1. Chapter 27 Stars and Galaxies The Universe is immense.
We are only now beginning to comprehend how big.
For Earth Day
U.S. Dept of the Interior: Engaging the next generation
Scale of the Universe
Scale (CERN version)

2. Stars vary tremendously in size
Red Giants
Dwarf
Stars vary in size and mass.
From smaller than earth to 1,000 × larger than our sun.
From 1/50th to 50 × the sun’s mass.
Star size comparison

3. 27.1 The Characteristics of Stars
Star – body of gases releasing radiant
energy in the form of light and heat.
Represents the most fundamental
building blocks of galaxies.
We characterize stars by...
composition
temperature
distance from earth
brightness.
What is the name of our closest star?

4. Composition of Stars Composition:
Using spectroscopy, we can determine elements based on wavelength
Composition:
Hydrogen is the most abundant element of stars
Our sun is 91.2% hydrogen and 8.7 % helium %
Most stars are 70% hydrogen and 28% helium
Hydrogen is the main element that fuels stars!

5. Temperature of Stars
The surface temperature of a star determines its color
Most stars are between
2,800° and 24,000°
OLD text p. 547
Blue stars can be as hot as 50,000°
Mnemonic devices:
Oh Be A Fine Girl, Kiss Me
Oh Boy, An F Grade Kills Me
Our sun has a surface temperature of ~ 5,500°

6. Distance from Earth
We measure small distance in space in Astronomical Units (A.U.),
the average distance between Earth and the Sun.
We measure large distances in space in Light Years.
Light year – the distance that light travels in one year.
The speed of light is nearly 300,000,000 m/s
(Actually 299,792,458 m/s or 670,616,629 mph in a vacuum; slows slightly through matter)
Other cool facts...
It takes ~ 8.3 minutes for the sun’s light to reach Earth (8 light minutes)
Proxima Centauri (closest star) is 4.3 light years away
Sirius (brightest star) is 9 light years away
Polaris is 700 light years away
Farthest we can see? Deneb is the 20th brightest star, but about
1,550 light-years away

7. How do we know distances?
Parallax – the apparent shift in location of an object
due to a change in the position of the observer
d = 1/p
d = distance to star in parsecs
p = parallax angle of the star in arc sec
1 Parsec = light years
New Text p. 702

8. Brightness Apparent magnitude – brightness
of a star as it appears from earth
Absolute magnitude – true brightness;
how bright a star would appear from
10 Parsecs, or 32.6 light years away.
New txt p. 703, Old txt p. 553

9. Classification of Stars
H-R Diagram
(Hertzsprung – Russell Diagram)
Plots surface temperatures against absolute magnitude.
Brightness of most stars increases as surface
temperature increases.
Stars pass through series of stages based on mass.
Most stars fall in the middle of diagram (Main Sequence)
New txt p. 704, Old txt p. 554
Main Sequence: cool, dim, red stars at lower right
to hot, bright, blue stars at upper left.
Giants & Supergiants: cool, bright in the upper right
White dwarfs: hot, dim in the lower left

10. Classification of Stars
Types of stars: