Our Galaxy, the Milky Way
Overview
The Milky Way is a galaxy that contains at least 200 billion (200,000,000,000) stars. It is an average-size galaxy of the barred spiral class. This means that there is a central region with a bar-like bulge with arms that extend from it in a spiral shape.
Our galaxy is approximately 100,000 light-years in diameter. From where we are, it takes approximately 240 million years to complete one orbit around the center, clockwise, even though we are moving at about 750,000 km per hour (465,000 mph). At that speed a space ship from Earth would only need 30 minutes to get to the moon.
Structure
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| Photograph of the Milky Way galaxy as seen in the night sky from Earth. Image courtesy of the Digital Sky, LLC. |
Deriving Structure: Determining the structure of our galaxy is fairly difficult because we lie within its disk. From Earth, our galaxy looks like the image above, spanning the night sky. If you live in an area far from light pollution, then you can see the heart of our galaxy filling the center of the sky during the summer and early fall months (from the Northern Hemisphere; this would be winter and early spring in the Southern Hemisphere).
From this edge-on view of our galaxy, there are still a few things that we can determine:
- During part of the year, we can see the bright center of the galaxy, and directly opposite it we can still see clouds of stars that belong to the galaxy. Because of this, we know that we are inside it, and that we are neither in the center nor at the outer edge.
- Our galaxy has a disk shape with a bulge in the center. If it were an elliptical galaxy, then we would not see the relatively thin stream of stars that forms it, but rather the galaxy would cover a much larger region of our sky.
From these observations, we have a model of what our galaxy looks like: A flattened disk with a central bulge, and we are located somewhere between the center and the edge.
To get any more information, the next step is to attempt to determine the location and distance to stars that we see in our galaxy. William Herschel (the man who discovered the planet Uranus) was the first to attempt this. He counted the number of stars in different parts of the sky in order to try to determine the general shape of the Milky Way.
It wasn't until the 1920s, with the discovery by Edwin Hubble that our galaxy is just one of millions, that we began to look at other galaxies in an attempt to fit ours to their templates.
However, it wasn't until the past few decades that we were finally able to determine the shape of the galaxy to any real accuracy. We originally thought that it was a large grand-design-type spiral galaxy, much like the Andromeda Galaxy (M33) or the Whirlpool Galaxy (M51). It was believed that there were five main arms that emanated from the center.
This was done through observations of radio waves. The main arms were called Norma, Scutum-Centaurus, Sagittarius, and Perseus, after the constellations that they were in the direction of. A minor or partial arm, Orion, was what was believed to contain our solar system.
But, in subsequent years, more evidence has piled up that suggests that there is a bar in the center instead of a bulge, shown through infrared studies. This is a fairly new result, only determined in the last decade, which is why science fiction shows such as Star Trek will show maps of the galaxy without a central bulge.
Our understanding of the galaxy's structure continues to be refined, and in 2008 it was determined - again through large infrared surveys - that instead of four or five main arms, there are only two, the Scutum-Centaurus and Perseus arms (NASA Press Release, 06/03/2008). The Sagittarius and Norma arms are thinner, minor arms, and our solar system lies in a small offshoot of the Sagittarius arm, the Orion Spur.
The diagram below illustrates our current model of the structure of the Galaxy.
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| Current model of the structure of the Milky Way galaxy. Coordinate system is centered on our position. This is based on infrared mapping from the Spitzer Space Telescope. Image courtesy of NASA. You can click on the image for a larger version. |
Our Current Model: It is currently believed that our barred spiral galaxy has the following characteristics:
- A main disk that contains:
- Two main arms, the Scutum-Centaurus and Perseus.
- Two minor arms, Sagittarius and Norma. The Sagittarius arm has a small spur off of it.
- The disk contains approximately 200 billion stars, though possibly as many as 400 billion. The upper limit is based on different estimates for the number of small, faint, low-mass stars.
- The disk is about 70,000-100,000 light-years in diameter. The bar is about 27,000 light-years long.
- The bar is surrounded by a ring of molecular hydrogen (H2) that extends about 15,000 light-years, and this is where most of the star formation takes place, besides the arms.
- The galaxy contains about 580 billion solar masses of material.
- There is a halo of material that surrounds the galaxy's disk in roughly a spherical shape:
- This halo contains mostly old stars as well as almost all of the globular clusters.
- The halo may extend up to 200,000 light-years, twice the size of the main disk.
- Our galaxy contains a large amount of dark matter, believed to be fairly uniformly distributed, that weighs at least as much as all the luminous material, but possibly up to 20 times more (600-12,000 billion solar masses).
Our Place
Our solar system is located within the Orion Spur, about half-way between the center and the outer "edge" of the galaxy's disk. We are located within that disk.
The sun orbits the galaxy about once every 250 million years. It does not stay in the same plane, but rather it oscillates up and down through the disk approximately once every 60,000 years.
Even though the sun is currently located in the Orion Spur, this may not always have been the case. Spiral galaxies do not have rigid arms that always contain the same stars, but rather they are density waves of stars that go through the galaxy. This is much like traffic on a highway where sometimes you are slowed down and right next to other cars, but other times there are large distances between you and other cars.
Satellites
Just as planets have moons as satellites and stars have planets as satellites, galaxies can have other small galaxies as satellites. In the case of the Milky Way, there are at least 13 known satellite galaxies (or parts of galaxies). Except for the Large and Small Magellanic Clouds, named after their European discoverer Ferdinand Magellan, the satellite galaxies are named for the constellation in which they are found.
- Sagittarius Dwarf
- Large Magellanic Cloud
- Small Magellanic Cloud
- Canis Major Dwarf
- Ursa Minor Dwarf
- Draco Dwarf
- Carina Dwarf
- Sextans Dwarf
- Sculptor Dwarf
- Fornax Dwarf
- Leo I
- Leo II
- Ursa Major Dwarf
Milky Way's Location
Our Milky Way galaxy is part of what is known as the "Local Group" of galaxies - a small collection of about 35 galaxies that are bound together by their mutual gravity; most of the galaxies are satellites of either the Milky Way or Andromeda. The Milky Way, Andromeda, and Triangulum galaxies are the main members of the group. The gravitational center of the Local Group is somewhere between the Milky Way and Andromeda galaxies. The Local Group is about 6 million light-years in diameter.
The Local Group is a member of a much larger group of galaxies known as the Virgo Supercluster, containing at least 1300 and possibly up to 2000 total galaxies. The Milky Way lies on the outer edge of the supercluster. The Virgo Supercluster is about 100 million light-years in diameter, the heart of it lying about 59±4 Mly (million light-years) away. It is believed to have about 1.2 trillion solar masses of material.
Our galaxy is thought to be moving in the direction of the constellation Hydra, though on the scale of many galaxies, a frame of reference becomes somewhat arbitrary.