The Atmosphere of Stars

As of now, you might have already become familiar with The spectral classification of stars and the famous Saha's equation. Today, I am giving you a glance into the atmospheres of beautiful stars that twinkle in our night sky. Let us try to understand what lies inside the stellar atmospheres by making use of the ionization theory.

Each element shows its presence in the star with its lines of spectrum.

We all know that certain elements present in the stellar atmospheres give rise to specific lines in the spectra of those stars as shown above. This depends not only on the composition of stars but also the stellar temperatures. Today, we shall understand how the ionization theory helps Astrophysicists gain knowledge about the interiors of stars just by looking at their respective spectra. We shall also see how spectral classification is based on the ionization theory. First, we need to know what is ionization energy.

What is Ionization Energy?

Ionization energy is the minimum energy required to remove the outermost loosely bound electron from an isolated gaseous metal. Some elements have high ionization energy, whereas, some metals have low ionization energy. The following figure depicts the trend of variation in ionization energy:

igure showing trend of ionization energies

Understanding Stellar Atmospheres

The ionization theory has led the "age of imagination" towards the "age of experimental science'. This theory established the Harvard spectral sequence of stars from O to M as a temperature sequence. I am going to discuss the composition and temperatures of stars based on ionization theory. First, let us differentiate the stars into two categories and understand the ionization processes associated with them:

Low-Temperature Stars

  • Only the coolest stars can produce the spectral bands corresponding to the molecules. These molecules can be - hydrocarbon (CH), cyanogen (CN), carbon molecules, titanium oxide (TiO), etc. These molecules can only exist in cooler atmospheres without being dissociated. Hence, the spectra of red and yellow stars that have the lowest surface temperatures show bands
  • The metals have low ionization and excitation energies. The cooler stars have sufficient stimulus to excite the neutral metallic lines. Hence, the low-temperature stars have a spectrum that shows the presence of neutral metals.

High-Temperature Stars

  • Now let us move from the spectra of lower-temperature stars to the higher-temperature stars. The neutral metal lines start getting weaker and the ionized metal lines become stronger. The partial ionization of metals at higher temperatures is the reason for this. As an example, the band line of Calcium-I appears in the spectra of coolest M stars. On other hand, Calcium-II spectral line becomes stronger in the spectrum of hotter (K or G) stars.
  • In the case of Helium lines, Helium has the highest ionization potential. Hence, Helium spectral lines can only appear in stars of very high temperatures. Helium I lines appear only in the stellar atmospheres of quite high temperatures, i. e., B stars.
  • Lines of neutral and ionized Helium, Oxygen, Carbon, Nitrogen, and Neon at various ionization stages appear in the spectra of O stars.

Author's Message

The aim of the previous and the current article was to give a flavor of stellar atmospheres. In the previous article, I wrote about the importance of Saha's equation in decoding the stars in the Universe. The field of stellar atmospheres is a widely researched topic in Astrophysics. It is quite vast and requires knowledge of plasma physics. I just wanted to introduce the concept of atmosphere of stars. We will leave this here and now move on in our series. These two articles were a bit tough, I confess. But, that's the core Astrophysics!


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