H-R Diagram because that StarsA Most necessary DiagramClassifying starsaccording to your spectrum is a an extremely powerful way to begin to understandhow they work. Together we stated last time, the spectral sequence O, B,A, F, G, K, M is a temperature sequence, with the hottest stars being oftype O (surface temperatures 30,000-40,000 K), and also the coolest stars beingof form M (surface temperatures approximately 3,000 K). Since hot starsare blue, and also cool stars room red, the temperaturesequence is likewise a shade sequence. The is occasionally helpful,though, to classify objects according to two various properties.Let"s to speak we shot to share stars follow to their apparent brightness,also. We could make a plot with shade on one axis, and apparent brightnesson the various other axis, prefer this:Figure 1: H-R diagram of obvious brightness matches star color (ortemperature). You can see the thisclassification system is not beneficial -- the stars room randomly scatteredon the plot.Obviously, plotting apparentbrightness versus color is not helpful, due to the fact that there space no patternsin the placement of the dots representing stars. They are scatteredaround randomly. This is since the starsare in ~ all different distances, therefore the nearby ones appear brighteven despite they may be intrinsically not therefore bright.But what if us look in ~ thissame plot, however somehow make certain that the stars space all in ~ the very same distance.You recognize that stars sometimes show up in clusters (because they to be allformed out of the same gigantic cloud, parts of which fell down to form alot that stars all approximately the very same time). Right here is a picture of thePleiades star cluster:Figure 2.If we plot the noticeable brightnessversus color for together a cluster, where all the stars are the same distance,you acquire a plot choose this:Figure 3.Now you deserve to see the thepoints representing the stars autumn along a clear line in the plot.Such a plot was first made by two astronomers working independently: EjnarHertzsprung (Denmark) and also Henry Norris Russell (Princeton, USA).This sort of chart was named after them, together the Hertzsprung-RussellDiagram, or H-R Diagram.It is one extremely an effective diagram because that classifying stars and also understandinghow stars work. We are going to spend the remainder of this class lookingin detail at this diagram. First, though, note the connection betweenapparent brightness and also absolute brightness that us talked about last time.We said that astronomers use absolute brightness, which is the apparentbrightness stars would have actually if they were every at the very same distance the 10parsecs. The diagram above uses apparent brightness (apparent magnitudes),but because that stars all at the exact same distance (the street to the Pleiades starcluster), so the is really a plot of pure brightness versus color.Or we can plot luminosity matches color, as below:Figure 4. Once we understand the ranges to stars, we can determine theirabsolute brightness, or luminosity.When us then plot luminosity (or absolute brightness) versus color(or temperature), the stars allfall along a narrow piece in the diagram. This is the H-R Diagram.So the right way to thinkabout one H-R Diagram. The is informing us that a star"s color (or temperature)and that luminosity room related. Blue stars are much more luminous thanred stars. To discover this out, though, wehave to know the ranges to the stars. Remember thestar catalog we verified one page of in the last lecture, from the NearbyStars catalog. We understand the ranges to this stars, by measuringtheir parallax. Here is the H-R diagram for that catalog:Figure 5.Now we view that over there isa brand-new region in the lower left, which exchange mail to faint-blue stars.If blue stars are so luminous, why space these so faint? this arefaint because they are really small! They room a course of stars calledWhiteDwarf stars. We can likewise look at the H-R diagram because that otherclusters. Here is one because that an old cluster of stars, M3, which is aglobular cluster:
Figure 6 a.

You are watching: A white-blue star is hotter than a red star.

Figure 6 b.
