on a hertzsprung-russell diagram, where would you find stars that are cool and luminous?

Physics 202

Intro to Astronomy: Lecture #17

Prof. Dale E. Gary
NJIT

H-R Diagram for Stars

A Virtually Important Diagram

Classifying stars according to their spectrum is a very powerful way to begin to sympathize how they work. As we said last time, the spectral sequence O, B, A, F, G, One thousand, M is a temperature sequence, with the hottest stars being of blazon O (surface temperatures 30,000-twoscore,000 K), and the coolest stars being of type M (surface temperatures around 3,000 Yard). Because hot stars are blueish, and absurd stars are carmine, the temperature sequence is also a color sequence . It is sometimes helpful, though, to allocate objects according to ii different properties. Permit'southward say we try to classify stars co-ordinate to their apparent effulgence, as well. We could brand a plot with color on one centrality, and credible brightness on the other axis, like this:
Figure 1: H-R Diagram of apparent brightness versus star color (or temperature). Y'all can encounter that this
classification scheme is not helpful -- the stars are randomly scattered on the plot.
Manifestly, plotting apparent brightness against color is not helpful, because at that place are no patterns in the placement of the dots representing stars. They are scattered around randomly. This is because the stars are at all unlike distances , and then the nearby ones announced bright even though they may be intrinsically non and so bright.

But what if we look at this same plot, but somehow make sure that the stars are all at the aforementioned distance. You know that stars sometimes announced in clusters (because they were all formed out of the aforementioned giant cloud, parts of which collapsed to form a lot of stars all around the same fourth dimension). 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 go a plot similar this:

Effigy 3.
Now you can come across that the points representing the stars fall forth a articulate line in the plot. Such a plot was offset made by two astronomers working independently: Ejnar Hertzsprung (Kingdom of 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 agreement how stars piece of work. We are going to spend the rest of this lecture looking in detail at this diagram. First, though, note the relationship between credible brightness and absolute brightness that we talked virtually last time. Nosotros said that astronomers utilise accented effulgence, which is the apparent brightness stars would have if they were all at the aforementioned distance of 10 parsecs. The diagram in a higher place uses credible brightness (apparent magnitudes), but for stars all at the same distance (the altitude to the Pleiades star cluster), so information technology is really a plot of accented brightness versus color. Or we could plot luminosity versus colour, as below:

Figure four. When we know the distances to stars, we can determine their accented brightness, or luminosity.
When nosotros 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 well-nigh an H-R Diagram. It is telling us that a star'due south colour (or temperature) and its luminosity are related. Blue stars are more than luminous than crimson stars. To discover this out, though, we have to know the distances to the stars . Call up the star catalog we showed one page of in the final lecture, from the Nearby Stars itemize. Nosotros know the distances to these stars, by measuring their parallax. Here is the H-R diagram for that catalog:

Effigy five.
Now nosotros run across that there is a new region in the lower left, which correspond to faint-blueish stars. If blue stars are so luminous, why are these so faint? These are faint because they are very small! They are a form of stars called White Dwarf stars . We can likewise look at the H-R diagram for other clusters. Hither is i for an former cluster of stars, M3, which is a globular cluster:

At present we see a new region of luminous red stars in the upper-correct! If red stars are fainter than blue stars, why are these ruby stars so luminous? It is because they are giant stars, like the star Betelgeuse, which I mentioned final fourth dimension is so large that, if it were at the distance of the Lord's day, it would engulf the Earth's orbit, and even the orbit of Mars. These are the Red Giant stars .

Patterns in the H-R Diagram

We run into that the H-R diagram tin can help u.s. classify different kinds of stars, co-ordinate to the design of where the stars fall in the diagram. The diagonal line that nosotros saw for the Pleiades star cluster represents what nosotros would call normal stars. The White Dwarfs and Red Giants are different classes of stars that the H-R diagram helps us to identify. So the H-R diagram tin can tell us something about the size (radius) of the stars. The fact that the H-R diagrams for the nearby stars, the Pleiades star cluster, and the M3 star cluster are all different leads us to look for other differences in these groups of stars that might explicate it. Information technology turns out that the departure is the historic period of the stars. The H-R diagram is going to help us acquire something about how stars modify equally they get older. Then yous tin can already run across that this is a very powerful diagram indeed.
Let's have a await at the overall H-R diagram, including all the different types of stars that we know of.

Figure seven.

The horizontal axis again shows the color of the stars, and the vertical axis shows the luminosity, in units of the solar luminosity. Note that the tick marks on this vertical, luminosity axis are a factor of 10 apart! A cistron of ten is called an order of magnitude . So the range of luminosity from bottom to summit in this diagram is enormous. Each star in the sky can be placed in a unique identify on this diagram. For example, the Sun is a xanthous star of 1 solar luminosity (by definition!), so you can find it about the center of the diagram. Information technology falls on the "normal star" line running diagonally from the lower right to the upper left. This is called the Main Sequence . Most stars fall along this line.

