Classification of Stars

Classification of Stars

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Stars are fascinating objects in the universe. To make it easier to study and understand them, we need a system to classify them. In this article, we will explore why classification of stars is important and what characteristics we can use to do it. We'll also take a quick look at some of the most significant classification systems for stars and how other objects fit into these categories. So, let's dive in!

What is a system of classification of stars?

In this section, we will study the definition of a system of classification of stars, why it is useful, and what physical quantities it may be based on.

Motivation and definition for classification of stars

Astrophysics and astronomy may seem like disciplines based solely on observation, but they are actually heavily reliant on statistics. Since stars and planets cannot be replicated on Earth, scientists use precise measurements and statistical analysis to gather information about them.

To make sense of all this data, scientists developed a system of classification for stars. By categorizing stars based on specific measurements, we can gain insights into the composition of our universe. This system helps us understand the unique characteristics of each star and how it fits into the bigger picture of our galaxy.

Relevant quantities for classification of stars

When it comes to classifying stars, there are several key quantities we can measure. Because stars emit radiation through nuclear processes, we can approximate their emitted radiation using black body radiation, which simplifies their study.

One important quantity is luminosity, or the amount of electromagnetic energy a star radiates per unit of time. However, due to the distance between stars and Earth, we can only measure a portion of the radiation emitted by a star. To deal with this, we use the concept of apparent magnitude, which is related to the electromagnetic radiation that we observe from Earth. Another useful measurement is absolute magnitude, which is a measure of the received electromagnetic radiation per unit of time measured by an observer ten parsecs away from the object. However, to use this measurement, we need to have alternate ways of measuring the distance from Earth to the object.

Temperature of emission is also a crucial measurement. According to thermodynamics, the frequency of radiation emitted by an object depends on its temperature. By analyzing the spectrum of a star's emission, we can estimate its temperature, with hotter stars appearing closer to blue and cooler stars appearing closer to red. By taking all of these measurements into account, we can gain a better understanding of the unique characteristics of each star and how they fit into the larger picture of our universe.

Spectral Classification of Stars

The Harvard stellar classification system, developed in the late 19th century, is a useful way of categorizing stars based on their spectral radiation. The system uses letters to represent different categories, ranging from O (the hottest and most massive stars) to M (the coolest and least massive stars). This system has proven to be a useful tool for astronomers, but it has its limitations in terms of the amount of new information it provides.

To gain a deeper understanding of stars, scientists developed the Hertzsprung-Russell (HR) diagram. This diagram plots a star's luminosity against its temperature, allowing scientists to better understand the relationship between these two key measurements. By analyzing the placement of stars on the HR diagram, scientists can gain insights into their life cycles and evolution. This diagram has proven to be an invaluable tool for astronomers and has allowed us to better understand the complex nature of our universe.

Classification of Stars on the HR Diagram

The HR diagram is a plot of a star's absolute magnitude (which is related to its luminosity) versus its temperature or spectral class (which is related to its color). By plotting these two variables, scientists can gain insights into the relationship between a star's luminosity and temperature.

The HR diagram has proven to be a powerful tool in astrophysics because it allows scientists to study two variables whose relationship is not fully understood. By analyzing the placement of stars on the diagram, scientists can gain insights into the different stages of a star's life cycle and learn more about the underlying physical processes that govern their behavior. This has led to many important discoveries in the field of astrophysics and has expanded our understanding of the universe as a whole.

Image 1:

Hertzsprung-Russell Diagram, which features a plotting of stars by luminosity against surface temperature in degrees
Hertzsprung-Russell Diagram, which features a plotting of stars by luminosity against surface temperature in degrees

In this figure (image 2), we see some shapes where stars are grouped together. The diagonal long line is called “main sequence”. It is where stars spend most of their life. The upper region of the diagram has the "giants branch" and the "supergiants branch". They are comprised of stars with huge radii and advanced age. We find the "white dwarfs branch" in the lower part of the diagram. These are stars at the very ends of their lives, with low and intermediate masses, and with very small radii and luminosity.

This diagram allows us to reliably predict the behaviour of a star; Its age, mass, and composition, due to statistical features extracted from stars that have already been catalogued.

The classification of other astronomical objects

We will briefly review three special astronomical entities, namely, supernovas, neutron stars, and black holes. Do they fall into similar categories as the ones we have studied already?


The Hertzsprung-Russell diagram is the map of the life of a star, but its death is not included. The development of a star is determined by its mass since that is indicative of how much nuclear fuel it contains. Once they achieve a mass over a certain value (around 8-15 solar masses; 1 solar mass is approximately 1.989 * 10 ^ 30 [kg]), stars suddenly explode after millions of years of life, forming new elements and sending them around the universe. Although these episodes have rarely been observed, they constitute a good laboratory for interstellar experiments. Their luminosity is known to correlate with time in a very precise way.

Neutron stars

A giant star undergoes some processes which can lead it to expel its outer layers. If this happens, its core, depending on the mass, maybe too massive to end as a white dwarf. In this case, a rapidly spinning body is formed which is believed to be made mainly of neutrons. These bodies have a high luminosity in the radio frequency. Their emitting properties are also very well known and serve various purposes upon measurement.

Black holes

These are the most famous and mysterious of astronomical objects. Supernovas may not be a full extinction of the star. A remnant may survive the explosion. Depending on the mass, again, a black hole may form. These are objects which do not let anything, not even light, escape their gravitational attraction. Their properties are all theoretical. It is almost impossible to do measurements with them. However, they are believed to play a very important role in the formation of galaxies and big structures in the universe.

Life cycle of a star, school sobservatory
Life cycle of a star, school sobservatory


KEY TAKE AWAY S Systems of classification of stars are one of the keystones of astrophysics. They allow us to efficiently predict the characteristics of faraway objects. There are many techniques of measurement in astrophysics, which yield different relevant quantities. Luminosity, absolute/apparent magnitude, and temperature are some examples of these important quantities. There is a relationship between the temperature of a star and its pattern of emission, which is related to the colour. The stellar spectral classification is a categorization of this relationship. The Hertzsprung-Russell diagram is a representation of the luminosity of stars against their temperature, which leads to the appearance of shapes which accurately classify stars throughout their lives.


Classification of Stars

Under which category of classification does the Sun fall?

The sun belongs to category G of the stellar spectral classification. It is a star in the main sequence.

How does the chemical composition of stars determine their classification?

As stars get older they produce heavier chemical elements. The content of heavy elements helps to estimate the age of a star and to categorise it.  

What is the spectral classification of stars?

What is the spectral classification of stars?

What are three ways that stars are classified?

By surface temperature, luminosity and mass. We can also classify them by radius and other characteristics.

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