From space, (as this diagram shows) the Sun’s spectrum peaks in the blue-green wavelengths. The Sun appears yellow to us because our atmosphere scatters the shorter wavelength blue light.
pp. 166-167
pp. 595 (Special Topic)
pp. 502-507
pp. 513-517
pp. 510-512
03 The Stars Mc Neely 2008
1.
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
2.
Distances to the Stars <ul><li>How can we measure the distances to the nearest stars? </li></ul><ul><li>Parallax Method : </li></ul><ul><ul><li>Stars close to our sun </li></ul></ul><ul><ul><li>Measure star’s position once, then 6 months later </li></ul></ul><ul><ul><li>Nearby stars appear to shift back and forth relative to more distance stars </li></ul></ul><ul><ul><li>Amount of shift can be used to calculate distances to stars </li></ul></ul>
3.
Stellar Parallax <ul><li>A star close to our sun appears to shift back and forth compared to the more distant stars in the background </li></ul><ul><li>This diagram is not to scale. The shifting of stars due to parallax is incredibly tiny and is measured in arc seconds </li></ul>Earth in summer Earth in winter Star close to our sun
5.
Arc Seconds <ul><li>Seconds of Arc : Stellar parallaxes are measured in arc seconds (“) </li></ul><ul><li>Arc seconds are tiny divisions of a degree </li></ul><ul><li>Remember that 360 ° makeup a circle </li></ul><ul><li>1º = 60’ = 3600” </li></ul><ul><li>1’ = 60” </li></ul><ul><li>1” = 1/3600 degree </li></ul><ul><li>1” = width of an aspirin tablet one mile away </li></ul>
6.
Parsec <ul><li>Distance in light years to a star showing one arc second (“) of parallax </li></ul><ul><li>Parsec (pc): Distance measure = 3.26 light years </li></ul><ul><li>The closest star is Alpha Centauri at 4.3 ly </li></ul><ul><li>To calculate a star’s distance from observed parallax: </li></ul><ul><li>Star’s Distance (in pc) = 1/parallax (“) </li></ul>
7.
Newton’s Prism Experiment (1665) <ul><li>Isaac Newton discovered that light could be broken down into component colors by using a prism </li></ul><ul><li>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 </li></ul><ul><li>The separated light is known as a spectrum </li></ul>
10.
Spectrum of Light <ul><li>Spectroscopy : Analysis of spectra </li></ul><ul><li>Spectroscope : Device attached to a telescope, splits light of a star into its spectrum of colors </li></ul><ul><li>Spectroscopy reveals what stars are made of </li></ul>
11.
Spectra of Stars <ul><li>3 Types of Spectra: </li></ul><ul><ul><li>Continuous : Complete array of all the rainbow colors. (incandescent light bulb) </li></ul></ul><ul><ul><li>Emission (Bright-Line): A pattern of bright-colored lines emitted by hot gas (neon light, overhead fluorescent bulb) </li></ul></ul><ul><ul><li>Absorption (Dark-Line): A pattern of dark lines across a continuous spectrum. Created when light passes through a cool gas. (Stars, the sun) </li></ul></ul>
12.
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
16.
Elements <ul><li>Atoms are the smallest unit of matter that still retains that matter’s known properties </li></ul><ul><li>Atoms create the spectra types </li></ul><ul><li>100 types of unique atoms are known, each is an element </li></ul><ul><li>Periodic table </li></ul>
17.
Bohr Model <ul><li>Bohr atom model : A nucleus made of positively charged protons surrounded by the same number of negatively charged electrons </li></ul><ul><li>Electrons are confined to a set of allowed orbits around the nucleus </li></ul>
18.
Bohr Model of Nitrogen <ul><li>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 </li></ul><ul><li>Nitrogen is the 7 th element in the periodic table due to its having 7 protons </li></ul>Nucleus Energy levels http://education.jlab.org/qa/atom_model.html
19.
Binding Energy <ul><li>Binding Energy : energy that holds electrons in place around the nucleus </li></ul><ul><li>Each element has its own unique set of allowed electron orbits or energy levels </li></ul>
20.
Jumping Electrons <ul><li>Binding Energy : energy that holds electrons in place around the nucleus </li></ul><ul><li>Each element has its own unique set of allowed electron orbits or energy levels </li></ul><ul><li>Ground State : Undisturbed atom, electrons in allowed orbits, lowest energy </li></ul><ul><li>Excited State : Electrons will jump to higher energy levels, release light particle (photon) when falling back </li></ul>
21.
Excite Me <ul><li>When an atom is excited, an electron jumps momentarily to a higher energy level or “orbit” </li></ul><ul><li>When the electron jumps back down to its previous level, the atom emits a photon of light </li></ul>
22.
Animation <ul><li>Animation 4.2: Absorption and Emission of a Photon </li></ul>
23.
Emission Lines <ul><li>Excited atoms create bright colored emission lines due to their jumping electrons </li></ul><ul><li>Each chemical element has its own unique set of bright emission lines </li></ul>Emission spectra of various elements
24.
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
25.
Absorption Lines <ul><li>Correspond to the bright emission lines </li></ul><ul><li>Unique dark absorption lines produced when an atom absorbs light, causes electrons to jump </li></ul><ul><li>Absorption lines represent light subtracted from the continuous spectrum </li></ul><ul><li>Observation of emission or absorption lines in spectra allows identification of the chemical element that produced them </li></ul>
26.
Spectra of Hydrogen <ul><li>ABSORPTION SPECTRUM </li></ul><ul><li>A pattern of dark lines across a continuous spectrum </li></ul><ul><li>Light passing through a cold gas </li></ul><ul><li>If plotted as a graph, absorption lines appear as dips </li></ul><ul><li>EMISSION SPECTRUM </li></ul><ul><li>A pattern of bright-colored lines with black gaps. </li></ul><ul><li>Hot, glowing gas </li></ul><ul><li>If plotted as a graph, emission lines appear as peaks </li></ul>
27.
Stellar Spectra <ul><li>Star light can be broken down with a spectrometer </li></ul><ul><li>Dark absorption lines can be observed </li></ul><ul><li>Absorption lines can be matched to specific chemical elements </li></ul><ul><li>Stellar 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 </li></ul>
28.
Absorption Spectra O B A F G K M Coolest stars Hottest stars <ul><li>The unique chemical elements in stars create dark, absorption lines, the “fingerprints of the stars”. </li></ul><ul><li>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 top </li></ul><ul><li>OBAFGKM is the spectral sequence of stars </li></ul>
30.
Chemical Composition of Stars <ul><li>Sun, first star to be analyzed (1814) </li></ul><ul><li>Fraunhofer (1814) recorded the strongest absorption lines, named Fraunhofer lines in his honor </li></ul><ul><li>Astronomers have since recorded thousands of dark lines in the sun’s spectrum </li></ul><ul><li>Comparison with spectral lines produced in laboratories on earth have enabled the identification of 70 different elements in the sun </li></ul><ul><li>Stars are primarily hydrogen and helium </li></ul><ul><li>The coolest stars allow actual molecules, compounds of more than one element, to survive </li></ul>
31.
Fraunhofer Lines Some of Fraunhofer’s original drawings of the sun’s spectrum
32.
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
33.
Betelgeuse Spectra <ul><li>Betelgeuse is so cool that the star allows complete molecules such as TiO to survive in its atmosphere </li></ul>
34.
Spectral Classes <ul><li>Absorption lines used to classify stars into 7 spectral classes </li></ul><ul><li>Originally in alphabetical order, Annie Cannon (1863-1941) rearranged them into the present form of O B A F G K M (“ O h B e A F ine G irl/Guy K iss M e”) </li></ul>
36.
Temperature of Stars <ul><li>The OBAFGKM sequence of spectral classes is also a temperature sequence </li></ul><ul><li>O stars are hottest (> 30,000 K), M stars coolest (< 3,500 K) </li></ul><ul><li>Vega & Sirius, O stars (10,000 K) </li></ul><ul><li>The sun is a G star (5-6,000 K) </li></ul><ul><li>Antares & Betelgeuse, M stars (3-3,500 K) </li></ul>
37.
Spectral Class and Star Color <ul><li>The diagram shows the star color that corresponds to each spectral class </li></ul><ul><li>Many stars have colors that are visible with the naked eye and in telescopes </li></ul>Click: Stellar Spectra Mini Exercise
38.
Star Colors: Big & Little Dippers Where is Polaris?
39.
Orion Betelgeuse Rigel <ul><li>Look for these star colors when you see Orion </li></ul><ul><li>Red areas represent glowing gas in space (nebulas), most is too faint to see with the unaided eye </li></ul>Belt Sword Orion Nebula in Sword of Orion
40.
Andromeda www.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
41.
Planck Curves and Blackbodies <ul><li>Blackbody is a theoretical object that absorbs all of the light that strikes it </li></ul><ul><li>Absorbed light heats the blackbody </li></ul><ul><li>The blackbody then reemits the light at different wavelengths </li></ul><ul><li>Planck curves are graphs of the types of light reemitted by blackbodies of different temperatures </li></ul><ul><li>The shape of a blackbody curve is a function of the blackbody’s temperature </li></ul><ul><li>Ideal blackbody curves (Planck curves) were first discovered by Max Planck in 1900 (photo) </li></ul>
42.
3 Planck Curves <ul><li>Three blackbodies at three temps </li></ul>
43.
<ul><li>Shown is a plot of intensity versus wavelength for blackbodies at different temperatures. </li></ul><ul><li>Blackbodies at different temperatures will appear as different colors or wavelengths. </li></ul><ul><li>At higher temperatures the most intense wavelengths are shorter. </li></ul><ul><li>The sun is very similar to the 6000 K curve. It’s peak wavelength is in the blue-green portion of the visible spectrum. </li></ul><ul><li>Very hot stars have peak emissions in the ultraviolet and beyond, very cool stars can peak in the infrared. </li></ul>Wien’s Law governs the peak wavelength, Stephan-Boltzmann governs the intensity
44.
Stars and Blackbodies (Planck Curves) <ul><li>Star color is related to its temperature </li></ul><ul><li>Hot stars are bluish white, cool stars are reddish </li></ul><ul><li>Light emitted by stars follows a Planck curve </li></ul><ul><li>A star’s Planck curve can be used to estimate a star’s temperature </li></ul>
45.
Blackbody Radiation Example <ul><li>A heated iron poker will begin to glow emitting photons. The intensity and wavelength of the radiation changes with temperature. </li></ul><ul><li>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. </li></ul>WHEN FIRST HEATED THE POKER GLOWS DIMMLY AND IS RED AS THE TEMPERATURE RISES, THE POKER BECOMES BRIGHTER AND GLOWS ORANGE AT HIGHER TEMPERATURES THE POKER BECOMES EVEN BRIGHTER AND GLOWS YELLOW
46.
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.
47.
Summary : Properties of Stars <ul><li>Method of parallax—distances to stars </li></ul><ul><li>Spectroscopy—Composition of stars </li></ul><ul><li>Planck curves—Star temperatures </li></ul>
48.
Apparent Motion of Stars <ul><li>Earth’s rotation: Stars rise and set </li></ul><ul><li>Earth’s revolution: Stars change with the seasons </li></ul><ul><li>Earth’s precession: Positions of stars change in a 26,000 year cycle </li></ul>
49.
Stars Move Through Space <ul><li>Stars in our galaxy revolve around the galaxy’s center </li></ul><ul><li>220 million years for the sun </li></ul><ul><li>Stars have high velocities </li></ul><ul><li>Stars are so distant that they appear still for thousands of years </li></ul><ul><li>Star motions are revealed by measuring and comparing positions over periods of time, and by analyzing spectra </li></ul>
50.
Sun’s Revolution in Milky Way http://www.envirotruth.org/images/graphics/suns_path.jpg <ul><li>The sun revolves around the center of the Milky Way galaxy every 220 my </li></ul><ul><li>The sun is just one of around 100 billion other stars in the Milky Way </li></ul><ul><li>The Milky Way is a flat disk of stars organized into spiral arms; the spiral rotates clockwise in this view </li></ul>
51.
Space Motions of Stars <ul><li>Space Velocity : True motion of a star in space, motion of a star with respect to our sun and earth </li></ul><ul><li>Space velocity exhibits two components : </li></ul><ul><ul><ul><li>Radial Velocity : Motion towards or away from us </li></ul></ul></ul><ul><ul><ul><li>Proper Motion : Motion at right angle to us </li></ul></ul></ul>
52.
Star Motions <ul><li>V = Space Velocity </li></ul><ul><li>V t = Proper Motion </li></ul><ul><li>V r = Radial Velocity </li></ul><ul><li>Yellow circle=1 st observation </li></ul><ul><li>Blue Arrowhead=2 nd observation </li></ul>star The star moved along the blue arrow in the observation period
53.
Doppler Shift <ul><li>Radial velocity revealed by Doppler shift </li></ul><ul><li>A star’s spectrum exhibits a Doppler shift if it is moving towards or away from us </li></ul><ul><li>Doppler shift: When a source of waves are approaching or receding, the observed wavelengths are changed. </li></ul><ul><li>Ex: A train rushing by, the pitch of the train’s whistle drops abruptly as it passes </li></ul>
54.
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
55.
Doppler Shift cont. <ul><li>Stars: Doppler shift is revealed by the positions of dark absorption lines </li></ul><ul><li>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 </li></ul>
56.
Blueshift and Redshift <ul><li>Blueshift : The star is approaching </li></ul><ul><li>Redshift : The star is moving away </li></ul>
57.
