Download Stars III The Hertzsprung

Stars III
The Hertzsprung-Russell Diagram
Attendance Quiz
Are you here today?
Here!
(a) yes
(b) no
(c) here is such a 90’s concept
Today’s Topics (first half)
•  Spectral sequence and spectral types
•  Spectral classification and Annie Jump Cannon
•  OBAFGKM
•  HR diagram
•  Patterns among the stars
•  Main sequence, giants and dwarfs
•  Variation of properties along the main sequence
•  Stellar clusters
•  Open clusters
•  Globular clusters
•  Cluster lifetimes
Spectral Sequence
•  In the late 1800s, Henry Draper and
later Edward Pickering and his team of
female assistants (“computers”) at
Harvard worked classifying thousands
of stars by their absorption spectra
•  At first they classified them by the
strengths of the hydrogen absorption
lines (A, B, C, …) from strong to weak
•  Later, Annie Jump Cannon, who
personally classified over 400,000 stars,
realized that the sequence could be
ordered and simplified as
OBAFGKM
(Oh, be a fine girl/guy, kiss me)
Annie Jump Cannon
Spectral Types
•  As temperature increases, atoms
• 
• 
• 
• 
in a stellar atmosphere become
more and more excited due to
more frequent collisions, so the
lines in the star’s absorption
spectrum change
Eventually, if the temperature is
high enough, some atoms are
ionized and their lines disappear
By knowing the energies of
excitation and ionization of the
various elements, it is possible to
infer the temperature of the
surface of a star
Spectral types are subdivided
using numbers, e.g., G0-G9, from
hotter to cooler within each type
This information can also be used
to determine the composition of
the star
>30,000 K
10-30,000 K
7,500-10,000 K
6,000-7,500 K
5,000-6,000 K
3,500-5,000 K
<3,500 K
Hertzsprung-Russell (HR) Diagram
•  In the early 20th century, two
astronomers independently had
the idea of plotting stars on a
temperature-luminosity plot
•  This diagram is named in their
honor a Hertsprung-Russell
diagram (HR diagram for short)
•  Note that the x-axis has
temperature increasing to the
left (backwards)
•  This is because HR actually
plotted the stars using a measure
of color (spectral type) from O
to M (blue to red)
Hertzsprung-Russell (HR) Diagram
•  Note that from the Stefan-
Boltzmann law (L ∝ T4R2),
we can draw lines of constant
radius (Interactive Figure 15.10)
•  As expected, stars in the upper
right of the diagram are larger
than those in the lower left
•  Thus, those in the upper right are
called giants or supergiants
•  Those in the lower left are called
dwarfs
Categorization of Stars
•  Stars are not distributed randomly
• 
• 
• 
• 
• 
around the diagram
>90% of all stars fall along a curved
line known as the Main Sequence
(Sun, Spica, Vega, Proxima Centauri)
These stars are undergoing fusion of H
to He in their cores
Stars in the upper right are red and
blue giants or red and blue
supergiants (Rigel, Deneb, Aldebaran,
Antares, Betelgeuse)
Stars in the lower left are white dwarfs
(Sirius B, Procyon B)
These categories represent an
evolutionary sequence
Lecture Tutorial: HR Diagram, pp. 117-118
• 
Note: there is another measure of stellar luminosity
called Absolute Magnitude. We are not learning
about it in this class, and you are not responsible to
know about it. For the LT, answer questions about
the Absolute Magnitude using the diagrams, but
otherwise, don’t worry about it.
• 
Work with one or more partners - not alone!
• 
Get right to work - you have 10 minutes
HR Diagram Quiz I
What do the colors of stars in the Hertzsprung-Russell
diagram tell us?
a)  The size of the star
b)  The luminosity of the star
c)  The surface temperature of the star
d)  The core temperature of the star
e)  The mass of the star
HR Diagram Quiz II
On an HR diagram, stars at the same temperature are
found
a)  aligned horizontally (i.e., next to each other)
b)  aligned vertically (i.e., one above the other)
c)  along the main sequence
d)  There is no relationship between their positions.
HR Diagram Quiz III
On an HR diagram, stars with the same luminosity are
found
a)  aligned horizontally (i.e., next to each other)
b)  aligned vertically (i.e., one above the other)
c)  along the main sequence
d)  There is no relationship between their positions.
HR Diagram Quiz IV
A red giant of spectral type K9 and a red main sequence
star of the same spectral type have the same
a)  luminosity
b)  temperature
c)  Both are the same.
d)  Neither is the same.
e)  Not enough information to tell.
