03 The Stars Mc Neely 2008
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Transcript of 03 The Stars Mc Neely 2008
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Astronomy
The Stars
Star cluster NGC 457, the Owl Cluster, in the
constellation of Cassiopeia
http://www.buytelescopes.com/gallery/view_photo.asp?pid=10298&sg=9&page=3
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Distances to the Stars
How can we measure the distances to the nearest stars?
Parallax Method:Stars close to our sunMeasure star’s position once, then 6 months laterNearby stars appear to shift back and forth relative
to more distance starsAmount of shift can be used to calculate distances
to stars
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Stellar Parallax
•A star close to our sun appears to shift back and forth compared to the more distant stars in the background
•This diagram is not to scale. The shifting of stars due to parallax is incredibly tiny and is measured in arc seconds Earth in
summerEarth in winter
Star close to our sun
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Parallax Animation
http://www.astro.washington.edu/labs/parallax/solar.htmlCool parallax demo:
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Arc Seconds
Seconds of Arc: Stellar parallaxes are measured in arc seconds (“)
Arc seconds are tiny divisions of a degree Remember that 360° makeup a circle1º = 60’ = 3600”1’ = 60”1” = 1/3600 degree1” = width of an aspirin tablet one mile away
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ParsecDistance in light years to a star showing one arc
second (“) of parallaxParsec (pc): Distance measure = 3.26 light
yearsThe closest star is Alpha Centauri at 4.3 lyTo calculate a star’s distance from observed
parallax:Star’s Distance (in pc) = 1/parallax (“)
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Newton’s Prism Experiment (1665)
•Isaac Newton discovered that light could be broken down into component colors by using a prism
•Newton isolated a single color and passed it through a second prism indicating that the prism wasn’t introducing false colors but that they were a true property of light
•The separated light is known as a spectrum
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Visual Spectrum
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Overhead Spectra Demo
Teacher demo
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Spectrum of Light
Spectroscopy: Analysis of spectraSpectroscope: Device attached to a telescope,
splits light of a star into its spectrum of colorsSpectroscopy reveals what stars are made of
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Spectra of Stars
3 Types of Spectra:1. Continuous: Complete array of all the
rainbow colors. (incandescent light bulb)2. Emission (Bright-Line): A pattern of bright-
colored lines emitted by hot gas (neon light, overhead fluorescent bulb)
3. Absorption (Dark-Line): A pattern of dark lines across a continuous spectrum. Created when light passes through a cool gas. (Stars, the sun)
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Three Types of Spectra
Solid array of rainbow colors
Mostly dark with a few brightly colored lines
Mostly continuous but with a few missing dark lines
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Spectral Tube Demo
Demo
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Animation
Animation 4.1: Spectra
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Spectroscope
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Elements
Atoms are the smallest unit of matter that still retains that matter’s known properties
Atoms create the spectra types100 types of unique atoms are known,
each is an elementPeriodic table
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Bohr Model
Bohr atom model: A nucleus made of positively charged protons surrounded by the same number of negatively charged electrons
Electrons are confined to a set of allowed orbits around the nucleus
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Bohr Model of Nitrogen
•The Bohr model of an atom of the element Nitrogen contains 7 protons (+) in the nucleus and 7 electrons (-) arranged in two energy levels surrounding the nucleus
•Nitrogen is the 7th element in the periodic table due to its having 7 protons
Nucleus
Energy levels
http://education.jlab.org/qa/atom_model.html
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Binding Energy
Binding Energy: energy that holds electrons in place around the nucleus
Each element has its own unique set of allowed electron orbits or energy levels
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Jumping Electrons
Binding Energy: energy that holds electrons in place around the nucleus
Each element has its own unique set of allowed electron orbits or energy levels
Ground State: Undisturbed atom, electrons in allowed orbits, lowest energy
Excited State: Electrons will jump to higher energy levels, release light particle (photon) when falling back
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Excite Me
•When an atom is excited, an electron jumps momentarily to a higher energy level or “orbit”
•When the electron jumps back down to its previous level, the atom emits a photon of light
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Animation
Animation 4.2: Absorption and Emission of a Photon
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Emission Lines
