Download Starlight And Atoms

Lecture 8
Starlight And Atoms
Announcements
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Homework 5 – Due Monday, Feb 26
• Unit
• Unit
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23:
24:
25:
55:
RQ
RQ
RQ
RQ
1,
1,
1,
3,
P 2, TY 3
2, TY 2
P2
P4
The Basic Idea
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Stars produce light.
Star light tells us a lot
about the star it
comes from.
NEED to use star light
to study stars because
the material from a
star has never been
collected for study!
Where Does Star Light Come
From?
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
The light we detect
from a star comes
from the star’s
photosphere.
The photosphere
consists of the
glowing gases in
the outer “surface”
of a star.
How Can A Cloud Have A Surface?

A photosphere is
made of a thin gas, so
how can it be a
surface?
• A photosphere isn’t a
solid surface like the
Earth’s surface, BUT…
• It glows so brightly you
can’t see through it to
the gases below.
• It’s a surface in the
sense you can’t see
under it, just like the
surface of the Earth.
Why Does A Photosphere Glow?
A photosphere glows because it is
very hot!
 The hotter something is, the more
light it produces AND the higher
energy (on average) that light
becomes.
 Makes hotter objects look BRIGHTER
and BLUER than cooler ones.

What is Heat?

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To understand why
hot things glow, we
need to understand
what heat is.
Heat is a way to
measure how quickly
and violently the
atoms or molecules in
a substance are
vibrating.
These vibrations cause
the atoms and
molecules to collide.
Heat Creates “Free Electrons”


Colliding
atoms
sometimes
create…
…a free electron

In any substance
some of the atoms
and molecules vibrate
faster than others.
The hotter something
is, the more violently
its atoms and
molecules vibrate on
average.
Some collisions
between
atoms/molecules are
violent enough to
“bump” electrons off
of their atoms. These
are called free
electrons.
More Heat Means More Free
Electrons

Since a hotter
substance has faster
moving atoms on
average:
• Collisions are more
frequent.
• Collision are, on
average, more violent.
• More collisions are able
to create free
electrons.

Cool gas … not many free
electrons.
So the hotter
something gets, the
more free electrons it
has.
Hot gas … lots more free
electrons.
Free Electrons Also Collide With
Atoms
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
In a hot gas, the
free electrons also
collide with each
other, and the
other atoms in the
gas…
These collisions
accelerate the
electrons (change
their speed and
direction).
The Light Comes From Accelerated
Electrons!

When an electron is accelerated it gives off
light.
• The more the electron is accelerated the more energy
the emitted light has.
• For light, higher energy makes bluer light.

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Very low energy light is invisible: infrared, microwaves, and
even radio waves.
Low energy light looks red to us.
Light with more energy than red light looks orange. Even
more energy makes it look yellow, then green, then blue,
then violet.
Light with even more energy than violet is ultraviolet (and
is invisible to us).
X-rays and gamma rays have even more energy than
ultraviolet.
More Accelerated Electrons Means
More Light!
REMEMBER: The hotter something is,
the more free electrons it has.
 So there are MORE collisions, and we
make MORE light.
 CONCLUSION:

• A hotter substance will produce (on
average):
BLUER light (higher energy light)
 MORE light (more photons because there
are more free electrons to make light)

Blackbody Radiation
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
The light (EM
radiation) produced by
free electrons in a hot
object is called
blackbody radiation.
“Hot” is relative!
Anything above
absolute zero
produces some
blackbody radiation.
The Radiation Laws
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
There are two natural laws (equations)
that describe Blackbody Radiation.
How Bright Is The Light?
• Stefan-Boltzmann Law:
E =  T4
• E = amount of light energy produced per
square meter from the object’s surface each
second (in Watts/m2)
• T = temperature of the surface (in Kelvin)
•  = 5.67 × 10-8
The Radiation Laws
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What Color Is The
Light?
• Actually, light of all colors
is produced, but the most
common color of light is
given by Wein’s Law:
max = 3,000,000 / T
• max is the wavelength of
the most common color of
light (more photons are
this wavelength than any
other). The units are
nanometers!
• T is the temperature
(again, in Kelvin)
What is this “Kelvin”?

