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Berkeley ASTRON 10 - 28. Multiple Stars and Prestellar Nebulae

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Multiple Stars and Prestellar Nebula, 4 November 2013!Double Stars: !1.Although our star is single, the majority of stars are in a multiple star A.systems.!The percentage of stars in multiple star systems is currently quoted at a.80%, though astronomers probable don't know enough for this to be accurate. !The percent of stars in multiple star systems changes with the 1.decades. !Fifty or sixty years ago, it was 50%. !A.In early 1980 it was 90%.!B.The percentage is probably around 66%, definitely more than half. !b.Astronomers have detected stars very near to one another (double stars) B.since the earliest telescopes. !Primitive statistical studies decided there were too many of these for a.them to all be accidental. !This was proven by Sir William Herschell in the 1700s-1800s. !1.You can see some double stars with the unaided eye (you can tell that b.two stars move around each other over the years).!The "wiggling" motions of double stars can reveal that a single point of light C.is a double star before telescopes are good enough to actually see its separate components. !If a star's motion is straight, it is a single star system. !a.If a star's motion wiggles up and down, then the star is being impacted by b.gravitational orbits and is a double star. !For example, Sirius's double star nature was revealed in 1838, although c.its components weren't seen distinguished until 1862.!Astronomers cannot tell the actual arrangement of double stars by looking D.at them.!The apparent shape of the orbit of a double star depends on the angle a.you see it from on earth. !If the earth is on the same plane of the orbit of two stars, you see 1.them shuffling back and forth side by side in the sky.!The few stars that happen to have orbits in the earth's line of sight/A.plane eclipse each other. They are called "eclipsing variable stars."!When the dimmer star eclipses the brighter, the brightness falls a.more than when the brighter star eclipses the dimmer. !These stars are mostly double stars with small orbits.!b.They were discovered when their brightness changed. !1.You can get the mass and motion of these stars readily. !2.If the earth directly faces the orbit of the two stars, you see the 2.widest open orbit. !At any other position, the orbit narrows. !3.There is some angle where the orbit appears circular.!A.NOTE: Astronomers conventionally depict the orbit of double stars with b.the brighter star in the middle and the dimmer star orbiting the brighter. In reality, both stars orbit around a common center of gravity.!This conventional orbit is easier to describe because you cannot see a 1.center of gravity. !There is some angle where you can see the brighter star of a double 2.star system as the center of orbit. !Astronomers can tell the actual physical arrangement of double stars by E.measuring how fast they move (their orbital period).!Stars orbit around a center of mass, and how fast they orbit is a.determined by the shape of their orbit and the masses of the two objects.!Everything has a center of mass, double stars included. !1.For example, a baton is twirled not from its center, but from a A.center of mass closer to the heavy end.!The more massive an object is, the closer it is to the center of mass. 2.The less massive object is farther away. !Stars move fastest when they are physically closest to the center of b.mass.!This can be, but is not usually when the stars appear closest in the sky. !1.This is because astronomers often see orbits from an angle.!A.You can see this by looking at a star, and monitoring its distance and 2.speed.!Orbits were successfully monitored as early as the mid 1800s. !c.Measurements of the orbits of double stars can be used with Kepler's laws F.to determine both of the stars' masses.!To do this, astronomers must first look at the double star with a a.telescope.!Then, they chart the positions of the two stars over years to find their b.orbital period (how long it takes them to complete one full orbit).!They compare this data to the star's distances from each other in c.Kepler's law p squared = a cubed to find the mass of the whole system. !Kepler's law: The period of a planet's orbit squared is equal to the 1.length of the planet's semimajor axis cubed. !By comparing the sizes of the two orbits, you can find the percent of the d.total mass each star of the double star has. This gives you the mass of each object.!This is the only way to determine the masses of objects in the universe, e.which is why astronomers don't know much about the masses of single stars, asteroids, and comets. These objects do not orbit other objects. !Problem with measuring double stars: You cannot know both the orbital G.period and the spectral types of stars in a double star system. !In a double star with a wide orbit, it is easy to get the brightness and a.spectrum of each star, but hard to get the orbital period. !The period is thousands of years longer than astronomers have been 1.measuring the two stars.!While you can get the surface temperature and luminosity of these 2.objects, you cannot get their masses.!In a double star with a small orbit, you can get the period of the objects, b.but not their brightness and spectral types. !They are so close that their spectrum are contaminated by each 1.other. !While you can get the masses of these objects, you cannot get their 2.surface temperature or luminosity. !Solution: Astronomers have to wait to measure the orbits of double c.stars with wide orbits. !Astronomers can tell the motion of components of a double star to a few H.meters per second by measuring red and blue shifts in their spectra.!This is the Doppler effect: !a.Stars emit longer (redder) wavelengths when moving away from the 1.earth and shorter (bluer) wavelengths when approaching the earth.!The stars might not actually appear red or blue, but their spectral A.lines are shifted towards that end of the spectrum. !When a star is moving across the earth's line of sight, there is no 2.doppler effect. !The "wiggling" motions of double stars can reveal their nature before I.telescopes are good enough to actually see its separate components. !If a star's motion is straight it is a single star system. !a.If a star's motion wiggles, then the star is being impacted by gravitational b.orbits and is a double star. !For example, Sirius's double star nature and therefore the masses of its c.components was revealed in 1838, although they weren't seen distinguished until 1862.!There are


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Berkeley ASTRON 10 - 28. Multiple Stars and Prestellar Nebulae

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