HR-DiagramNear & Bright StarsTypes of Stars Binary Stars DistancesHome PagePROPERTIES OF STARS
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Q1. How can we tell that red giant stars are very large stars, from just their position in the HR diagram? Answer
Q2. Describe the observed properties of stars, as described by the HR diagram. What are the three general types of stars identified in the diagram? What are the ranges of properties they have in the diagram? Answer
Q3. What are the three main groups of stars identified in the HR diagram? How do we know that some stars must be very large, just from their place in the HR diagram? Answer
Q4. What effect does distance have on the position of a star in an HR diagram plotted with apparent brightness and color? How can distance be measured for nearby stars? Answer
Q5. From purely observational grounds how do we know that a massive star will have a short lifetime? Answer
Q6. From their position in the HR diagram, how can we be so sure that red giant stars are truly giant stars? Answer
Q7. Describe the appearance and properties of main sequence stars in the HR diagram. Answer
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Q1. What are the ranges of properties of stars, for radius, surface temperature, brightness, and mass? Answer
Q2. What is the distinction between apparent brightness and true brightness? Which is more useful for studying the properties of stars, and why? Answer
Q3. Describe the observable properties of most common type of star, in comparison to the sun. Answer
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Q1. Compare the distribution of the nearby stars in an HR diagram to the distribution of the apparently bright stars. What is the significance of the difference between these two groups of stars? Answer
Q2. What is the most common type of star in our galaxy? How is this determined? Answer
Q3. What is the most common type of star in our galaxy? Why don’t we see very many of them in our night-time sky? Answer
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Q1. What do we learn about the properties of main sequence stars from observations of binary stars? Answer
Q2. What is a binary star? What are the three ways of observing the properties of the orbits of binary stars? What is the most important thing learned from such studies? Answer
Q3. Why is the study of binary stars important for the understanding of one of the basic properties of stars? What have we learned about stars from such observations? Answer
Q4. In what way does the analysis of binary star orbits lead to a determination of the mass of the stars? Answer
Q5. What is the distribution of star masses along the main sequence? Answer
HR-Diagram Answers
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A1. If a red giant star is compared to a star of the same color on the main sequence, both stars will have the same temperature but the red giant will be vastly brighter. Since both stars emit the same amount of energy from each square foot of their surface, the red giant must have a much larger surface than the main sequence star.
A2. The observed properties of stars in an HR diagram are brightness (either true if random stars are plotted or apparent if only stars in one group at a fixed distance are plotted) and surface temperature or color. True brightnesses of stars range from a million times brighter than the sun to a million times dimmer than the sun. Surface temperatures range from about 2,500 K up to more than 30,000 K. Stars in the HR diagram are identified as either main sequence stars (the majority), red giants, or white dwarfs.
A3. The three types of stars are main sequence, red giant, and white dwarf stars. We can tell that red giants are very large, compared to main sequence stars of the same color because both stars must emit the same amount of energy from each square foot of their surfaces, since they have the same color and surface temperature, and yet the giant is many millions of times brighter than the main sequence star. The only way this could occur is for the giant to have a much larger surface than the main sequence star.
A4. A more distant star will appear fainter to us than a comparable nearby star. That will place it lower in the HR diagram. Its color, on the other hand, is unaffected by its distance. The distance to nearby stars is measure by the parallax method. When the star is observed at six month intervals, its position in the sky is slightly different due to Earth’s motion around the sun. This shift in position depends upon the distance to the star.
A5. Massive stars are very bright stars, at the top of the main sequence in the HR diagram. They are consuming their nuclear fuel at a much faster rate than less massive stars because of these extremely high brightness. Thus, even though they have more fuel than less massive stars, they use it so fast that they will run out much sooner.
A6. Let’s compare a red giant star to a star of the same color on the main sequence. Both stars have the same color; therefore they must have the same surface temperature. That means that each square inch of surface emits the same amount of energy on both stars. Yet the red giant star is millions of times brighter than the red main sequence star. That can only occur if the red giant star has millions of times the surface area – it must be a big star.
A7. In the HR diagram, the main sequence stretches from the lower right (cool and dim stars) to the upper left (hot and bright stars) in a diagonal band across the diagram.
Types of Stars Answers
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A1. Surface temperatures of stars range from about 2500K up to roughly 30000K. Their true brightnesses can be a million times less than the sun or a million times greater than the sun. The mass of stars ranges from as little as 8 % of the sun up to about 60 times the mass of the sun. Radii can be as little as 1 % of the sun’s radius (roughly the size of Earth) and as large as a thousand times the size of the sun.
A2. Apparent brightness is a measure of how bright the star is in our nighttime sky. The true brightness measure how much light is actually emitted by the star. The difference depends upon how far away the star is from us. For example a fairly bright star (in true brightness) may appear rather dim to us if it is very far away. If we want to understand the properties of stars, we must first determine their true brightnesses so we can study the amount of energy they produce.
A3. The most common type of star in our galaxy is a cool, dim star at the bottom of the main sequence. These stars are smaller than the sun, have less mass, are cooler than the sun, and much dimmer.
Near & Bright Stars Answers
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A1. The nearby stars are all relatively faint, usually fainter than the sun and most on the bottom of the main sequence. In contrast, the apparently bright stars in the sky are in the top half of the HR diagram, either at the top of the main sequence or in the red giant region. The apparently bright stars are not typical of stars in general, but shine brightly in our sky only because of their great brightness. The nearby stars represent a more typical distribution of stars.
A2. The most common type of star in our galaxy is a cool main sequence star at the bottom of the main sequence. To reach this conclusion, we must observe a random population of stars that is large enough to avoid bias from special groups of stars. We select to observe a complete sample of stars surrounding the sun.
A3. The most common type of star in our neighborhood is a cool dim star at the bottom of the main sequence. We do not see many of these stars because they are very dim in our sky even when they are very close to us. That makes them hard to notice against the background of brighter but more distant stars.
Binary Stars Answers
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A1. The study of binary stars allows us to determine the mass of the stars in the system. For main sequence stars, we find that low mass stars are at the bottom of the main sequence, with progressively higher masses at systematically higher positions on the main sequence.
A2. A binary star is actually two (or more) stars which orbit each other. Orbital motion is observed directly for both stars in a visual binary. Only the changing velocity of the stars toward or away from us is seen for a spectroscopic binary. A change of brightness is observed when one star passes directly in front of the other in an eclipsing binary star. In the latter two cases, only a single point of light can be seen for the two stars. The study of binary star orbits allow us to determine the mass of the stars in the binary star system.
A3. Binary stars are important to astronomy because their study presents the only way to directly determine the mass of stars. Since the mass of a star determines most of its other properties, it is important that we are able to measure it directly. A study of binary stars has shown that, in general, the brightness of stars increases with mass. Hence, the position of a star on the main sequence is determined by its mass.
A4. The motion of a star in its orbit around another star depends upon the strength of gravity it experiences from its companion. For example, a stronger force of gravity requires faster motion to maintain a stable orbit. The force of gravity the star experiences depends only upon its distance from its companion and the mass of the companion. Hence, an observation of the orbit – size of orbit and speed of motion – is enough to determine the mass of the companion.
A5. Star mass increases as one looks up the main sequence from the cool dim stars at the bottom (low mass) to the hot bright stars at the top (high mass).