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Stars Characteristics
5/22/2017
What are the characteristics of a star?
Stars differ in…
Mass
Size
Temperature
Color
Luminosity
*Stars can be plotted using these
characteristics on a chart or HR diagram
Star Mass and Composition
Mass and composition determine most
of the properties of a star.
The more massive a star is, the greater
the gravity, the hotter and denser a star
must be.
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Star Size
The diameters of stars range from as
little as 1/10 the Sun’s diameter to
hundreds of times larger.
The mass of stars can be from less
than 1/100 to 20 or more times that of
our Sun.
Stars over 50 times the Sun’s mass are
extremely rare.
Our Sun is a medium sized star.
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Star Energy
The enormous pressure and heat in a
star’s core converts matter into energy.
Stars consist of controlled atomic
reactions called nuclear fusion in which
hydrogen (nuclei) atoms fuse to form
helium (nuclei) atoms.
During each step of the process, mass
is lost and energy is released.
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Star Energy cont.
The energy released causes the star (Sun) to
shine and gives the star its high temperature.
Star stability – the energy from fusion
stabilizes a star by producing the outward
pressure needed to counteract the inward
force of gravity.
Fusion>Gravity = expansion
Fusion<Gravity = contraction
Fusion=Gravity = stable
NOTE: Stable stars are plotted on the HR
diagram.
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Star Temperature
Stars in the sky show tinges of different
colors which reveal the stars’ temperatures.
Blue stars shine with the hottest
temperatures and red stars shine with the
coolest temperatures.
Our Sun is a yellow star having a surface
temperature of about 5,500ºC
Stop and Think
The hottest flame color in a campfire is?
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Star Spectral Classification
Astronomers place stars in
spectral (color) class categories
based on their surface
temperature.
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The Spectral Class Letters are
OBAFGKM
Oh Be A Fine Girl/Guy Kiss Me
Star colors from Hottest to Coolest:
Blue, Blue-White, White, Yellow-White, Yellow, Orange, Red
O or blue is the hottest (>25,000oC) and
M or red is the coolest (3,500-2,000oC)
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Classification of Stars
Spectral Class
Color
Surface
temp (ºC)
Examples
Blue
Above 30,000
10 Lacertae
O
Blue-white
10,000 – 30,000 Rigel, Spica
B
White
7,500 – 10,000
Vegas, Sirius
A
Canopus,
Procyon
F
Yellow-white 6,000 – 7,500
Yellow
5,000 – 6,000
Sun, Capella
G
Orange
3,500 – 5,000
Arcturus,
K
Aldebaran
Red
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Less than 3,500
Betelgeuse,
Antares
M
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Star Luminosity/Brightness
The energy output from the surface of a
star per second measured in watts.
The brightness of a star depends upon
the distance and its luminosity.
Stop and Think
The star Rigel in Orion is about 60,000
times larger than our Sun.
Why does our Sun appear brighter than
Rigel?
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Stop and Think
If person A has a pen flashlight and
person B has a mega flashlight and
both were standing at the front of the
classroom, which light would appear
brighter?
If person A (pen light) stood in the classroom and
person B (mega light) stood at the baseball fields
across the street, which light would appear brighter?
You cannot tell by looking in the sky how bright a star
truly is. The farther away the star is, the less bright it
will appear.
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Apparent Magnitude
The brightness of a star as it appears
from Earth with the naked eye.
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As you can see, the magnitude numbers
are bigger for faint stars, and magnitudes
are negative for very bright stars.
Absolute Magnitude
The true brightness of a star if all stars
were at a uniform distance from Earth
The absolute magnitude of stars is reported in
comparison to our Sun.
Ex. AM of Sun = 1
AM < 1 : brighter than Sun
AM > 1 : less bright than Sun
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Apparent and Absolute Magnitudes for
the ten brightest stars in the night sky
Star
Apparent mag.
Absolute mag.
Sirius
-1.44
1.45
Canopus
-0.62
-5.53
Arcturus
-0.05
-0.31
Rigel Kent
-0.01
4.34
Vega
0.03
0.58
Capella
0.08
-0.48
Rigel
0.18
-6.69
Procyon
0.40
2.68
Betelgeuse
0.45
-5.14
Achernar
0.45
-2.77
The larger the negative magnitude a star has, the brighter it is;
but the larger positive magnitude, the fainter the star.
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What is the Absolute Magnitude and
Luminosity of Procyon? Rigel? Sun?
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The Hertzsprung-Russell Diagram
An H-R diagram plots stars according
to their luminosity and temperature
(or spectral class)
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Location of Stars on an H-R Diagram
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1. Main Sequence Stars
90% of stars occur along the main sequence
Middle aged, stable stars
Main Sequence runs from upper left to lower right
Upper left stars are
Large, hot, luminous
Ex. Blue Giants
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Lower right stars are
Small, cool, dim
Ex. Red Dwarfs
Locate Main Sequence Stars
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2. Upper Right Stars
Red Giants & Super Giants
(Super Super Giants, too)
Large
Cool
Bright
1% of stars
Our sun in 5 billion years will
be 2000 times brighter and
100 times larger (large enough
to expand past Mars orbit)
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Locate Red Giants & Super Giants
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3. Lower Left Stars
White Dwarfs (9% of stars)
Small
Hot
Dim
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Locate White Dwarfs
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Star Mass
Position on H-R depends on mass
The larger the amount of mass in a star,
the more luminous it is.
Giants: more mass, more luminous
Dwarfs: less mass, less luminous
During its lifetime a star will evolve on the
H-R diagram depending on its mass.
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Bibliography
http://www.le.ac.uk/ph/faulkes/web/images/hrcolour.jpg
http://www.unitarium.com/temperature
http://physics.uoregon.edu/~jimbrau/BrauImNew/Chap17/FG17_23.jpg
http://abyss.uoregon.edu/~js/images/40EridanusB.jpg
http://upload.wikimedia.org/wikipedia/commons/thumb/1/15/Redgiants.svg/280px-Redgiants.svg.png
http://startswithabang.com/wp-content/uploads/2008/05/sn1.jpg
http://www.creationofuniverse.com/images/atom/helium.gif
http://www.lancs.ac.uk/ug/hussainw/fusion.jpg
http://www.aip.org/png/images/sn1987a.jpg
http://www.cksinfo.com/clipart/construction/tools/lights/flashlight-large.png
http://www.wildwoodchapel.com/sitebuildercontent/sitebuilderpictures/campfire.jpg
http://projectsday.hci.edu.sg/2001/web%20reports/cat5/14/mstarstructure.jpg
http://webhome.idirect.com/~rsnow/aboutstars.htg/H-RDIAGRAM.gif
http://www.bramboroson.com/astro/images/hrdiagram.jpg
http://outreach.atnf.csiro.au/education/senior/astrophysics/images/stellarevolution/hrwhitecompsml.jpg
http://webs.mn.catholic.edu.au/physics/emery/images/HR%20Evol%20Tracks.jpg
en-US:official%26sa%3DN
http://www.globe.gov/fsl/scicorngifs/Fahrenheit_to_Celsius.jpg
http://images.google.com/imgres?imgurl=http://physics.uoregon.edu/
http://images.google.com/imgres?imgurl
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