Lecture 15 - Patterns in the Stars

What are the two axes of the Hertzsprung-Russell (HR) Diagram?

Surface temperature and luminosity

What is the Main Sequence?

The Main Sequence is a long line on the HR diagram that most stars fall on. It runs from the upper left (hot and luminous) to the lower right (cold and dim)

What are the Giants and Supergiants? What size must they have?

The Giants and Supergiants are stars with very high luminosities but cooler temperatures than the stars on the Main Sequence. They must have very large radii to have high luminosities despite their cool temperatures

What are White Dwarfs? What size must they have?

White Dwarfs are stars that are very hot but very dim. They must very small to be dim despite their hot temperatures.

How does star size vary on the HR diagram? Do stars on the Main Sequence change in size?

Diagonal lines of constant radius on the plot are almost parallel to the Main Sequence, but not quite. Thus stars on the Main Sequence do change in size

How does lifetime vary along the main sequence, and why?

Lifetime decreases as we move up the Main Sequence because larger stars are more massive because they have more hydrogen, but they are burning it at a much faster rate

How does mass vary along the Main Sequence?

Mass increases with hotter temperatures along the Main Sequence

Why is mass the most fundamental property of a star?

The mass of a star determines its luminosity and temperature. Higher mass stars need higher pressures and temperatures to balance the inward pull of gravity, and these lead to higher luminosity because there is more fusion

What is the relationship between mass and lifetime of stars?

High mass have shorter lifetimes than low mass stars because despite having more hydrogen, they are burning it at a much faster rate

Compare the characteristics of a high mass star and a low mass star

A high mass star has high luminosity, a short lifetime, a larger radius, and is blue. A low mass star has low luminosity, a long lifetime, a smaller radius, and is red.

How do stars move off the main sequence?

All stars become larger and redder after exhausting their core hydrogen, so they become giants and supergiants. Most stars end up small and white after fusion has ceased, so they become white dwarfs.

What are pulsating stars? Where do they lie in the HR diagram?

Pulsating stars are unstable because their inward pull of gravity is out of balance with the outward pressure from their core. They lie in the instability strip near the giant and supergiant area on the HR diagram.

What are the two types of star clusters?

Globular clusters and open clusters

What are the characteristics of open clusters?

Open clusters contain a few thousand stars, are often young, found in the disk of our galaxy, and span a few tens of light years across

What are the characteristics of globular clusters?

Globular clusters contain a million stars, are very old, found in the halo of our galaxy, and span 150 light years across

How can we determine the age of a star cluster via the HR diagram?

We can estimate the age of a star cluster by analyzing the main sequence cutoff point, because stars that have exhausted their hydrogen cores will have moved off the main sequence.