Age from Star Clusters - Guided Notes update
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Guided Notes – Main Sequence Lifetime and Age from Star Clusters
Name:
Leah Walker
These notes cover slides 35 – 47 of
Video Lecture - Properties of Stars
.
Please type or handwrite notes as you watch the video lecture and answer the
included questions.
Main Sequence Lifetime:
The main sequence is a group of stars that form a distinct and continuous band on a plot of stellar color
versus brightness. These stars are commonly known as main-sequence stars or dwarf stars. Their
position on or off the band is believed to indicate their physical properties and their progress through
different stages of their life. Main-sequence stars, including the Sun, form the majority of true stars in
the universe. Hertzsprung-Russell diagrams are color-magnitude plots named after Ejnar Hertzsprung
and Henry Norris Russell. The Main Sequence lifetime is based on:
1. Lifetime
2. Fule/luminosity
Main Sequence Fuel:
Main-sequence stars generate energy in their cores through hydrogen fusion, which is also known as
"hydrogen burning." This process helps to maintain the hydrostatic equilibrium of the star's core, where
outward thermal pressure is balanced by the inward pressure of gravitational collapse. The rate of
energy generation depends on the temperature and pressure of the core, which are determined
primarily by the mass of the star, but also based on its chemical composition and age. The energy
generated makes its way to the surface and is radiated away at the photosphere, carried by either
radiation or convection.
Calculating the amount of usable fuel in a star:
Only applies to main sequence stars
Calculating Lifetimes:
Your Turn: Stellar Lifetime
A. Calculate the main sequence lifetime of a star with a mass of 1.5 solar masses (M
⊙
) and 12,000 solar
luminosities (L
⊙
)
Your answer:
M=1.5M
L=12,000L
T= 10yr
T/T=ML
⊙
/M
⊙
L
T= 1.5/12000X10^10YR=1.25X10^6YR
B. Is this in fact a main sequence star? How can you tell?
Your answer: No
L= (1.5)^3.5
L
⊙=
4.13
⊙
But
given L
⊙
= 12,000L
⊙,
this is not the main sequence of a start
Making Sense of Equations:
Formula
Name
Variables
Units
What is it used for?
T
ms
=.0007Mc
2
/L
Click or tap here to enter
text.
T
ms
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tap here
to enter
text.
Click or tap here to enter text.
Click or tap here to enter
text.
M
Click or
tap here
to enter
text.
Click or tap here to enter text.
Click or tap here to enter
text.
c
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tap here
to enter
text.
Click or tap here to enter text.
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text.
L
Click or
tap here
to enter
text.
Click or tap here to enter text.
Lifetimes:
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Stars are formed from collapsing clouds of gas and dust called nebulae or molecular clouds. Nuclear
fusion powers a star for most of its existence. As the core becomes more helium-dominant, stars like the
Sun begin to fuse hydrogen along a spherical shell surrounding the core. When a star has used up all its
nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary
nebula. Stars with around ten or more times the mass of the Sun can explode in a supernova as their
inert iron cores collapse into an extremely dense neutron star or black hole.
Mass of Star
Lifetime
0.08 M
☉
12.6 trillion years
0.5 M
☉
13.6 billion years
1 M
☉
10 billion years
2 M
☉
1 billion years
8 M
☉
107 million years
20 M
☉
10 million years
40 M
☉
3 million years
60 M
☉
1.5 million years
120 M
☉
300,000 years
Star Clusters:
Star clusters are groups of stars that are held together by their mutual gravitational attraction. There are
two main types of star clusters: globular and open clusters. Globular clusters are spherical clusters of
stars that contain tens of thousands to millions of old stars. On the other hand, open clusters are more
loosely clustered groups of stars that generally contain fewer than a few hundred members and are
often very young. Open clusters are located in the galactic plane and are almost always found within
spiral arms. In contrast, globular clusters are located in the galactic halo and are distributed spherically
around the galactic center. Star clusters provide astronomers with crucial insight into stellar evolution
through comparisons of stars' ages and compositions.
Test Yourself – Clusters
Consider a cluster of stars that form together. What should happen to the average color of the cluster
over time?
A. It should get bluer
B. It should get redder
C. It shouldn’t change
Your answer:
B. It should get redder
Open Clusters:
Open clusters are groups of stars formed from the same cloud with similar age. They are loosely bound
by gravity and can contain tens to thousands of stars. Over 1,100 such clusters have been discovered in
the Milky Way, but many more exist. Close encounters with other clusters and gas clouds can cause them
to lose members and disperse into the galaxy. Open clusters typically last a few hundred million years,
while more massive globular clusters can survive for billions of years.
Globular Clusters:
Globular clusters are spherical groups of stars held together by gravity, with a higher concentration of
stars towards their centers. They contain tens of thousands to millions of member stars orbiting in a
compact formation. These symmetrical clusters contain the oldest stars in a galaxy and are located in the
galactic halo. Their age is determined by their almost complete lack of metals, and Omega Centauri is
the largest cluster associated with our Milky Way.
Clusters and Ages:
Determining age of a cluster:
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Test Yourself – Cluster Age
Which cluster is older?
A. M67
B. NGC 188
Your answer:
B. NGC 188