Properties of Main Sequence Stars
•  As noted previously, there are
links between a stars mass, radius,
temperature, and luminosity
•  All the stars on the Main Sequence
are undergoing fusion of H to He
in their cores
•  The “sequence” goes from
•  Upper left: hot (10-30,000+ K), bright
(100-10,000+ L!), blue, massive
(5-30+ M!) stars
•  Lower right: cool (3-4,000 K), dim
(0.001-0.01 L!), red, low-mass
(0.08-0.3 M!) stars
•  The Sun is somewhere in the middle
(5,800 K, 1 L!, 1 M!, yellow)
Properties of Main Sequence Stars
There are many more cool, red,
low-mass stars than hot, bright,
high-mass stars
•  There are two reasons for this:
1.  It is harder to assemble the
material needed for a high-mass
star (10-30 M!) than for a lowmass star (0.1 M!)
2.  High-mass stars live a much
shorter time on the Main
Sequence (and overall) than lowmass stars
• 
Lifetimes of Main Sequence Stars
lifetime ∝
amount of stuff to create energy
rate stuff is used up
amount of stuff ∝ mass of star ∝ M
rate stuff used up ∝ lum. of star ∝ L
L ∝ M 4 (high - mass)
L ∝ M 3 (low - mass)
so
M
1
∝
for high - mass stars
4
3
M
M
M
1
lifetime ∝ 3 ∝ 2 for low - mass stars
M
M
lifetime ∝
Lifetimes of Main Sequence Stars
Recall t! ≈ 10 billion years
M
1
lifetime ∝ 4 ∝ 3 for high - mass stars
M
M
so t(10 M!) ≈ (10 billion) × (1/10)3
≈ (10 billion)/1000 ≈ 10 million years
lifetime ∝
M
1
∝
for low - mass stars
3
2
M
M
so t(0.1 M!) ≈ (10 billion) × (1/0.1)2
≈ (10 billion) × 100 ≈ 1 trillion years
≈ 70 × (age of the Universe)
Main Sequence Quiz I
If two stars are on the main sequence, and one is more
luminous than the other, we can be sure that the
a)  more luminous star will have the longer lifetime
b)  less luminous star is the more massive
c)  more luminous star is the more massive
d)  more luminous star will have the redder color
Main Sequence Quiz II
Stars that begin their lives with the most mass live longer than less
massive stars because it takes them a lot longer to use up their
hydrogen fuel.
a)  Yes, with more hydrogen to burn, massive stars can live for
billions of years.
b)  Yes, low mass stars run out of hydrogen very quickly and have
very short lifetimes.
c)  No, stars have similar lifetimes despite their different masses.
d)  No, more massive stars are much more luminous than low mass
stars and use up their hydrogen faster, even though they have
more of it.
Homework
•  For homework, complete the ranking tasks,
Luminosity, Exercises 2-4, and Stellar
Evolution 1 (download from class website)
Star Formation
Attendance Quiz
Are you here today?
Here!
(a) yes
(b) no
(c) here is such a 90’s concept
Today’s Topics II
•  The battle against gravity
•  Interstellar medium
•  Interstellar dust
•  Molecular clouds
•  How stars form from molecular cloud cores
•  The role of gravity
•  Stellar masses
•  Low-mass limit and Brown dwarfs
•  High-mass limit
•  Stellar mass distribution
The Battle Against Gravity
•  Recall that a star like the Sun is
balancing between the crushing
force of gravity, that would like to
squeeze it smaller, and the outward
pressure of the radiation caused by
the nuclear fusion in its core
•  This battle is the life-story of every
star, from before its birth to its end
as a white dwarf, neutron star, or
black hole
•  The balance of a main sequence star
is simply an extended interlude in
this battle
The Interstellar Medium
•  To understand the formation of stars, we have to ask, “What are they made of?”
Answer: About 99% hydrogen and helium gas
•  Where might this gas come from?
Answer: The space between the stars is not empty!
The Interstellar Medium
•  There are clouds of gas and dust between the stars
•  Like the stars themselves, they are made primarily of hydrogen and helium
•  Since they are colder than stars, we can’t see them in visible light
•  The dark patches below are dust particles absorbing the light of background stars
The Interstellar Medium
•  Dust particles (carbon and silicate particles the size of cigar smoke or smaller)
absorb and scatter light creating dark patches in the sky
•  What do you notice about the image below (besides the “hole”)?