Redshift <ul><li>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 </li></ul><ul><li>The degree of shift is indicative of the speed of the star’s motion away from us </li></ul>Reference Twin spectral lines
58.
Proper Motion <ul><li>Proper motion, star motion perpendicular to our line of sight to a star </li></ul><ul><li>Average proper motion for all visible stars is less than 0.1” per year </li></ul><ul><li>Proper motions are very tiny </li></ul>
59.
Slow Motion <ul><li>Big Dipper stars will appear much different in 50,000 years due to a high proper motion </li></ul><ul><li>Barnard’s Star has the highest observed proper motion. Comparison of telescope sketches or photos over a lifetime would reveal its motion </li></ul><ul><li>61 Cygni is also a high-proper motion star </li></ul>
61.
Barnard’s Star <ul><li>Highest proper motion star, lies at a distance of 6 light years from earth </li></ul><ul><li>Photo a montage displaying 4 years of the star’s proper motion </li></ul>
63.
Hipparcos Web Site <ul><li>Proper Motion Demo </li></ul><ul><li>http://www.rssd.esa.int/Hipparcos/TOUR/tour.html </li></ul>
64.
Luminosity <ul><li>Luminosity, measure of a star’s total light output </li></ul><ul><li>Luminosity is the amount of light a star shines into space each second </li></ul><ul><li>Sun’s luminosity (L), L = 3.85x1026 watts, equivalent to 3850 billion trillion 100-watt light bulbs </li></ul><ul><li>Rigel in Orion is about 60,000 times more luminous than the sun </li></ul><ul><li>Why does our sun appear much more luminous than Rigel? </li></ul>
65.
Propagation of Light <ul><li>Propagation, how light travels through space </li></ul><ul><li>Light moving away from a star becomes dimmer </li></ul><ul><li>Amount of starlight drops off as the square of the distance away from the star (inverse square relationship) </li></ul>
66.
Propagation <ul><li>The light from a star is spread further and further apart as it travels into space </li></ul><ul><li>At 1 AU, the star’s light is spread into a 1x1=1 area </li></ul><ul><li>At 2 AU, the star’s light is spread into a 2x2=4 area </li></ul><ul><li>At 3 AU, the star’s light is spread into a 3x3=9 area </li></ul><ul><li>This behavior is an inverse square relationship </li></ul>Each sphere represents the same amount of light from the star Star
67.
Apparent Magnitude <ul><li>Apparent magnitude, a measure of a star’s brightness from earth </li></ul><ul><li>Greek astronomer Hipparchus </li></ul><ul><li>Traditional magnitude scale is 1—6, represents all stars visible to the unaided eye </li></ul><ul><li>“ 1” were the brightest, “6” were the faintest </li></ul>
68.
Modern Magnitude Scale <ul><li>Modern magnitude scale: A 1 st magnitude star is exactly 100 times brighter than a 6 th magnitude star </li></ul><ul><li>Astronomers found that some stars were brighter than 1, requiring zero and negative magnitudes </li></ul><ul><li>The smaller the magnitude number, the brighter the star </li></ul>Magnitude scale from a star map
69.
Magnitude Scale <ul><li>Magnitude 6 and less equal naked eye objects </li></ul><ul><li>Binoculars can see to magnitude 11; An 8-inch aperture telescope can see down to magnitude 14 </li></ul><ul><li>Interestingly, the Hubble Space Telescope can see nearly as faint as our sun is bright </li></ul>> Mag = < Bright The magnitude scale can be considered a type of number line 8-in BN
70.
Little Dipper Magnitudes <ul><li>Little Dipper star map, identify some magnitudes </li></ul><ul><li>LD is circumpolar </li></ul>Polaris is a 2 nd magnitude star
71.
Absolute Magnitude <ul><li>Absolute magnitude measures a star’s true brightness </li></ul><ul><li>Absolute magnitude is the magnitude that a star would have at a distance of 10 parsecs (32.6ly) </li></ul>Sirius is the brightest star in our sky, which star, Sirius or Polaris, is truly the brightest? Star Polaris Sirius Apparent Magnitude +2.3 -1.5 Absolute Magnitude -4.6 +1.4 Distance 240 Parsecs 2.7 Parsecs
72.
Apparent & Absolute Magnitudes http://media.skytonight.com/images/Sirius_Mags_m.gif <ul><li>Canis Major how it appears in our sky—note the brightness of Sirius, the brightest star in apparent magnitude </li></ul><ul><li>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 </li></ul>
73.
Comparisons Star App Mag Abs Mag Spectral Class Parallax 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”
74.
