Star Qualities (Life of Stars, Part II)
- Dylan Lee
- Jun 15, 2019
- 5 min read
Dylan Lee, Stellar Astronomy
In the last post, we explored the creation stars: huge balls of gas that use the fusion of hydrogen to radiate heat and light. Once the fusion process begins, the stars reach a state of hydrostatic equilibrium, a balance between the gas pressure produced by fusion and the gravitational attraction of the star itself. The star will now remain in this harmonious state for millions or even billions of years. In this post, we'll examine this peaceful period in the life of a star, known as the main sequence.
The Main Sequence
The main sequence of a star begins as soon as it reaches hydrostatic equilibrium. It has departed from the turbulent compression and heating that is characteristic of the birth of stars, and now emits a colossal amount of energy (provided that it didn't become a brown dwarf). However, each star releases this energy in a different way. While some devour their hydrogen swiftly and furiously, others consume it more slowly and in moderation.
The cause of these differences is, in fact, caused by different stars having different masses. If a star has a high mass, the inward gravitational attraction is incredibly strong, which means that the hydrogen must be fused incredibly fast to provide the gas pressure necessary to balance gravity. On the other hand, if a star has a low mass, the force of gravity is relatively weak, which means that the hydrogen can be consumed at a slower rate.
Now that we know the causes of different rates of hydrogen fusion, we can further our understanding by looking at its observable effects. Recall that nuclear fusion emits energy in the form of heat and light. It would then make sense that if fusion in a star occurs at a faster rate, heat and light would also be emitted from the star at a faster rate; if fusion occurs at a slower rate, heat and light would be emitted at a slower rate. In this way, the rate at which hydrogen fusion takes place in a star controls both the temperature and luminosity of that star (luminosity is a measure of the rate at which a star emits light energy). Furthermore, recall that the rate of hydrogen fusion is dependent on the mass of the star. Therefore, a star with a greater mass has a higher temperature and a higher luminosity, while a star with less mass has a lower temperature and a lower luminosity.
Additionally, the rate of hydrogen fusion also affects a star's color, formally called its spectral type. Firstly, note that when objects are heated, they may first adopt a red glow, before turning orange, and then yellow. If it is heated further, it will then emit a white glow, and eventually even a blue glow. Now, recall that more massive stars fuse hydrogen more quickly, and thus have a higher temperature. This means that more massive stars will appear more blue or white since they are hotter, while less massive stars will appear orange or red since they are cooler. Those such as our Sun, which have a moderate amount of mass, are yellow. Listed from the hottest to the coolest, the commonly accepted spectral types are O, B, A, F, G, K, M (click on the link for acronyms to remember the types).
The last important property that the rate of hydrogen fusion affects is the absolute magnitude, or brightness, of a star (absolute magnitude is more negative for brighter stars and more positive for dimmer stars; for more on how absolute magnitude works, visit the hyperlink). This makes sense since the rate of hydrogen fusion also affects the rate of light energy that is emitted from a star. The faster a star fuses hydrogen, the brighter it is, and therefore the more negative its absolute magnitude. It is important to note that the brightness represented by absolute magnitude is not the brightness of the star as it appears from Earth. The issue with that definition of brightness would be that it doesn't take into account how far away certain stars are. A star could be so far away that it appears dim, even if it is actually very bright. Therefore, absolute magnitude refers to the intrinsic brightness of a star - how bright it would appear at a given distance (the exact distance used in the definition of absolute magnitude is 10 parsecs, which is about 32.6 light years).
The H-R Diagram
As revealed above, stars can have many different properties: temperature, luminosity, spectral type, and absolute magnitude. In order to organize these results, astronomers use the Hertzsprung-Russell Diagram (often abbreviated as H-R Diagram). This diagram plots either stars' temperature on the horizontal axis against their luminosity on the vertical axis, or their spectral class on the horizontal axis against their absolute magnitude on the vertical axis. The latter arrangement is called the observational H-R Diagram since spectral type and absolute magnitude can be more easily observed than temperature and luminosity, which must often be calculated. On the horizontal axis, the hottest or bluest stars are plotted toward the left while the coolest or reddest stars are plotted toward the right. On the vertical axis, the stars with the highest luminosity (therefore most negative absolute magnitude) are plotted toward the top while the stars with the lowest luminosity (therefore the most positive absolute magnitude) are plotted toward the bottom.
Notice that when main sequence stars are plotted on the diagram, they lie along a path that stretches from the top left of the diagram to the bottom right of the diagram (the other groups on the diagram will be covered in later posts). This illustrates how the luminosities and temperatures of main sequence stars are both dependent on the rate of hydrogen fusion, and in turn dependent on the masses of stars. For stars with a high mass, both the luminosity and temperature are high. None of the main sequence stars have a high luminosity but a low temperature, or vice versa, which supports the idea that temperature and luminosity both depend on a single factor (the mass) of a star. Furthermore, it can be concluded that the stars at the top left corner are the most massive (with the highest luminosity and temperature), while the stars at the bottom right are the least massive (with the lowest luminosity and temperature).
The last thing to note about the H-R Diagram is that stars actually move on the diagram depending on what stage in life they are at. For example, recall that protostars, the precursors to stars do not even fuse hydrogen, so they do not emit light or heat. However, some are still very bright due to the brightness of the gas clouds they come from (recall how nebulae are beautifully lit). This means that they are incredibly cold but may be bright or dim, so they occupy the far right of the H-R Diagram. As they compress and begin to fuse hydrogen, their temperatures increase, which causes them to move left on the H-R Diagram (though not always in a straight path), before taking their place as a main sequence star. When they reach hydrostatic equilibrium, the rate at which fusion occurs stabilizes, and the star stops moving on the diagram. It spends nearly the rest of its life at its position on the main sequence, before it meets its inevitable and beautiful end, which we will cover in the next post.
Fraknoi, Andrew, et al. Astronomy. Houston, Texas, OpenStax, Rice University, 2017.
COSMOS. "Hertzsprung-Russell Diagram", The SAO Encyclopedia of Astronomy, Swinburne University of Technology, http://astronomy.swin.edu.au/cosmos/H/Hertzsprung-Russell+Diagram.
Tate, Jean. "Main Sequence", Universe Today, WordPress, 27 January 2010, https://www.universetoday.com/52252/main-sequence/.
Photos are linked to sources.
Cover photo: star cluster NGC 3572: https://en.wikipedia.org/wiki/Open_cluster
Low-Mass vs. High-Mass Star diagram created in Google Drawings
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