Excited atoms create bright colored emission lines due to their jumping electrons
Each chemical element has its own unique set of bright emission lines
Emission spectra of various elements
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Emission Spectrum of an Element
By analyzing the light of a burning element in the laboratory, its unique bright spectral lines can be observed and recorded
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Absorption Lines
Correspond to the bright emission linesUnique dark absorption lines produced
when an atom absorbs light, causes electrons to jump
Absorption lines represent light subtracted from the continuous spectrum
Observation of emission or absorption lines in spectra allows identification of the chemical element that produced them
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ABSORPTION SPECTRUM• A pattern of dark lines across a
continuous spectrum• Light passing through a cold gas• If plotted as a graph, absorption
lines appear as dips
EMISSION SPECTRUM• A pattern of bright-colored lines with
black gaps.• Hot, glowing gas• If plotted as a graph, emission lines
appear as peaks
Spectra of Hydrogen
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Stellar Spectra
Star light can be broken down with a spectrometer
Dark absorption lines can be observedAbsorption lines can be matched to specific
chemical elementsStellar absorption spectra are created when light
created inside a star passes through relatively cooler layers of gas in the outer atmosphere of the star before traveling into space
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Absorption Spectra
O
B
A
F
G
K
M Coolest stars
Hottest starsThe unique chemical elements in stars create dark, absorption lines, the “fingerprints of the stars”. Various stellar spectra are shown in the image, each band is a different star, and they range from cool stars at the bottom to hot stars at the topOBAFGKM is the spectral sequence of stars
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Stellar Spectra
Spectral Classes
Cooler stars display more absorption lines
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Chemical Composition of StarsSun, first star to be analyzed (1814)Fraunhofer (1814) recorded the strongest absorption
lines, named Fraunhofer lines in his honorAstronomers have since recorded thousands of dark
lines in the sun’s spectrumComparison with spectral lines produced in laboratories
on earth have enabled the identification of 70 different elements in the sun
Stars are primarily hydrogen and heliumThe coolest stars allow actual molecules, compounds of
more than one element, to survive
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Fraunhofer Lines
Some of Fraunhofer’s original drawings of the sun’s spectrum
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Stellar Elements
Comparison of the sun’s absorption spectrum with the emission spectrum of iron allows the identification of iron in the sun’s outer atmosphere
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Betelgeuse Spectra
•Betelgeuse is so cool that the star allows complete molecules such as TiO to survive in its atmosphere
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Spectral Classes
Absorption lines used to classify stars into 7 spectral classes
Originally in alphabetical order, Annie Cannon (1863-1941) rearranged them into the present form of O B A F G K M (“Oh Be A Fine Girl/Guy Kiss Me”)
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Politically Incorrect
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Temperature of Stars
The OBAFGKM sequence of spectral classes is also a temperature sequence
O stars are hottest (> 30,000 K), M stars coolest (< 3,500 K)
Vega & Sirius, O stars (10,000 K)The sun is a G star (5-6,000 K)Antares & Betelgeuse, M stars (3-3,500 K)
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Spectral Class and Star Color
•The diagram shows the star color that corresponds to each spectral class
•Many stars have colors that are visible with the naked eye and in telescopes
Click: Stellar Spectra Mini Exercise
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Star Colors: Big & Little Dippers
Where is Polaris?
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Orion
Betelgeuse
Rigel
•Look for these star colors when you see Orion
•Red areas represent glowing gas in space (nebulas), most is too faint to see with the unaided eye
Belt
Sword
Orion Nebula in Sword of Orion
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Andromedawww.scienceandart.com
Galaxy (M31)
The Andromeda Galaxy is visible to the naked eye—at 2.3 million light years it is the most distant object visible to the naked eye
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Planck Curves and Blackbodies
Blackbody is a theoretical object that absorbs all of the light that strikes it
Absorbed light heats the blackbodyThe blackbody then reemits the light at different
wavelengthsPlanck curves are graphs of the types of light reemitted
by blackbodies of different temperaturesThe shape of a blackbody curve is a function of the
blackbody’s temperature Ideal blackbody curves (Planck curves) were first
discovered by Max Planck in 1900 (photo)
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3 Planck Curves
Three blackbodies at three temps
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•Shown is a plot of intensity versus wavelength for blackbodies at different temperatures.
