Back To CourseBasics of Astronomy
28 chapters | 325 lessons
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To find the size of a shoe, you look at the label on the box or, in the worst case scenario, you take a ruler and just measure it. To find the circumference of your waist, you can use a waist tape measure. But finding the size of a star is hard to do. Yes, it's because they're far away, and we can't reach them directly. Yes, we don't have a ruler big enough to measure them. Yes, we'd be burned alive if we even tried to get too close even if we had a big enough ruler.
But there's another problem: no matter what telescope you use, stars look like a point of light, making it difficult to figure out their true size. This is why astronomers figured out a workaround for this problem, a workaround using a very famous, very important diagram, a workaround that can tell us the size of a distant star. This workaround includes things we'll cover in this lesson, including something called the H-R diagram, temperature, and luminosity.
To find out the size of a star, you first need to know its temperature and luminosity. Luminosity refers to the total energy a star radiates in one second. In simple terms, it is a star's intrinsic brightness. A separate lesson on luminosity and brightness exists for more information on this.
In the 20th Century, two astronomers independently discovered a very basic relationship between the temperatures and luminosities of stars. These astronomers were Henry N. Russell of the U.S. and Ejnar Hertzsprung of Denmark. Their work came to be known as the Hertzsprung-Russell (H-R) diagram. As you can tell below, it's a diagram that plots a star's luminosity vs. surface temperature. Carefully note how the temperature gets hotter as we move left along the horizontal axis and luminosity gets larger as we move up the vertical, or y-, axis. By the way, the luminosity of a star is measured relative to the sun.
Looking at the diagram, you can tell what the H-R diagram plots out and what important relationship was discovered: a star with a high temperature and luminosity is larger in size. A star with lower temperature and lower luminosity is smaller in size. To understand why this is the case, you must understand that surface area and temperature affect luminosity.
Think of a candle flame burning a couple of feet away from you. A small candle flame has a pretty small surface area. If you touch your finger to the flame, it would burn you because it's hot. But because the flame is small, it can't radiate a lot of heat outwards, towards you. Remember: luminosity is the total energy a star radiates in one second. This means you can comfortably sit there and not worry about the small flame making it uncomfortable for you to sit near it.
Now, think of another candle flame as hot to the touch as the small one but 20 feet tall! Its surface area is humongous, so huge that its luminosity would make it extremely uncomfortable for you to do anything but run away from the flame.
When several thousand stars are chosen at random and plotted on the H-R diagram, they will fall into specific regions. Such patterning shows us that there is a true connection between a star's temperature and luminosity. Had there been no meaningful connection between temperature and luminosity, our randomly chosen stars should have been scattered all over the graph in a random, rather than specific, fashion. But they weren't and they aren't. So there.
Approximately 90% of stars lie on a band that is called the main sequence. A star whose characteristics place it in the main sequence is also called a main-sequence star. Stars in the sequence include our sun and red dwarfs. The majority of the remaining ten percent of stars are found elsewhere. One percent are found in the upper right region of the H-R diagram, representing cool, bright giants and supergiants. Conversely, nine percent can be found in the lower left corner, where they are hot white dwarfs with low luminosity.
Why does this grouping occur? Well, in the upper right hand side, the stars are cool and luminous at the same time. A cool star will radiate a lot less light per unit of surface area than a hot star. Thus, in order for these cool stars to be so luminous, to make up for that coolness and yet still be very luminous, they have to be huge like a huge candle flame. So we call them giants. The largest members of this class of stars are called supergiants, and they have a diameter about 1,000 times that of the sun!
The cooler members of this group of stars are known as red giants since they have a reddish color to them. To understand why they're red and not something like purple, check out the lesson on how light helps us determine the temperature of a star. Overall, the giant stars have temperatures ranging from 3,000 to 6,000 kelvin (K), compared to our sun's surface temperature of 5,800 K. But giants are about 10 to 100 times larger than our sun. They are also about 100 to 1,000 times more luminous than the sun.
Now on the flipside, we have hot stars in the lower left corner of the H-R diagram. Even though they are hot, their luminosities are low. The only way for this to occur is for them to be small. Therefore, they are called white dwarfs. How small are they? About the same size as our very planet! That's pretty small compared to something relatively gigantic as our sun. They are so dim that we cannot see them without a telescope, unlike some gigantic stars that can be easily seen with the naked eye. White dwarfs are the dying remnants of giant stars.
To find out the size of a star, you first need to know its temperature and luminosity. Luminosity refers to the total energy a star radiates in one second. Luminosity and temperature are used on the Hertzsprung-Russell (H-R) diagram. Again, it's a diagram that plots a star's luminosity vs. surface temperature.
Approximately 90% of stars lie on a band on the H-R diagram that is called the main sequence. A star whose characteristics place it in the main sequence is called a main-sequence star. Remember that surface area and temperature affect luminosity. This will help you understand the positioning of specific sizes of stars on the H-R diagram.
Cool stars like red giants have to be huge in order to be luminous. This is because a cool star radiates a lot less light per unit of surface area than a hot star. On the other hand, small hot stars, like white dwarfs, have very low luminosities despite their high temperatures precisely because they are so small. Therefore, a star's temperature and luminosity help us predict its size.
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons