the Milky Way Galaxy are an important reference point for the interpretation of other galaxies.

Astrophysicists know that the stars in the Milky Way are distributed as a disk with a central bulge dominated by a long bar-shaped arrangement of stars. Outside this central area, stars are located along spiral arms.

In addition to the two main spiral arms in the inner galaxy, two weaker arms exist. These arms end about 10,000 light-years from the galaxy's center. The Sun is located about 25,000 light-years from the galactic center. One of these arms has been known for a long time but has always been a mystery because of its large deviation from circular motion. The new model explains the deviation as a result of alternations to its orbit caused by the bar's gravitational pull. The other symmetric arm on the farside of the galaxy was recently found in gas data.

The discovery of this second arm was a great relief for Englmaier: "Finally it is clear that our model assumption of symmetry was correct and the inner galaxy is indeed quite symmetric in structure."

It’s very difficult to figure out what the Milky Way looks like exactly, because we’re embedded inside it. If you’d never been out of your house, you wouldn’t know what it looked like from outside. But you’d get a sense by looking at other houses in your neighborhood.

Our Sun is located in the Orion Arm, a region of space in between the two major arms of the Milky Way. The spiral arms are formed from density waves that orbit around the Milky Way. As these density waves move through an area, they compress the gas and dust, leading to a period of active star formation for the region.

Astronomers estimate that there are between 100 and 400 billion stars in the Milky Way, and think that each star has at least one planet. So there are likely hundreds of billions of planets in the Milky Way, and at least 17 billion of those are the size and mass of the Earth.

Our Sun is located about 27,000 light years from the galactic core. At the heart of the Milky Way is a supermassive black hole, just like all of the other galaxies. This monster is more than 4 million times the mass of the Sun.

Our Sun takes about 240 million years to orbit the Milky Way once. Just imagine, the last time the Sun was at this region of the galaxy, dinosaurs roamed the Earth, and the Sun has only made 18-20 trips around in its entire life.

The Milky Way, like all galaxies, is surrounded by a vast halo of dark matter. Nobody knows what it is, but its mass helps keep the galaxy from tearing itself apart as it rotates.

It’s believed that our galaxy formed through the collisions of smaller galaxies, early in the Universe. These mergers are still going on, and the Milky Way is expected to collide with Andromeda in 3-4 billion years. The two galaxies will combine to form a giant elliptical galaxy, and their supermassive black holes might even merge.

The Milky Way and Andromeda are part of a larger collection of galaxies known as the Local Group. And these are contained within an even larger region called the Virgo Supercluster.


A spiral galaxy like the Milky Way has 3 basic components to its visible matter: (1) the disk (containing the spiral arms), (2) the halo, and (3) the nucleus or central bulge. In addition to these visible components, the galaxy also contains at least three other components that are &invisible&: the galactic magnetic field, charged particles trapped in the galactic magnetic field, and a halo of "dark matter" that is of unknown composition but that makes itself felt by its gravitational influence on the visible matter.

The Disk of the Galaxy

Most of the gas and dust of the Milky Way is contained in the disk. This material between the stars is often termed the interstellar medium. The gas is primarily hydrogen and helium, and the dust makes many regions of the disk opaque. For example, we cannot see the center of the galaxy in visible light because of intervening dust clouds in the disk.

The disk is not a clearly defined thing, because it depends on what objects that we use to define it, and because portions of it are blocked from our view at visible wavelengths. In addition, as one goes vertically in the disk (perpendicular to the plane), there is no sharp boundary for the disk. Rather, the density of stars gradually gets smaller. Therefore, astronomers sometimes refer to more than one disk for the galaxy. For example, the &disk& defined by the hottest young stars and their associated dust clouds is about 5-10 times thinner than the &disk& defined by older stars like the Sun, which is approximately 1000 parsecs thick.

The disk is quite prominent in our own galaxy and in other spiral galaxies because of its spiral arms, which contain many hot young stars and therefore is luminous. These younger stars are often contained in associations, which are groups of typically 10-100 young stars that are moving together through space because they have been recently formed from the same nebula, and open clusters, which contain 100-1000 stars and are more strongly bound together gravitationally than associations.

The Halo of the Galaxy

The halo of the galaxy is rather spherical in shape and contains little gas, dust, or star formation. It is difficult to measure precisely, but the halo appears to extend beyond the disk. The clusters found in the halo are globular clusters(approximately 100 of them), so the halo is population II, and contains very old stars. Dating of globular clusters by their turnoff points indicates that they may be as old as 15 billion years and are the oldest components of the galaxy. This implies that the galaxy itself is at least 15 billion years old.

The stars in the disk of the galaxy are on nearly circular orbits lying in the plane of the galaxy, but the stars of the halo are on more elliptical orbits that are randomly oriented. Thus, the halo stars pass through the disk and the nucleus of the galaxy, but spend the majority of the time far above or far below the plane of the galaxy.

The Nucleus of the Galaxy

The nuclear bulge or core contains the highest density of stars in the galaxy. Although some hot young stars may be found in the nucleus, the primary population of stars there is similar to the old stars found in the halo. Although at visible wavelengths the core of the galaxy is obscured by dust, gas, and stars, we can observe it at other wavelengths and there is some evidence that violent processes may be taking place there. As we shall discuss later, many galaxies may contain very>massive black holes at their cores, and our own galaxy may be no exception.

The Galactic Magnetic Field and Cosmic Rays

The disk of the galaxy is permeated by a magnetic field. This field is weak, being only about 1/50,000 of the strength of the Earth's magnetic field at the surface, but it influences the motion of charged particles in the galaxy. One important consequence of the magnetic field is that it can bend the path of and even trap the high-energy charged particles that we call cosmic rays. Thus the galaxy is filled with cosmic rays and because of the effect of the magnetic field we cannot tell with certainty where they come from, though the strongest arguments favor supernova explosions for their source.

The Dark Matter Halo

As we shall discuss in more detail later, there is abundant evidence that the vast majority of matter in the Universe does not show up in our telescopes but we can infer its presence by its gravitational influence. We refer to this as dark matter, and at present we do not know what it is, though there is fairly strong evidence that it is not the ordinary matter of stars, gas, dust, and planets. In our own galaxy, the observed rotation of the stars and gas clouds indicates that the visible matter is surrounded by a halo of this dark matter containing the major portion of the total galaxy mass and extending very far beyond the visible matter. Some indirect means suggest that the dark matter halo may extend as far as 100,000 parsecs from the center of the galaxy.


New Information

January 5, 2009

Fasten your seat belts - we're faster, heavier, and more likely to collide than we thought. Astronomers making high-precision measurements of the Milky Way say our galaxy is rotating about 100,000 miles per hour (161,000 kilometers) faster than previously understood.

That increase in speed, said Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CfA), increases the Milky Way's mass by 50 percent, bringing it even with the Andromeda Galaxy. "No longer will we think of the Milky Way as the little sister of the Andromeda Galaxy in our Local Group family."

The larger mass means a greater gravitational pull that increases the likelihood of collisions with the Andromeda Galaxy or smaller nearby galaxies.

Our solar system is about 28,000 light-years from the Milky Way's center. At that distance, the new observations indicate we're moving at about 600,000 miles per hour (966,000 km/h) in our galactic orbit, up from the previous estimate of 500,000 miles per hour (805,000 km/h).

The scientists are using the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope to remake the map of the Milky Way. Taking advantage of the VLBA's unparalleled ability to make extremely detailed images, the team is conducting a long-term program to measure distances and motions in our galaxy.

The scientists observed regions of prolific star formation across the galaxy. In areas within these regions, gas molecules are strengthening naturally occurring radio emission in the same way that lasers strengthen light beams. These areas, called cosmic masers, serve as bright landmarks for the sharp radio vision of the VLBA. By observing these regions repeatedly at times when Earth is at opposite sides of its orbit around the Sun, the astronomers can measure the slight apparent shift of the object's position against the background of more distant objects.

"The new VLBA observations of the Milky Way are producing highly-accurate direct measurements of distances and motions," said Karl Menten of the Max Planck Institute for Radio Astronomy in Germany, a member of the team. "These measurements use the traditional surveyor's method of triangulation and do not depend on any assumptions based on other properties, such as brightness, unlike earlier studies."

The astronomers found that their direct distance measurements differed from earlier, indirect measurements, sometimes by as much as a factor of two. The star-forming regions harboring the cosmic masers "define the spiral arms of the galaxy," Reid said. Measuring the distances to these regions provides a yardstick for mapping the galaxy's spiral structure.

"These direct measurements are revising our understanding of the structure and motions of our galaxy," Menten said. "Because we're inside it, it's difficult for us to determine the Milky Way's structure. For other galaxies, we can simply look at them and see their structure, but we can't do this to get an overall image of the Milky Way. We have to deduce its structure by measuring and mapping," he said.

The VLBA can fix positions in the sky so accurately that the actual motion of the objects can be detected as they orbit the Milky Way's center. Adding in measurements of motion along the line of sight, determined from shifts in the frequency of the masers' radio emission, the astronomers are able to determine the full 3-dimensional motions of the star-forming regions. Using this information, Reid said that "most star-forming regions do not follow a circular path as they orbit the galaxy; instead we find them moving more slowly than other regions and on elliptical, not circular, orbits."

The researchers attribute this to what they call spiral density-wave shocks that can take gas in a circular orbit, compress it to form stars, and cause it to go into a new, elliptical orbit. This, they said, helps to reinforce the spiral structure.

Reid and his colleagues found other surprises, too. Measuring the distances to multiple regions in a single spiral arm allowed them to calculate the angle of the arm. "These measurements," Reid said, "indicate that our galaxy probably has four, not two, spiral arms of gas and dust that are forming stars." Recent surveys by NASA's Spitzer Space Telescope suggest that older stars reside mostly in two spiral arms, raising a question of why the older stars don't appear in all the arms. Answering that question, the astronomers say, will require more measurements and a deeper understanding of how the galaxy works.

The VLBA, a system of 10 radio-telescope antennae stretching from Hawaii to New England and the Caribbean, provides the best ability to see the finest detail of any astronomical tool in the world. The VLBA can routinely produce images hundreds of times more detailed than those produced by the Hubble Space Telescope. The VLBA's tremendous resolving power, equal to being able to read a newspaper in Los Angeles from the distance of New York, is what permits the astronomers to make precise distance determinations.