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The H-R Diagram

The H-R Diagram

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The H-R Diagram

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  1. The H-R Diagram • In 1913, the American astronomer Henry Russel plotted the luminosities of stars versus their spectra class (related to the temperature) • Similar to work done in 1911 by Danish astronomer Ejnar Hertzsprung • They found that the luminosities and temperatures of star are related • H-R Diagram Lecture 18

  2. Features of the H-R Diagram • Note that the luminosity is plotted on a logarithmic scale and the temperature is plotted backwards • The majority of these stars are located along a narrow sequence from the upper left (hot, bright) to the lower right (coll, dim) • Main sequence • Hotter stars are more luminous that cooler stars • Stars above and to the right of the main sequence are giants and supergiants • Stars below and to the left of the main sequence are white dwarfs Lecture 18

  3. More H-R Diagram • The previous H-R Diagram was plotted for stars whose distances are well known • Instead, we could plot intensities and temperatures for all stars • Astronomers find that 90% of all stars are on the main sequence • Models of stellar evolution show that the main sequence is a sequence of stellar mass Lecture 18

  4. Extremes • There are superluminous stars in the top left of the H-R diagram • The cool supergiants in the upper right corner are as much as 10,000 times a luminous as the Sun and are very much larger than the Sun • The red, cool, low-luminosity stars in the lower right hand corner are about 80 times denser than the Sun • The white dwarfs in the lower left have very high densities Lecture 18

  5. White Dwarfs • The first white dwarf was discovered in 1862 which is a binary star with Sirius • Hundreds of white dwarfs are now known • A typical white dwarf is 40 Eridani B • 12,000 K temperature • L = 1/275 Lsun • 1.4% the diameter of the Sun • Density = 200,000 g/cm3 • 1 teaspoon has a mass of 50 tons Lecture 18

  6. Imaging of Stars using Optical Interference • In 1996, the highest resolution picture of 2 binary stars was taken using the Navy Prototype Optical Interference (NPOI) (from Scientific American, March, 2001) Lecture 18

  7. Units of Distance • The first sets of measurements were based on human dimensions • The metric system of measurements was adopted in France in 1799 and is now used by every country in the world EXCEPT the US • The basic unit of length was defined at the meter and was based on a bar of platinum-iridium metal • In 1960, the definition of the meter was changed to be 1,650,763.73 wavelengths of an atomic transition in krypton-86 • In 1983, the meter was again redefined in terms in terms of the frequency of a cesium-133 clock and the speed of light • 1 light second = 299,792,458.6 m Lecture 18

  8. Distances in the Solar System • Copernicus and Kepler established the relative distances of the planets • Absolute distances are measured with radar telescopes • Radar measurements have been made of the distance to Venus, Mercury, Mars, satellites of Jupiter, the rings of Saturn, and several asteroids • The distance from the Earth to the Sun is taken as 1 astronomical unit (AU) • 500 light seconds (LS) • 8.3 light minutes (LM) • 1.5 x 1011 m • 150 million km Radar Telescope Lecture 18

  9. Triangulation • Measuring the distance to stars cannot be done by radar • A good method is triangulation • Your brain does triangulation to get depth perception using your two eyes • Triangulation is consists of using a baseline and two angles • Parallax is the angle that lines AC and line BC make • Measuring the angles at A and B allow the calculation of a triangle and the distance to the tree • To study the distances to stars, the baseline becomes the diameter of Earth’s orbit around the Sun Lecture 18

  10. Distance to Stars • As the Earth travels around the Sun, it provides us with a baseline of 2 AU or 300 million km • Even with this baseline, the parallax for stars is still too small to see with the naked eye • Early (Greek) observers were confused by the lack of parallax for the stars • They could not conceive how far way the stars are • The first successful measurement of the parallax of stars were done in 1838 Lecture 18

  11. Units of Stellar Distance • We define the unit parsec as the distance a star would be if it had a parallax of 1 arcsec and a baseline of 1 AU • “the distance at which we have a parallax of one second” • pc • 3.1 x 1013 km • 1 pc = 3.26 LY, 1 LY = 0.31 pc • The distance of a star is just the reciprocal of its parallax • R = 1/p Lecture 18

  12. The Nearest Stars • No star is within 1 LY or even 1 pc • The closest stars are three stars the make up a multiple system in the constellation of Centaurus • Alpha Centauri • Not visible in the northern hemisphere • A binary system • 4.4 LY from Earth • Proxima Centauri • 4.3 LY • The closest stars visible without a telescope from the US is Sirius • 8 LY • Binary system • Even with the Hipparchus we can only measure distances out to 300 LY, which is only 1% of the size of own galaxy and very small on the scale of the universe Lecture 18

  13. Standard Bulbs Revisited • Stars come in many luminosities • If astronomers could tell what the luminosity of a star was, they could calculate the distance to the star knowing the apparent brightness • Most stars shine steadily but some star have variable brightness • The amount of light as a function of time is called the light curve Cepheid Light Curve For Delta Cephei Lecture 18

  14. Variable Stars • There are two type of variable stars • Cepheids • RR Lyrae • Both types of variable stars actually change their diameters with time • Cepheids are large, yellow, pulsating stars • Named for the constellation in which they were first found, Delta Cephei (confusing!) • Several hundred cepheids are known • Polaris is a cepheid varying 10% in visual luminosity with a period of 4 days Lecture 18

  15. The Period-Luminosity Relationship • The period and average luminosities of cepheid variables are related • The period is easy to measure and give the astronomer the luminosity of the star • Using the luminosity and the apparent brightness, the astronomer can calculate the distance to the star • The relationship between period and luminosity was discovered by Henrietta Leavit in 1908 • Leavit found that the brighter cepheids always had longer periods • These cepheids were all in the Large Magellanic Cloud and so were all about the same distance Lecture 18

  16. Cepheids in the Large Magellanic Cloud • The cepheids that Leavit found were all in the Large Magellanic cloud • Later the distance to the Large Magellanic cloud was measured by other means and absolute distances were attached to the period-luminosity-apparent brightness distances Lecture 18

  17. Very Distant Cepheids • Image of part of M100 galaxy taken with HST • Inset shows single cepheid going through its cycle of brightness Lecture 18

  18. H-R Diagram and Cosmic Distances • Variable stars are useful but not all stars are variable • We can use the H-R Diagram to measure the distance to non variable stars Lecture 18

  19. Luminosity Classes • Knowing the the spectra index gives the astronomer the luminosity and thus the distance using the variable star information • We can also get the pressure so we know whether we are dealing with giant stars or main sequence stars • 6 luminosity classes • Ia: Brightest supergiants • Ib: Less-luminous supergiants • II: Bright giants • III: Giants • IV: Subgiants • V: Main-sequence stars Lecture 18

  20. RR Lyrae Stars • A related group of stars is the RR Lyrae variables • More common but less luminous than cepheids • Thousands are known in our galaxy • Periods are less than a day • Brightness changes by less than a factor of two • All RR Lyrae stars have the same luminosity • 50 Lsun • RR Lyrae stars can be detected out to about 2 million LY Lecture 18