February 7, 2006 Astronomy Chapter 17: The Stars: A Celestial Census

February 7, 2006Astronomy The Lives of Stars Stars live for a very long time, 100 million years and up. We can’t possibly observe a star this long! How can we learn about the stages in a star’s life? We perform a census, getting a snapshot of many stars at different stages of their life. We infer the stages a star goes through from the data we assemble in the census. Cuidado! We can be misled if the star sample in the census is biased. (Like political surveys.)

February 7, 2006Astronomy A Stellar Census We measure distances in light years (LY). More details in Ch. 18 Small stars are less luminous, and therefore harder to see. If not corrected for, we will have a biased sample of stars. Careful observation reveals that small stars (brown dwarfs) are more common than large stars. While less numerous, large stars are easier to see at large distances. Most of the stars visible to the naked eye are large.

February 7, 2006Astronomy All Stars are Different colors: blue-white to red brightness: bright to very faint Orion: Constellation with many different star types Betelgeuse: orange-red supergiant Rigel: blue-white supergiant

February 7, 2006Astronomy Key Properties of Stars color = surface temperature Rigel (blue) 10,000 K Sun (yellow) 6000 K Betelgeuse (orange-red) 3, Betelgeuse (orange-red) 3,000 K spectrum = chemical composition Rigel B Sun G Betelgeuse M luminosity Rigel 60,000 L Sun Betelgeuse 50,000 L Sunmass Rigel 16 solar masses Betelgeuse 20 M Sundiameter Rigel 7 solar diameters Betelgeuse 1000 D Sun

February 7, 2006Astronomy Measuring Stellar Masses Mass is one the key parameters of stars Determines the behavior and life cycle of the star Determined for binary stars – most common case: spectroscopic binary stars orbit depends on mass of two stars

February 7, 2006Astronomy center of mass Orbit of Binary Stars  Masses Kepler's third law: (modified) orbital period P and semimajor axis D of ellipse related to masses M 1 and M 2 D 3 = (M 1 +M 2 )P 2 D 3 = (M 1 +M 2 )P 2 D in AU, P in years, M 1 and M 2 in solar masses (M sun ) each star orbits a common point – the center of mass Star’s distance from center of mass determines the star’s individual mass. D

Visual Binary: Sirius A and B

February 7, 2006Astronomy Visual Binaries: Wobbling Motion

February 7, 2006Astronomy Sirius A and B Sirius A normal star (class A main sequence) Sirius B white dwarf companion orbits are drawn to scale exaggerated sizes of the two stars Sirius A is considerably larger than the Sun while the white dwarf Sirius B is about the size of the Earth.

February 7, 2006Astronomy Spectroscopic Binary Stars most known binaries are spectroscopic binaries distance too great to resolve the two stars individually binary nature is indicated in the periodic shift of their spectral lines as they orbit around each other can measure their speeds from the Doppler shifted lines speed determines the mass

February 7, 2006Astronomy The Range of Stellar Masses How large and small can stars be? Stars with 100 solar masses are known, and we believe there may be stars up to about 200 solar masses (200 M sun ). True stars must be heavier than 1/12 M sun. Objects between 1/100 M sun and 1/12 M sun may produce energy by fusion for a short time and are called brown dwarfs. Objects less than 1/100 M sun are planets.

February 7, 2006Astronomy The Mass-Luminosity Relation As shown in Fig. 17.8, there is a correlation between mass and luminosity. More massive stars are more luminous (give off more energy). For a few stars this relation is violated. These exceptions are the white dwarfs.

Mass vs. Luminosity each point on this plot represents the absolute magnitude (luminosity) and color (temperature) of a main sequence star Sun: luminosity 1.0 mass 1.0

February 7, 2006Astronomy Diameters of Stars Moon crosses in front of star – eclipse brightness of the star decreases gradually during the eclipse time for decrease depends on size of star Eclipsing binary stars Time Brightness

February 7, 2006Astronomy Eclipsing Binary System Stars orbiting each other in a plane parallel to the line of sight: orbit is seen edge-on. One star periodically eclipses the other: total brightness of the combined stars decreases during the eclipse. The reduction in brightness depends on the luminosity and relative size of the two stars.

February 7, 2006Astronomy Interferometry Combine the light from two or more telescopes in a special way that yields the resolution of a much larger telescope. Regularly done with radio waves, very difficult to do with light.

February 7, 2006Astronomy H-R Diagram Relationship between temperature (color) and luminosity (absolute magnitude) for 90% of the stars 90% of stars lie along a band called the main sequence Plot of luminosity vs. temperature is called the Hertzsprung-Russell diagram or just H-R diagram for short. The following slides show different examples of H-R diagrams.

HR Diagram each point on this plot represents the absolute magnitude (luminosity) and color (temperature) of a star Sun: +4.8 magnitude B-V color index 0.62

temperature (Kelvin) spectral class luminosity (solar units) classes: familiar stars

February 7, 2006Astronomy H-R Diagram classes: main sequence 90% of stars large blue stars medium yellow stars small red stars supergiantsgiants white dwarfs

February 7, 2006Astronomy Main Sequence Stars mass Rigel (blue giant) ~ 16x Sun Proxima Centauri (red dwarf) ~ 0.4x Sun size Rigel ~ 7x Sun, Proxima ~ 0.6x Sun color = surface temperature Rigel (blue) 28,000 K, Sun (yellow) 6000 K, Proxima (red) 3,500 K spectrum = chemical composition Rigel B, Sun G, Proxima Centauri M luminosity Rigel 27,000x Sun, Proxima 0.05x Sun

February 7, 2006Astronomy Main Sequence Star Properties colorclass solar mass solar size Temper- ature (K) exampleprominent lines bluest O 20 – ,00010 Lacertaeionized helium bluish B ,000Rigel, Spica neutral helium, neutral hydrogen blue- white A ,000Siriusneutral hydrogen white F ,000Canopus neutral hydrogen, ionized calcium yellow- white G ,500 Sun,  -Centauri neutral hydrogen, strongest ionized calcium orange K ,000Tau Ceti neutral metals (calcium, iron), ionized calcium red M ,000 Proxima Centauri molecules and neutral metals

February 7, 2006Astronomy Color-Magnitude H-R diagram also called a color-magnitude diagram All stars visible to the naked eye (magnitude =< +5) & all stars within 25 parsecs. Luminous stars easier to observe rarer in the galaxy. Mostly in top half of the H-R diagram. Faint stars harder to see more common in the galaxy. Mostly in bottom half of the H-R diagram.

February 7, 2006Astronomy The Other 10% of Stars About 10% of stars don't follow the mass-luminosity relationship don't lie on the main sequence Giant and Supergiant stars upper right of the HR diagram. large in diameter because very luminous even though they are cool. White dwarfs lower left of the HR diagram. small diameter (Earth-sized) hot but dim Betelgeuse

February 7, 2006Astronomy Main Sequence: Typical Stars Is the Sun an "average'' or "typical''? The meaning of "average'' depends on how one chooses the sample! Compared to the nearby stars, the Sun is luminous, hot, and big. Compared to the apparently bright stars, the Sun is dim, cool, and small. Compared to the stars in globular clusters, the Sun is very young. Compared to the stars in open (galactic) clusters, the Sun is very old.

February 7, 2006Astronomy Our Sun compared to… From Hipparcus survey. From Hipparcus survey. Most stars that appear bright in our sky are also intrinsically luminous Near stars are all within 7.63 parsecs of the Sun. Near stars are mostly cool and faint. the 100 (apparent) brightest stars in our sky and the 100 nearest stars.

February 7, 2006Astronomy More comparisons… Bright Stars: Most of the apparently bright stars are hot and luminous A and B-type stars. Includes a few of the very hot O-type stars. All but one of the K-type stars in the bright star sample are giants or supergiant stars. All of the M-type stars are giants or supergiants. Proportions of the spectral types for each group.

February 7, 2006Astronomy Proportions of the spectral types for each group. Near Stars Look different from bright stars. Majority of near stars are cool and faint K and M-type stars. Only one star in the entire sample is a giant star. Rest are main sequence stars. More comparisons…

February 7, 2006Astronomy Representative Sample Which of these samples is more representative of the entire population of stars in our galaxy? A representative sample includes all parts of the population of the objects your are investigating in their proper proportions. The relative proportion of common things will be greater than the relative proportions of rare things. In fact, the uncommon things may not be found in a small representative sample because they are so rare!

February 7, 2006Astronomy Summary A careful census of stars leads to a number of conclusions: Stars have a wide range of masses, luminosities, temperatures, and sizes. The most common stars are smaller and less luminous than the Sun. The stars organize into an understandable pattern on the H- R diagram. Binary star systems are common and useful for measuring masses. Several techniques exist for measuring diameters of stars.