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Learning objectives

By the end of this section, you will be able to:

  • Explain how the H–R diagram    of a star cluster can be related to the cluster’s age and the stages of evolution of its stellar members
  • Describe how the main-sequence turnoff of a cluster reveals its age

In the previous section, we indicated that that open clusters are younger than globular clusters, and associations are typically even younger. In this section, we will show how we determine the ages of these star clusters. The key observation is that the stars in these different types of clusters are found in different places in the H–R diagram, and we can use their locations in the diagram in combination with theoretical calculations to estimate how long they have lived.

H–r diagrams of young clusters

What does theory predict for the H–R diagram of a cluster whose stars have recently condensed from an interstellar cloud? Remember that at every stage of evolution, massive stars evolve more quickly than their lower-mass counterparts. After a few million years (“recently” for astronomers), the most massive stars should have completed their contraction phase and be on the main sequence, while the less massive ones should be off to the right, still on their way to the main sequence. These ideas are illustrated in [link] , which shows the H–R diagram calculated by R. Kippenhahn and his associates at Munich University for a hypothetical cluster with an age of 3 million years.

Young cluster h–r diagram.

Hypothetical H-R Diagram of a Young Cluster. In this plot titled “M 2001 Age: 3 million years,” the vertical axis is labeled “Luminosity (LSun),” and goes from 0.1 at the bottom to 100,000 at the top. The horizontal axis is labeled “Surface Temperature (K)”, and goes from 40,000 on the left to 3,000 on the right. The zero-age main sequence is drawn as a red diagonal line starting just above 100,000 LSun at the top of the graph down to about 4000 K at the bottom. The “Present position of Sun” is indicated at 5500 K and 1 LSun. Over-plotted on the graph are black dots representing the individual stars in the cluster. About half of the dots lie neatly along the red line until about 10000 K and 100 LSun. At this point, the remainder of the dots lie above the red line, meaning these stars have yet to reach the main sequence.
We see an H–R diagram for a hypothetical young cluster with an age of 3 million years. Note that the high-mass (high-luminosity) stars have already arrived at the main-sequence stage of their lives, while the lower-mass (lower-luminosity) stars are still contracting toward the zero-age main sequence (the red line) and are not yet hot enough to derive all of their energy from the fusion of hydrogen.

There are real star clusters that fit this description. The first to be studied (in about 1950) was NGC 2264, which is still associated with the region of gas and dust from which it was born ( [link] ).

Young cluster ngc 2264.

Image of the Young Cluster N G C 2264. This youthful cluster derives its name from the shape outlined by its brightest stars. The “Christmas Tree” is upside down in this image. The brightest star at the top of the frame is the base of the tree. The top of the tree is the star above the dark v-shaped lane in the nebula just left of the center at the bottom of the image.
Located about 2600 light-years from us, this region of newly formed stars, known as the Christmas Tree Cluster, is a complex mixture of hydrogen gas (which is ionized by hot embedded stars and shown in red), dark obscuring dust lanes, and brilliant young stars. The image shows a scene about 30 light-years across. (credit: ESO)

The NGC 2264 cluster’s H–R diagram is shown in [link] . The cluster in the middle of the Orion Nebula (shown in [link] and [link] ) is in a similar stage of evolution.

Ngc 2264 h–r diagram.

In this plot the vertical axis is labeled “Luminosity (LSun)” and goes from 0.1 at the bottom to 100,000 at the top. The horizontal axis is labeled “Surface Temperature (K)” and goes from 40,000 on the left to 3,000 on the right. The zero-age main sequence is drawn as a red diagonal line starting just above 100,000 LSun at the top of the graph down to about 4000 K at the bottom. Over plotted are the observed values of stars in N G C 2264, shown as black dots. Stars lie on the line until about 10000 K and 10 LSun, below which the stars reside above the main sequence.
Compare this H–R diagram to that in [link] ; although the points scatter a bit more here, the theoretical and observational diagrams are remarkably, and satisfyingly, similar.

As clusters get older, their H–R diagrams begin to change. After a short time (less than a million years after they reach the main sequence), the most massive stars use up the hydrogen in their cores and evolve off the main sequence to become red giants and supergiants. As more time passes, stars of lower mass begin to leave the main sequence and make their way to the upper right of the H–R diagram.

