Star Classification
To understand the longevity of a star’s life cycle, it is important that we be able to classify it. We can achieve this by using the spectral classes on the Hertzsprung Russell Diagram (a plot of stars’ luminosity against its temperature) into the categories from hottest (highest mass, shortest life) to coolest (lowest mass, longest life) span based on their spectral classification type: O, B, A, F, G, K, M.114Originally, the stars were classed by just using alphabetical order.115 These letters came as astronomers were able to study the stars more closely and were able to note details and important information about them, which then led to the narrowing of these specific letters shown.116 We know now that the temperature scale runs hottest to coolest from O to M.117
On the HR diagram, stars can be charted by multiple categories, these categories include Color, Temperature since color is related to temperature. Stars are classified by Spectral type as stated above, that gives an idea of their temperature and luminosity.118 Each of these properties of a star, help astronomers to determine the longevity of its cycle span, and how the star will behave over time.119 These stars are also charted and classed, based on how large they are: super giants, giants, main-sequence stars and white dwarfs.120 In addition, there are different types of stars in different age stages we will learn about such as: dwarfs and red giants.121 During this curriculum unit, students will engage in stem learning, by creating a concept of their own star and charting it where it would go on the Hertzsprung-Russel diagram if it were to become a real star. We will keep a classroom map of these charted stars during our planning process.
Star Color and Temperature
Figure 2: The Colors of the Stars; Photo credit: nasa.gov
Star Colors are listed and generally span from shades of red, orange, yellow, white and blue. It is in this order that the stars actually move from coolest to hottest as well; cooler stars being red and orange K and M types, while the O and B type stars that are blue are the hottest.122 Therefore, there is a clear indication that temperature and color in terms of stars, are related and we can determine the temperature by its color.123 The core of a star has a much higher temperature than its surface layer.124 We typically see as only small white lights in the sky when we look at the stars, when in reality, these small dots of what look like white light are actually a variety of colors that our human eye can’t see from far away.
It is also possible to measure the temperature of a star, through the colors of the rainbow. 125 This is accomplished through a spectrum, which is the rainbow of all the light that a star gives.126 This is organized in a specific way. According to the Lives of Stars by Croswell, “…they are organized by order of wavelength: purple light has the shortest wavelength, then blue light, then green, then yellow, then orange and finally red, which has the longest…”.127 These colors, allow astronomers to determine the star’s temperature.128 These wavelengths of spectrum color classification, measured and charted. These spectra of stars are recorded with the star classification types: O, B, A, F, G, K, M.129
Star Mass
Mass matters when classifying stars, as it generally determines the other characteristics of the star, and points to the longevity the star will have.130 A star mass is how much material the star has within it.131 It is not an easy thing to determine the mass of a stellar object, but it is possible.132 Many stars are binary stars two stars that orbit each other.133 “About half of all stars are binary stars.” Within these binary stars, the star with the greater mass, can give mass to the star with less mass.134 Stars also have a range of mass. “Stellar masses range from about 1/12 to more than 100 times the mass of the sun”.135 Stars that have more mass than the Sun do exist, just as stars that have less mass than the Sun exist.136 Stars that have a very small amount of mass, compared to the Sun, are considered brown dwarfs.137 Brown dwarfs are stars which so low a mass that they do not reach temperatures high enough for hydrogen fusion to take place. There is a relationship between luminosity and mass as well, as it is the case that high mass stars are also very bright.138
Luminosity vs Apparent Brightness
The categories of Luminosity and Apparent Brightness are charted on the Hertzsprung-Russell Diagram that was also referenced above.139 Studies have shown that there are two type A stars, one type F star, seven type G stars, seventeen type K stars, ninety-four type M stars, eight white dwarfs, and thirty-brown dwarf stars that are in the neighborhood of our Sun.140 There are many more stars that can be seen with the naked eye, but they are farther away.141 According to OpenStax Astronomy, “Luminosity is the rate at which a star or other object emits electromagnetic energy into space; the total power output of an object”.142 This means that luminosity is how bright the star actually is.143 Apparent Brightness on the other hand, is how bright the star appears to be.144 Just as a car’s headlight appears bright when it is close, but dim that is far away, a star of a given luminosity will appear brighter if it is closer to us, but dimmer if it is far away. Many of the stars around us that are closest are actually low in luminosity.145 Some of these stars have low temperatures, and not as bright, therefore there may be more we have not seen yet that are close to Earth.146 That is to say, that very few stars can actually be seen with the human eye without a telescope unless they have the right distance and/or luminosity.147 Again, the luminosity of a star, is not dependent on how close the star is, but by energy it emits.148 Both distance from us and a star’s luminosity explain why some stars appear most illuminated to us.149 According to luminosity data, “The most luminous of the bright stars listed emit more than 50,000 times more energy than the sun”.150
Location, Distance and Census of Stars
In addition to thinking about the life cycle of stars and their characteristics, it is important to consider how we see the stars and chart them. Technology has advanced so much that we know have ways of charting new star births and charting this process for our solar system. Scientists chart stars in a census, and use of measurements taken when the Earth is in different parts of its orbit to determine how far away stars are since their distance is partially related to in which direction they appear to be when.151 The unit used to measure how distances to stars is the “lightyear”, which is the distance travelled by light in one year.152 According to OpenStax Astronomy, “The closest star to us, is more than 4 light years away”.153 This, of course, is a star other than our Sun. Stars can appear to shift directions as astronomers observe them in relation to the Sun.154 This is called parallax; which is what makes a star look like it has shifted directions in relation to stars that are very distance because the Earth is in a different part of its orbit.155 As we use the Hertzsprung-Russell diagram, and determine the spectral classes of stars, we can also use the topic of luminosity discussed above, to determine the closeness of stars to us and to our Sun.156 This luminosity as discussed above, is a result of the energy produced inside the star due to nuclear fusion and has no direct correlation of luminosity with distance ¾ some nearby stars are very luminous, as are some stars that are very far away. For example, Vega, Sirius, Altair, Alpha Centauri, Fomalhaut and Procyon are the only extremely bright stars that are close in proximity to earth, although there are more of these visible stars, we can see just as brightly.157 This information about luminosity and distance can help us further classify stars and understand what we are seeing or not seeing in the night sky around us.
Star Sounds
There are some things that we will learn in life, that almost never become fathomable; things that are so hard to believe, even with evidence. An example of one of these things is the fact that stars make sounds! In fact, we can listen to the sounds that stars make, and use this as a classification tool.158 The actual term for listening to and analyzing star sounds is Asteroseismology, i.e., the seismic study of stars.159 Just as geophysicists use seismic waves on Earth to study what is happening inside the Earth and earthquakes, astronomers use seismic data of the Sun and other stars to study what is going on side those objects. These star sounds give astronomers a great deal of information such as: size, and mass, and can also be used to monitor internal changes in the stars.160 This information is yet another amazing way we can study the stars around us.
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