15 The Milky Way Galaxy

1. Our Parent Galaxy

Looking in the opposite direction (blue arrow), we still see the Milky Way band, but now it is fainter because our Sun lies far from the Galactic center, so we see more stars when looking toward the center than when looking in the opposite direction.

Looking perpendicular to the disk (red arrows), we see far fewer stars.

The faint stars of the halo, completely surrounding the disk and bulge, cannot be seen here. The white stars sprinkled all across this image are foreground stars in our own Galaxy about a thousand times closer. 

2. Measuring the Milky Way

Variable Stars: A New Yardstick

RR Lyrae Period: 0.5 - 1 days

Cepheid Period: 1 - 100 days

Distance Calculation

Understanding Period vs Luminosity Graph

Variable Stars on Distance Ladder

Application of the period–luminosity relationship of Cepheid variable stars allows us to determine distances out to about 25 Mpc with reasonable accuracy.

3. Galactic Structure

Infrared view of Milky Way

It shows much more detail of the galactic center than the visible-light view does, as infrared is not absorbed as much by gas and dust.

Stellar Populations in Our Galaxy

Artist’s conception of a (nearly) edge-on view of the Milky Way Galaxy (The brightness and size of our Sun are greatly exaggerated for clarity). It schematically shows the distributions of 

The galactic halo:

The galactic disk:

The galactic bulge:

Population Classes

Orbital Motion

Around the Sun:

4. Formation of Milky Way

(a) The Milky Way Galaxy possibly formed through the merger of several smaller systems.

(b) In early stages, our Galaxy was irregularly shaped, with gas distributed throughout its volume.

When stars formed during this stage, there was no preferred direction in which they moved and no preferred location in which they were found. Their orbits carried them throughout an extended three-dimensional volume surrounding the newborn Galaxy.

(c) In time, the gas and dust fell to the Galactic plane and formed a spinning disk. The stars that had already formed were left behind in the halo.

(d) New stars forming in the disk inherit its overall rotation and so orbit the Galactic center on ordered, circular orbits.

Cloud's angular momentum effecting galactic evolution 

Cloud Density effecting galactic evolution

5. Galactic Spiral Arms

Radio Maps

Galactic Model

Spiral Structure

Milky Way Spiral Structure. An artist’s conception of our Galaxy seen face-on. 

Persistence of the Spiral Arms

Spiral Density Waves

Illustration of Density Wave

6. Mass of the Milky Way

This mass is concentrated at the center of the Galaxy. However Galactic Matter is distributed along the Galaxy.

Dark Matter

7. The Galactic Center

Galactic Center Close-Up

(a) An infrared image of part of the Galactic plane shows many bright stars packed into a relatively small volume surrounding the Galactic center (white box). The average density of matter in this boxed region is estimated to be about a million times that in the solar neighborhood.

(b) The central portion of our Galaxy, as observed in the radio part of the spectrum. This image shows a region about 100 pc across surrounding the Galactic center (which lies within the bright blob at the bottom right). The long-wavelength radio emission cuts through the Galaxy’s dust, providing a view of matter in the immediate vicinity of the Galaxy’s center.

(c) A recent Chandra image showing the relation of a hot supernova remnant (red) and Sgr A*, the suspected black hole at the very center of our Galaxy.

(d) The spiral pattern of radio emission arising from Sagittarius A itself suggests a rotating ring of matter only a few parsecs across.

The Galactic Center

Apparently, there is an enormous black hole at the center of the galaxy, which is the source of these phenomena.