Radius:
Call up that last lecture we said that if we know the temperature and distance to a star we can determine its size. Equally it turns out, the red stars on the Chief Sequence are smaller than the Dominicus, and the stars get bigger as you go along the Main Sequence toward the hotter (bluer) end. Stars on the Main Sequence that are hotter than the Dominicus are also larger than the Lord's day. So hot blueish stars are more luminous (and therefore announced higher in this diagram) for ii reasons: they are hotter, and hot objects are more luminous than absurd objects, merely they are also larger. In fact, if a hot star were to become cooler without changing its radius, its luminosity would drop and its colour would become more crimson so that it would follow the diagonal lines in the higher up diagram. Notice that the White Dwarfs, in the lower left part of the diagram, are parallel with these constant radius lines. From this nosotros might expect that White Dwarfs get cooler, but stay the same size, as they become older, and nosotros would be right! Other stars also get hotter or libation during their lifetimes, but they also change size at the same time, and so they exercise non follow these lines.

The Red Giant and Ruby-red Supergiant parts of the diagram show that these stars are 30 to several hundred times larger in radius than the Sun. Nosotros will learn adjacent time that such stars are one-time, and that the Sun, as information technology nears the finish of its lifetime, will also swell upwardly and become a scarlet giant star.
Lifetimes:
Notice that there are time markers along the Main Sequence. These are the lifetimes of the stars that are establish there. At the spot where the Dominicus is located, with 1 solar luminosity and a surface temperature of 6,000 One thousand, stars live for about 10 ¹⁰ years, or ten billion years. Stars that are hotter and more luminous than the Sunday live for shorter times, while stars that are cooler and less luminous alive for longer times. This seems reasonable, since more luminous stars must be putting out energy at a higher rate, then they utilise up their hydrogen "fuel" faster. The hottest stars, of type O and B, live only for 10 one thousand thousand years or less! It is a good thing for u.s.a. that the Dominicus is not this kind of star, or else life would never have had time to develop on Earth.

Masses:
There is a single parameter that accounts for all of the patterns we encounter on the Principal Sequence, and that is the star's mass. If a star develops out of a 10 solar mass deject, it will become a B star, its surface temperature will exist nigh 20,000 K, it will have a luminosity of well-nigh 10,000 Sunday's, and it will alive for just almost 20 meg years. All of these characteristics of the star are determined by the initial mass of the cloud, with very little dependence on annihilation else! So this is the principal point to keep in mind. The Main Sequence is a mass sequence. Higher mass stars will have surface temperatures and luminosities that identify at the upper-left end of the Main Sequence, and lower mass stars will have parameters that place them at the lower-right.

Numbers of Stars vs. Mass:
As it turns out, a behemothic cloud of gas of hundreds or thousands of solar masses will collapse non to form a single giant star, but volition plummet in several places at once (several dense centers) to form many stars. Typically, but a few high-mass stars are formed, and many more than of the lower-mass variety are formed. Such a deject volition form a cluster of stars. Because of the lifetime difference, if we look at a young cluster nosotros will see all masses of stars but if nosotros look at an old cluster we volition see only the smaller mass stars. Why? Because the high-mass stars have already lived their lives out and died (we will discuss how stars die later on). Compare the young Pleiades cluster (figures 2 and 3, higher up), with the much older M3 cluster (effigy 6 a and b). The Pleiades has a few very bright stars and lots of less luminous (lower-mass) stars. The M3 cluster has only fainter stars on the main sequence. It too has lots of Red Giants, but that is another story. If we look at the stars in our neighborhood (figure 5), we encounter far more low-mass stars. And then near stars in the milky way today are depression-mass stars, for 2 reasons: ane) more low-mass than loftier-mass stars are born in each deject, and ii) depression-mass stars live much much longer than high-mass stars.

Primary Sequence Turn-off:
If you expect at the M3 cluster H-R diagram (figure 6b), y'all run across that the main sequence only extends part way to the upper-left, and so the stars appear off the chief sequence to the upper correct, in the Cerise Giant surface area of the H-R diagram. This is considering when stars historic period, they get cooler (which makes them turn blood-red) and larger (which makes them more luminous), so they actually become Carmine Giants. If nosotros await at an H-R diagram for several clusters of different ages, here is what we see:

Figure 8 Really young clusters similar the Double Cluster h and chi Persei have high-mass O stars at the upper cease of the Chief Sequence. Older clusters similar the Pleiades have B stars starting to historic period off the Main Sequence. The Hyades, even holder, is starting to take A stars get out the Main Sequence, and the much older NGC 188 has F stars leaving the Primary Sequence. This aging off the Main Sequence is called the Main Sequence Turn-off, and we tin can use it to actually tell how old clusters are. The oldest clusters in our galaxy are well-nigh 14 billion years old, which is one way we know how old the Universe is.