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Classification of Stars

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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 colour of the 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:

Class

Chromaticity

Temperature (Kelvin)

O

Blue

≥ 30000

B.

Blue-white

10000-30000

A.

White

7500-10000

F.

Yellow-white

6000-7500

G

Yellow

5200-6000

K

Light orange

3700-5200

M.

Orange-red

2400-3700

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.

Classification of Stars, Hertzsprung-Russell Diagram, StudySmarterImage 1: Hertzsprung-Russell Diagram, which features a plotting of stars by luminosity against surface temperature in degrees. Their colour is also included since it is related to the temperature, wikipedia

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.

Classification of Stars, Life cycle of a star, StudySmarterImage 2: Life cycle of a star, schoolsobservatory

KEY TAKEAWAYS

  • 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.


Frequently Asked Questions about Classification of Stars

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

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?

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

Final Classification of Stars Quiz

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The absolute magnitude measures the luminosity of a star or astronomical object as perceived by an observer 10 parsecs away from the object.

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White dwarves have a blue spectrum and low luminosity.

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Massive stars turn into red supergiants at the ends of their lives.

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Supernovas are rarely observed in the universe.

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A stars are hotter than M stars.

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What is a system of classification of stars?

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It is a categorisation of stars according to certain physical characteristics.

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Do stars emit black body radiation?

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Black body radiation is an ideally perfect emission of radiation that cannot be achieved, but is a really good approximation of the radiation profile of stars

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Are the temperature of a star and its colour related?

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Yes. The hotter a star is, the more blue its spectrum is. The colder a star is, the redder its spectrum is.

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Where are most stars located on the Hertzsprung-Russell diagram?

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Most stars belong to the main sequence. This is the principal group found in the HR diagram since stars spend most of their lives in it.

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Does the life cycle of stars depend on their mass?

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Yes. Under a certain threshold, they will become white dwarves. Beyond that mass threshold, they may end up as neutron stars or black holes

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 What is one characteristic of neutron stars?

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They are objects spinning very fast.

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What is the definition of a black hole?

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A black hole is a huge amount of mass in a small space, having a very strong gravitational attraction that does not allow anything to escape its influence, even light.

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What is a supernova?

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 It is the big explosion of a star that causes the formation and spreading of many new chemical elements.

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 Are supernovas useful for measurements?

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Yes, since the time dependence of their luminosity is a well-studied relationship.

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Is astrophysics a statistical science?

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Yes. It is hard (or impossible) to reproduce the conditions of stars on Earth, and their features can only be extracted by looking for statistical correlations.

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A star is an astronomical object.

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Supernovae create most of the heavy elements of the universe.

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Neutron stars exist thanks to the degeneracy pressure.

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The spectral analysis of a supernova yields useful information.

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The radius of a black hole depends only on its mass.

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What is a black hole?

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It is an astronomical object generated after a supernova that is comprised of a point of infinite density that attracts everything, even light.

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What is a supernova?

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A supernova is an astronomical object caused by the collapse of the core of a star. Some are also generated by the explosion of a white dwarf.

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What is a neutron star?

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A neutron star is a very small and dense astronomical object left behind after a supernova that is mainly composed of neutrons.

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What is the current model for the active nuclei of galaxies?

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They are believed to be supermassive black holes with mass orbiting them.

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What is the name of the distance threshold beyond which not even light can escape the influence of a black hole?

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Horizon event.

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Does general relativity completely describe black holes?

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No, it fails to properly describe all their properties.

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Can we extract information from inside a black hole?

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No, not even light can leave its inside.

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Is the rotation of neutron stars precise or erratic?

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It is very precise, which is why they can be used as clocks.

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Do neutron stars have an intense magnetic field?

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Yes, they do.

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Does the radius of a black hole depend on its density?

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No, it only depends on the total mass of the black hole.

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The characteristics of thermal radiation strongly depend on the temperature.

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The higher the temperature, the bigger the intensity of the most intense frequency emitted.

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Infrared waves have a smaller wavelength than radio waves.

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F stars are hotter than G stars.

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O spectral type stars can belong to either the main sequence or the white dwarf region.

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What is a black body?

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A black body is a body that absorbs and emits radiation perfectly, without any losses.

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What is Wien’s law?

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Wien’s law is the law that shows the dependence between the maximum intensity of emission of a frequency and the temperature of the emitting body.

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Name three regions of the electromagnetic spectrum.

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Regions of the electromagnetic spectrum include visible, infrared, radio, ultraviolet, gamma, X-ray, microwave.

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What are the stellar spectral types of stars from higher temperature to lower temperature?

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O, B, A, F, G, K, M.

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Hotter stars are bluer than colder ones. True or false?

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True.

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Does the stellar spectral classification have any information about luminosity?

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No, the stellar spectral classification only has information about the spectrum/temperature.

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What is the Hertzsprung-Russell diagram?

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The Hertzsprung-Russell diagram shows how the stars are distributed according to their luminosities and temperatures/spectral properties.

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Does the stellar spectral type remain constant for a star throughout its lifetime?

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No, as can be seen from the Hertzsprung-Russell diagram, they vary a lot.

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What are the elements associated with the typical emission lines we find in hot stars?

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We usually find emission lines for hydrogen and helium.

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Luminosity is the intensity of electromagnetic radiation emitted in all frequencies per unit of time.

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The perceived luminosity is spread over spherical surfaces and is lost due to extinction.

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Magnitudes are a measure of the perceived luminosity, that is, luminosity per unit area.

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Stars emit approximately like black bodies.

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Luminosity depends on both the size of a star and its temperature.

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Why is Sirius brighter in the sky if the luminosity of Antares is higher?

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Because the distance to Antares is much greater than to Sirius.

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