There is a
temperature that is so
cold that atoms and
molecules stop
moving completely:
absolute zero.
• At -273ºC (or -459ºF)

Lord Kelvin invented a
new temperature
scale, based on the
Celsius scale, where
the zero point is
absolute zero.
The Kelvin Scale
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On the Kelvin scale, there are no
negative temperatures (no such thing as
a temperature below absolute zero).
Subtract 273 from the Kelvin
temperature to get the Celsius
temperature.
Unlike common temperature scales,
which are ratios based on two fixed
points, the Kelvin is an absolute unit of
measure and so is not expressed in
“degrees”.
Using The Radiation Laws
The sun’s surface temperature is
5,800 K. How much light energy
comes from each square meter of
the sun’s surface each second?
 Solution:
E = ( = 5.67 × 10-8)(5,800 K)4
E = 64,164,532 W/m2
(The W stands for “Watts”)
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Using The Radiation Laws
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What is the most common color of light
the sun produces?
Solution:
max = 3,000,000 / (5,800 K)
max = 517 nm
(This is actually green light, but it is
mixed in with enough yellow that our
eyes see this color as yellowish)
Daily Grade 8 – Question 1

The wavelength of maximum intensity
that is emitted by a black body is
A. proportional to temperature.
B. inversely proportional to temperature.
C. proportional to temperature to the fourth
power.
D. inversely proportional to temperature to the
fourth power.
What color are the stars?
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We can use Wein’s
Law to figure this one
out…
First, all stars produce
light at all colors! And
we see all colors
together as white!
But, the fact that
more photons of one
color are produced
than any other gives
the white light from
the star a tinge of
color.
What color are the stars?

The coolest stars are
around 3,000 K
• max = 1,000 nm
• That’s infrared
light, which means
that in visible light
the star will
produce more red
photons than any
other color.
• The star will look
reddish-white.
What color are the stars?

The sun has a
temperature of
about 5,800 K.
• max = 517 nm
• Tinges the sun’s
white light
yellow.
• The star will look
yellow-white.
What color are the stars?

The hottest stars
have temperatures
over 40,000 K.
• max = 75 nm
• This is ultraviolet
light.
• In visible light the
star produces more
blue light than any
other color.
• Makes the star look
blue-white.
The Color Index
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So very hot stars
are bluish, and
very cool stars are
reddish.
But how do we
quantitatively
measure a star’s
color.
In astronomy, use
the color index.
The Color Index

Procedure:
• Take a picture of a star
through two different
color filters.

Two common ones to use
in astronomy is a blue (B)
filter and a green “visual”
(V) filter.
• Measure the magnitude
of the star in each filter
(i.e. how bright it looks in
each filter).
• Take the two magnitudes
and subtract them:
B–V
This number is called the
“B-V Color Index”
B band
V band
The Color Index
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Remember, the brighter a star is the
smaller its magnitude!
A blue star:
• Looks brighter through the B filter than the V
one.
• B-magnitude is smaller than V-magnitude.
• B – V = small number – large number
= negative number
• Blue stars have a negative color index
The Color Index
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Remember, the brighter a star is the
smaller its magnitude!
A red star:
• Looks brighter through the V filter than the B
one.
• V-magnitude is smaller than B-magnitude.
• B – V = large number – small number
= positive number
• Red stars have a positive color index.
Daily Grade 8 – Question 2
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The B - V color index of a star indicates
its
A.
B.
C.
D.
total mass.
radius.
chemical composition.
surface temperature.
A Primer on Atoms
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An atom contains:
• A positive nucleus
composed of two types
of particles:
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Protons – actually
have the positive
charge.
Neutrons – no
charge, but same mass
as protons.
Contains most of the
atom’s mass.
• Electrons (negative
charge) that orbit the
nucleus.

Electrons have very
little mass compared to
protons and neutrons.
Daily Grade 8 – Question 3

Which subatomic particle has a negative
charge?
A.
B.
C.
D.
The electron.
The proton.
The neutron.
Both the neutron and the proton.
Atomic Density
If you could fill a teaspoon
just with material as dense as
the matter in an atomic
nucleus, it would weigh
~ 2 billion tons!!
Different Kinds of Atoms
• The kind of atom
depends on the
number of
protons in the
nucleus.
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Most abundant:
Hydrogen (H), with
one proton (+ 1
electron).
Next: Helium (He),
with 2 protons (and
2 neutrons + 2 el.).
Different
numbers of
neutrons ↔
different
isotopes
Daily Grade 8 – Question 4

Which of the following is true of an
atomic nucleus?
A. It contains more than 99.9% of an
atom's mass.
B. It contains all of an atom's positive
charge.
C. It contains no electrons.
D. All of the above.
Next Time
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Read Units 25 and 55 (yes, I mean 55,
way towards the back. I did say we’d
be jumping around in this book).