•Blackbodies at different temperatures will appear as different colors or wavelengths.
•At higher temperatures the most intense wavelengths are shorter.
•The sun is very similar to the 6000 K curve. It’s peak wavelength is in the blue-green portion of the visible spectrum.
•Very hot stars have peak emissions in the ultraviolet and beyond, very cool stars can peak in the infrared.
Wien’s Law governs the peak wavelength, Stephan-
Boltzmann governs the intensity
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Stars and Blackbodies (Planck Curves)
Star color is related to its temperatureHot stars are bluish white, cool stars are
reddishLight emitted by stars follows a Planck
curveA star’s Planck curve can be used to
estimate a star’s temperature
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•A heated iron poker will begin to glow emitting photons. The intensity and wavelength of the radiation changes with temperature.
•As the object heats up, it gets brighter, emitting more photons of all colors (wavelengths), and the color of its greatest light output changes from orange to yellow to blue.
WHEN FIRST HEATED THE WHEN FIRST HEATED THE POKER GLOWS DIMMLY POKER GLOWS DIMMLY AND IS REDAND IS RED
AS THE TEMPERATURE AS THE TEMPERATURE RISES, THE POKER RISES, THE POKER BECOMES BRIGHTER BECOMES BRIGHTER AND GLOWS ORANGEAND GLOWS ORANGE
AT HIGHER TEMPERATURES AT HIGHER TEMPERATURES THE POKER BECOMES EVEN THE POKER BECOMES EVEN BRIGHTER AND GLOWS BRIGHTER AND GLOWS YELLOWYELLOW
Blackbody Radiation Example
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Stars emit light that is close to an ideal blackbody. We can estimate the surface temperature of a star by examining the intensity of emitted light across a wide range of wavelengths.
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Summary: Properties of Stars
Method of parallax—distances to starsSpectroscopy—Composition of starsPlanck curves—Star temperatures
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Apparent Motion of Stars
Earth’s rotation: Stars rise and set Earth’s revolution: Stars change with the
seasonsEarth’s precession: Positions of stars
change in a 26,000 year cycle
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Stars Move Through Space
Stars in our galaxy revolve around the galaxy’s center
220 million years for the sunStars have high velocitiesStars are so distant that they appear still
for thousands of yearsStar motions are revealed by measuring
and comparing positions over periods of time, and by analyzing spectra
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Sun’s Revolution in Milky Way
http://www.envirotruth.org/images/graphics/suns_path.jpg
•The sun revolves around the center of the Milky Way galaxy every 220 my
•The sun is just one of around 100 billion other stars in the Milky Way
•The Milky Way is a flat disk of stars organized into spiral arms; the spiral rotates clockwise in this view
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Space Motions of Stars
Space Velocity: True motion of a star in space, motion of a star with respect to our sun and earth
Space velocity exhibits two components :1. Radial Velocity: Motion towards or away from us
2. Proper Motion: Motion at right angle to us
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Star Motions
V = Space Velocity
Vt = Proper Motion
Vr = Radial Velocity
Yellow circle=1st observation
Blue Arrowhead=2nd observation
star
The star moved along the blue arrow in the observation period
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Doppler Shift
Radial velocity revealed by Doppler shiftA star’s spectrum exhibits a Doppler shift
if it is moving towards or away from usDoppler shift: When a source of waves
are approaching or receding, the observed wavelengths are changed.