Comparisons Cont. <ul><li>Which star is: </li></ul><ul><ul><li>(a) hottest? __________ </li></ul></ul><ul><ul><li>(b) coolest? __________ </li></ul></ul><ul><ul><li>(c) brightest looking? __________ </li></ul></ul><ul><ul><li>(d) faintest looking? ___________ </li></ul></ul><ul><ul><li>(e) actually most luminous? _________ </li></ul></ul><ul><ul><li>(f) actually least luminous? __________ </li></ul></ul><ul><ul><li>(g) closest? __________ </li></ul></ul><ul><ul><li>(h) most distant? __________ </li></ul></ul>
75.
Comparisons Cont. <ul><li>Which star is: </li></ul><ul><ul><li>(a) hottest? Altair </li></ul></ul><ul><ul><li>(b) coolest? Barnard’s Star </li></ul></ul><ul><ul><li>(c) brightest looking? Alpha Centauri </li></ul></ul><ul><ul><li>(d) faintest looking? Barnard’s Star </li></ul></ul><ul><ul><li>(e) actually most luminous? Altair </li></ul></ul><ul><ul><li>(f) actually least luminous? Barnard’s Star </li></ul></ul><ul><ul><li>(g) closest? Alpha Centauri </li></ul></ul><ul><ul><li>(h) most distant? Thuban </li></ul></ul>
76.
H-R Diagram <ul><li>Hertzprung-Russell Diagram, a plot of luminosity (absolute magnitude) versus temperature (spectral class) </li></ul><ul><li>When plotted, stars fall into definite regions, not random </li></ul><ul><li>Relationship between luminosity and temperature </li></ul><ul><li>The diagram was independently created in 1910 by Ejnar Hertzsprung and Henry Norris Russell </li></ul>
77.
<ul><li>Main Sequence : About 90% of stars, runs from upper left to lower right </li></ul><ul><li>Sun is a main sequence star </li></ul><ul><li>Upper left—blue giants </li></ul><ul><li>Lower right—red dwarfs (most common star) </li></ul><ul><li>Upper right—cool giants and supergiants </li></ul><ul><li>Lower left—white dwarfs </li></ul>H-R Diagram cont.
79.
H-R Diagram again Click http://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram
80.
HR Diagram Regions http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html
81.
Main Sequence <ul><li>Star’s position on H-R Diagram determined by its mass </li></ul><ul><li>Main sequence, stars decrease in mass from upper left to lower right </li></ul><ul><li>Mass-luminosity relation : More massive a star, the more luminous it is </li></ul><ul><li>After a star forms, it quickly joins the main sequence where it spends most of its life </li></ul>
82.
Mass-Luminosity Relationship <ul><li>M sun =Mass of our sun </li></ul><ul><li>Masses of stars decrease from upper left to lower right of HR diagram </li></ul>http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html
83.
Sizes of Stars <ul><li>Star size: From luminosity and temperature (stellar spectra) </li></ul><ul><li>Sun = 864,000 miles, the same as 109 earths placed end to end </li></ul><ul><li>Blue-white giants are 25 times the sun’s radius, supergiant stars such as Betelgeuse are 400 times the sun’s radius! </li></ul><ul><li>If our sun were replaced by Betelgeuse, its radius would extend beyond the orbit of Mars </li></ul><ul><li>White dwarfs are about the size of the earth </li></ul>
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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 <ul><li>Binary Star : Pair of stars revolve around a common center of gravity. Twins </li></ul><ul><li>Binary stars are useful in calculating the masses of stars </li></ul><ul><li>Many visual binaries visible in telescopes, display color and brightness differences </li></ul><ul><li>Famous Double Stars : Mizar in Ursa Major, Albireo in Cygnus </li></ul><ul><li>Optical Double : Apparent double star, one member is actually much more distant, lined up by coincidence </li></ul>
87.
Albireo <ul><li>Albireo is a famous double star located at the foot of the Northern Cross (Cygnus) </li></ul><ul><li>Albireo consists of two stars in orbit about each other, the brighter star displays an orange tint, and the fainter companion is blue </li></ul><ul><li>The separation between the two stars, and their colors, are easily seen in a small telescope </li></ul>
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Northern Cross <ul><li>Cygnus represents a swan in flight </li></ul><ul><li>Many refer to the central portion of the constellation as the Northern Cross </li></ul><ul><li>Albireo lies at the foot of the Cross </li></ul>
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Dog Star and Pup <ul><li>Sirius, the night sky’s brightest star, is also a double </li></ul><ul><li>Seeing the companion in a small telescope is extremely challenging </li></ul>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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