Answer: Classifying stars according to their spectrum is a very powerful way to begin to understand how they work. As we said last time, the spectral sequence O, B, A, F, G, K, M is a temperature sequence, with the hottest stars being of type O (surface temperatures 30,000-40,000 K), and the coolest stars being of type M (surface temperatures around 3,000 K). Because hot stars are blue, and cool stars are red, the temperature sequence is also a color sequence. It is sometimes helpful, though, to classify objects according to two different properties. Let's say we try to classify stars according to their apparent brightness, also. We could make a plot with color on one axis, and apparent brightness on the other axis, like this:
Figure 1: H-R Diagram of apparent brightness versus star color (or temperature). You can see that this
classification scheme is not helpful -- the stars are randomly scattered on the plot.
Obviously, plotting apparent brightness against color is not helpful, because there are no patterns in the placement of the dots representing stars. They are scattered around randomly. This is because the stars are at all different distances, so the nearby ones appear bright even though they may be intrinsically not so bright.
But what if we look at this same plot, but somehow make sure that the stars are all at the same distance. You know that stars sometimes appear in clusters (because they were all formed out of the same giant cloud, parts of which collapsed to form a lot of stars all around the same time). Here is a photograph of the Pleiades star cluster:
Figure 2.
If we plot the apparent brightness versus color for such a cluster, where all the stars are the same distance, you get a plot like this:
Figure 3.
Now you can see that the points representing the stars fall along a clear line in the plot. Such a plot was first made by two astronomers working independently: Ejnar Hertzsprung (Denmark) and Henry Norris Russell (Princeton, USA). This kind of diagram was named after them, as the Hertzsprung-Russell Diagram, or H-R Diagram. It is an extremely powerful diagram for classifying stars and understanding how stars work. We are going to spend the rest of this lecture looking in detail at this diagram. First, though, note the relationship between apparent brightness and absolute brightness that we talked about last time. We said that astronomers use absolute brightness, which is the apparent brightness stars would have if they were all at the same distance of 10 parsecs. The diagram above uses apparent brightness (apparent magnitudes), but for stars all at the same distance (the distance to the Pleiades star cluster), so it is really a plot of absolute brightness versus color. Or we could plot luminosity versus color, as below:
Figure 4. When we know the distances to stars, we can determine their absolute brightness, or luminosity.
When we then plot luminosity (or absolute brightness) versus color (or temperature), the stars all
fall along a narrow strip in the diagram. This is the H-R Diagram.
So the right way to think about an H-R Diagram. It is telling us that a star's color (or temperature) and its luminosity are related. Blue stars are more luminous than red stars. To find this out, though, we have to know the distances to the stars. Remember the star catalog we showed one page of in the last lecture, from the Nearby Stars catalog. We know the distances to these stars, by measuring their parallax. Here is the H-R diagram for that catalog:
Answers & Comments
Answer:
These are the brightest stars. Stars like Sirius, Canopus, and others are first magnitude stars.
-Hope I helped
Answer: Classifying stars according to their spectrum is a very powerful way to begin to understand how they work. As we said last time, the spectral sequence O, B, A, F, G, K, M is a temperature sequence, with the hottest stars being of type O (surface temperatures 30,000-40,000 K), and the coolest stars being of type M (surface temperatures around 3,000 K). Because hot stars are blue, and cool stars are red, the temperature sequence is also a color sequence. It is sometimes helpful, though, to classify objects according to two different properties. Let's say we try to classify stars according to their apparent brightness, also. We could make a plot with color on one axis, and apparent brightness on the other axis, like this:
Figure 1: H-R Diagram of apparent brightness versus star color (or temperature). You can see that this
classification scheme is not helpful -- the stars are randomly scattered on the plot.
Obviously, plotting apparent brightness against color is not helpful, because there are no patterns in the placement of the dots representing stars. They are scattered around randomly. This is because the stars are at all different distances, so the nearby ones appear bright even though they may be intrinsically not so bright.
But what if we look at this same plot, but somehow make sure that the stars are all at the same distance. You know that stars sometimes appear in clusters (because they were all formed out of the same giant cloud, parts of which collapsed to form a lot of stars all around the same time). Here is a photograph of the Pleiades star cluster:
Figure 2.
If we plot the apparent brightness versus color for such a cluster, where all the stars are the same distance, you get a plot like this:
Figure 3.
Now you can see that the points representing the stars fall along a clear line in the plot. Such a plot was first made by two astronomers working independently: Ejnar Hertzsprung (Denmark) and Henry Norris Russell (Princeton, USA). This kind of diagram was named after them, as the Hertzsprung-Russell Diagram, or H-R Diagram. It is an extremely powerful diagram for classifying stars and understanding how stars work. We are going to spend the rest of this lecture looking in detail at this diagram. First, though, note the relationship between apparent brightness and absolute brightness that we talked about last time. We said that astronomers use absolute brightness, which is the apparent brightness stars would have if they were all at the same distance of 10 parsecs. The diagram above uses apparent brightness (apparent magnitudes), but for stars all at the same distance (the distance to the Pleiades star cluster), so it is really a plot of absolute brightness versus color. Or we could plot luminosity versus color, as below:
Figure 4. When we know the distances to stars, we can determine their absolute brightness, or luminosity.
When we then plot luminosity (or absolute brightness) versus color (or temperature), the stars all
fall along a narrow strip in the diagram. This is the H-R Diagram.
So the right way to think about an H-R Diagram. It is telling us that a star's color (or temperature) and its luminosity are related. Blue stars are more luminous than red stars. To find this out, though, we have to know the distances to the stars. Remember the star catalog we showed one page of in the last lecture, from the Nearby Stars catalog. We know the distances to these stars, by measuring their parallax. Here is the H-R diagram for that catalog:
Explanation: