Star Properties
(Chapter 8)
Student Learning Objectives
• Classify stars
• Explain how star properties are related.
What is the parallax method for measuring distance?
 Stellar parallax is a change in an object’s apparent
position caused by a change in the observer’s position.
 Distance in parsecs is inversely related to the
parallax angle in arcseconds.
d= 1
p
Example: Sirius is a very bright star in our sky.
It has a parallax angle of 0.379 arcseconds.
Practice
Calculate the distance in Light Years
(1 pc = 3.26 LY)
1) Parallax angle = 0.2 arcseconds
2) Vega: Parallax angle = 0.12 arcseconds
3) Polaris: Parallax angle = 0.01 arcseconds
How are stellar properties related?
Magnitudes are a
measure of brightness,
and brightness is the
direct result of energy
output.
Brightness refers to the
number of photons.
Intrinsic to Star
Apparent on Earth
Mathematical relationship between
brightness & energy
B= L
4pd2
Magnitudes & Distance
Absolute Magnitude is Magnitudes can be used
the magnitude a star
to determine distance.
would have at a distance
of 10 parsecs.
m – M = 5log10(d)
Luminosity and Flux measure energy from the star.
L = (Area)sT4
The more mass a star
has, the greater the
energy output.
L = M3.5
Mass → Temperature → Energy → Brightness
Practice
Star A: Large Surface Area at 6,000 K
Star B: Small Surface Area at 6,000K
1) Which would have a greater luminosity?
2) Which would have a greater flux?
3) Which would be brighter if both were the same
distance from us?
What does a star’s spectral class indicate?
The spectral class is determined from a star’s
atomic spectrum.
All stars have similar compositions
Stars have different temperatures
OBAFGKMLT
Type
Color
Temperature
O
B
Blue-Violet
Blue
30,000+ K
18,000 K
A
F
G
K
Blue-White
White
Yellow
Orange
10,000 K
7,000 K
5,500 K
4,000 K
M
L
T
Orange-Red
Red
Dark Red
3,000 K
2,000 K
700-1,300 K
http://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectral_class.html
What is shown by an H-R diagram?
An H-R
diagram is a
graph of
luminosity
compared to
temperature.
 The locations of the stars
on the graph indicates
where they are in the
stellar evolution process.
HR Graph Shows
Distance
Radius
Mass
Luminosity Class
Age of Cluster
Globular Cluster M55
Cluster Ages
 The HR Diagram is further divided into Luminosity
Classes based on line width.
I
Super
Giants
II
Bright
Giants
III
Giants
IV
Sub
Giants
V
MainSequence
Why are binary stars important?
The mass of a binary system can be determined from
the orbital period.
(m1 + m2)P2 = a3
Visual Binaries
See orbital periods
Spectroscopic Binaries
Doppler shift leads to orbital period
Eclipsing binaries
Light curves lead to orbital periods