Brightness and Color of Stars
Introduction to the Nature of Stars
What factors contribute
to a star’s brightness ?
(What factors determine how bright a star looks when we go outside and look up?)
What factors contribute to a star’s brightness ?
• distance
• temperature
• size
Proper
Motion
• Which stars will
have the highest
proper motion?
• Can we use the
doppler effect?
What kind of relationship is this?
Magnitude
• An inverse logarithmic brightness scale that
roughly parallels the untrained eye’s ability to
perceive brightness changes
• Originally qualitative; later based on 5 mag
difference = 100x change in brightness
• Scale is relative
• Was “zeroed” by using Vega
• There are several ways to define magnitude: two
most common are apparent visual magnitude
and absolute magnitude
How do astronomers
• Apparent
measure stars
magnitude –
brightnesses?
how bright an
object appears
from earth
• Absolute
magnitude –
how bright at
ten parsecs
Measure a star’s absolute
magnitude. This is a good
approximation of overall
luminosity
Sub-2 is the fainter star; sub-1 is the brighter star
This equation
compares two
stars – why
would we do
this?
Distance Modulus
How are distance and
magnitudes related?
m – M = 5log(d) – 5
m= apparent magnitude
M= absolute magnitude
d= distance in parsecs
1 parsec is the distance where the PARallax = 1 arcSEC (3.26 ly)
How do we determine how size and temperature
contribute to a star’s brightness?
• Determined
by
experiment
• Blackbody
• Area of
radiating
object
• T4
What factors contribute to a star’s brightness ?
L=
2
4πr
4
σT
The Stefan-Boltzmann Law – Where
did this come from? What does it
mean?
The Sun’s Surface
UBV (Johnson Morgan) System
• A series of color
indices
• U,B,V,R,I
• There are other
systems in use
Stars do have different
temperatures!
Who are these dudes?
What is the H-R diagram?
• Began as an experiment.
• What was the question??
What is the H-R diagram?
• Began as an experiment.
• What was the question??
The H-R diagram
Is now used as an
important tool
for discovery.
How?
The H-R
Diagram
bright
• Aka color
magnitude
diagram
• Plot of
luminosity vs
surface
temperature
faint
hot
cool
What factors contribute to a star’s brightness ?
L=
2
4πr
4
σT
The Stefan-Boltzmann Law – Does
the H-R diagram conform to this
law?
Comparison of Main
Sequence Star Sizes
Location on
Main
Sequence
Depends on
Mass
• Many systems are
multiple
• Visual – observe
over time
• Spectroscopic –
see doppler shifts
from relative
motion
• Spectrum –
overlapping
spectrum of two
stars
• Eclipsing – light
curves
Using
Kepler’s Laws
we can
determine
the masses –
this is still the
only direct
way we have
of
determining
the masses of
stars.
(M1 + M2) P2 = a3
Eclipsing Binary
Vs. Brightest
Deneb: 19th 1500ly 60,000 Ls
Vega: 5th 25ly 35 Ls
Altair: 12th 17ly 10 Ls
Arcturus: 4th 37ly 210Ls
Antares: 16th 600ly 65,000Ls
Rigel: 7th 770ly 66,000Ls
Capella: 6th 42ly 78Ls
Pollux: 17th 34ly 32Ls
Procyon: 8th 11ly 8Ls