A Hertzsprung-Russell Diagram

A Hertzsprung-Russell diagram is a very useful tool in explaining the properties of stars. These properties include surface temperature and luminosity. In the graph, surface temperature is on the x-axis, starting out with the highest temperature and blue line, and as the temperature decreases, the color goes from white to yellow, and finally to red. The y-axis most commonly uses solar units as luminosity. This is described compared to the luminosity of our sun, and is graphed as a multiple of that. A star can be classified in these diagrams by seven different spectral types, those being: M, K, G, F, A, B, and O, in ascending order. This is based mainly on surface temperature. In this way, the spectral type of a star can be used on the x-axis in place of surface temperature. Our own sun is considered to be a class G star. Stars in the upper right portion are most of the largest stars, while stars in the lower left portion of the graph are most of the smallest of our stars.  This is because as luminosity increases along with the surface temperature, lines of constant radius are formed diagonally through the graph. These also group the stars into these categories, starting with the bottom left, and going towards the top right of the graph: White dwarfs, main-sequence stars, and giants (along with the subcategory supergiants). Within the main-sequence stars, mass is correlated in which as luminosity increases, so does mass.

We can use the Doppler Effect to see how fast things move towards or away from us. The Doppler Effect is known as the effect made by a moving source of waves. As an object moves, the wavelengths in the direction of the moving object are shorter than those in the opposite direction. In order to notice Doppler shifts with light, we need high speeds. An upward shift in frequency is known as a blue shift, while a downward shift in frequency is known as a red shift.  They are named this way because an upward shift in frequency is shifted towards the blue end of the spectrum, and towards the red end of the spectrum for the opposite. We can use emission lines to tell whether an object is moving towards or away from us, and at what speed. By comparing an object’s wavelengths while moving to an object’s wavelength while stationary, we can calculate speed.

Another way to find out about an object is spectroscopy. To figure out the composition of an object, you can use emission lines. Since each element has a unique emission line spectrum, you can use it to figure out exactly what elements are contained in an object, such as an interstellar cloud of gas. These emission lines are produced by photons moving at different speeds. Opaque objects tend to have a continuous spectrum, while transparent objects have more defined emission lines. An opaque object’s emission lines are based on temperature. Hotter objects emit most of their radiation at shorter wavelengths, and contain more overall radiation.