Now we check out a newregion of luminous red stars in the upper-right! If red stars arefainter 보다 blue stars, why room these red stars for this reason luminous? Itis due to the fact that they are giant stars, prefer the star Betelgeuse, which ns mentionedlast time is so huge that, if it to be at the street of the Sun, that wouldengulf the Earth"s orbit, and also even the orbit that Mars. These are theRedGiant stars.Patterns in the H-R DiagramWe view that theH-R chart can help us classify various kinds of stars, according tothe pattern of wherein the stars fall in the diagram. The diagonalline the we observed for the Pleiades star cluster represents what we would contact normalstars. The White Dwarfs and also Red Giants are different classes the starsthat the H-R diagram helps us to identify. Therefore the H-R diagram cantell united state something about the dimension (radius) the the stars. The factthat the H-R diagrams because that the adjacent stars, the Pleiades star cluster,and the M3 star cluster are all various leads us to look for other differencesin these groups of stars that could explain it. It transforms out thatthe difference is the period of the stars.The H-R diagram is going to aid us find out something about how stars changeas they get older. So you can already see the this is a very powerfuldiagram indeed. Let"s take it a look at theoverall H-R diagram, including 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 mirrors the luminosity,in devices of the solar luminosity. Note that the mite marks on thisvertical, luminosity axis are a aspect of 10 apart! A aspect of 10is called an order of magnitude.So the range of luminosity native bottom to optimal in this chart is enormous.Each star in the sky have the right to be placed in a unique place ~ above this diagram.For example, the sunlight is a yellow star that 1 solar luminosity (by definition!),so you can discover it close to the center of the diagram. It falls on the"normal star" line to run diagonally from the reduced right to the upperleft. This is referred to as the key Sequence.Most stars autumn along this line.Radius:Remember the last lecturewe stated that if we know the temperature and also distance to a star we deserve to determineits size. Together it turns out, the red stars on the key Sequence aresmaller than the Sun, and also the stars get bigger as you go follow me the MainSequence toward the hot (bluer) end. Stars on the main Sequencethat are hotter 보다 the sunlight are likewise larger 보다 the Sun. Therefore hotblue stars are more luminous (and as such appear greater in this diagram)for two reasons: they room hotter, and also hot objects are more luminous thancool objects, however they are additionally larger. In fact, if a hot star wereto get cooler without transforming its radius, that luminosity would certainly drop andits shade would become more red so that it would certainly follow the diagonal line linesin the over diagram. Notification that the White Dwarfs, in the lowerleft component of the diagram, are parallel through these consistent radius lines.From this we can expect the White Dwarfs obtain cooler, however stay the samesize, together they get older, and we would certainly be right! other stars alsoget name is or cooler during their lifetimes, however they also change sizeat the same time, so they execute not follow these lines.The Red huge andRed Supergiant components of the diagram present that this stars space 30 come severalhundred times bigger in radius 보다 the Sun. Us will discover next timethat together stars space old, and that the Sun, together it nears the finish of that is lifetime,will also swell up and also become a red gigantic star.Lifetimes:Notice that there space timemarkers along the key Sequence. These room the lifetimes that the starsthat are discovered there. At the spot whereby the sunlight is located, with1 solar luminosity and also a surface ar temperature that 6,000 K, stars live forabout 1010 years, or 10 billion years.Stars that space hotter and much more luminous than the sun live for shorter times,while stars that room cooler and also less luminous live for longer times.This appears reasonable, since much more luminous stars need to be placing out energyat a higher rate, therefore they usage up your hydrogen "fuel" faster. Thehottest stars, of form O and B, live just for 10 million year or less!It is a great thing for us that the sun is not this sort of star, or elselife would never ever have had time to develop on Earth.Masses:There is a single parameterthat accounts for all of the trends we view on the key Sequence, andthat is the star"s mass. If a star establishes out of a 10 solar masscloud, that will end up being a B star, its surface temperature will certainly be about 20,000K, that will have actually a luminosity of around 10,000 Sun"s, and it will live foronly around 20 million years. Every one of these features of thestar are determined by the initial massive of the cloud, with an extremely littledependence on something else! so this is the main suggest to save inmind. The key Sequence is a fixed sequence. Higher mass starswill have actually surface temperatures and also luminosities that location at the upper-leftend the the main Sequence, and lower fixed stars will have parameters thatplace them in ~ the lower-right.Numbers of Stars vs. Mass:As it transforms out, a giantcloud of gas of hundreds or hundreds of solar masses will collapse notto type a single giant star, however will please in several places at once(several dense centers) to kind many stars. Typically, only a fewhigh-mass stars room formed, and also many much more of the lower-mass range areformed. Such a cloud will type a cluster of stars. Becauseof the lifetime difference, if us look in ~ a young swarm we will watch allmasses that stars however if we look at an old cluster we will certainly see just the smallermass stars. Why? since the high-mass stars have currently livedtheir resides out and also died (we will discuss how stars dice later). Comparethe young Pleiades swarm (figures 2 and also 3, above), through the lot olderM3 swarm (figure 6 a and also b). The Pleiades has a couple of very brightstars and lots of less luminous (lower-mass) stars. The M3 clusterhas just fainter stars top top the key sequence. It also has too many ofRed Giants, but that is one more story. If us look in ~ the stars inour ar (figure 5), we check out far more low-mass stars. For this reason moststars in the galaxy today are low-mass stars, for 2 reasons: 1) morelow-mass 보다 high-mass stars are born in every cloud, and also 2) low-massstars live much much much longer than high-mass stars.Main sequence Turn-off:If girlfriend look at the M3 clusterH-R chart (figure 6b), you view that the main sequence just extends partway come the upper-left, and then the stars show up off the key sequenceto the top right, in the Red large area the the H-R diagram. Thisis because when stars age, they gain cooler (which makes them revolve red)and larger (which renders them much more luminous), therefore they actually end up being RedGiants. If us look at an H-R diagram for several clusters that differentages, right here is what us see:Figure 8Really young clusters favor theDouble swarm h and chi Persei have high-mass O stars at the upper endof the key Sequence. Larger clusters prefer the Pleiades have actually B starsstarting to period off the main Sequence. The Hyades, even holder, isstarting to have A stars leaving the main Sequence, and also the lot older NGC188 has actually F stars leave the main Sequence. This aging off the MainSequence is referred to as the main Sequence Turn-off, and also we can use it to actuallytell how old clusters are. The earliest clusters in our galaxy areabout 14 billion year old, which is one means we know just how old the Universeis.