Are Galaxies Mostly Stars? Mass and Luminosity Determination of Spiral Galaxies and Stellar Groups

In order to determine what spiral galaxies are composed of, we have to find their mass and their luminosity to produce a mass/luminosity ratio. This will help us determine what spiral galaxies are composed of because we can compare their mass/luminosity ratios to known stellar mass/luminosity ratios. By comparing these two different mass/luminosity ratios, we can determine if galaxies are composed mostly of stellar material. In this report, I will be analyzing 26 different spiral galaxies by finding their mass, their luminosity, and their mass/luminosity ratio. By using these techniques, I will show that only \(10\%\) of the total galactic mass is made of stellar material and that \(90\%\) of a galaxy is composed of non luminous “dark matter.”


A very important question in astronomy is if spiral galaxies are composed entirely of stellar material - stars. This question has puzzled astronomers for years and in this report, I will explain how to find the answer to this question. I will be finding the mass and luminosity of 26 different galaxies in order to find their mass/luminosity ratio. I will be using the Inital Mass Function to find the mass/luminosity ratio of stars and I will compare the two mass/luminosity values in order to find the percent of stellar mass in a galaxy. I will be using data from an astronomy Java program for the galaxy data and Guy Worthey’s Interpolation Medel (an Initial Mass Function program) for the stellar data.

This paper will be outlined as follows: Section 1: How to find the mass/luminosity relationship for Galaxies, Section 2: How to Determine a mass/luminosity relationship for Stars using the IMF, Section 3: Comparing the two different mass/luminosity relationships and what that means for Spiral Galaxies, and finally, Conclusions.

Section 1: How to find the mass/luminosity Relationship for Galaxies

Data and Analysis

The Java applet used for galaxy data provides the flux of the galaxy in \(Watt/ m^2\), the velocity of the galaxy in \(km/s\), and the X and Y position of the velocity data in arc seconds ("). The velocity of the galaxy was produced by the applet by taking the doppler shift of the galaxy along the line of nodes. The line of nodes is along the major axis of the galaxy (the pink dots in the photo below). It is important to use the line of nodes because they are along the major axis and that means that the points are where the radial velocity is at a maximum. The radial velocity comes directly from the doppler shift, which is measured by how fast stars are moving toward or away from Earth as they revolve around the galaxy. If a star is moving away from Earth, the spectrum from the star is ’redshifted’ and if the star is moving toward Earth, the spectrum is ’blueshifted.’ By measuring how much the data point is blue or red shifted, astronomers can calculate a radial velocity for that point. That means that the center point of the line of nodes will have a velocity of \(0 \textrm{ km/s}\) (if the galaxy is neither moving toward or away from Earth) because the center point does not revolve around the galaxy since it is the center point. The galaxy does have a velocity at the center point because the entire galaxy is moving away from Earth and therefore the whole galaxy is redshifted by a certain amount.

It is important to understand that the only reason we can measure a doppler shift (and therefore a radial velocity) is because the galaxy has an inclination toward or away from Eath. The only reason doppler shifts happen is because the star are moving toward or away from Earth in their respective galactic orbit. For example, an inclination of 90 degrees is edge on, and an inclination of 0 degrees is face on, and any angle in between will describe the tilt from Earth. If the galaxy is face on, we can not measure a doppler shift, and therefore a radial velocity, because the stars are not moving toward or away from Earth, they are always the same distance away.