Star Classification

Astronomers classify stars based on size and temperature. In the Hertzsprung-Russell diagram (Figure 3-15), giant blue stars are in the upper left, the sun is in the center, and small red stars are in the lower right. The sun is a middle of the road G star.

Figure 3‑15. Hertzsprung Russell diagram with main sequence phase from upper left to lower right and paths to star death (giants and supergiants) in upper right section. Credit: European Space Agency.

The Hertzsprung-Russell (HR) diagram shows the relationship between mass, temperature, color, and luminosity. The main sequence phase is the line of stars from upper left to lower right in the HR diagram (Figure 3‑15). Stars remain on the main sequence line as long as they are burning hydrogen in their core. They become larger and redder as they die. The lower axis in the HR diagram is temperature, which is directly related to mass during the main sequence phase.

Prior to the ability to collect spectrographic and photographic images of stars, scientists did not know that there were specific mathematical and physical relationships that governed star color and luminosity. Annie Jump Cannon first attempted to classify stars based on their hydrogen absorption lines, which are the result of gases around the star absorbing light emitted by the star. Annie Jump Cannon. Max Planck determined that stars have different colors based on their temperature. Cecelia Payne Gaposchkin then showed that star spectra were a function of their temperature and the elements (mostly hydrogen) in the atmospheres that surrounded the stars. Payne Gaposchkin rearranged the star classifications based on temperature and arrived at the current O-B-A-F-G-K-M classification scheme. Hertzsprung and Russell then independently developed the Hertzsprung-Russell diagram. O stars have the highest temperature (x-axis) and the highest luminosity (y-axis). Each of the seven spectral classifications have 10 subtypes (G0, G1, G2, etc.). With the aid of the HR diagram, astronomers don’t need to measure the temperature directly, instead they look at the color spectrum in order to determine the classification and temperature of the star.

The main factor that determines a star’s life span is its mass. It is counterintuitive, but large stars, which are the blue stars in the upper left of the HR diagram, have much shorter lifespans. They burn much hotter and thus have lifespans of millions of years rather than billions of years. Large stars have spectacular endings as supergiants and supernovae. Small stars on the lower right of the HR diagram are much cooler and are red. They last for tens of billions of years, which means none of them have ever burned out. Scientists can determine that clusters of stars are old if the only stars left in the cluster are red. All of the hotter stars already burned out.

Figure 3‑16. O, M, and G stars.

Energy moves from the core to the surface of stars, but the type of energy transfer differs between large and small stars. Figure 3‑16 shows the energy transfer mechanisms in O (large), G (medium), and M (small) stars. The inner core (blue) is where nuclear fusion takes place, yellow represents a radiative zone, and multicolor represents a convective heat transfer zone. Convection is the movement of energy by mass movement such as wind. M stars (red dwarfs) never burn out because they burn very slowly, and because they are convective, they keep cycling outer layers filled with hydrogen (H) into the core. The inner part of the sun is radiative, so restocking of H into the core does not take place. Even though the inner part of O stars is convective and cycles hydrogen into the core, they burn out quickly because the core is hot.

Our sun is a “middle of the road” G2 star. Scientists classify the sun as a low-mass star even though it is in the 88th percentile of all-stars and is the fourth largest of the 50 closest stars. At the end of its main sequence phase after 9 billion years, the sun will drift off from the main sequence line and become large and red.

Banner

Messier 80 globular cluster. All stars in the cluster are 12 billion years old. Clusters enable astronomers to study the size, color, and lifespans of stars since they are all known to be the same age. Credit: NASA.

Page updated

Report abuse