Ch. 28 The Stars Properties of Stars ??? Are all stars the same? What’s different about them? List a few differences….

Luminosity- Brightness 3 factors control brightness of a star: How big (size) it is How hot (temperature)it is How far away (distance)it is

Apparent Magnitude Hipparchus, 2nd century B.C. Classified all the 1000 stars visible to the naked eye on a scale of 1 to 6. 1 = brightest Relative: ranking of brightness Brightness or a star as it appears when viewed from Earth

Absolute Magnitude The “true” brightness (LUMINOSITY) Measurable: the amount of energy/sec Distance: if the stars were the same distance from Earth. (32.6 ly or 10 pc)

Star brightness scale The magnitude scale is logarithmic: a change in 5 magnitudes = change of 100x’s the brightness

Binary (double) Stars Most stars are binary (two stars rotating around each other due to gravitational attraction) or multiple systems. Binary stars in the constellation Scorpio Rigel A and Rigel B in the constellation Orion

Variable Stars A stars that changes its luminosity over time. Either in a regular way or erratically They “pulse” – like a heart A star is unstable: it cools and shrinks, then heats and expands. Due to varying density in the star: the energy can out easier or slower.

Parallax to measure distances of stars Measured in PARSECs For very distance stars, the parallax is so small, it is very difficult to measure

Sirius is the brightest star in the sky at 2.6 pc distance. Pollux (10 pc) and Rigel (240 pc) appear to have similar brightness, Yet Rigel is 24 x’s FARTHER DISTANCE away! How can that be? Pollux Rigel Sirius is the brightest star in the sky at 2.6 pc distance.

Our stellar neighborhood Nearest stars to our Sun Alpha Centauri (triple star system) Has the largest stellar parallax 270,000 AU or 4.3 light-years 300,000 times distance from Earth to Sun! http://www.youtube.com/watch?v=vg3noqtm0L0&safety_mode=true&persist_safety_mode=1 Sheldon star steps

Distance to our Nearest Neighbor 1. Proxima Centauri is the closest It has the largest stellar parallax 1.3 pc Parsecs or parallax second 19 trillion miles 270,000 AU, astronomical units distance from Earth to Sun or 93 million miles 4.3 ly light-year (distance light travels in a year: 6 trillion miles!

Mass of Stars Measured in ‘solar masses’ Calculated indirectly, using its gravitaitonal affect on nearby objects (Newton’s law of gravity) Measured in ‘solar masses’ Other stars are compared to our Sun’s mass: 1 SM Can be up to 2000 SM !!

Stellar Size If the star is close enough, we can measure the size by parallax. If we know a stars luminosity, and temperature, Scientists can indirectly determine the radius (size) of a star. Compared to our Sun’s radius: 1R

Yet it’s not the brightest star, So its size must be…. Aldebaran Cool temp. = 4000 K Low luminosity = 2.3 x 1025 W Its COOL and DIM So…. for us to see it, Its size must be….. Procyon B High temp. = 8500K Large luminosity = 1.3 x 1029 W Its HOT and BRIGHT Yet it’s not the brightest star, So its size must be…. BIG!! 40 R Sun radius Small .01 R

Of the light from the photosphere Sun’s Composition What is the sun made of and how do scientists know?????? Spectral Analysis Of the light from the photosphere

ENERGY FROM THE SUN http://www.classzone.com/books/earth_science/terc/content/visualizations/es2601/es2601page01.cfm?chapter_no=26

Spectrum

Electromagnetic Spectrum and Wavelength

Each element has its own spectra that can be used to identify the element Suns’ Spectra Hydrogen Helium Combined, what COLOR does the Sun appear to us?

Our Sun is yellow and has a surface temp. of 6000K

Our eyes evolved to see the wavelengths of yellow

Every element has a unique spectra that can be used to identify it.

Temperature = Color H-R diagram graph

30,000 Rigel Temperature ºC Sirius Sun blue yellow (RED) Color

Hertzsprung-Russell Diagram (pg.663) bright Cool and bright = red giant Luminosity / absolute brightness 1- Hot and dim = blue dwarf dim 30,000 K 3000 K Hotter /blue Surface temperature / color Cooler (RED)

Astronomers classify stars in spectral classes by color/temperature

3000°

The Hertzsprung-Russell diagram isn’t just an orginazational chart for the the stars, but the life cycle of the stars!

H-R diagram is the Life Cycle of the stars!

Color and wavelength