Classification of Stars

In this article, we will review why we need a system to classify stars, and which characteristics we should use to do so. We will also briefly study some of the most important systems of classification of stars, and how other objects fall into similar classifications.

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

Contrary to what you may expect, both astrophysics and astronomy are mainly statistical sciences. Like all scientific disciplines, they must rely on measurements and experimentation. However, due to the fact that the subjects of experimentation, the stars or planets, cannot be replicated on Earth, it is necessary to refine the measurements and use statistics to extract information unknown to us.

With all this in mind, we define a system of classification of stars as a scheme where stars are associated with certain measured quantities. This allows us to categorize them. The statistical features of this categorization give us information about the composition of our universe.

Relevant quantities for classification of stars

What are the useful quantities we can measure in order to obtain a useful classification? Stars are essentially bodies that emit radiation due to nuclear processes occurring inside of them. We approximate their emitted radiation by black body radiation. This is a good approximation that enormously simplifies their study, by considering them as perfectly-emitting bodies. (This implies that the majority of relevant quantities we should be worried about having to do with their spectral radiation.)

Some of these quantities are:

• Luminosity: the amount of electromagnetic energy per unit of time radiated by a star or other astronomical object.

• Apparent magnitude: the problem with luminosity is that we cannot measure all the radiation emitted by an object very far away. We can only measure the portion which reaches us and the telescopes or devices we may have placed in space. To deal with that, we use the concept of apparent magnitude, which is related to the electromagnetic radiation we observe from Earth.

• Absolute magnitude: also a measure of the received electromagnetic radiation per unit of time, but measured by an observer ten parsecs away from the object. In order to use this quantity, though, we need to have alternate ways to measure the distance from the earth to the object.

• Temperature of emission: according to the laws of thermodynamics, the frequency of the radiation emitted by an object depends on the temperature of the object. The hotter an object, the closer to blue its colour; the colder an object, the closer to red its colour. This allows us to estimate the temperature of stars by analyzing the spectrum of emission.

Spectral Classification of Stars

As we mentioned, knowing the star's spectrum of radiation is useful to determine its temperature (and also its composition). Historically, some of the differences in colour were observed very early, but it was not until the late nineteenth century that a system was created to rigorously classify stars. This system, named Harvard stellar classification, is based on categories named after letters and is summarized in the following table:

Although this system is very useful and has more precise variants, it does not have a lot of new information. That brings us to the Hertzsprung-Russell diagram (or HR diagram).

Classification of Stars on the HR Diagram

In the Hertzsprung-Russell (HR) diagram, stars are plotted according to their absolute magnitude, which is related to their luminosity, and their temperature / spectral class, which is related to their colour. The usual way to proceed in astrophysics is to study two variables whose dependence (if any) is not known in order to extract conclusions.

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?

Supernovas

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.