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Astronomy 112 Light & The HR Diagram (1)

by: Ashley Childers

Astronomy 112 Light & The HR Diagram (1) AST 112

Marketplace > University of Southern Mississippi > Physics 2 > AST 112 > Astronomy 112 Light The HR Diagram 1
Ashley Childers

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About this Document

These are the first sections on light and stars including many images and graphs, include black body radiation, types of stars, spectral lines and more.
General Astronomy
Christopher Sirola
stars, astronomy, spectral lines
75 ?




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This 9 page Bundle was uploaded by Ashley Childers on Monday February 15, 2016. The Bundle belongs to AST 112 at University of Southern Mississippi taught by Christopher Sirola in Spring 2016. Since its upload, it has received 22 views. For similar materials see General Astronomy in Physics 2 at University of Southern Mississippi.


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Date Created: 02/15/16
Light  Virtually all our information comes to us via light (“electromagnetic radiation”)   Visible light is a wave phenomenon  It ranges from red (longest wavelength) to violet (shortest wavelength)  To express wavelength, the unit used is called a nanometer o 1nm = 1x10^-9meters  visible light ranges from 400 nm (violet) to 700 nm (red)  light travels faster (through a vacuum) than anything else  it has its own special symbol c o its speed c = 3.00x10^8 meters per second  All waves obey the same relationship between speed, wavelength and frequency o The speed of light is much fast than the speed of sound  Wavelength of visible light are much smaller than those of audible sound  Frequencies of light o Visible light 400-700nm Electromagnetic waves  How is an EM wave created o The most common way is to shake a charged particle back & forth o Because charges can have a wide range of frequencies so can EM waves o If an object has a high density (like a solid or thick gas) the object will have a blending of vibrations o This is the origin of Black body (thermal) radiation o Different wavelengths reveal different aspects of objects o Any acceleration charge will emit EM waves o But a real electron doesn’t orbit the nucleus of an atom o Rather electrons exist in different levels of set amounts of energy o Since the electrons aren’t orbiting, the aren’t accelerating o Instead EM waves are emitted (or absorbed) when an electron changes energy levels  o Electrons naturally live on the lowest (1 ) level  For example the first level for hydrogen has en energy of -13.6 eV  eV “electron volt” o  What we see on the ground also depends on absorption from the Earths atmosphere  Again imagine a point source emitting waves The amplitude of the waves of a star decrease as the waves spread out The power (luminosity) of the source is constant, but the intensity “flux” which is power per unit area increases In other word the brightness decreases as you move away Trig parallax - The most difficult problem in astronomy is to find distances - Distances to the nearest stars are determined by the trig parallax Modern magnitude system proposed by Borman Pogson in 1856 (England) - Magnitudes to be logarithmic - 5 mag difference is defined equivalent to 100 times brightness o One mag difference is 2.512 times brightness - Vega originally chosen to be standard star (mag 0) o Now use a suite of standard stars  The Hr Diagram o Plots luminosity (vertical) vs. temperature (horizontal) for stars o The History  Ejnar Hertzsprung  Tabulated luminosities and star colors in 1905  Henry Norris Russell  Graphed luminosity vs. color in 1913 o Magnitudes rd  Hipparchus (3 century BCE) first suggested ranking of stars  1 magnitude stars are like kings, down to 6 th magnitude like peandnts  Others like Ptolemy (2 century CE) associated magnitudes with size  Tycho Brahe (c. 1600) tired to measure the angular sizes of stars with the naked eye (not possible)  Even with telescopes, astronomers continued to associated size with brightness  Early telescopes were subject to unrecognized distortions o Stars would show what we now call an “Ariy disk” due to diffraction effects o Astronomers mistook Airy disk for real star sizes  William Herschel (1800) determined disks were spurious  But astronomers still linked size to magnitude until mid 1800s  With better telescopes & photographs, astronomers started talking about magnitudes as indicators of brightness  Modern magnitude system proposed by Norman Pogson England in 1856  