Distances to Stars: Parsecs and Light Years. The Stars are other Suns Or…the Sun is the closest star “L’amor che move il Sole E le altre stelle”… Dante,

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Presentation transcript:

Distances to Stars: Parsecs and Light Years

The Stars are other Suns Or…the Sun is the closest star “L’amor che move il Sole E le altre stelle”… Dante, end Of Paradiso Think about them: Demo of “Starry Night”

Basic Questions about Stars How far away are they? How hot are they? How massive are they What are they made of? Do they have planets too? How do they change with time (stellar evolution) 16 Cygni

Most Basic Method of Stellar Distance Determination Trigonometric parallax: an ordinary surveying technique Figure 16.1 of textbook, and Section 16.2 DEMO

The concept of parallax and distance measurement

How big is the angular shift? s r Th If Th << 45 degrees, Th (in radians) = s/r 90 degrees make a right angle 60 arcminutes made one degree 60 arcseconds make one arcminute

The values of stellar parallax Smaller parallax > greater distance Largest parallax = 0.75 arcseconds 1 arcsecond = dime at 1.3 miles

Alpha Centauri, the closest star

How close/far is Alpha Centauri, and what units do we use to describe its distance? 1 parallax-second = 1 parsec = 3.08E+16m =206,265 au. Alpha Centauri is 1.33 parsecs If 1 au were 1cm, Alpha Centauri at a distance of 1.5 miles The Light Year = (2.9979E+08 m/sec)*(3.156E+07 sec) = 9.461E+15 m. Alpha Centauri is 4.3 light years away 1 parsec = 3.26 ly

Think back to descriptions of solar system distances (light minutes to light days) and compare the situation for nearby stars to get an idea of the immensity of interstellar space

Sources of Parallax Measurements Pre-1990: ground- based telescopes, ~ 1000 stars measured Hipparchus: 118,000 stars measured See book for breakdown on precision (section 16.2 of book)

What distances do we have for the nearest stars? Appendix 11 of book Abbreviated Version StarDist. (ly)Dist. (pc) Alpha Cent Sirius Altair Vega268 Arcturus3611

Look at Appendix 12 (distances to brightest stars) and think about the significance of those data

What are the distances to the stars in Cygnus?

Distances to the Stars of Cygnus 990pc 3.5 pc 17pc 52 pc 467 pc125 pc 22 parsecs