Oh Be A Fine G… (Girl ή Guy), Kiss Me.
Such a lovely little poem! Easy to remember!
That is a smart way to remember the spectral types of stars! The letter sequence OBAFGKM is a classification of the stars according to their surface temperature, from the hottest blue-white supergiants attributed to the spectral type O or B, to the coolest red dwarfs attributed to the spectral type K and M.
As the following picture shows, the spectral type of the stars is equivalent to the stars’ temperature. If we mark out -on the horizontal axis- the star’s spectral type (which is related to its temperature) and -on the vertical line- its luminosity, then what we have is the Hertzsprung-Russell diagram.
Stars’ luminosity is plotted along the y-axis (vertical axis) and stars’ spectral types OBAFGKM (meaning stars’ surface temperatures) are plotted on the x-axis (horizontal axis). Most stars are gathered in a band that runs from the top-left to the bottom-right of the chart, these stars are in the, so-called, ‘main-sequence‘ stage of their lives. We can also spot the red giants and the red supergiants that fall in the top-right of the chart. In the bottom-left of the chart, we can spot the hot stars which are dimmer than main-sequence stars. These are white dwarf stars.
The evolution of stars!
This diagram is so important because it helps us better understand stars’ evolution, from birth to death! Yes, it is true! Stars are born and die!
Take the Sun for example. It has lived 4.5 billion years already and is estimated to double it. Sun is middle-aged!
Stars of spectral type O are huge, warm blue stars. Zeta Puppis -formally named Naos (ζ Puppis)– of spectral type O4, is a good example of a bright and warm blue star.
The Sun, on the other hand, is of spectral type G. Sun’s surface temperature is about 6.000 Celsius.
The Orion Constellation
A very well-known, cold, red star is Betelgeuse. This super-giant star is spotted among other super-giants in the Hertzsprung-Russell diagram.
Betelgeuse is one of the four stars that form the trapezium in the Orion constellation.
We can easily discern the four brightest stars that form a large, roughly rectangular shape. Two of them serve as Orion’s shoulders: Betelgeuse, the red star, establishes the right shoulder, and Bellatrix, the blue one, serves as Orion’s left shoulder. The other two stars serve as Orion’s foot. In the small picture below, we see the three stars that form Orion’s Belt.
Red for Cold and Blue for Hot?
The red stars, like Betelgeuse, are cold and the blue stars are hot! Yes, you heard right! This is the exact opposite of what we experience when we turn on our water faucets. But how do we remember that?
- Will I get burned if I put my hands in cold water (blue)? – NO
- Will I get burned if I go near a blue star? – YES
Nuclear Fusion & Heat
As we saw in the Hertzsprung-Russell diagram, most stars are gathered in a band that runs from the top-left to the bottom-right of the chart, the so-called main sequence. These stars fuse hydrogen in their cores. Smaller clusters of stars are those of red giants (and red super-giants) as well as those of white dwarfs.
Nuclear fusion is the process by which two or more atomic nuclei combine to form a single heavier one. When the forming nucleus’ size is smaller than the size of iron’s core, massive amounts of energy are released. This is exactly what happens at the stars which are clustered in the diagram’s main sequence. It is this nuclear process that is responsible for our Sun’s heat.
So why don’t we use this process to generate energy, to warm us up? This is a great question, right? Because by applying this method, we will put an end to environmental harm that is caused, among others, by the combustion of fossil fuels. Even though scientists are trying to generate controlled and efficient fusion, the truth is that we are not ready, yet! It is highly possible, though, that by the second half of the 21st century, people will be able to use fusion to heat their houses.
Let’s take a look again at the Hertzsprung-Russell diagram where the white dwarf stars are spotted in the bottom-left, these dwarf stars are dimmer than the main-sequence stars.
This effect occurs when the light elements are used up and other nuclei reactions are generated so the fusion can continue its process. Very recently, an astrophysicist, remarked that “even stars go through an energy crisis.” The only thing that is certain, is the fact that stars create something beautiful and spectacular in their effort to keep existing. If you are unconvinced about the star’s beauty, please take a look at the following image.
We will talk about the star’s spectacular death in the near future! Until then, let’s travel in time to watch two episodes of an old Greek TV show from the 80s, named “Red Giants and White Dwarfs”