In order to study the physical properties of stars, we can classify them by using their emission properties and their temperature. The goal of stellar spectral classification is to understand why these quantities are relevant and how they are related.
The importance of temperature and thermal radiation
Stars are bodies that emit electromagnetic radiation due to the nuclear reactions happening inside them. We aren’t going to study how these reactions make the existence of stars possible, but we do explore temperature and thermal radiation in more detail.
- Temperature is a quantity that measures the mean kinetic energy of the particles of a certain substance, i.e. the average movement energy they have. When studying systems in a gas state, such as stars, we can assume that all the particles are moving more or less freely (unlike solid-state systems).
- Thermal radiation is the electromagnetic radiation created from the electromagnetic interactions between the many particles of a substance with an absolute temperature greater than zero. Since the temperature measures the mean kinetic energy of these particles, the characteristics of the emission of thermal radiation are related to the temperature. It is the primary source of the radiation of stars.
Wien’s law
Usually, systems with many particles are hard to characterise. Even when we focus on collective properties (thermodynamic properties), the models used are typically complex. However, for emitting bodies, there is an approximation that simplifies the computations and models: the black body approximation.
A black body is a name given to a perfectly emitting and absorbing system. Quite remarkably, these objects have many thermodynamic properties that stars follow almost exactly, which suggests that the approximation is almost true for these astronomical bodies.
In this explanation, we explore the relation between the intensity of emission of frequencies and the temperature. Put simply: if a star is at a certain temperature T, which frequencies will it emit more intensely?
Below is a diagram showing the frequency profiles for different temperatures and their maxima, which are described by Wien's law. You can see a decrease in the wavelength peak with an increase in temperature. Also, note that the dashed lines show you the visible spectral range, and the slanted dotted line shows Wien’s law.
Wien’s law states that objects of different temperatures will emit spectra of different wavelengths. It shows the dependence between the maximum intensity of emission of a frequency and the temperature of the emitting body.
Planck’s radiation law, Wikimedia Commons
Check out our explanation on Black Body Radiation. Note that the higher the temperature, the smaller the wavelength of the thermal radiation (an inversely proportional relationship).
What is stellar spectral classification?
There are many stellar spectral classifications, but we only study the most widely used in this explanation. Although stars emit radiation with all kinds of frequencies, this classification is based on visual properties by relating the electromagnetic properties to the colour in the visible range.
Stellar spectral classes and the stellar classification table
The stellar spectral classification categorises stars according to their spectral properties. The adjective spectral refers to the spectrum of emission of thermal radiation of stars. The visible region has to do with which colours are emitted and with which intensity, but, in general, it refers to frequencies in the whole spectrum, which can be X-ray, radio, ultraviolet (UV), infrared (IR), gamma, etc.
Below is the general stellar spectral classification chart of the classification system, also known as Harvard spectral classification.
Class | Chromaticity | Temperature (Kelvin) | Prominent absorption lines |
O | Blue | >30,000 | Helium |
B | Blue-White | 10,000-30,000 | Hydrogen + Helium |
A | White | 7500-10,000 | Hydrogen + Ionised metals |
F | Yellow-White | 6000-7500 | Weaker hydrogen + Ionised metals |
G | Yellow | 5200-6000 | Hydrogen + Ionised metals + Neutral metals |
K | | 3700-5200 | Neutral metals |
M | Orange-Red | 2400-3700 | Oxide molecules + Neutral molecules |
As we can see, Wien’s law allows us to relate the chromaticity to the temperature of stars, which allows us to establish a solid classification. The left column of the table shows the spectral types or spectral classes.
The seven spectral types of stars are O (blue), B (blue-white), A (white), F (yellow-white), G (yellow), K (light orange), and M (orange-red).
Refined stellar spectral classification
Although this classification is very helpful on its own, sometimes further refinement is needed in astronomy and astrophysics in the spectral types for stars. This requires the addition of distinctive elements to the already shown names for the spectral types of stars. For instance, by studying their spectrum and characterising their frequencies, we can assign numbers in addition to the letters to reference specific wavelengths.
How is stellar spectral classification related to the stages of a star’s life?
The reason for using methods of classification of stars is to study their properties collectively and extract helpful information thanks to statistical analysis. The features of the spectral classification of stars are strongly related to the stages of a star's life, which allows us to deduce properties, like age, from observational properties, like colour.
Luminosity classes and the Hertzsprung-Russell diagram
The Hertzsprung-Russell diagram (H-R diagram) is a useful diagram that captures the statistical distribution of stars regarding their luminosities and temperatures. The main strand is called the main sequence, where stars spend most of their lives. The upper region is related to the late phases of stars, and the lower region is connected to the final stages of a star’s life that was not very massive.
We can see that luminosity and colour (related to spectral characteristics) correlate. This correlation depends on the star’s stage of life but can be accurately described by many models that have been developed and produce the HR diagram.
Hertzsprung-Russell diagram, ESO CC 4.0
Spectrum, luminosity, and the stages of life of a star
Having seen the Hertzsprung-Russell diagram, we can now make general statements about the spectrum of stars and the meaning of stellar spectral classification.
For instance, we see that most stars with lower temperatures (last spectral types of stars) are the stars in the giant region or the main sequence. Their luminosity class (which, in this case, is strongly correlated to their size) determines which classification they fall under.
On the other hand, hotter stars are the very bright stars in the main sequence (usually young massive stars) or white dwarves, which have a high surface temperature due to the process that allows them to exist as astronomical objects. As we can see, both luminosity and spectral considerations are essential to describe the characteristics of a star and understand its stage of life.
Stellar Spectral Classes - Key Takeaways
- Stellar spectral classification categorises stars according to their spectrum of emission. The categories are called spectral types of stars.
- The spectrum of emission depends strongly on the temperature. This relation is very well known for a simplified model that stars fit very well: a black body.
- Stellar spectral classes give us information on many features of stars, such as the presence of certain elements or molecules.
- Other classification systems can complement stellar spectral classification with more spectral types for stars to obtain a very accurate description of the properties of a star.
ImagesHertzprung-Russell diagram, https://commons.wikimedia.org/wiki/File:Hertzsprung-Russell_Diagram_-_ESO.png
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