Ex: A train rushing by, the pitch of the train’s whistle drops abruptly as it passes
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Doppler Shift of Sound S = A moving source of
sound such as a train whistle
Observer 2 hears a higher pitch whistle, and the pitch drops suddenly when the
source passes
Observer 1 hears a lower pitch whistle than observe 2
For observer 2, the waves are pushed together
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Doppler Shift cont.
Stars: Doppler shift is revealed by the positions of dark absorption lines
Spacing between individual absorption lines of an element remains constant, yet the entire set of lines can be shifted right or left compared to the background spectrum
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Blueshift and Redshift
Blueshift: The star is approachingRedshift: The star is moving away
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Redshift
•Compared to the reference at top, the twin absorption lines are progressively moved towards the red end of the spectrum as a star’s radial velocity increases
•The degree of shift is indicative of the speed of the star’s motion away from us
Reference
Twin spectral lines
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Proper MotionProper motion, star motion perpendicular
to our line of sight to a starAverage proper motion for all visible stars
is less than 0.1” per yearProper motions are very tiny
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Slow Motion
Big Dipper stars will appear much different in 50,000 years due to a high proper motion
Barnard’s Star has the highest observed proper motion. Comparison of telescope sketches or photos over a lifetime would reveal its motion
61 Cygni is also a high-proper motion star
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Big Dipper Proper Motion
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Barnard’s Star
•Highest proper motion star, lies at a distance of 6 light years from earth
•Photo a montage displaying 4 years of the star’s proper motion
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61 Cygni Proper Motion
http://www.almanak.hi.is/61cygni.html
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Hipparcos Web Site
Proper Motion Demohttp://www.rssd.esa.int/Hipparcos/TOUR/
tour.html
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Luminosity
Luminosity, measure of a star’s total light outputLuminosity is the amount of light a star shines
into space each secondSun’s luminosity (L), L = 3.85x1026 watts,
equivalent to 3850 billion trillion 100-watt light bulbs
Rigel in Orion is about 60,000 times more luminous than the sun
Why does our sun appear much more luminous than Rigel?
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Propagation of Light
Propagation, how light travels through space
Light moving away from a star becomes dimmer
Amount of starlight drops off as the square of the distance away from the star (inverse square relationship)
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Propagation•The light from a star is spread further and further apart as it travels into space
•At 1 AU, the star’s light is spread into a 1x1=1 area
•At 2 AU, the star’s light is spread into a 2x2=4 area
•At 3 AU, the star’s light is spread into a 3x3=9 area
•This behavior is an inverse square relationship
Each sphere represents the same amount of light from the star
Star
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Apparent Magnitude
Apparent magnitude, a measure of a star’s brightness from earth
Greek astronomer HipparchusTraditional magnitude scale is 1—6,
represents all stars visible to the unaided eye
“1” were the brightest, “6” were the faintest
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Modern Magnitude Scale
Modern magnitude scale: A 1st magnitude star is exactly 100 times brighter than a 6th magnitude star
Astronomers found that some stars were brighter than 1, requiring zero and negative magnitudes
The smaller the magnitude number, the brighter the star
Magnitude scale from a star map
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Magnitude Scale
•Magnitude 6 and less equal naked eye objects
•Binoculars can see to magnitude 11; An 8-inch aperture telescope can see down to magnitude 14
•Interestingly, the Hubble Space Telescope can see nearly as faint as our sun is bright
> Mag = < Bright
The magnitude scale can be considered a type of number line
8-inBN
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Little Dipper Magnitudes
Little Dipper star map, identify some magnitudes
LD is circumpolar
Polaris is a 2nd magnitude star
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Absolute Magnitude
Absolute magnitude measures a star’s true brightness
Absolute magnitude is the magnitude that a star would have at a distance of 10 parsecs (32.6ly)
Star
Polaris
Sirius
Apparent
Magnitude
+2.3
-1.5
Absolute
Magnitude
-4.6
+1.4
Distance
240 Parsecs
2.7 Parsecs
Sirius is the brightest star in our sky, which star, Sirius or Polaris, is truly the brightest?
