H-R Diagram because that StarsA Most crucial DiagramClassifying starsaccording to your spectrum is a very powerful means to start to understandhow they work. Together we stated last time, the spectral succession O, B,A, F, G, K, M is a temperature sequence, through the sexy stars being oftype O (surface temperature 30,000-40,000 K), and also the coolest stars beingof form M (surface temperatures about 3,000 K). Since hot starsare blue, and cool stars space red, the temperaturesequence is also a shade sequence. The is occasionally helpful,though, to classify objects according to two various properties.Let"s to speak we try to share stars according to their obvious brightness,also. We can make a plot with color on one axis, and also apparent brightnesson the other axis, choose this:Figure 1: H-R chart of evident brightness versus star color (ortemperature). You can see that thisclassification plan is not valuable -- the stars space randomly scatteredon the plot.Obviously, plot apparentbrightness versus color is not helpful, because there space no patternsin the location of the dots representing stars. They are scatteredaround randomly. This is due to the fact that the starsare in ~ all various distances, for this reason the surrounding ones show up brighteven though they may be intrinsically not so bright.But what if us look in ~ thissame plot, however somehow make sure that the stars space all at the exact same distance.You know that stars sometimes appear in clusters (because they were allformed out of the same giant cloud, components of which fell down to form alot that stars all roughly the very same time). Right here is a picture of thePleiades star cluster:Figure 2.If us plot the obvious brightnessversus shade for such a cluster, where all the stars room the exact same distance,you acquire a plot favor this:Figure 3.Now you can see the thepoints representing the stars fall along a clear line in the plot.Such a plot was an initial made by two astronomers working independently: EjnarHertzsprung (Denmark) and Henry Norris Russell (Princeton, USA).This kind of diagram was named after them, as the Hertzsprung-RussellDiagram, or H-R Diagram.It is an extremely powerful diagram because that classifying stars and also understandinghow stars work. We room going to spend the rest of this lecture lookingin information at this diagram. First, though, keep in mind the connection betweenapparent brightness and also absolute brightness that us talked around last time.We stated that astronomers usage absolute brightness, which is the apparentbrightness stars would have actually if they were all at the very same distance the 10parsecs. The diagram over uses obvious brightness (apparent magnitudes),but for stars all at the same distance (the distance to the Pleiades starcluster), so the is really a plot of absolute brightness versus color.Or we can plot luminosity matches color, as below:Figure 4. As soon as we know the distances to stars, we have the right to determine theirabsolute brightness, or luminosity.When us then plot luminosity (or pure brightness) matches color(or temperature), the stars allfall along a narrow piece in the diagram. This is the H-R Diagram.So the right method to thinkabout one H-R Diagram. That is telling us that a star"s color (or temperature)and that luminosity space related. Blue stars are more luminous thanred stars. To uncover this out, though, wehave to understand the distances to the stars. Mental thestar directory we proved one page of in the critical lecture, native the NearbyStars catalog. We know the distances to these stars, through measuringtheir parallax. Right here is the H-R chart for that catalog:Figure 5.Now we watch that over there isa brand-new region in the lower left, which exchange mail to faint-blue stars.If blue stars space so luminous, why are these so faint? this arefaint due to the fact that they are really small! They room a class of stars calledWhiteDwarf stars. We can also look in ~ the H-R diagram because that otherclusters. Right here is one because that an old cluster of stars, M3, i m sorry is aglobular cluster:
 Figure 6 a.You are watching: What type of star has a high temperature but a low luminosity Figure 6 b.
Now we watch a newregion the luminous red stars in the upper-right! If red stars arefainter than blue stars, why space these red stars therefore luminous? Itis because they are giant stars, choose the star Betelgeuse, which i mentionedlast time is so large that, if it were at the distance of the Sun, the wouldengulf the Earth"s orbit, and also even the orbit that Mars. These room theRedGiant stars.Patterns in the H-R DiagramWe view that theH-R chart can help us classify various kinds that stars, according tothe pattern of where the stars loss in the diagram. The diagonalline that we experienced for the Pleiades star swarm represents what us would speak to normalstars. The White Dwarfs and Red Giants are different classes that starsthat the H-R diagram helps us to identify. So the H-R diagram cantell united state something around the dimension (radius) of the stars. The factthat the H-R diagrams for the adjacent stars, the Pleiades star cluster,and the M3 star cluster are all various leads united state to watch for other differencesin these teams of stars that could explain it. It turns out thatthe difference is the period of the stars.The H-R diagram is walking to assist us learn something around how stars changeas they obtain older. So you can currently see the this is a really powerfuldiagram indeed. Let"s take a look in ~ theoverall H-R diagram, consisting of all the different species of stars that weknow of.

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