Our Celestial Home: The Milky Way

If you go outside on a dark summer night, you can look overhead and see a faint, cloud-like white river running from the southwest corner of the sky to the northeast corner. To the ancients, this "cloud" looked like a long splash of milk across the black sky background. Not knowing what is was, they named the cloud the "Milky Way." As time progressed and astronomical techniques improved, astronomers discovered that the cloud was actually innumerable stars that were so crowded together, they gave the perspective of being a giant cloud. In time, astronomers began to unravel the secrets of the Milky Way and generate a map of this giant collection of stars. The panorama photograph of the Milky Way is courtesy of John Gleason and represents what you look at when you pan the swath of stars running through the summer sky.

For those living in the northern hemisphere, every star that you see with your naked eye is a part of this galactic home we call the Milky Way. If your night sky is really dark, you might be able to spot the Andromeda Galaxy (M31) (in photograph to the left) is the late summer and autumn sky, and this object is not part of our galaxy, but a distant galaxy like ours, situated 2.65 million light years from here. It is the most distant object you can see with your naked eye. Other than this object, the rest of the stuff you see in the night sky are all stars and gas clouds within our own galactic confines.

 

Those who live in the southern hemisphere can glimpse two large patches of stars that are actually companion galaxies of ours, and are irregular in shape. These two places are called the Large Magellanic Cloud (179,000 light years away) and the Small Magellanic Cloud (210,000 light years away). The images below, from the Malin collection show pictures of these neighbors left and right respectively. Recently, astronomers have discovered a galaxy opposite of our line of sight, and quite invisible for our naked eyes that the Milky Way may be consuming. The Sagittarius Dwarf Galaxy is about 80,000 light years from us, and is being incorporated into our Galaxy's structure.

The Milky Way is a huge group of an estimated 150 to 200 billion stars, all moving in a circular motion defined by a beautiful spiral shape. In the pair of images below, you can see a depiction of the structure of our Galaxy. This depiction is based on a detailed study of the stars in our galaxy. Astronomers have used various distance measuring techniques to determine the location of many, many stars relative to our own Sun and relative to each other. The result of this distance measurement system is the three-dimensional map that you see below.

The sheer dimensions of the Milky Way are staggering. Looking at the galaxy from an edge-on standpoint, light would require a time of 75,000 - 90,000 years to go across the diameter of the Galaxy. The central bulge of the Galaxy is a dense cluster of older stars that are predominantly of spectral class M. Splaying out from the central bulge is a series of "arms" of younger stars that are of all spectral classes. Since the Galaxy is spinning, the entire structure is flattened like a disk. However, the gravitational attraction of the Galaxy itself holds a spherical influence, and although the majority of the material is in the disk, there is a "halo" of objects called globular clusters that independently orbit the central bulge. These small clusters of stars contain anywhere from a few thousand to a million stars, and act like planets orbiting the Sun. The difference is that the globular clusters are orbiting independent of the flattened disk, with some moving in a "north-south" orientation and others in an "east-west" orientation. Some stay outside of the spiral arms, while others move right through them. Some orbit in a clockwise fashion, while others move counterclockwise. And, the stars of these globular clusters are mostly M class stars with tiny white dwarfs giving evidence to the advanced age of the clusters.

Our Sun occupies a spot in the so-called "Orion Arm," that is bracketed by the neighboring Perseus and Centaurus arms. As stated earlier, the entire Galaxy is rotating, so all of the stars are moving in an elliptical path around the center of the Galaxy. Along with the other arms of this spiral galaxy, the Sun orbits the central bulge in a period of 225 million years. Over the estimated 4.5 billion years of our planet, Earth has taken about 20 trips around this merry-go-round. As you can see from the second image below, the Sun is at an estimated distance of 25,000-30,000 light years from the galactic center.

It should be emphasized that there are almost as many stars between the spiral arms as in the spiral arms. The reason why the arms of spiral galaxies are so prominent is that the brightest stars are found in the spiral arms. Spiral arms are the major regions of star formation in spiral galaxies and this is where most of the major nebulae are found.


The few paragraphs in the smaller font below are from the SEDS (University of Arizona) website that teaches a very nice unit on the Milky Way. I have brought these words and the following image of the central bar as well as the image of galaxies similar to our own from that site. You are encouraged to check out this website for your viewing pleasure and to increase your learning.

The Shape of the Milky Way - The Evidence
The first good map of the spiral form of the Milky Way was produced by Oort, Kerr and Westerhout in 1958 (an early version of their map exists here.) They mapped the neutral hydrogen (HI) in the plane of the Galaxy. Later, in 1976, Yvonne and Yvon Georgelin produced a map of many of the major HII regions (bright nebulae of ionized hydrogen) showing how they are distributed along the spiral arms. In 1993, Taylor and Cordes produced an updated version. Finally in 1995 J Vallee examined the evidence and concluded that the Galaxy was a four-arm logarithmic spiral depicted here.

It has long been suspected that the middle of the Milky Way may have a bar, but the first conclusive evidence was produced by Blitz and Spergel in 1991. Information on the exact shape of the central bar was published recently in July 2001 by López-Corredoira, Hammersley, Garzón, Cabrera-Lavers, Castro-Rodríguez, Schultheis and Mahoney, (available as a postscript file here.) They conclude that the central bar of the Milky Way looks much like the bar in the spiral galaxy M95.

If we put all this data together then we get a map like this one below. There is very little data available about the far side of the Galaxy but spiral galaxies are usually fairly symmetrical. Features on one side of a galaxy are often repeated on the other side.

Best estimates of the statistics of the Milky Way

Diameter of Galaxy

75,000 - 90,000 light years

Number of stars

200 billion

Mass of Galaxy

600 billion solar masses

Length of central bar

25,000 light years

Distance from Sun to the center

26,000 light years

Thickness of Galaxy at Sun location

2000 light years

Rotation period of the Sun around the Galaxy

225 million years

Below - four galaxies which look like the Milky Way. NGC 3953 (top left) is 55 million light years away and 95,000 light years in diameter. NGC 5970 (top right) is 105 million light years away and 85,000 light years in diameter. NGC 7329 (bottom right) is even further at a distance of 140 million light years but it is larger with a diameter of 140,000 light years. NGC 7723 (bottom right) is 80 million light years away with a diameter of 90,000 light years.

end of SEDS material

To better understand our place in space, astronomers utilize telescopes that are attenuated to wavelengths of the electromagnetic spectrum other than the visible band. A stunning comparison of these photographs taken from NASA is shown below, and by click on the image itself, you can see it in a larger format.

 

Mystery of the Milky Way Structure

What this image set above demonstrates is that more than visible light is pouring out of our Galaxy. There are large x-ray and gamma ray sources, and bright gaseous regions glowing in the Infrared. Scientists are still trying to determine just what holds our Galaxy together. The visible light spectrum indicates the presence of 150 to 200 billion stars, but the structure holds a mystery. The spiral arms do not have enough visible mass to maintain their structure. Although the entire galaxy rotates every 225 million years, it is actually rotating so fast that the stars should be flung off into intergalactic space. The fact that these stars remain in their arms alludes to the presence of additional mass of a kind not yet identified. This so-called "Dark Matter" is theorized to exist, but has yet to be found or described.

Star Types Within Our Galaxy

Our galaxy has two different kinds of stars. The central bulge and halo globular clusters are dominated by M class stars burning Hydrogen into Helium, and are as old as the Galaxy itself. These stars are reddish in color and are depicted in the drawings according to that color. Meanwhile, the spiral arms are composed of these same M class stars, but also hold a lot of bright, hot, blue and white stars. Spiral galaxies are unusual in that they are regions of intense starburst activity. The term "starburst" means new stars are forming from old stars. Stars that are formed from recycled dead star material are called Population I stars, while those that are original to the galaxy are Population II stars. The Pop II stars are all rich in Hydrogen, poor in metals, and long-lived. Pop I stars are more rich in metals and short-lived.

Starburst activity occurs due to an interesting invisible structure in the spiral arms called a Density Wave, depicted in the drawings above. Orbiting the central bulge of the Milky Way, and other spiral galaxies like ours, are these dense arms of gas and dust that orbit at a slower rate than the visible star component arms. As a spiral arm of stars and gas catches and moves through a density wave, the gas clouds are affected in such a manner that new stars collapse from them. Therefore, looking at a typical spiral arm of the galaxy, the trailing part of the arm that has just passed through a density wave will hold the youngest stars that were newly formed, while the leading edge that has yet to move through the next density wave will hold older stars that were formed at a more distant time. In this manner, the spiral arms continually recycle stellar material. It is this very recycling of material that possibly gave rise to our own Sun ... a second generation Population I star rich in metals, and with a planet rich in metals and now life.

In conclusion, the picture below shows the Andromeda Galaxy with the subtle differences between colors of stars. The core of the galaxy is primary composed of Population II stars which are older, more evolved, and less-frequently recycled. They appear reddish in color. The arms of the galaxy have recycled stars that are younger, often hotter, and therefore blueish in color. All of the other stars in this image are actually stars in our own galaxy that are in front of this much more distant object. The two larger bright spots are companion galaxies that are gravitational associated with Andromeda.

Formation of Our Galaxy

The key to understand how our Galaxy formed is the location, ages and chemical composition of the various stellar populations. The oldest stars are in the halo and bulge. The most metal rich stars are in the disk and bulge. From this we deduce that the halo formed first, followed by the bulge then disk. All the gas is located in the disk (which is rotating) because gas clouds can undergo inelastic collisions.

Inelastic collisions occur when two objects collide and share momentum as a single body. Stars are too small to collide within the Galaxy (their cross section is very, very low). But gas clouds are large and can 'stick' together.

Given the above facts, it is theorized that the formation and evolution of our Galaxy must have taken place through a series of continuous stages. First, the Galaxy began as a large single gas cloud a few hundred of thousand light-years across. Passage near other proto-galaxies caused this large cloud to spin. This rotation was far from organized as currents and smaller clouds formed within the proto-Galaxy.

Spheres of gas containing about a million solar masses of material collapsed first, and these became the halo globular clusters. These first clouds were very weak in chemical abundance, but the first supernovae in the halo stars began to enrich the interstellar medium.

Cloud-cloud collisions steadily eliminated those clouds with the greatest inclination and those moving in the opposite directions until the distribution of gas clouds became flatter and flatter. Most of the gas was directed to the bulge regions where the high densities produced a highly dense core region.

Lastly, the remaining gas settled into the disk where the rotation slowed the formation of new stars until spiral density wave formed to dominate the appearance of the Galaxy today. This is the dominant theory for the formation of our galaxy. To learn more details about the formation of other galaxies, please go to Galaxy Formation in the Universe for a more detailed explanation.

Later, you should move forward to Galaxy Structure, and then to various Galaxy Images and an attempt to unravel the mysteries of the Cosmos itself, or return to the Galaxy Introduction page

or go to the Syllabus .


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