Chapter 11
Surveying The Stars
Properties of Stars

Distance

Speed

Radius

Brightness (Intensity)

Temperature

Luminosity ( L ) - the amount of energy a star radiates
per unit time = power
(e.g. Lsun=
4 x 1026 Watts.)

Intensity ( I ) – Power per unit area (Power/Area)
Intensity is measured in Watts/m2.

Stefan-Boltzmann Law - a star of temperature T
radiates an amount of energy each second (Power)
equal to sT4 per square meter ( Intensity )
Definitions
I = Power/area = sT4.
L = Power output of the star.
I = (Luminosity)/(surface area of a sphere).
I = L/4d2
Measuring A Star’s “Brightness”

Inverse-Square Law - the apparent brightness
(Intensity) of a star decreases with increasing
distance from Earth
L
BI 
2
4d
The Luminosity of
the star can be
written as:
L  4R sT
2
4
Where:
L = Luminosity
R = Radius of the star
s= Stefan-Bolzmann
constant
T = Surface
temperature in K
The Inverse Square Law for Light
power
I
area
L

2
4d
area = 4d2
The Magnitude System

Apparent Magnitude - logarithmic scale of
brightness for stars (e.g. the size of the dots
on star charts)

Absolute Magnitude - the apparent
magnitude that a star would have if it were
10 parsecs away from Earth
Measuring a Star’s Distance

Parallax - the apparent change in the position
of a star due to the motion of the Earth

Nearby objects exhibit more parallax that
remote ones.
Distance in Parsecs =
1
Parallax Angle
d
1
p
Stellar
Parallax
Stellar Parallax
Stellar Distances

Parsec - the distance corresponding to a parallax angle
of exactly 1” (1 arc second) and the baseline is 1A.U.
(distance between the earth and sun)

1 parsec = 3.26 light years

1 arc second = 1/3600 degrees

Light-year - the distance that light travels in one year.

Proxima Centauri is 4.2 light years from Earth
(24 trillion miles).
Stellar Motion
Measuring A Star’s Temperature

A star’s surface temperature can be
determined from its color using Wien’s
Law.
• Red
 coolest star
• Orange
• Yellow
• White
• Blue
 hottest star
Hubble
Space
Telescope
view
through
the
Galaxy
reveals
that stars
come in
different
colors
Stellar Spectroscopy


Stellar Spectroscopy - the study of the
properties of stars by measuring absorption
line strengths
Spectral Class - classification of star
according to the appearance of their spectra
O B A F G K M
Binary Stars

Optical Double - two stars that just happen
to lie in the same direction as seen from
Earth

Visual Binary - two stars that are orbiting
one another and can both be seen from
Earth
Binary Stars

Spectroscopic Binary - two stars that are
found to orbit one another through
observations of the Doppler effect in their
spectral lines

Eclipsing Binary - two stars that regularly
eclipse one another causing a periodic
variation in brightness

Light Curve - a plot of a variable star's
apparent magnitude versus time
Sirius A and Sirius B at ten year intervals
reveals a binary system
Apparent Brightness Of An Eclipsing Binary System
Spectral Lines of a Binary System – Alternating Doppler Shift
Compare these spectra.
Spectrum of Hydrogen in Lab
Spectrum a Star
What do these spectra tell us about the star?
Compare these spectra.
Spectrum of Hydrogen in Lab
Spectrum a Star
What do these spectra tell us about the star?
Compare these spectra.
Spectrum of Hydrogen in Lab
Spectrum a Star…..Day 1
Spectrum a Star…..Day 2
Spectrum a Star…..Day 3
Spectrum a Star…..Day 4
What do these spectra tell us about the star?
Mystery Star Properties
1. The star appears as a point of light through a telescope.
2. The absorption lines appear split and move over a
4 day period.
3. The brightness of the star also varies over 4 days.
Question: Why do you think the brightness of the
star is varying?
Answer: This could be an eclipsing binary star
system that cannot be resolved by a
telescope!
The H-R Diagram

Hertzsprung-Russell Diagrams - plots of
luminosity versus temperature known stars

Most stars on the H-R diagram lie along a
diagonal curve called the main sequence.
Stellar Luminosity Classes







CLASS
Ia
Ib
II
III
IV
V
DESCRIPTION
Bright supergiants
Supergiants
Bright giants
Giants
Subgiants
Main-sequence stars/dwarfs
Along the main
sequence, more
massive stars are
brighter and
hotter but have
shorter lifetimes
Stellar Lifetimes

Star
Spectral Mass Central Luminosity Estimated
Type
(Solar) Temp(K) (Solar Lum) Lifetime
•





Rigel
Sirius
-Centauri
Sun
P-Centauri
(106 Years)
B8Ia
A1V
G2V
G2V
M5V
10
2.3
1.1
1.0
0.1
30
20
17
15
5.0
44,000
23
1.4
1.0
0.00006
20
1,000
7,000
10,000
>1,000,000
Variable Stars

Stars that have a change in brightness over
time are called variable stars.

Examples:
– Eclipsing binary stars
– Cepheid variables
– RR Lyra variables
A Typical Light Curve For A Cepheid Variable Star.
~ 15 days
H-R
diagram
with the
instability
strip
highlighted
Star Clusters

Open Clusters: Loosely bound collection of tens to
hundreds of stars, a few parsecs across, generally found
in the plane of the Milky Way.

Globular Clusters: Tightly bound, roughly spherical
collection of hundreds of thousands , and sometimes
millions, of stars spanning about 50 parsecs. Globular
Clusters are distributed in the halos around the Milky
Way and other galaxies.
H-R Diagram for the Pleiades.
Missing
upper main
sequence
stars
B6
Main Sequence
turn off –
Pleiades ~ 100
million yrs old
Open Cluster
Globular Cluster
Star
Clusters
H-R Diagram for the globular cluster Palomar 3.
Main
sequence
turnoff
indicates age
~ 12-14
billion years
Matching Questions
1. The temperature of a star can be determined from its_____________.
2. The pattern of the absorption spectral lines for a star contains information about
a star’s________________.
3. The Doppler shift of a star's spectral lines tells us something about the
star’s_______________.
4. The distance of a star from Earth can be determined from the
star’s_______________.
5. The radius of a star can be determined from its ________________.
(a)
(b)
(c)
(d)
(e)
composition.
parallax shift.
motion.
luminosity and temperature.
color.
H-R Diagram Questions
1. What property is measure along
the horizontal axis?
2.
… along the vertical axis?
3. Where are the red giants?
4.
… the white dwarfs?
5.
… the hottest stars?
6.
… the coolest stars?
7.
… the largest stars?
8.
… the smallest stars?
H-R Diagram Questions
9. Where are O class stars?
10.
… M class stars?
11.
… G class stars?
12. Where is the Sun?
13. Where are the high-mass mainsequence stars?
14. Where are the low-mass mainsequence stars?
15. Where are the oldest stars?
16. Which stars along the main-sequence
live the longest?
End of Chapter 13