THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland, Maine 04103
(207) 780-4249      usm.maine.edu/planet
43.6667° N    70.2667° W  Founded January 1970
2022-2023: LXIX
Sunrise: 6:04 a.m.
Sunset: 5:40 p.m.
Civil twilight begins: 5:36 a.m.
Civil twilight ends: 6:08 p.m.
Sun's host constellation: Aquarius
Moon phase: Waning gibbous moon (96% illuminated)
Moonrise:: 8;01 p.m.
Moonset: 7:26 a.m. (03/10/23)
Julian date: 2460013.29
           "Beware how  you take hope away from a human being."    -Oliver Wendell Holmes

THE DAILY ASTRONOMER
Thursday, March 9, 2023
Age of the Seven Sisters

Look high in the western sky early this evening and you'll find the  Pleiades,  a thumb-smudge of light pressed against the back of Taurus the Bull.     Considered to be the most eastern of the winter star patterns, the  Pleiades is now descending in the western sky and will vanish into the dusk by late April.      Before it disappears, we wanted to answer a subscriber's question about  its age:

"You mentioned that the Pleiades Star Cluster was about 120 million years old.   How can astronomers possibly know  a star cluster's age?"
                -Perplexed in Portland


Greetings!  
You have brought up one of our favorite aspects of astronomy:  how do astronomers know what they know about the objects in our Universe?   In order to answer your splendid question, we have to put a lot of wrenches together, so please bear with us.

Wrench  # 1: THE ASTRONOMER'S STONE
It is also known as the H-R Diagram, or the Hertzsprung-Russell Diagram.    
 

The H-R Diagram.  The diagram relating a star's temperature and luminosity. Image by Cornell.edu


This immensely powerful diagram relates a star's luminosity (energy output per second) and its spectral class or temperature.     As stellar temperature relates to the star's color, the graphic above is color coded.     The rainbow colored band extending between the upper left and lower right of the chart is the "main sequence," which contains more than 90% of all stars in the Universe.    Red dwarf or M stars, those that just became hot enough to ignite and sustain thermonuclear fusion reactions, are clustered at the lower right.      At the upper right we find the extremely hot blue-white, or O-type stars.      Our Sun, is a G-type star tucked away within the part of the main sequence that intersects with the horizontal line extending from the "1" at the left hand side of the page.   Those numbers denote a star's luminosity in terms of the Sun's energy output.   So, a red dwarf star's luminosity will be much less than that of the Sun, while an O type star can be tens or even hundreds of thousands of times more luminous than the Sun.  

We notice that, at least on the main sequence, the coolest stars are the least luminous.  This correlation isn't surprising, as a star's size and temperature determine its luminosity.      

Wrench # 2: MASS-LUMINOSITY

While cheerfully ignoring the tricky details, we now must explain that a star's mass determines its luminosity.      When an active star first forms, it begins its life cycle on the main sequence.  Its mass will therefore determine its position along the main sequence.   A low mass star might be a red dwarf.    A high mass star could be an O or a B star.     Nevertheless, no matter how massive, the star will ALWAYS start its life on the main sequence.

Wrench # 3:  STELLAR EVOLUTION

All stars on the main sequence are "hydrogen burners," meaning that they are fusing hydrogen into helium within their cores.     When a star depletes its core hydrogen reserves, it will "evolve" into the red giant stage as its outer layers expand and cool. Meanwhile, the star's core contracts, its temperature increases  and it will eventually progress to the helium burning stage.     However, what is important to us is the star's expansion.       When a star expands after moving off the main sequence, it shifts to the right on the H - R diagram as a consequence of its reduced temperature.   


When a star "evolves" off the main sequence, it expands and
its outer layers cool.  The star then moves "right" on the main sequence. 
Image:  Tutorvista.com

Wrench  # 4:    LIFE SPAN AND LUMINOSITY
Even though a highly massive star contains a lot more "fuel," than a low mass star, the high mass stars expends its fuel reserves far more rapidly.   Whereas a red dwarf star could remain on the main sequence for more than a trillion years, a high mass O or B star lives for only tens of millions of years.    The upper left of the main sequence is a kaleidoscope of butterflies, the lower right is a bale of turtles.   

Wrench # 5:     PIECING IT ALL TOGETHER
So, now, in this vibrant and fantastic universe of the mind, we're going to imagine that we decided to form our very own star cluster.     All we need is a vast gas dust cloud and a precipitating event, such as a nearby supernova explosion that will eject cast off material into the cloud.   The cloud collapses and after a few million years, we have a star cluster.    Next, we gather together a cluster of astronomers to study this cluster of stars.     They hurriedly arrange all the stars onto an H-R Diagram and produce the following:


In this brand new cluster, all the stars are on the main sequence.   The cluster is devoid of red giants because, as we learned earlier, all stars start their lives on the main sequence because all stars begin by fusing hydrogen into helium.    Granted, red dwarfs are far more numerous than the O stars, but still, all the stars in our new cluster are on the main sequence.

Since we're now floating in the universe of our minds, we can cause the astronomers to teleport ahead into the future ten million years.    They then study the cluster again and  they produce an updated H-R Diagram:



These astronomers realize that the most massive stars have evolved off the main sequence into the red giant stage.     Most of the stars still remain on the main sequence.      Now,  let's pretend that these astronomers continue to teleport into the future another ten million years.    What do they notice?   More stars have evolved....those that were slightly less massive than the first stars that evolved off the main sequence.   

You can probably see where this is going now.       As time elapses, more stars will evolve off the main sequence.   The evolution will progress downward along the main sequence:  the most massive stars first...the least massive stars last.   By noting the cut off point, an astronomer can estimate a star cluster's age:




Is this matter messier and more complicated?   Yes, of course.     The devil is always in the details.   Hopefully, however, this answer will help explain how astronomers can estimate the age of star clusters.



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