patterns among stars. an h-r diagram (hertzsprung – russell) plots stars according to: luminosity...
TRANSCRIPT
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?