Astr 1020 1st Edition Lecture 7Current lecture(Reading Questions)Q: What is a Main Sequence?A: A set of stars, which are fusing hydrogen in their coresQ: Star cluster ages can be determined fromA: Main sequence turnoff How does our Sun compare to other stars? (properties) -luminosity -temperature -mass -age -compositionHertzsprung-Russell Diagram (H-R diagram) -how stars relate to each otherhigh |||Luminosity ||low ||___________________________________hot Temperature coolThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Inverse Square Law of Brightness + Stellar Luminosity -Apparent brightness decreases as surface area increases:Apparent Brightness = Luminosity / (4pi x distance^2)Luminosity = Apparent Brightness x 4pi x distance^2WHAT WE MEASUREo Apparent Brightness –how bright it appears to us here on Eartho Distance –measured with parallax CAN INFER (absolute) LUMINOSITY o How much power is emitted Q: Star Harry is twice as far away from us as Star Hermione. Harry is twice the luminosity as Hermione. How does Harry appear compared to Hermione in sky?A: Harry appears half as brightMeasuring the Temperature of Stars1. Peak of thermal spectrum (Wien’s Law) Hotter=bluer, cooler = redder2. Spectral linesa. Complex atoms or molecules (more electrons, more atoms) are fragileb. Fragile types are easily destroyed by collisions in high temperature regions High temp=high energy of particlesIf there are signs of fragile atoms and molecules, the temperature must be lowHISTORY Classifying Stars –Harvard College observatory- Women “computers” hired to help with survey of stars in the Milky Way- Paid .50/hr, about half what men would have earned- Most had studied astronomy, but were not allowed to work as scientists- Many became important turn-of-century astronauts Devising the strange temperature code- 1890’s –original classification of spectra was:o A =strongest hydrogen featureo B =less strong hydrogeno C, D, E….o O =least hydrogen featuresAnnie Jump Cannon sequence based on presence of various atoms/molecules made more sense O B A F G K M increasing temperature Cecilia Payne-Gaposchkin: Temperature is the key- Showed that surface temperature was the big factor (not composition) - Higher temps=more atoms ionized, molecules disassociated- Cooler temps = more atoms and molecules can survive  O B A F G K M -decreasing temperature Oh Be A Fine Guy, Kiss Me !Spectral Type- So hot that everything is ionized- Strong H lines, but nothing else- Moderate H lines, some other elements as well- Cool enough for lots of molecules to existO, B =hottest, bluestA, F, G =middle type, yellow (Sun is a G2 star)K. M =coolest, reddest Each class is actually divided into 10 subdivisions: O0-O9, B0-B9, etc… How do we find mass?- Can’t measure for single stars- Binary stars are fairly commono 80% of stars in galaxy- massgravitymotion (Newton’s Version of Kepler’s Thirds Law): p^2 =[(4pi^2) / (GM)] a^3Three variables: p, a, M (If we know two, we can solve for the third!)- p= orbital period- a=average orbital distance or angular separation (between centers) /SEMI-MAJOR AXIS (from parallax)- M=mass of the orbiteeMass = (4pi^2/G) a^3/p^2 kgWhat if the stars have similar masses? p^2 =[(4pi^2) / (G (M1 + M2)] a^3Main sequence stars- Fusing hydrogen in their cores- More massive stars have:o Hotter Temps –more gravitational pressure=higher To Higher Luminosity –higher T =much higher fusion ratesStars with Greater Mass Have Sorter Lifetimes- Available hydrogen fuel is greater for the most massive stars…- But luminosity is MUCH higherMore massive (more luminous) main sequence stars run out of fuel soonerEx: Most massive O star:M =100L = 10^6 LsunM/L = 10^2 /10^6 =10^-4 times the SunLifeO-star = 10^10 yrs x 10^-4 = 10^6 yearsQ: Which best describes how the lifetimes compare between 1 solar mass star and a 3 solar mass star?A: The 1 solar mass star will live more than three times longer than the 3 solar mass starLifetimes on Main Sequence- Stars spend 90% of their lives on MSfusing hydrogen into helium in their core- For Sun (G type), this is about 10 billion years- For more massive stars (OBA), lifetime is shorter (millions years)- For less massive stars (K,M), lifetime is longer (trillions)Stellar lifetimeLooking at collections of stars: Star Clusters- Groups of 100’s to millions of stars formed from the same interstellar gas cloud- All at about the same distance- All about the same age- Range of different mass stars- As stars run out of hydrogen fuel they move off the main sequence- In general, they cool off slightlyo Top end of main sequence starts to “turn off” towards the right- Important note: Stars do NOT move along the main sequence as they liveANALOGY: different foods have different shelf livesMain-sequence turn off of a cluster tells us its age***Which stellar parameter has the most extreme range between largest and smallest stars?