•  The bright stars are blue = hot = massive = young (~106 yrs)
•  The stars near the edge of the dark patch look reddish
Interstellar Dust
•  Although interstellar dust particles make up only about 1% of the interstellar
medium (which is itself about 1% of the mass of the stars in the Galaxy), they
play a disproportionate role in how we study stars and star formation
•  Dust grains vary in size from 20-1000 nm (0.02-1 µm), similar to the wavelength
of visible light (400-700 nm or 0.4-0.7 µm)
Interstellar Dust
•  Since the size of a dust grain is approximately the same as the wavelength of
light, dust grains are very efficient at absorbing and scattering visible (and even
IR) light
•  Furthermore, the degree to which light is scattered is a function of wavelength:
bluer light is scattered much more than redder light (Scattering Demo)
Interstellar Dust
•  Thus, dust grains redden light from stars and other objects as the light passes
through interstellar clouds (explaining the reddened stars at the edge of the cloud)
•  If there is enough dust, everything can be blocked, causing dark patches
•  It is not a coincidence that there are young, blue stars near this dark cloud
•  Stars are forming in these dense clouds of dust and gas (see next slide)
Dark Globule Barnard 68
Infraredlight
Visible
light
Interstellar Dust Emission
•  These same dust particles can emit thermal radiation
•  Typical temperatures are 10-30 K (~300-1000× cooler than a star), so emission
peaks 300-1000× longer in wavelength (by Wein’s Law), at 100-300 µm
•  The left hand image is the familiar constellation of Orion (note the nebula)
•  The right hand image is a 100 µm image of the same part of the sky
Molecular Clouds
•  Although dust is often easiest to see in the interstellar medium, most (99%) of
• 
• 
• 
• 
the material between the stars is gas, primarily hydrogen
The densest clouds allow molecules to form, primarily H2: molecular clouds
Other molecules: CO (shown below), OH, H2O, NH3,…, CH3OC2H5, HC11N
The clouds below contain millions of solar masses of material
As of May 2016, over 200 interstellar molecules have been detected in space,
mostly at radio (rotation) and infrared (vibration) wavelengths
Star Formation in Molecular Clouds
•  How do stars form in molecular clouds?
•  As a cloud (or part of a cloud) contracts under gravity, the gas heats up
•  A combination of thermal pressure and pressure from magnetic fields keeps a
cloud from collapsing for a time
•  As the cloud continues to contract, it passes a threshold where it keeps collapsing
until the densities and temperatures are high enough for nuclear fusion to begin
•  This computer simulation shows how a cloud fragments into small, dense clumps
over time. These clumps fragment into dense cores from which stars form
Lecture Tutorial: Star Formation and
Lifetimes, pp. 119-120
• 
Work with one or more partners - not alone!
• 
Get right to work - you have 10 minutes
• 
Read the instructions and questions carefully.
• 
Discuss the concepts and your answers with one another.
Take time to understand it now!!!!
• 
Come to a consensus answer you all agree on.
• 
Write clear explanations for your answers.
• 
If you get stuck or are not sure of your answer, ask another
group.
• 
If you get really stuck or don’t understand what the Lecture
Tutorial is asking, ask me for help.
Stellar Lifetime Quiz
Consider the information given below about the lifetime of
three main sequence stars A, B, and C.
Star A will be a main sequence star for 2 million years
Star B will be a main sequence star for 70 million years
Star C will be a main sequence star for 45,000 million years
Which of the following is a true statement about these stars?
a) Star A has the greatest mass
b) Star C has the greatest mass
c) Stars A, B and C all have approximately the same mass
d) None of the above
Stellar Masses
•  If M < 0.08 M! ≈ 80 Mjupiter, then the
temperature will never be high enough to
start fusion
•  Such objects, called Brown Dwarfs, are
warm from the heat of gravitational
contraction but are much fainter than stars
Brown Dwarfs
Stellar Masses
•  If M < 0.08 M! ≈ 80 Mjupiter, then the
temperature will never be high enough to
start fusion
•  Such objects, called Brown Dwarfs, are
warm from the heat of gravitational
contraction but are much fainter than stars
•  As such cooler objects have been found 2
new spectral classes (L, T) have been added
OBAFGKMLT
Oh Be A Fine Girl/Guy, Kiss My Lips Tenderly
Only Boring Astronomers Find Gratification in
Knowing Mnemonics Like This
Stellar Masses
•  If M > 150 M!, then the large
• 
• 
• 
• 
amount of radiation pressure actually
blows off some of the material trying
to form the star
Thus stellar masses are limited to
0.08 M! < M < 150 M!
Because of the details of how
molecular clouds fragment into
smaller clumps and cores, many
more low mass stars are formed than
high mass stars
The same phenomenon occurs
throughout nature (e.g., in a rock pile
or bag of cookies)
The paucity of high mass stars is
made more stark by the short lives
they lead