Questions & Answers

hi, I am yamini, I am in class 8 but very much interested in astronomy and go to NASA, what are the subjects in which I can master and lead to NASA.
yamini Reply
Hello Yamini, Im Ken and I'm avery intrested in joining the NASA too. Is nice to meet you.
Ken
very*
Ken
nice to meet u ken
yamini
What I know about the subjects is that you have to be a master on science and math, also if you know about aviation is better too
Ken
I read that the Russian language is very important, is not a requirement but it's like an extra point!
Ken
yeah like physics, chemistry and maths, they are my most favorite.
yamini
than you can go free of cost
Arush
Yes, That's right!
Ken
how
yamini
Oh! Free of cost?
Ken
hello yamini nice to meet you
Burak
what is time
Abdul Reply
Time is relative
mrunal
pls elaborate
sakshi
the clear defination.I know that.
Abdul
In planet mars there the life exits or not and is there water there
Eshwarsa Reply
see till now nothing can be found as u know that the curiosity rover has struck in mars
Maya
It has been proven that there are water molecules on Mars but not enough that most lifeforms could thrive upon.
Ariana
There are huge amounts of water in the ice caps and under the surface. The surface and chemistry indicate that Mars had cosiderable amounts of water on its surface in the past.
Julius
what is your opinion about the theory of Vedas about modern physics..
Manish Reply
i think in some ways vedas are also correct but not everytime
Maya
I agree
sakshi
hmm even I agree
Samuel
Is there any patened theory about time relativitg in growth and development?
donot Reply
some astronomer's says that there is no alien exist but why search for extra terrestrial intelligence center is established
Eshwarsa Reply
No One Knows That For Absolute Fact, The Universe Is Too Huge To Have Any Type Of Idea About What Exist In The Far Reaches Of Our Universe.....
Adam
Check Out The Drake Equation.....
Adam
their should be aliens as like ours there would be another planet
Maya
which could have existed life on it
Maya
adam i want to ask a question
Maya
can kepler 1st law be applied on all the planets of the universe
Maya
hello, anyone home?
Denise
sjskskfhjkkktewqqw and try?
Lanika Reply
what is this ?
Samuel
hi I am Samuel from India mumbai
Samuel
nice to meet you
Samuel
thats my question, what is this?
penzias and wilson's a discovery of the cosmic microwave background is a nice example of scientific serendipity-something that is found by chance but turns out to have a positive outcome
Jacqueline Reply
how should I make my carrier in astronomy
Ayush Reply
I think that Newton's third law is not appropriate if any also thinks like this please reply me
Ayush
Can you explain your reasoning
Huh
why u think so
joseph
yes Ayush u are right
Yoganshu
I think when we apply force to a object it start moving but , a/c to Newton's third law every action has equal and opposite reaction,so object should also exert equal force on us and it should not move due to balanced force
Ayush
if I am not right then reply me
Ayush
no
Zack
because of friction that opposes that force and help us to move ahead
Manish
but this is not satisfied as third law say another thing
Ayush
you are telling why object moves
Ayush
you have to think a/c to third law
Ayush
its because of its mass
Maya
because it is applying equal and opposite force but also our mass is also less in comparison to the object
Maya
which is why we cant move the object but it can make move us
Maya
manish is too correct in his place because we need to apply force which would overcome the frictional force
Maya
My dear friends, can u plz tell me that among u guys who are in the field of cosmology
Madhav Reply
😢I am not there in cosmology
Samuel
Just A Science Fan.....
Adam
Adam even I am 😥😅😅😂😂
Samuel
I am also not in cosmology but I am just a fan or we can say science and part of NASA is my dream
Yoganshu
yoganshu Arya same here
Samuel
you are from which country
Yoganshu
hi yoganshu
Samuel
India
Samuel
which state
Samuel
I am also from India
Yoganshu
from delhi
Yoganshu
and u...?
Yoganshu
I am from Maharashtra
Samuel
from which state?
Yoganshu
You are a ASTRONOMER ...
Yoganshu
or a scientist..
Yoganshu
or just a member
Yoganshu
What is time...? not about Newton= time is constant..... that all scientists openions n point of view I m knowing. . what can be the Perfect Definition of Time
Madhav Reply
time is what clock reads
Ayush
Who is the best astronomer of India at present time
Gian Reply
Jayant Narlikar, Proponent Of Steady State Cosmology.....
Adam
What is the real colour of sun rays
Gian Reply
white.. so white it becomes violet.. so violet it become ultraviolet
Tom
white and red and yellow
Bianca
Vibgyor
Samuel
the real colour of sunlight is White
Madhav
the Sun's has a variety of waves all throughout the elextromagnetic spectrum.
Jacie
we only see it as a few bc of how some of them get redshifted (? can that term be applied for something so local?) by some particles in our upper atmosphere
Jacie
Vibgyor will be when, the white light will pass through the clouds ( prism ) then Refraction phenomenon leads us to 7 colours splitting from a single colour "White " light
Madhav
so the sun rays r of White colour
Madhav
Taurus in astronomy and horoscope?
Yasser Reply
how to put E=MC2
Gospel Reply
What Do You Mean By How To Put?
Adam
What Do You Mean By How To Put?
Adam
What Do You Mean By "How To Put E=MC2?
Adam
yep
Gospel
Hi guys
Samuel
i mean how NASA came to know the mass and diameters of Stars.how?
Gospel
how did they do using E=MC2
Gospel
thats my questioning
Gospel
that's easy formula's derivation
Madhav
Practice Key Terms 1

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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