Often Only Seeing a Point of Today: Measuring the Stars Light - PowerPoint PPT Presentation

ASTR 1120 General Astronomy: Last two classes Stars & Galaxies • The Sun - Energy by Fusion � OMEWORK #2 – Solar Structure • Chromosphere On Mastering Astronomy due TODAY, by 5pm • Corona • Solar Wind � OMEWORK #3 – Solar Activity • Sunspots • Flares On Mastering Astronomy available by today, 5pm, and • Coronal Mass Ejections due on TUESDAY 09/29, by 5pm Often Only Seeing a Point of Today: Measuring the Stars Light • Stars are so small 1. Measuring distances compared to their distance to us that we almost never have the 2. Measuring stellar resolution to see their luminosities sizes and details directly– “point sources” 3. Measuring temperatures • We deduce everything by measuring the amount of 4. Measuring masses light (brightness) at different wavelengths (color, spectra) Angular size of Alpha Centauri = .004 arcsec

One of the Most Basic Problems in Astronomy • Stars take millions or even billions of years to go through their life stages- we rarely see a single star change • Observing many Star of given APPARENT BRIGHTNESS different stars lets us could be either figure out the sequence of a single A. very luminous star far away star’s life B. low luminosity star closer by DISTANCE to the star matters! Clicker Question Inverse Square Law of Brightness If you quadruple (x4) your distance to a light and look again, how much dimmer does it appear? Apparent Brightness = A. one-half as bright as originally L o B. one-fourth as bright ------------------- C. one-eighth as bright 4 (distance) 2 D. one-sixteenth as bright E. unchanged, since really same light

Clicker Question Stellar Luminosity • What we measure: If you quadruple (x4) your distance to a APPARENT light and look again, how much dimmer BRIGHTNESS does it appear? – how bright it appears to us here on Earth A. one-half as bright as originally • What we want to know: B. one-fourth as bright (absolute) LUMINOSITY C. one-eighth as bright – how much energy is D. one-sixteenth as bright emitted (Joules/sec or watts) E. unchanged, since really same light • Need to know DISTANCE to the star Determining Distance Using Parallax How Do We Measure the Distances to Astronomical Objects? • Measure the apparent • We’ll keep asking this question again movement of stars over a and again over the semester year – Movement is caused by Earth’s movement around • Several techniques, each valid for the Sun different objects at different distances • Technique #1: Parallax

Self-demo of parallax Parallax • Your nose is the Sun • Parallactic angle (p) = • Your left eye is the Earth in January 1/2 of the change in • Your right eye is the Earth in June angular position over • Your thumb (placed six inches from your face) is 6 months a nearby star • Larger for closer • Watch the apparent motion of your thumb objects against a distant reference point as you take • Smaller for farther measurements in January and June objects – Repeat experiment with a further star (thumb at arm’s length) Which “move” more -- closer or farther objects? Clicker Question Parallax formula What is parallax? • New Distance Unit invented for just this method of distance measurement!! A. The total amount of power that a star emits into – Parsec = (parallax+arcsecond) space. • An object at a distance of one parsec has a parallax of 1 B. A measurement of the separation of two stars in a arcsecond visual binary. • Distance (parsecs) = 1/p (arcsec) C. A classification of a star based on its temperature. 1 parsec = 1 pc = 3.26 light years D. The shift of a star’s apparent position due to the motion of the Earth. E. a statement that is seemingly contradictory or Remember 1 arcsecond = 1/3600 degree! opposed to common sense and yet is perhaps true

Clicker Question Clicker Question The biggest ground-based telescopes with What is parallax? adaptive optics can measure stars’ positions to accuracies of about 0.1 arcseconds. How far A. The total amount of power that a star emits into away can they map the positions of stars via space. parallax? B. A measurement of the separation of two stars in a visual binary. A. 1 pc C. A classification of a star based on its temperature. B. 10 pc D. The shift of a star’s apparent position due to the motion of the Earth. C. 100 pc E. a statement that is seemingly contradictory or D. 1000 pc opposed to common sense and yet is perhaps true Clicker Question Parallax Brad and Angelina are two stars that • B. maximum distance is have the same apparent brightness. set by the accuracy with Brad has a larger parallax angle which you can measure positions in the sky (space than Angelina. Which star is more does better than ground) luminous? A. Brad Distance (pc) = d (in parsecs) = 1 / 0.1 arcsec B. Angelina 1 / p (in arcsec) = 10 pc = 32.6 ly C. Not enough information to know

