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Astrophysics 1

by: Stephan Kuvalis

Astrophysics 1 ASTR 3730

Stephan Kuvalis

GPA 3.89

Philip Armitage

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Philip Armitage
Class Notes
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This 9 page Class Notes was uploaded by Stephan Kuvalis on Thursday October 29, 2015. The Class Notes belongs to ASTR 3730 at University of Colorado at Boulder taught by Philip Armitage in Fall. Since its upload, it has received 22 views. For similar materials see /class/231954/astr-3730-university-of-colorado-at-boulder in Astronomy at University of Colorado at Boulder.


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Date Created: 10/29/15
Stars basic observations Basic properties of stars we would like to know in order to compare theory against observations 39 Stellar mass M 39 Stellar radius R Surface temperature effective temperature Te Bolometric luminosity L Recall definition of effective temperature L 472 x 0T which relates L R and Te so only three independent quantities to measure mass plus two of luminosity radius and effective temperature For the Sun can also determine the age and many other properties ASTR 3730 Fall 2003 Effective temperature Can be determined for a single star without knowledge of the distance Measure spectrum of the star then compare with theory to get the effective temperature eg for a blackbody spectrum the peak of FV is at hvmX 282kTe so a measure of where FV has a maximum determines Te Real spectra are more complicated but same independence from distance applies No spectrum Can use the color of the star as a measure of the effective temperature ASTR 3730 Fall 2003 Radius Angular diameter of the Sun at a distance of 10 pc is 2Rmn1x4x flmn 10pc 3x1d9mn cf Hubble resolution of about 01 arcsec 5 gtlt10 9 rad 10 3 arcsec Very difficult to measure the radii of single stars even nearby ones directly Hubble image of Betelgeuse Some other stellar radii measured using interferometers to moderate accuracy ASTR 3730 Fall 2003 Luminosity Measure the energy flux from the star integrated over all wavelengths inverse square law L 4nd2 gives the bolometric luminosity if the distance d is known Can determine the distance directly if we can measure the parallax apparent stellar motion due to orbit of Earth around Sun For small angles 1au 1au d 80 d 1 pc P if p is measured in arcsec ASTR 3730 Fall 2003 Since nearest star is gt 1 pc away and groundbased telescopes have a resolution of 1 arcsec might seem impossible to measure p and thus d to any useful precision eg for 10 error in d need p to 01 arcsec for nearest star to 001 arcsec for a useful sample Actually possible to measure p to substantially better accuracy than the resolution of the telescope 1838 Bessel measured p 0316 for star 61 Cygni modern value p 029 current groundbased best errors of 0001 arcsec Hipparcos satellite measured 105 bright stars with errors also of 0001 arcsec GAIA satellite will measure positions of 109 stars with an accuracy of microarcsecs this is a reasonable fraction of allthe stars in the Milky Way Currently measure d accurately to 100 pc ASTR 3730 Fall 2003 The HertzsprungRussell diagram Measuring stellar masses is difficult What can we learn using only observations of Te and L Plot Te against L theorists or color eg BV against absolute magnitude observers H39R dia ram Plot lines of constant M stellar radius on the 3909 L 39 m HR diagram using L 4JIR20T64 quot Individual star is a quot small R single point in this plane log Te HOT BLUE stars COOL RED stars ASTR 3730 Fall 2003 HR diagram from Hipparcos measurements An HR diagram for the 5000 stars with the best distance determinations from Hipparcos Hertzsprung Russell DRE 5 lt 005 Observational axes color and absolute magnitude 5 Colors indicate the density of stars in that part of the diagram Most important point most of the possible combinations of L and Te are not populated by real stars most stars occupy a thin strip in the diagram the main sequence 15 05 00 quot5 LO L5 20 25 30 V I lmag AS TR 3730 Fall 2003 Since stars with large radius fall in the upper right corner of the HR diagram can determine which stars lie in different areas of the plot Observational diagram S 39 ts looks a bit different 104 depending on what Giants colors bands are plotted 100 LLsun 1 Main sequence 102 Where do binaries fall in this plot 104 White dwarfs quot 20000 10000 5000 Te K ASTR 3730 Fall 2003 Having determined where the main sequence is from the Hipparcos HR diagram can obtain more information from HR diagrams of clusters without parallax distances apparent magnitude g39ants main sequence color Use Hipparcos results to determine absolute magnitude M on the main sequence for stars of given color Compare to apparent magnitude m of stars of same color in cluster ie can estimate mM510g10 distance d 10 c p ASTR 3730 Fall 2003


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