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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Figure 7.The horizontal axis againshows the color of the stars, and the vertical axis reflects the luminosity,in units of the solar luminosity. Keep in mind that the mite marks ~ above thisvertical, luminosity axis space a variable of 10 apart! A factor of 10is called an bespeak of magnitude.So the variety of luminosity from bottom to top in this diagram is enormous.Each star in the sky deserve to be placed in a distinct place on this diagram.For example, the sunlight is a yellow star of 1 solar luminosity (by definition!),so girlfriend can uncover it close to the facility of the diagram. It falls on the"normal star" line running diagonally native the reduced right come the upperleft. This is referred to as the key Sequence.Most stars autumn along this line.Radius:Remember that last lecturewe said that if we know the temperature and distance come a star we deserve to determineits size. Together it turns out, the red stars on the key Sequence aresmaller than the Sun, and the stars gain bigger together you go along the MainSequence toward the hot (bluer) end. Stars ~ above the main Sequencethat are hotter 보다 the sun are additionally larger than the Sun. For this reason hotblue stars are an ext luminous (and because of this appear higher in this diagram)for two reasons: they are hotter, and hot objects are an ext luminous thancool objects, however they are likewise larger. In fact, if a hot star wereto obtain cooler without an altering its radius, its luminosity would certainly drop andits color would become an ext red so that it would certainly follow the diagonal linesin the above diagram. Notification that the White Dwarfs, in the lowerleft component of the diagram, room parallel with these continuous radius lines.From this we might expect that White Dwarfs obtain cooler, but stay the samesize, together they obtain older, and we would be right! various other stars alsoget name is or cooler during their lifetimes, but they also readjust sizeat the very same time, for this reason they do not follow these lines.The Red large andRed Supergiant components of the diagram display that these stars space 30 to severalhundred times larger in radius than the Sun. We will find out next timethat such stars room old, and also that the Sun, as it nears the end of its lifetime,will likewise swell up and also become a red large star.Lifetimes:Notice the there room timemarkers along the key Sequence. These are the lifetimes of the starsthat are uncovered there. At the spot where the sunlight is located, with1 solar luminosity and a surface ar temperature of 6,000 K, stars live forabout 1010 years, or 10 billion years.Stars that are hotter and much more luminous than the sun live for shorter times,while stars that are cooler and less luminous live for longer times.This appears reasonable, since much more luminous stars should be putting out energyat a greater rate, for this reason they use up your hydrogen "fuel" faster. Thehottest stars, of form O and also B, live just for 10 million year or less!It is a an excellent thing for united state that the sun is no this kind of star, or elselife would never ever have had actually time to build on Earth.Masses:There is a solitary parameterthat accounts for all of the trends we check out on the main Sequence, andthat is the star"s mass. If a star establishes out that a 10 solar masscloud, that will come to be a B star, its surface temperature will certainly be around 20,000K, it will have actually a luminosity of about 10,000 Sun"s, and also it will certainly live foronly about 20 million years. Every one of these qualities of thestar are figured out by the initial fixed of the cloud, with very littledependence on something else! for this reason this is the main suggest to save inmind. The key Sequence is a fixed sequence. Greater mass starswill have surface temperatures and also luminosities that ar at the upper-leftend that the main Sequence, and also lower massive stars will have parameters thatplace them at the lower-right.Numbers that Stars vs. Mass:As it turns out, a giantcloud the gas that hundreds or thousands of solar masses will certainly collapse notto form a solitary giant star, yet will please in several locations at once(several dense centers) to type many stars. Typically, just a fewhigh-mass stars are formed, and many more of the lower-mass variety areformed. Together a cloud will type a cluster of stars. Becauseof the lifetime difference, if us look in ~ a young swarm we will view allmasses the stars yet if us look at an old swarm we will certainly see only the smallermass stars. Why? because the high-mass stars have already livedtheir stays out and also died (we will talk about how stars dice later). Comparethe young Pleiades cluster (figures 2 and 3, above), v the much olderM3 cluster (figure 6 a and also b). The Pleiades has actually a few very brightstars and lots of much less luminous (lower-mass) stars. The M3 clusterhas just fainter stars ~ above the main sequence. It likewise has lots ofRed Giants, but that is one more story. If us look in ~ the stars inour ar (figure 5), we watch far an ext low-mass stars. Therefore moststars in the galaxy today room low-mass stars, for 2 reasons: 1) morelow-mass than high-mass stars are born in every cloud, and 2) low-massstars live much much much longer than high-mass stars.Main sequence Turn-off:If girlfriend look in ~ the M3 clusterH-R diagram (figure 6b), you see that the key sequence only extends partway to the upper-left, and also then the stars appear off the main sequenceto the top right, in the Red gigantic area the the H-R diagram. Thisis due to the fact that when stars age, they get cooler (which renders them rotate red)and bigger (which renders them an ext luminous), for this reason they actually end up being RedGiants. If us look in ~ an H-R chart for several clusters of differentages, right here is what us see:Figure 8Really young clusters like theDouble cluster h and chi Persei have high-mass O stars at the top endof the main Sequence. Larger clusters like the Pleiades have B starsstarting to age off the main Sequence. The Hyades, also holder, isstarting to have A stars leave the main Sequence, and the lot older NGC188 has F stars leave the key Sequence. This aging off the MainSequence is referred to as the key Sequence Turn-off, and also we have the right to use it to actuallytell just how old swarm are. The earliest clusters in our galaxy areabout 14 billion years old, i beg your pardon is one way we know how old the Universeis.