Download MEASURING THE STARS

MEASURING THE STARS Big Picture 1) stars are born from clouds of interstellar gas Big Picture 1) stars are born from clouds of interstellar gas 2) stars are powered by nuclear fusion for millions or billions of years Big Picture 1) stars are born from clouds of interstellar gas 2) stars are powered by nuclear fusion for millions or billions of years 3) stars die, someGmes in dramaGc ways Big Picture 1) stars are born from clouds of interstellar gas 2) stars are powered by nuclear fusion for millions or billions of years 3) stars die 4) some stars are like the sun and others are not MEASURING THE STARS Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii Luminosity •  Luminosity is a rate •  It is defined as the energy radiated into space per unit Gme •  It can be stated in waQs, like for light bulbs •  The Sun’s luminosity is Lsun=380,000,000,000,000
,000000,000,000 waQs! Luminosity •  Luminosity is a rate •  It is defined as the energy radiated into space per unit Gme •  It can be stated in waQs, like for light bulbs •  The Sun’s luminosity is Lsun=380,000,000,000,000
,000000,000,000 waQs! Luminosity •  Luminosity is a rate •  It is defined as the energy radiated into space per unit Gme •  It can be stated in waQs, like for light bulbs •  The Sun’s luminosity is Lsun=380,000,000,000,000
,000000,000,000 waQs! Luminosity •  Luminosity is a rate •  It is defined as the energy radiated into space per unit Gme •  It can be stated in waQs, like for light bulbs •  The Sun’s luminosity is Lsun=380,000,000,000,000
,000000,000,000 waQs! Time out to think Compare the two stars and select the correct statement B A A – Star A is more luminous than star B. B – Star B is more luminous than star A. C – Star A and B are equally luminous. D – There is not enough informaGon. Luminosity is defined as the energy radiated into space per unit Gme Luminosity The apparent brightness of a star does not tell us its luminosity, because apparent brightness depends on distance 45 waQ 75 waQ Luminosity-­‐Distance Formula luminosity apparent brightness = 4 π (distance)2 Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii Distance 1) How do we measure distance? Time out to think Everyone: 1.  Close your right eye. 2.  Cover the red line on the screen with your thumb at arm’s length. 3.  What happens to your thumb when you look at it with your right eye instead? A – The thumb appears to have moved to the leb of the line. B – The thumb remains lined up with the line. C – The thumb appears to have moved to the right of the line. The parallax angle is the semi-­‐
angle between two lines of sight. parallax angle Background stars are far away and do not appear to move Star we want to measure appears to move like our thumb Us January July parallax angle, p measured in arcseconds 1 arcsecond = 1 degree / 3600 The moon subtends an angle of 0.5 degrees or 1800 arseconds! Time out to think 1)  What will happen to your thumb if you repeat the experiment but with your thumb closer to your nose? A – The thumb will appear to have moved less B – The thumb will appear to have moved the same amount C – The thumb will appear to have moved more January 1 2 July greater distance ó smaller parallax angle distance to star, d It is convenient to measure the distance to stars in parsecs instead of miles. A point located at a distance of 1 parsec has a parallax of one arcsecond. p d d in parsecs p in arcseconds d=1/p •  We have just learned about how use trigonometric parallax to measure distances •  There are other techniques for measuring distance, but we won’t cover them today Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii The Magnitude System •  The magnitude system devised by Greek astronomer Hipparchus originally classified stars according to how bright they looked to our eyes The Magnitude System •  The magnitude system devised by Greek astronomer Hipparchus originally classified stars according to how bright they looked to our eyes •  On a scale of 1 to 6 –  1 was the brightest –  6 was the faintest The Magnitude System •  The magnitude system devised by Greek astronomer Hipparchus originally classified stars according to how bright they looked to our eyes •  On a scale of 1 to 6 –  1 was the brightest –  6 was the faintest •  6 is the faintest magnitude that