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Apparent & Absolute Magnitudes
Canis Major how it appears in our sky—note the brightness of Sirius, the brightest star in apparent magnitude
Canis Major in absolute magnitude, as if all of its stars were brought within 10pc of our sun. The true brightness of the stars is shown
http://media.skytonight.com/images/Sirius_Mags_m.gif
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ComparisonsStar App Mag Abs Mag Spectral
ClassParallax
Alpha Centauri
-0.3 4.1 G 0.750”
Thuban 4.7 5.9 K 0.176”
Barnard’s Star
9.5 13.2 M 0.545”
Altair 0.8 2.3 A 0.202”
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Comparisons Cont.
Which star is:(a) hottest? __________ (b) coolest? __________(c) brightest looking? __________ (d) faintest looking? ___________ (e) actually most luminous? _________(f) actually least luminous? __________ (g) closest? __________ (h) most distant? __________
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Comparisons Cont.
Which star is:(a) hottest? Altair(b) coolest? Barnard’s Star(c) brightest looking? Alpha Centauri(d) faintest looking? Barnard’s Star (e) actually most luminous? Altair(f) actually least luminous? Barnard’s Star (g) closest? Alpha Centauri (h) most distant? Thuban
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H-R Diagram
Hertzprung-Russell Diagram, a plot of luminosity (absolute magnitude) versus temperature (spectral class)
When plotted, stars fall into definite regions, not random
Relationship between luminosity and temperature
The diagram was independently created in 1910 by Ejnar Hertzsprung and Henry Norris Russell
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Main Sequence: About 90% of stars, runs from upper left to lower right
Sun is a main sequence starUpper left—blue giants Lower right—red dwarfs (most common
star)Upper right—cool giants and supergiantsLower left—white dwarfs
H-R Diagram cont.
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H-R Diagram
Sun
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H-R Diagram again
Clickhttp://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram
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HR Diagram Regions
http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html
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Main Sequence
Star’s position on H-R Diagram determined by its mass
Main sequence, stars decrease in mass from upper left to lower right
Mass-luminosity relation: More massive a star, the more luminous it is
After a star forms, it quickly joins the main sequence where it spends most of its life
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Mass-Luminosity Relationship
Msun=Mass of our sun
•Masses of stars decrease from upper left to lower right of HR diagram
http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html
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Sizes of Stars
Star size: From luminosity and temperature (stellar spectra)
Sun = 864,000 miles, the same as 109 earths placed end to end
Blue-white giants are 25 times the sun’s radius, supergiant stars such as Betelgeuse are 400 times the sun’s radius!
If our sun were replaced by Betelgeuse, its radius would extend beyond the orbit of Mars
White dwarfs are about the size of the earth
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Main Sequence Star Sizes
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Giant Star Example
The star V838 Monocerotis would extend beyond the orbit of Mars in our solar system
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Double Stars
Binary Star: Pair of stars revolve around a common center of gravity. Twins
Binary stars are useful in calculating the masses of stars
Many visual binaries visible in telescopes, display color and brightness differences
Famous Double Stars: Mizar in Ursa Major, Albireo in Cygnus
Optical Double: Apparent double star, one member is actually much more distant, lined up by coincidence
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Albireo•Albireo is a famous double star located at the foot of the Northern Cross (Cygnus)
•Albireo consists of two stars in orbit about each other, the brighter star displays an orange tint, and the fainter companion is blue
•The separation between the two stars, and their colors, are easily seen in a small telescope
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Northern Cross
•Cygnus represents a swan in flight
•Many refer to the central portion of the constellation as the Northern Cross
•Albireo lies at the foot of the Cross
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Dog Star and Pup
•Sirius, the night sky’s brightest star, is also a double
•Seeing the companion in a small telescope is extremely challenging
Orbital diagram of the Sirius system. Closest separation occurred in
1997
Sirius
Sirius B (“Pup”)
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Canis Major
http://www.winshop.com.au/annew/CanisMajor.html
Sirius
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150 Binary Stars
The orbits of 150 visual binaries