Best parallax measurer: Hipparcos satellite (1989-1993) • Space measurements not • Brad has a larger PARALLAX ANGLE. affected by atmosphere Thus, he is closer to us. • Measurement made many times until accurate to 0.001 • If they both have the same APPARENT arcsec ( � 3300 light years) BRIGHTNESS, but Brad is closer… • 100,000 stars mapped • (2.5 million to slightly lesser accuracy) • B. Angelina must be more luminous. Magnitudes: Crazy Units Magnitudes: The modern version • Dates back to the original Hipparchus (the person! • Later calculated more precisely and found our eye 190-120 B.C.) sees on a semi-logarithmic scale. – Brightest stars were ‘of first magnitude” – Linear difference of 2.5 in magnitude is a factor of 10 in apparent brightness – Dimmest stars were ‘of sixth magnitude” • Mag 1 is 100 times brighter than mag 6 (diff=5=2*2.5) – Everything else sorted in between. Sirius (brightest star in the night sky) = -1.5 Apparent magnitude = 1 are the brightest stars in the Sun = -26.7 sky Mag 30 = faintest ever detected (with Hubble) Mag = 6 is faintest naked eye can see. NOTE THE BACKWARDS SCALE! NOTE THE BACKWARDS SCALE! Bigger number is fainter! Bigger number is fainter!

Clicker Question Clicker Question Tom is a magnitude 5 star and Katie Tom is a magnitude 5 star and Katie is a magnitude 12.5 star. Who is is a magnitude 12.5 star. Who is brighter and by how much? brighter and by how much? A. Tom is 7.5 times brighter than Katie. A. Tom is 7.5 times brighter than Katie. B. Katie is 7.5 times brighter than Tom. B. Katie is 7.5 times brighter than Tom. C. Tom is 100 times brighter than Katie. C. Tom is 100 times brighter than Katie. D. Tom is 1000 times brighter than Katie. D. Tom is 1000 times brighter than Katie. E. Katie is 1000 times brighter than Tom. E. Katie is 1000 times brighter than Tom. Astronomer’s Toolbox: Two ways to measure What do we know how to do temperature now? 1) Thermal spectrum (i.e. Wien’s Law, Chapter 5) • Measure Distance: Hotter = bluer; cooler = redder – parallax…good to nearby stars but not beyond • Measure Luminosity: – measure apparent brightness and distance, infer luminosity Next: Surface Temperature

Spectra help classify stars 2.) Spectral lines even better! • Different atoms and molecules can be characterized as “tough” or “fragile” • The more complex an atom or molecule (more electrons, more atoms), the more fragile it is. – Fragile types are more easily ionized or knocked apart by collisions in high temperature regions � If there are signs of fragile atoms and molecules, the temperature must be low Clicker Question Clicker Question Which star is hotter? Which star is hotter? A. This star near A. This star near the top (less “fragile” the top (less “fragile” atoms) atoms) B. This star near B. This star near the bottom (more the bottom (more “fragile” atoms) “fragile” atoms) C. We don’t have C. We don’t have enough information. enough information.

Devising the strange temperature code A bit of history: Classifying Stars • Original classification of World War I, Harvard spectra (1890) was: College observatory A = strongest hydrogen feature Women were hired by the observatory B = less strong director as hydrogen …C, D, etc. “computers” to help with a new survey of the Milky Way • Annie Jump Cannon realized that, visually, a Most had studied different sequence astronomy, but were made more sense not allowed to work as � O B A F G K M !! (~1910) scientists OBAFGKM • Important: the different spectral lines seen are • Spectral (color) NOT primarily because classification stars are made of different elements O = hottest, bluest • Most stars are made mostly of hydrogen G = middle type, yellow (Sun) • The variety in spectra is due to temperature via the survival of electrons M = coolest, reddest attached to atoms and Cecelia Payne-Gaposchkin figured this out molecules at the star’s surface

O B A F G K M O B A F G K M Oh Be A Fine Girl/Guy, Kiss Me How to remember the sequence? Spectral Classification: O B A F G K M Hottest stars: O B mostly helium, little hydrogen Hot stars: A F helium, hydrogen Cooler stars: G hydrogen, heavier atoms Coolest stars: M molecules, (complex absorption bands)