One of the most recognizable schemes for classifying elements in the world of science is the periodic table. In a simplified way, the Hertzsprung-Russell diagram is like a periodic table, but for stars. With this diagram, we can locate a group of stars and see where they are classified according to their characteristics. Thanks to this, significant progress has been made in the observation and classification of the different groups of stars that exist.
Therefore, we are going to dedicate this article to tell you all the characteristics and importance of the Hertzsprung-Russell diagram.
Features and operation
Let's try to understand how the Hertzsprung-Russell diagram works and what it consists of. The two axes of the graph measure different things. The horizontal axis measures two scales that can be summarized into a single one. At the bottom, we see a scaling of the star's surface temperature in degrees Kelvin, ranging from the highest temperatures to the lowest.
At the top, we see something different. There are a series of sections, each marked with a letter: O, B, A, F, G, K, M. This represents the spectral type, which indicates the star's color. Just like the electromagnetic spectrum, it ranges from bluish to red. Both scales indicate the same thing and are consistent with each other because the spectral type is determined by the star's surface temperature. As the temperature increases, the star's color also changes, transitioning from red to bluish, passing through orange and white tones along the way. These types of diagrams allow for easy comparison of the temperatures associated with each color of the star.
On the other hand, the vertical axis of the Hertzsprung-Russell diagram measures the same concept, expressed on different scales, such as luminosity. On the left, luminosity is measured relative to the sun. This facilitates a fairly intuitive identification of the luminosity of other stars, using the sun as a reference point. It's easy to see if a star is more or less luminous than the sun, given its visual similarity. The right-hand scale measures luminosity in a slightly more precise way. It can be measured using absolute magnitude. For example, when we look at stars in a forest, one star might appear brighter than others. Obviously, this often occurs because the stars are at different distances, not because one is inherently brighter than another.
Star shine
When we look at the sky, we see that some stars appear brighter, but this only happens from our perspective. This is called apparent magnitude, although there's a slight difference: the apparent magnitude of a star is determined by fixing the value that its luminosity would have outside our atmosphere, not within it. Therefore, the apparent magnitude doesn't represent the star's actual luminosity. Consequently, a scale like the one on the Hertzsprung-Russell diagram cannot be used.
In order to be able to measure the luminosity of a star well, the absolute magnitude must be used. It would be the apparent magnitude that a star would have 10 parsecs away. The stars would all be at the same distance, and therefore the apparent magnitude of a star would be converted into its actual luminosity.
The first thing we should notice when looking at the graph is a large diagonal line running from the top left to the bottom right. This is known as the main sequence, and it's where most stars, including the Sun, are located. All stars produce energy by fusing hydrogen to create helium in their cores. This is the common factor they all share, and what differentiates their luminosity is their mass. In other words, the more massive a star is, the faster the fusion process occurs, resulting in greater luminosity and surface temperature.
Therefore, it follows that stars with greater mass are located further to the left and higher up, and thus have higher temperatures and greater luminosity. These are the blue giants. We also have stars with lower mass located to the right and lower down, and therefore have lower temperatures and luminosity; these are the red dwarfs.
Giant stars and supergiants of the Hertzsprung-Russell diagram
If we move away from the main sequence, we can see other sectors within the diagram. At the top are the giants and supergiants. Although they have the same temperature as many other main sequence stars, they have a much greater luminosity. This is due to their size. These giant stars are characterized by having long since burned through their hydrogen reserves, so they have had to start using different fuels, such as helium, for their function. It is then that their luminosity decreases because the fuel is not as powerful.
This is the fate that holds a large number of stars that are located in the main sequence. It depends on the mass they have, they can be gigantic or super-gigantic.
Below the main sequence are white dwarfs. The ultimate fate of most of the stars we see in the sky is to become a white dwarf. During this phase, the star becomes very small and extremely dense. As time passes, white dwarfs move further to the right and down the diagram. This is because they constantly lose luminosity and temperature.
Basically, these are the main types of stars that appear in this chart. There is some current research attempting to highlight and focus on some of the extremes of the chart to gain a deeper understanding.
I hope that with this information you can learn more about the Hertzsprung-Russell diagram and its characteristics.