Magnitudes to be logarithmic  5 mag differences is deafened to be 1000 time brightness o 1 is 2.512 times brightness  Vega originally chosen to be standard star (mag=0) o We now use suite of standard stars  Quirks of the mag system  The smaller (or more negative) the number the brighter the star  Uses a version of base-ten logarithms  Apparent mag (m ) iv how bright an object appears to be  Absolute Mag (Mv) is how bright an object really is  Distance in parsecs is given by the formula  d(pc)=10 x o Where x is called the distance modulus  x=(1/5)(m -v +v) o Color index  Astronomers recognized different colors for stars for centuries  But astronomers did not know why until recently  Physicists described “black body” objects first in the late 1800s  A black body is not necessarily black in color  Rather an object is called a black body because it absorbs all light that touches  The color is determined by the average surface temperature of the black body  Stars are very close to being black bodies  Astronomers put filters on their telescopes in order to let only certain colors through  B for blue  V for visual (yellow)  R for red   Results are expressed as a difference in mag o For example one might see B-V=0.31 o The more negative B-V, the more blue the more positive the more yellow o Spectral type  Current spectral types based on Harvard University’s system first invented in the 1890’s  Originally based upon strengths of (visible) spectral lines of hydrogen from stars  Type “A” had the strongest lines, type “B” the second-strongest and so on  Hydrogen lines dominated spectra for types A – D  The creation of spectral types originated with the Henry Draper catalog (“HD”)  Work began when Draper first photographed distinguishable spectral lines from Vega in 1872   Funding for the project donated by Draper’s Widow (Mary Anne)  Project taken over by Edward Charles Pickering in 1885  Antonia Maury rearranged lettering system in 1897  Brought to fruition by Annie Jump Cannon in early 1900s o Modern Spectral types  The major spectral types run from O (hottest) to M (coolest)  Note this talks about temperature, not luminosity   Common mnemonic for spectral types:  Oh  Be A  Fine  Girl/Guy  Kiss  Me  Overseas  Broadcast  Announcement!  Flash!  Godzilla  Kills  Motha!   Letters (M,O,A,B etc. are usually followed by an number)  Interoperations of the HR Diagram o Stars are not distributed randomly on the graph  90% of stars are on a diagonal from “Main Sequence”  Most of the rest are in a horizontal segments “Giant Branch”  Another noted section is a small diagonals to the lower left “White Dwarfs” o Main Sequence  Eddinton in 1920  Main sequence stars are where stars spend the majority of their lives  MS stars can live for hundreds of millions of years or more  In accord with biology & geology understanding of times scales  Made before nuclear fusion processes were understood  Luminosity Classes o Stars grouped in different sections belong to different stellar life stages o “Luminosity Class”  Ia & Ib: super-giants (middle age or old)  II: Bright giants  III: Giants (old)  IV: Sub-giants (getting old)  V: Main Sequence (middle age)  wd: White Dwarfs (dead)  Forming stars (most formation occurs in infrared light)  Other dead stars  Neutron Stars  Black Holes o Main Sequence Stars  Ms stars turn hydrogen into helium in their cores  Stars spend 90% of their lives on the MS  Or 90% of all stars are on the MS o Sub-giant  Sub-giants are stars that are running out of fuel  Hydrogen begins to turn into helium in a shell surrounding the core  The presence of fusion in the shell pushes the rest of the star out  Sub-giants are thus larger across than MS stars o Giants  Former MS stars that now turn helium into carbon instead  Hydrogen also turns into helium in a shell surrounding the core  The outer layers of the star are forces outward  In the process the temperature of the surface drops  10% of all stars are giants o Super Giants  Super giants are stars that begun with much more mass than a normal MS star  Super giants are blue when burning hydrogen  Super giants alter colors back and forth when hydrogen is depleted in their cores  Super giants go super nova and that makes all heavy elements o White Dwarfs  White Dwarfs are the remnants of the stars that have completely run out of fuel  They are essentially the exposed cores of former MS stars  As such they are extremely small and extremely hot  They are no longer sustained by gas pressure but by special properties of electrons


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