With a casual glance at the night sky, all stars may be similar. They all appear to be a similar color but with varying brightness. However, scientists have developed a complex and intricate classification system to classify stars. In this article, we will explore how do scientists classify stars.
The only information we receive directly from stars is their light. This light contains much information we can decode to determine certain star properties. In a way, stars broadcast details about themselves. Scientists have developed techniques to decipher these messages.
While visually stars appear similar colors imaging them displays a high degree of color variation.
The scientific classification of stars is based on the star’s surface temperature and its brightness (scientists call this luminosity). These properties depend only on the star’s mass and, to a lesser extent, the star’s position in its lifeline.
In this article, we will describe the classification schemes scientists use for stars.
How do Scientists Classify Stars?
Scientists use the Hertzsprung – Russell (H-R) Diagram to classify stars. On the diagram, star luminosities are placed on the Y-axis (i.e., the vertical axis) and spectral type on the X-axis. Broadly speaking, the spectral type is dependent on surface temperature. The luminosity increases in the upward direction. Unlike convention, the surface temperature decreases from left to right.
The Hertzsprung – Russell Diagram is based on the work of Hans Oswald Rosenberg and Henry Norris Russell in the early 1900s (Hertzsprung–Russell diagram). However, before delving into the diagram we need to look at how a star’s spectral class and luminosity are determined. A star’s spectral class is a function of its surface temperature.
How to Determine Star Surface Temperature
An important part of the H-R diagram is that it is based on surface temperature. As we can’t measure it with a thermometer, how do we do this?
As stated earlier, light from stars contains very important information. It turns out that the color of a star is directly related to its surface temperature. Stars with cool surfaces are red, while hot stars are blue. Scientists call the range of light that is emitted its spectrum. A star’s spectrum comprises a range of colors (or, more accurately wavelengths). If you wish to delve into the reason why this is so, see Blackbody Radiation.
The diagram below illustrates the relationship between a star’s surface temperature and the light it emits. The visible part of the spectrum is also shown. The left graph shows the spectrum emitted by a cool star (black line). Cool stars emit most of their light to the right of the visible range (i.e. in the infrared part of the spectrum). On the right is the graph from a hot star. The peak of its spectrum is in the ultraviolet part of the spectrum. As a result, it will have a blue light. As a comparison, the Sun’s spectrum is in the center, and its peak is in the visible part of the spectrum.
Therefore, to determine the surface temperature of stars, scientists measure their color. This is achieved by passing the star’s light through filters. For example, we could use a red filter and a blue filter. A cool star would be a lot brighter when viewed through the red filter than the blue filter. The converse is true for a hot blue star.
Also of note in the diagrams is that stars with a higher surface temperature emit light at a much higher intensity over a wider range of frequencies than cooler stars.
UBV Photometry
For more accurate results, scientists use more than two filters. A common set of filters scientists use are called ultraviolet, blue and visual. The process of using these filters in unison is called UBV Photometry. Obviously, U represents ultraviolet, B blue and V for visual.
The ultraviolet filter allows light in the ultraviolet part of the spectrum to pass while blocking other parts of the spectrum. It follows that the other two filters perform the same function for the respective part of the spectrum. The blue filter allows light at and near blue to pass while th visual filter peaks in the middle part of the visible light range.
Using the graphs in the previous section, we can see that the star on the left will be very bright in the visual filter, less bright in the blue filter, and very dim (if visible at all) in the ultraviolet filter. Conversely, the star on the right will be very bright in the ultraviolet filter, dimmer in the blue filter and dimmer again in the visual filter.
While the process described is only qualitative, scientists quantify star surface temperatures using ratios. For example, by using ratios of the brightness of a star in the visual filter (bV) to that using the blue filter (bB) and the blue filter to Ultraviolet filter (bU) the surface temperature can be determined. For example, the Sun’s bV/bB is 1.87 and bB/bU is 1.17. This data indicates that the Sun’s surface temperature is 5800 K and is a yellow star.
So we now know of one technique to determine the surface temperatures of stars. However, there are confirming techniques.
Star Spectral Classification
While a star’s spectrum can tell us a lot about the star, what the light doesn’t contain also tells us a story. Light emitted from a star is what is called a continuous spectrum. This means that there are no gaps in the spectrum. However, on close inspection, there are gaps. So what is this about?
A star’s light is emitted from a hot region of the star. This is called a photosphere, an excited gas in a state called plasma. Beyond the photosphere, the light can travel more or less unhindered away from the star. However, it must pass through cooler gas in the outer regions of the star. This gas leaves a fingerprint on the light.
As light travels through a cool gas it will encounter gas atoms. If the energy of the light is right, atoms can absorb the light. Different types of atoms will absorb different frequencies of light. For example, hydrogen absorbs different wavelengths than does helium.
Once an atom absorbs light, it enters an excited state. At a later time, it may emit light to regain its unexcited state. The re-emitted light may not be at the same frequency the atom absorbed. If all of the light at a specific frequency is absorbed in this fashion, the spectrum will have a gap at that frequency. These gaps are called absorption lines.
