The Galactic Disk
The Galactic Disk
Goals: Examine the disk component of the Milky Way, including the Galactic center, the bar, and the spiral arms.
Before starting, turn on: galaxy
You will be using: center command, bar, h2
If you're just starting the Milky Way Atlas, fly away from the Sun so that you see the entire Galaxy. Up to this point, we have discussed the data sets that make up the Galaxy. In this tutorial, we will discuss the Galaxy as an object itself.
Even today, astronomers are challenged to understand the structure of our Galaxy. Because we live within the Galaxy, we cannot see the entire star system. This often makes the structure of galaxies millions of light-years away easier to understand than the structure of our own Milky Way.
From your vantage point in the Atlas, you should see the two-dimensional image we use to represent our Galaxy. Of course, we have never seen the Galaxy from this perspective. This image is the galaxy named NGC 1232, a galaxy thought to resemble the Milky Way.
The Sun Is 26,000 Light-Years from the Galactic Center
The Galactic disk is just less than 100,000 light-years across. The Sun orbits 26,000 light-years (8,000 parsecs) from the Galactic center and is about 50 light-years (15 parsecs) above the disk midplane. The disk is dominated by the bright center and the outlying spiral arms.
To examine this structure, change the Point of Interest to the center of the Galaxy. Right now it is set to the Sun's position at (x, y, z) = (0, 0, 0) parsecs. The center of the Galaxy is at (x, y, z) = (8000, 0, - 15) parsecs, so set the Point of Interest to this location with the command
center 8000 0 -15To confirm the shift, increase the center size using the Censize Slider. You will see Cartesian axes appear at the Galactic center, as well as a half-axes element at the Sun. Upon confirmation, you can reduce the center size back to zero to hide it from view.
Now you can orbit the center of the Galaxy. Bring the Galactic center to the middle of your Graphics Window using the Fly Flight Mode if necessary, then switch back to Orbit Flight Mode.
The Galactic Center
The center of our Galaxy is tumultuous. Within the bright region on the image are billions of stars with densities approaching 50,000 stars per cubic parsec (compare that with the density around the Sun of 0.05 stars per cubic parsec). These high densities ensure stellar interactions and even collisions. Astronomers cannot see the Galactic center in the visible spectrum because of the intervening gas and dust. However, these regions can be probed in the infrared and radio, which are less susceptible to light scattering from dust.
At the core of the Galaxy, astronomers observe a swirling ring of gas only a few parsecs across with a strong magnetic field and extended filaments. The energy required to sustain this activity must come from a massive object. But because the gas features are so small, the object driving this activity must be small too. Astronomers currently believe that a super-massive black hole exists at the Galactic center. A black hole is a region of space where gravity is so high that nothing escapes, not even light.
Our Barred Galaxy
Surrounding the core is the Galactic bar. Astronomers observe noncircular orbits among the stars near the Galactic center and therefore believe our Galaxy is a barred spiral. Turn on the Galactic bar to see the size and shape of this feature.
The bar is an ellipsoid with a size of (ax, ay, az) = (5500, 2050, 1350) light-years and its long axis tipped 14o with respect to the Sun. Inside this ellipsoid are billions of stars and the most active star formation in the Galaxy. This is the Galactic powerhouse, and much of the Galaxy's activity is driven from this area.
Beyond the bar, there is the Galactic disk. The image shows spiral arms with bright HII regions (star-forming regions like the Orion Nebula) sprinkled throughout them. Depending on what kind of object you consider, the disk varies in thickness. If you consider young stars, gas, and dust in the vicinity of the Sun, the disk is only 300 parsecs (978 light-years) thick. However, if you look at older disk stars in the range of 7 billion to 10 billion years, the disk is around 2 to 3 kpc (6,500-9,800 light-years) thick. These stars formed in the disk but over time found their way out of the disk from interactions with molecular clouds and other stars.
These are the main elements in the disk component of our Galaxy. Let's now explore the spherical components of the Galaxy in the next tutorial.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott