H-R Diagram for StarsA Most Important DiagramClassifying starsaccording to their spectrum is an extremely powerful method to begin to understandexactly how they job-related. As we shelp last time, the spectral sequence O, B,A, F, G, K, M is a temperature sequence, via the hottest stars being ofkind O (surface temperatures 30,000-40,000 K), and the coolest stars beingof type M (surface temperatures approximately 3,000 K). Since hot starsare blue, and also cool stars are red, the temperaturesequence is additionally a shade sequence. It is occasionally helpful,though, to classify objects according to two various properties.Let"s say we try to classify stars according to their evident brightness,likewise. We can make a plot through shade on one axis, and evident brightnesson the various other axis, prefer this:Figure 1: H-R Diagram of apparent brightness versus star shade (ortemperature). You can watch that thisclassification scheme is not beneficial -- the stars are randomly scatteredon the plot.Obviously, plotting apparentbrightness versus shade is not useful, because there are no patternsin the placement of the dots representing stars. They are scatteredroughly randomly. This is bereason the starsare at all different distances, so the nearby ones appear brighteven though they may be intrinsically not so bideal.But what if we look at thisexact same plot, but someexactly how make sure that the stars are all at the same distance.You recognize that stars occasionally appear in clusters (because they were alldeveloped out of the same giant cloud, parts of which collapsed to form alot of stars all roughly the exact same time). Here is a photograph of thePleiades star cluster:Figure 2.If we plot the noticeable brightnessversus shade for such a cluster, where all the stars are the same distance,you acquire a plot choose this:Figure 3.Now you can see that thepoints representing the stars autumn along a clear line in the plot.Such a plot was first made by two astronomers functioning independently: EjnarHertzsprung (Denmark) and also Henry Norris Rusoffer (Princeton, USA).This kind of diagram was named after them, as the Hertzsprung-RussellDiagram, or H-R Diagram.It is a very effective diagram for classifying stars and understandingjust how stars work-related. We are going to spend the remainder of this lecture lookingin information at this diagram. First, though, note the connection betweenevident brightness and also absolute brightness that we talked around last time.We shelp that astronomers use absolute brightness, which is the apparentbrightness stars would have if they were all at the very same distance of 10parsecs. The diagram over provides evident brightness (apparent magnitudes),however for stars all at the same distance (the distance to the Pleiades starcluster), so it is really a plot of absolute brightness versus color.Or we can plot luminosity versus color, as below:Figure 4. When we recognize the distances to stars, we deserve to recognize theirabsolute brightness, or luminosity.When we then plot luminosity (or absolute brightness) versus color(or temperature), the stars allautumn along a narrowhead spilgrimage in the diagram. This is the H-R Diagram.So the right method to thinkabout an H-R Diagram. It is telling us that a star"s color (or temperature)and also its luminosity are associated. Blue stars are even more luminous thanred stars. To discover this out, though, wehave to recognize the distances to the stars. Remember thestar magazine we verified one page of in the last lecture, from the NearbyStars catalog. We recognize the ranges to these stars, by measuringtheir parallax. Here is the H-R diagram for that catalog:Figure 5.Now we watch that tbelow isa new region in the reduced left, which correspond to faint-blue stars.If blue stars are so luminous, why are these so faint? These arefaint because they are very small! They are a class of stars calledWhiteDwarf stars. We have the right to likewise look at the H-R diagram for otherclusters. Here is one for an old cluster of stars, M3, which is aglobular cluster:
Figure 6 a.

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Figure 6 b.
Now we check out a newarea of luminous red stars in the upper-right! If red stars arefainter than blue stars, why are these red stars so luminous? Itis bereason they are giant stars, choose the star Betelgeuse, which I mentionedlast time is so big that, if it were at the distance of the Sun, it wouldengulf the Earth"s orlittle bit, and also also the orbit of Mars. These are theRedGiant stars.Patterns in the H-R DiagramWe check out that theH-R diagram have the right to assist us classify various kinds of stars, according tothe pattern of wbelow the stars fall in the diagram. The diagonalline that we experienced for the Pleiades star cluster represents what we would certainly speak to normalstars. The White Dwarfs and Red Giants are various classes of starsthat the H-R diagram helps us to determine. So the H-R diagram cantell us something about 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 us to look for other differencesin these teams of stars that might define it. It turns out thatthe distinction is the age of the stars.The H-R diagram is going to assist us learn something about how stars changeas they gain older. So you deserve to already see that this is a really powerfuldiagram indeed. Let"s take a look at theas a whole H-R diagram, including all the different types of stars that weunderstand of.