we can see with our naked eye Modern Magnitude System •  Scale has been extended to include –  negaGve numbers –  numbers larger than 6 –  FracGons E.g., brightest star in night sky, Sirius, has magnitude of -­‐1.46 Modern Magnitude System •  Scale has been extended to include –  negaGve numbers –  numbers larger than 6 –  FracGons E.g., brightest star in night sky, Sirius, has magnitude of -­‐1.46 •  Defines absolute magnitude as a way of describing stellar luminosiGes Absolute Magnitude = Apparent Magnitude of star if it were at a distance of 10 parsecs Compression •  Magnitudes are a compression system so that we don’t have to write all the zeros as in Lsun=380,000,000,000,000,000000,000,000 waQs Compression •  Magnitudes are a compression system so that we don’t have to write all the zeros as in Lsun=380,000,000,000,000,000000,000,000 waQs •  Absolute magnitude of sun is 4.8 Compression •  Magnitudes are a compression system so that we don’t have to write all the zeros as in Lsun=380,000,000,000,000,000000,000,000 waQs •  Absolute magnitude of sun is 4.8 •  A difference of 5 in magnitude corresponds to a factor of 100 in brightness Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii Surface temperature and color It is the surface temperature of a star that causes it overall color From now on, when I say temperature, you should assum that I am talking about the surface temperature of a star If a star is cool it will look red If a star is hot it will look blue Colors and Temperature Colors and Temperature Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii Spectra of stars of different surface temperatures/colors blue red Spectra of stars of different surface temperatures/colors blue red The hydrogen absorpGon lines are weak because the hydrogen is ionized, so this most be a hot star The emission and absorpGon lines in a star’s spectrum provide an independent and a more accurate way to determine temperature O blue red B A F G K M Depending on which lines are present in the spectra of stars, astronomers assign them a spectral type given by leQers O,B,A,F,G,K,M O blue red B A F G K M Depending on which lines are present in the spectra of stars, astronomers assign them a spectral type given by leQers O,B,A,F,G,K,M O blue red B A F G K M Stars were first classified in order A, B, etc, based on only the hydrogen lines by Williamina Fleming (1857-­‐1911). O Oh B Be A A F Fine G Girl K Kiss M Me However, Annie Jump Cannon (1863-­‐1941) discovered that a beQer way of classifying the stars was by not only using solely the hydrogen lines, which led to the current ordering, which everyone uses, nowadays. O Oh B Be A A F Fine G Girl K Kiss M Me The leQers are subdivided into numbers from 0 (hoQer) to 9 (cooler) in order to be more precise. A B0 star is hoQer than a B5 star. Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii Hertzprung-­‐Russell Diagram •  Independently, Hertzprung & Russell took a collecGon of stars and ploQed –  Color/Spectral Type/Temperature on the x-­‐axis –  Luminosity on the y-­‐axis •  Such a plot is called the H-­‐R diagram •  H & R discovered that the stars were not randomly distributed in the H-­‐R diagram Hertzprung-­‐Russell Diagram •  Stars spend most of their lifeGme burning hydrogen into helium on the Main Sequence •  When the fuel starts to run out, they expand and become Red Giant Stars •  They then find a new source of fuel and become Blue Horizontal Branch Stars •  Eventually they burn out and become White Dwarfs H-­‐R DIAGRAM The exact evoluGon of a star depends on its mass The main families of stars associated with their evoluGonary phase are ploQed here Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii STELLAR LUMINOSITY CLASS Gives the evoluGonary family of the star I Supergiants II Bright giants III Giants IV Subgiants V Main Sequence Sun = G2 V star Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii RADII Stars of same surface temperature (T) have different luminosiGes (L) if their radii (R) are different sqrt(L) R=-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐ sqrt(4πσT4) σ=5.7x10-­‐8 waQ/(m2 x K2) Topics Stellar Luminosity Distance to the stars The magnitude system Stellar surface temperature The spectral classificaGon of stars Hertzprung-­‐Russel Diagram Stellar Luminosity Classes Stellar Radii