giant nebula NGC 2014 and its neighbour NGC 2020
Credit: NASA, ESA, and STScI

Star Classification – How Do Scientist Classify Stars?

Star Classification

Take a look up at a dark night sky, and you’ll find it illuminated by hundreds or even thousands of individual twinkling points of light. While they might seem, to an untrained eye, to all be the same — except for, perhaps, some appearing brighter than others — a closer look reveals a number of intrinsic differences between them. Some of them appear redder or bluer than others; some are intrinsically brighter or fainter, even if they’re the same distance away; some have larger physical sizes than others; some have greater or lesser percentages of heavy elements in them.

Stars are huge celestial bodies made mostly of hydrogen and helium that produce light and heat from the churning nuclear forges inside their cores. Aside from our sun, the dots of light we see in the sky are all light-years from Earth. They are the building blocks of galaxies, of which there are billions in the universe. It’s impossible to know how many stars exist, but astronomers estimate that in our Milky Way galaxy alone, there are about 300 billion.

For a long time, scientists didn’t know how stars worked or what made one type different from another. Yet at the start of the 20th century, the pieces all came together to figure out exactly how the different stars should be classified, and we owe it all to a woman you might not have heard of: Annie Jump Cannon.

Annie Jump Cannon examines a photographic plates of the night sky.
Annie Jump Cannon examines a photographic plates of the night sky. She created the stellar classification system still used today. (Image: © Harvard-Smithsonian Center for Astrophysics )

With either good enough skies and a trained observer, or with a quality telescope, a look at the stars immediately shows that they come in different colours. Because temperature and colour are so closely related — heat something up and it glows red, then orange, then yellow, white and eventually blue as you turn up the temperature — it makes sense that you’d classify them based on colour.

But where would you make those divisions, and would those divisions encapsulate all the important physics and astrophysics going on? Without more information, there wouldn’t be a good, universal system that everyone would agree on. But the study of colour in astronomy (photometry) can be augmented by breaking up the light into individual wavelengths (spectroscopy).

If there are either neutral or ionized atoms in the outermost layers of the star, they’ll absorb some of the light at particular wavelengths. These absorption features can add an extra layer of information, and led to the earliest useful classification system.

Star Classification - The solar spectrum
The solar spectrum shows a significant number of features, each corresponding to absorption properties of a unique element in the periodic table. Image credit: Nigel A. Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF.

Star Classification

Different stars can be categorised into certain groups, depending on their mass and temperature. Over the centuries, the classification of stars has evolved into seven distinct classes or groups. These groups are known as O, B, A, F, G, K and M. Stars classified in the ‘O’ group are the most massive and hottest, with temperatures exceeding 30,000°C, whilst those in the ‘M’ group are the smallest and coolest, with temperatures less than 3,000°C.

Comparison of star types
Comparison of star types

Stars of different temperatures appear to shine with different colours. This is similar to what happens when you heat up a lump of metal to very high temperatures. After heating the metal for some time, it will start to glow red. As it gets hotter still, that red will evolve into yellow, then white and eventually the metal will be glowing a bright blue colour. In the same way, it turns out that blue stars are very hot and are therefore classed as ‘O’ stars, whereas the cooler, red stars, are placed into the ‘M’ class.

When we think about our star, the Sun, we picture it as being yellow. It is therefore not surprising to discover that the sun is classed as a ‘G’ star, with a temperature of approximately 5,500°C. The following table lists the different classes of stars, along with their approximate temperatures and colours.

Star Classification can be more accurately categorised under this system, by the addition of a number between 0-9 to the group letter. For example, G2 (the Sun’s more precise spectral class) is hotter than G7 but cooler than a G0. Similarly, a B9 star is cooler than a B4.

Star Classification – The Hertzsprung-Russell Diagram

There are a few hundred billion stars in our galaxy, the Milky Way and billions of galaxies in the Universe. One important technique in science is to try and sort or classify things into groups and seek out trends or patterns. Astronomers do this with stars.

The Hertzsprung-Russell Diagram - Star Classification
Hertzsprung-Russell diagram. A plot of luminosity (absolute magnitude) against the colour of the stars ranging from the high-temperature blue-white stars on the left side of the diagram to the low temperature red stars on the right side. / Image Credit:Richard Powell

So far we have discussed the luminosity and colour or effective temperature of stars. These can be plotted to form what is one of the most useful plots for stellar astronomy, the Hertzsprung-Russell (or H-R) diagram. It is named after the Danish and American astronomers who independently developed versions of the diagram in the early Twentieth Century.

In an H-R diagram the luminosity or energy output of a star is plotted on the vertical axis. This can be expressed as a ratio of the star’s luminosity to that of the Sun; L*/Lsun. Astronomers also use the historical concept of magnitude as a measure of a star’s luminosity. Absolute magnitude is simply a measure of how bright a star would appear if 10 parsecs distant and thus allows stars to be simply compared. Just to confuse things, the lower or more negative the magnitude, the brighter the star. By definition, a star of magnitude 1 is 100 × brighter than one of magnitude 6. Our Sun has an absolute magnitude of + 4.8.

Hertzsprung-Russell diagram, cycle of stars.
Hertzsprung-Russell diagram, cycle of stars. Range of axes that can be used for an H-R diagram. . Note how the temperature scale is reversed on the horizontal axis. Also take care if using magnitude to work upwards to negative values.

The effective temperature of a star is plotted on the horizontal axis of an H-R diagram. One quirk here is that the temperature is plotted in reverse order, with high temperature (around 30,000 – 40,000 K) on the left and the cooler temperature (around 2,500 K) on the right. In practice astronomers actually measure a quantity called colour index that is simply the difference in the magnitude of a star when measured through two different coloured filters. Stars with a negative colour index are bluish whilst cooler orange or red stars have a positive colour index.

The third possible scale for the horizontal axis is a star’s spectral class. By splitting the light from a star through a spectrograph its spectrum can be recorded and analysed. Stars of similar size, temperature, composition and other properties have similar spectra and are classified into the same spectral class. The main spectral classes for stars range from O (the hottest) through B, A, F, G, K and M (coolest). Our Sun is a G-class star. By comparing the spectra of an unknown star with spectra of selected standard reference stars a wealth of information, including its colour or effective temperature can be determined.

If we now plot a Hertzsprung-Russell diagram for a few thousand nearest or brightest stars we see the following:

Hertzsprung-Russell diagram - Star Classification

Each dot represents a star.

As we can see, stars do not appear randomly on the plot but appear to be grouped in four main regions. This is highly significant as it suggests that there may be some relationship between the luminosity and temperature of a star. Whilst not surprising (indeed we have already seen that a hotter star emits more energy per unit surface area than a cooler star) the relationship is complicated by the presence of these four groups. Let us examine these more closely.

Most stars seem to fall into group A. It shows a general trend from cool, dim stars in the lower right corner up to hot, extremely bright stars in the top left corner which fits in with our expected relationship between temperature and luminosity. This group is called the Main Sequence so stars found on it are main-sequence stars. Our Sun is one such example. Others include α Cen, Altair, Sirius, Achernar and Barnard’s Star.

Stars in group B are mostly 6,000 K or cooler yet more luminous than main-sequence stars of the same temperature. How can this be? The reason is that these stars are much larger than main-sequence stars. Although they emit the same amount of energy per square metre as main sequence stars they have a much greater surface area (area ∝ radius2) the total energy emitted is thus much greater. These stars are referred to as giants. Examples include Aldebaran and Mira.

The stars in group C are even more luminous than the giants. These are the supergiants, the largest of stars with extremely high luminosities. A red supergiant such as Betelgeuse would extend beyond the orbit of Jupiter if it replaced the Sun in our solar system.

The final group of interest are those stars in group D. From their position on the H-R diagram we see that they are very hot yet very dim. Although they emit large amounts of energy per square metre they have low luminosity which implies that they must, therefore, be very small. Group D stars are in fact known as white dwarfs. Sirius B and Procyon B are examples. White dwarfs are much smaller than main-sequence stars and are roughly the size of Earth. The diagram below shows the main groups labelled together with example stars in each group.

Hertzsprung-Russell Diagram - Graph of temperature (x-axis) vs luminosity (y-axis)
Hertzsprung-Russell Diagram identifying many well known stars in the Milky Way galaxy. Graph of temperature (x-axis) vs luminosity (y-axis). Temperature labels go from 3,000 to 30,000 K, increasing to the left. Luminosity labels increase upward from 10^-5 (0.00001) to 10^6 (one million) Lsun. A "main sequence" of stars from top left to bottom right is seen. At top left they are blue and medium size, at bottom right they are red and small. At top right a number of large stars are shown, from left to right in this region their colors change from blue/white to red. At the very top there is a label "supergiants" and just below that, but above the main-sequence and toward the right of the diagram there is another label "giants". At bottom left, there is a region labeled white dwarfs, which contains only two tiny stars. On the x-axis, the spectral sequence is also labeled, from left to right: O, B, A, F, G, K, M. / Image Credit: ESO

Having identified the existence of different types of stars based on measurable properties in the next section we will explore some of their characteristics and the sources of energy in the stars.

Ancient cultures looked to the sky for all sorts of reasons. By identifying different configurations of stars—known as constellations—and tracking their movements, they could follow the seasons for farming as well as chart courses across the seas. There are dozens of constellations. Many are named for mythical figures, such as Cassiopeia and Orion the Hunter. Others are named for the animals they resemble, such as Ursa Minor (Little Bear) and Canus Major (Big Dog).

Today astronomers use constellations as guideposts for naming newly discovered stars. Constellations also continue to serve as navigational tools. In the Southern Hemisphere, for example, the famous Southern Cross constellation is used as a point of orientation. Meanwhile, people in the north may rely on Polaris, or the North Star, for direction. Polaris is part of the well-known constellation Ursa Minor, which includes the famous star pattern known as the Little Dipper.

Hertzsprung-Russell diagram - Star Classification
This is an example of a Hertzsprung-Russell diagram; a plot of luminosity versus spectral class for a group of stars. The diagonal band labelled "main sequence" is where dwarf stars such as the Sun spend most of their active lifespan. Red giants and supergiants are evolved stars with a mass greater than a red dwarf, that are burning elements heavier than hydrogen. However, White Dwarfs are quite dense, non-luminous, but are still less in mass than supergiants. Once this supply of fuel is exhausted, these stars will migrate to the lower left on this diagram, becoming white dwarfs. / Image Credit: R.J. Hall

Bottom Line

Thanks to Payne and Cannon’s work, we learned that stars were made out of mostly hydrogen and helium, and not out of heavier elements like Earth is. Cecilia Payne’s work would have been impossible without Annie Jump Cannon’s data; Cannon herself was responsible for Star Classification, by hand, more stars in a lifetime than anyone else: around 350,000.

She could classify a single star, fully, in approximately 20 seconds, and used a magnifying glass for the majority of the (faint) stars. Her legacy is now nearly 100 years old: on May 9, 1922, the International Astronomical Union formally adopted Annie Jump Cannon’s stellar classification system. With only minor changes having been made in the 94 years since it is still the primary system in use today.

This is not meant to be a formal definition of Star Classification, like most terms we define on but is rather an informal word summary that hopefully touches upon the key aspects of the meaning and usage of Star Classification term that will help our readers to expand their word mastery.
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