The Hertzsprung-Russell Diagram

The Hertzsprung-Russell Diagram

If you plot the brightness versus color, or spectral type, of stars you get a scatter plot – these quantities are not correlated, and they do not form a pattern together. If you look at this diagram, you will see points scattered all over the place.

The points are now drawn to indicate stellar colour (point colour) and apparent brightness (point size). However, there is still no intrinsic pattern to the distribution. (By definition, large points are found at the top while small points are found at the bottom, and the points are dispersed according to colour from right to left.) [NMSU, N. Vogt]

But, if we use a sample of stars for which we have known distances, we can calculate their intrinsic luminosities. A plot of luminosity vs spectral type (or colour or temperature) is called a Hertzsprung-Russell (H-R) Diagram, and there are some definite patterns within it.
- What patterns do you see when you look at the H-R diagram below?
- Do you find stars everywhere in the diagram, or are there regions which are sparsely populated and regions which are heavily populated?
- Is there a characteristic pattern which most of the stars follow?
- Do you see a trend between spectral type and luminosity?
  
  [NMSU, N. Vogt]
- The H-R diagram has played a crucial role in developing our understanding of stellar structure and evolution. A major focus of astronomy during the 1940's and 1950's was constructing stellar models that would accurately predict the luminosity - temperature relation seen in the H-R diagram.
- For now, we will note some interesting things about the differences between the nearby stars and the brightest stars and use the H-R diagram to demonstrate one more property of stars.
- Most stars fall along a sequence that is called the Main Sequence. For this sequence, there is a strong correlation, in the sense that hotter stars are also more luminous.
Here is another version of the H-R diagram, showing the distribution of many nearby stars. A dot is placed on the diagram for each observed star, and the color of the dot corresponds to the stellar temperature. Note how the bulk of the stars lie along the Main Sequence, while the giant and dwarf populations are sparser (more rare). Where does the Sun lie on the H-R diagram?
Measure the masses for as many stars as we can find (using the techniques we discussed for binary systems) and we will discover the mass-luminosity relation for Main Sequence stars.

[NMSU, N. Vogt]

L = M^3.5
Mass and luminosity are proportional to either – intrinsically bright stars are also very massive, and intrinsically faint stars tend to be rather low mass. This tells us that the Main Sequence in the H-R Diagram is not only a luminosity sequence, but also a mass sequence!

[NMSU, N. Vogt]
A more massive star generates a stronger gravitational field than a less massive one. This creates higher pressures in the stellar core, which means that the star converts its fuel into energy faster and more efficiently. Though high mass stars have larger fuel supplies than low mass stars, they burn through it more quickly (hence they are much brighter), so their Main Sequence lifetimes are far shorter.

[NMSU, N. Vogt]

In a stellar cluster (a densely packed group of hundreds of stars) , the stars are all born at roughly the same time, with similar chemical compositions; they differ only in mass. By examining how many short-lived high-mass and intermediate-mass stars are still present in a cluster, we can estimate its age. In the figure above, four stages of cluster evolution are shown:

I: A young cluster, with plenty of hot, blue, high-mass stars

II: A fairly young cluster, missing only the highest of high-mass stars

III: A moderate-age cluster, lacking O and B stars

IV: An older cluster (6 billion years old), with many intermediate-mass

stars evolving off the Main Sequence and turning into giant stars

Thanks to Mike Bolte (UC Santa Cruz) for the base contents of this slide.

I:	A young cluster, with plenty of hot, blue, high-mass stars
II:	A fairly young cluster, missing only the highest of high-mass stars
III:	A moderate-age cluster, lacking O and B stars
IV:	An older cluster (6 billion years old), with many intermediate-mass
	stars evolving off the Main Sequence and turning into giant stars