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Figure 7.The horizontal axis againshows the color of the stars, and also the vertical axis reflects the luminosity,in devices of the solar luminosity. Keep in mind that the tick marks on thisvertical, luminosity axis are a variable of 10 apart! A aspect of 10is referred to as an order of magnitude.So the range of luminosity from bottom to peak in this diagram is massive.Each star in the sky can be placed in a distinctive area on this diagram.For example, the Sun is a yellow star of 1 solar luminosity (by definition!),so you deserve to find it close to the center of the diagram. It drops on the"normal star" line running diagonally from the lower best to the upperleft. This is called the Key Sequence.Most stars loss along this line.Radius:Remember that last lecturewe said that if we know the temperature and also distance to a star we can determineits dimension. As it transforms out, the red stars on the Key Sequence aresmaller than the Sun, and also the stars acquire bigger as you go alengthy the MainSequence towards the hotter (bluer) finish. Stars on the Key Sequencethat are hotter than the Sun are additionally larger than the Sun. So hotblue stars are even more luminous (and therefore show up better in this diagram)for two reasons: they are hotter, and also hot objects are more luminous thancool objects, but they are also bigger. In truth, if a warm star wereto get cooler without transforming its radius, its luminosity would certainly drop andits shade would come to be more red so that it would follow the diagonal linesin the over diagram. Notice that the White Dwarfs, in the lowerleft part of the diagram, are parallel through these constant radius lines.From this we can mean that White Dwarfs gain cooler, however remain the samesize, as they gain older, and we would be right! Other stars alsoacquire hotter or cooler in the time of their lifetimes, however they also adjust sizeat the same time, so they do not follow these lines.The Red Giant andRed Supergiant parts of the diagram show that these stars are 30 to severalhundred times larger in radius than the Sun. We will certainly learn next timethat such stars are old, and that the Sun, as it nears the finish of its lifetime,will certainly additionally swell up and become a red giant star.Lifetimes:Notice that tright here are timemarkers along the Main Sequence. These are the lifetimes of the starsthat are discovered tright here. At the spot where the Sun is located, with1 solar luminosity and also a surchallenge temperature of 6,000 K, stars live foraround 1010 years, or 10 billion years.Stars that are hotter and also more luminous than the Sun live for shorter times,while stars that are cooler and also less luminous live for much longer times.This seems reasonable, since more luminous stars should be placing out energyat a greater price, so they usage up their hydrogen "fuel" quicker. Thehottest stars, of kind O and also B, live just for 10 million years or less!It is an excellent point for us that the Sun is not this sort of star, or elselife would certainly never have actually had actually time to build on Planet.Masses:Tbelow is a solitary parameterthat accounts for every one of the fads we check out on the Key Sequence, andthat is the star"s mass. If a star establishes out of a 10 solar masscloud, it will become a B star, its surface temperature will be about 20,000K, it will have a luminosity of about 10,000 Sun"s, and it will live foronly about 20 million years. All of these characteristics of thestar are figured out by the initial mass of the cloud, via extremely littledependence on anypoint else! So this is the main point to save inmind. The Key Sequence is a mass sequence. Higher mass starswill certainly have surconfront temperatures and luminosities that area at the upper-leftend of the Main Sequence, and lower mass stars will certainly have parameters thatlocation them at the lower-appropriate.Numbers of Stars vs. Mass:As it transforms out, a giantcloud of gas of hundreds or hundreds of solar masses will certainly collapse notto develop a solitary huge star, however will collapse in a number of places at once(a number of dense centers) to develop many kind of stars. Generally, just a fewhigh-mass stars are formed, and also many type of even more of the lower-mass selection arecreated. Such a cloud will certainly form a cluster of stars. Becauseof the lifetime difference, if we look at a young cluster we will certainly see allmasses of stars but if we look at an old cluster we will certainly see just the smallermass stars. Why? Since the high-mass stars have actually currently livedtheir lives out and also died (we will certainly talk about just how stars die later). Comparethe young Pleiades cluster (numbers 2 and also 3, above), through the much olderM3 cluster (figure 6 a and also b). The Pleiades has a few exceptionally brightstars and several much less luminous (lower-mass) stars. The M3 clusterhas actually only fainter stars on the major sequence. It also has lots ofRed Giants, yet that is one more story. If we look at the stars inour area (figure 5), we check out far even more low-mass stars. So moststars in the galaxy today are low-mass stars, for two reasons: 1) morelow-mass than high-mass stars are born in each cloud, and also 2) low-massstars live much much much longer than high-mass stars.Main Sequence Turn-off:If you look at the M3 clusterH-R diagram (figure 6b), you view that the major sequence just exhas a tendency partmeans to the upper-left, and then the stars show up off the primary sequenceto the top appropriate, in the Red Giant location of the H-R diagram. Thisis because when stars age, they gain cooler (which makes them rotate red)and also bigger (which makes them more luminous), so they actually become RedGiants. If we look at an H-R diagram for numerous clusters of differentperiods, below is what we see:Figure 8Really young clusters like theDouble Cluster h and chi Persei have high-mass O stars at the top endof the Key Sequence. Older clusters like the Pleiades have actually B starsstarting to age off the Main Sequence. The Hyades, even holder, isstarting to have A stars leave the Main Sequence, and also the a lot older NGC188 has F stars leaving the Key Sequence. This aging off the MainSequence is called the Key Sequence Turn-off, and we have the right to usage it to actuallytell exactly how old clusters are. The earliest clusters in our galaxy areabout 14 billion years old, which is one means we recognize exactly how old the Universeis.