As different types of atoms absorb light at different wavelengths absorption lines can be used to identify elements existing in a star’s atmosphere and its temperature. This is the basis of spectral classification.
Spectral Classification Scheme
Spectral classification is done by studying the absorption lines in star spectra. The diagram below shows the spectra of selected star classifications. The key is to look at the appearance and disappearance of the various absorption lines (dark areas within the spectra).
Using spectral classification stars are classified by the spectrum into seven groups designated by letters of the alphabet. These are O, B, A, F, G, K and M. Class O stars are the largest and hottest stars, while M are the cool and small stars. Each of these is further divided into ten sub-categories, for example, G0, G1….G9.
OK, now that we have an understanding of how the surface temperature is determined, let’s move on to luminosity.
Absolute Luminosity
Stellar luminosity is a measure of the amount of energy stars emit. This is a function of the surface temperature and size of the star.
While it is beyond the scope of this post, it is important to understand absolute luminosity. When you look at the night sky, you will see that some stars are brighter than others. We can not use this observation to determine absolute brightness because some stars are much farther from us than others. What we see is the star’s apparent brightness. What we want to know is how bright they are.
To determine the absolute brightness of a star, we must know how far it is from us. On a superficial level, this is a simple process. We must first measure its apparent luminosity and determine how far it is. We then use a mathematical relationship: M = m – 5(log10(d/10)). In this expression, M is the absolute magnitude, m is the apparent magnitude, and d is the distance in parsecs.
That is as far as we are going to go with this. If you want a deeper understanding, see Absolute Magnitude.
Stellar Luminosity Classes
Scientists also classify stars into luminosity classes. Luminosity classes are based on differences in spectral lines. This classification is useful to separate stars that are at different points in their lives.
Most stars are classified as main sequence stars. They are steadily converting hydrogen to helium and are at equilibrium. This is the state where stars spend most of their lives. These stars belong in the V luminosity class.
Five other luminosity classes are called Ia, Ib, II, III and IV. These are classifications in which stars enter towards the end of their lives.
The Sun is classified into the G spectral classification (and into the 2 sub-classification) and is in the main sequence. In short-hand it is a G2V star.
Why this is important will become apparent in the next section on the H-R diagram.
Hertzsprung – Russell (H-R) Diagram
As already stated, the Hertzsprung – Russell (H-R) Diagram is a plot of stellar magnitude and their spectral type. The spectral type is analogous to surface temperature. Below is a H-R diagram on the left and on the right a representation of the surface temperatures and classifications. The relationship between luminosity and surface temperature indicates how big stars are. This is shown on the diagram on the right.
As stated earlier, stable stars plot along the main sequence line. Along this line, the hot and bright stars are found in the upper left and the cooler, dimmer stars are on the bottom right.
As large stars come to the end of their lives, they expand, and as a result, their surface temperature decreases. In this situation they occupy the upper right of the H-R diagram. This is where the stellar luminosity classification is helpful. As can be seen, a spectral type can describe the state of a star of two completely different luminosities. Knowing the luminosity classification the star’s absolute luminosity can be determined. The luminosity classes of Ia, Ib, II, III and IV are found in this part of the diagram.
Of note is that only the stars on the main sequence and upper right are placed into luminosity classes. The stars on the lower left (i.e. marked as white dwarfs on the right diagram) are not classified under that scheme. White dwarfs are the corpses of smaller stars. As such, they do not have sustained nuclear fusion, so are not technically stars.
Final Thoughts
As we have seen, scientists classify stars in several ways. They use the light emitted from stars to determine many of their characteristics. From the analysis of the light, we can determine a star’s surface temperature and luminosity. We can also determine what elements are contained in the star’s atmosphere.
A star’s surface temperature is determined by the color of the emitted light and gaps in its continuous spectrum.
A simple equation can determine the absolute luminosity of a star if the distance to the star is known.
By determining the surface temperature of a star and its luminosity, we can place it onto a Hertzsprung – Russell (H-R) Diagram.
While not discussed on this page, scientists can explain the nature of stars and describe stellar evolution using a H-R diagram.
We hope that you have found this article interesting and have gained some new knowledge.
We have discussed the emission of light from stars but have not mentioned where the energy comes from. If the source of the energy interests you then you may be interested in our article called How Do Stars Shine?
I found astronomy while working in dark rural locations. Initially, I explored the night sky and learnt the constellations before purchasing a pair of binoculars to further my knowledge of the sky.
My first telescope was a 200 mm Newtonian reflector on an equatorial mount. I found that this telescope had a steep learning curve but was a rewarding experience.
As time progressed, I became interested in astrophotography. This resulted in purchasing a 110 mm refracting telescope and a dedicated monochrome-cooled astronomical camera. This resulted in another very rewarding steep learning curve that far surpassed the experience with my first telescope.
I have joined Telescope Guru to share my knowledge of telescopes and astronomy.
