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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