Patterns Among Stars

An H-R diagram

(Hertzsprung – Russell)

plots stars according to:

• Luminosity (vertical axis)

• Surface temperature

(horizontal axis)

When you plot many stars this way, patterns

start to emerge

This is going to be a sort of road map of the lives

of stars

Which is more luminous, a giant star or a white dwarf?

A) A giant star

B) A white dwarf

C) Their luminosities are the same

D) Cannot answer without more information

Which has a hotter surface, a giant star or a white dwarf?

A) A giant star

B) A white dwarf

C) Their surface temperatures are the same

D) Cannot answer without more information

A complete description of a star would include not only its spectral class, but

also its “luminosity class”

Our Sun is a G2 V star

(Don’t worry about Ia vs Ib)

Surface temperature and luminosity are related

by radius:

So radius can be determined for any star

from luminosity and temperature

24 4

areaarea

energy energy Total

rTL

Which star would be the most luminous?

A) R = 0.01 RSun

T = 30,000 K

B) R = 1 RSun

T = 10,000 K

C) R = 100 RSun

T = 3000 K

D) They’re all the same luminosity

There are MANY moresmall reddish stars

than

large blue starsfor two reasons:

1. Small stars live longer

2. More small stars form

By simple numbers of stars, our Sun is larger than average!

How long a star can fuse hydrogen

(“Lifetime”)depends on the mass of

the star:

The more massive the star, the faster it uses up

its fuel!

10 MSun 10,000 LSun

Lives only 1/1000 as long as the Sun

10,000,000 yrs

0.3 MSun 0.01 LSun

Lives 30 times longer than the Sun

300,000,000,000 yrs

The life stages of a star…

More details later!

birth

productive life

old folks home

graveyard

Some stars change over short periods of time

“variable stars”

A layer of ionized Helium at just the right depth is opaque.

Heat can’t get out, causing the upper layers to expand.

It expands past the equilibrium point, but eventually the density gets

low enough for the heat to escape. The outer layers cool and contract

again.

It contracts past the equilibrium, making the He+ layer opaque again, trapping heat and starting the cycle

over.

A typical Cepheid variable light curve

Cepheid variable stars are bright and their period is closely related to their luminosity,making them a great distance measuring tool!

Star clusters

Globular clusterM80

Open clusterThe Pleiades or Seven Sisters

Stars in a cluster:• Are about the same distance from us• Formed at about the same time

Found in the haloUp to 1,000,000 stars or more

60 – 150 ly across

Found in the diskUp to 3 or 4 thousand stars

~ 30 ly across

What would happen if we plotted all the stars in a cluster on an H-R diagram?

What would happen if we plotted all the stars in a cluster on an H-R diagram?

They lie along the main sequence only up to a certain

point

“Main sequence turnoff point”

The lifetime of stars at this point is the age of the cluster!

This allows us to compare the ages of various clusters…

Comparing various cluster ages

Open clusters are rarely more than 5 billion years old, usually much younger.

Globular clusters can be much older…

Which cluster is oldest?

A) h + c Persei

B) Pleiades

C) Hyades

D) NGC 188

Globular clusters (this is M4) are found to be up to 13 billion years old!

These clusters started forming before the universe was 1

billion years old

Astro-Cash Cab!

Kaitlin Hoffman

Jake Heaton

Sabra Ross

Brenna Hunter

Mason Harvey

1) What are the axes on the HR diagram?

a) Mass, Radius c) Brightness, Distance

b) Temperature, Luminosity d) Mass, Temperature

2) What do we call this group of stars?

3) Which main sequence star has a higher luminosity?

4) Which star will run out of hydrogen fuel first?