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ASTRONOMY Stars

OBJECTIVES By the end of this presentation, students will be able to describe six properties of stars that can be determined from the earth; Explain some of the difficulties astronomers have in measuring these properties.

STELLAR PROPERTIES 1.Distance - using parallax Earth orbiting the sun. Astronomers photograph a star against a background of stars. Earth six months later. Astronomers photograph the star against a different set of background stars.

STELLAR PROPERTIES 1.Distance - using parallax The apparent shift in position of the star is the parallax angle. D a

STELLAR PROPERTIES 1.Distance - using parallax D a Knowing this angle and using simple trigonometry, the distance D can be calculated.

STELLAR PROPERTIES 1.Distance - using parallax The parallax angle measures the stellar distance in parsecs. D a

STELLAR PROPERTIES 1.Distance - using parallax D a This is converted into more familiar units - the mile and the light-year.

STELLAR PROPERTIES 1.Distance - using parallax D a The nearest star to the sun is 4.3 ly away, or 25,284,000,000,000 miles.

STELLAR PROPERTIES 1.Distance - using parallax D a There are stars within 20 parsecs, or 65 ly, of the earth.

STELLAR PROPERTIES 1. Distance - using Cepheid variable stars, et al. Cepheid variable stars brighten and dim at regular intervals, due to a regular swelling and shrinking of the star. The brighter the star, the longer the period of brightening and dimming.

STELLAR PROPERTIES 1. Distance - using Cepheid variable stars, et al PERIOD (days) ABSOLUTE MAGNITUDE POP. TYPE 1 POP. TYPE 2 RR LYRAE

STELLAR PROPERTIES 1. Distance - using Cepheid variable stars, et al. Periods of Cepheid variables vary from 7 hours to 100 days. Knowing their absolute brightness and comparing this to their apparent brightness, the distance to the star may be calculated.

STELLAR PROPERTIES 1. Distance - using Cepheid variable stars, et al. Ap.Mag - Abs. Mag. = 5 x log(dist÷10) Technique has an error margin of at least 10%. Problem: to calculate distances to other stars, their brightness must be known, but to determine their brightness, their distances must be known!!

STELLAR PROPERTIES 1. Distance - using red shift in galaxies The Doppler Effect is used to determine the speed of approach or recession of stars and galaxies based on the shifts in characteristic frequencies of the light they emit.

STELLAR PROPERTIES 1. Distance - using red shift in galaxies Blue Shifting - a shift in the emission spectra of a star towards more energetic values Indicates the star is moving towards the observer. The greater the blue shift, the faster the star is moving.

STELLAR PROPERTIES 1. Distance - using red shift in galaxies Red Shifting - a shift in the emission spectra of a star towards less energetic values Indicates the star is moving away from the observer. The greater the red shift, the faster the star is moving.

STELLAR PROPERTIES 1. Distance - using red shift in galaxies The shifted frequencies, (f s ) are compared to the “at rest” (f o ) and the speed of the star can be calculated: v = (f o 2 - f s 2 ) c (f o 2 + f s 2 )

STELLAR PROPERTIES 1. Distance - using red shift in galaxies Problem : the red shift might be due to the recession of the galaxy or due to other influencing effects… which?

STELLAR PROPERTIES 2. Luminosity - the total amount of energy a star radiates each second. Apparent Magnitude – the brightness of a star as viewed by an observer on the earth. determined by comparing the brightness of various stars on photographs. Apparent Magnitude – the brightness of a star as viewed by an observer on the earth. determined by comparing the brightness of various stars on photographs. Apparent Magnitude – the brightness of a star as viewed by an observer on the earth. determined by comparing the brightness of various stars on photographs. 2. Luminosity – the total amount of energy a star radiates each second. STELLAR PROPERTIES

2. Luminosity – the total amount of energy a star radiates each second. The first brightest stars have a magnitude of +1. The next brightest stars have magnitudes of +2 Each magnitude of brightness is 2.5 times brighter (or dimmer) than the one before it.

STELLAR PROPERTIES 2. Luminosity – the total amount of energy a star radiates each second. Stars of a magnitude greater than +6.0 are too dim to be seen without optical aide. Stars of a magnitude greater than +9.0 are too dim to be seen using small telescopes or binoculars. The sun has a magnitude of – 26.

STELLAR PROPERTIES 2. Luminosity – the total amount of energy a star radiates each second. Once the absolute magnitude has been determined, the luminosity can be calculated and compared to the luminosity of our sun. Abs. Mag o – Abs. Mag. s = 2.5 log L s L o Luminosity of the sun is 4x10 26 watts.1 = L o

STELLAR PROPERTIES 2. Luminosity – the total amount of energy a star radiates each second. Luminosity of all visible stars range from 1/1,000,000 the luminosity of the sun to 1,000,000 time the luminosity of the sun. 90% of the stars are not as bright as the sun.

STELLAR PROPERTIES 3. Stellar Temperatures - using color from spectrographs and Wien’s Law. Wien’s Law - the wavelength of the most intense light emitted from a star is proportional to the star’s temperature. (the hotter the star, the bluer it looks.) Temperature = 345 x (wavelength)

STELLAR PROPERTIES 3. Stellar Temperatures - using color from spectrographs and Wien’s Law. The range of temperatures for observable stars are from 1/3 as hot as our sun to 10 time as hot as our sun The sun’s surface temperature is 5500ºC or 9900ºF

STELLAR PROPERTIES 4. Stellar Diameter – using Luminosities, temperature and... Stephan-Boltzman Law L s = 7.2x10 -7 R 2 T 4 Stars range in size from 1/100 the size of the sun to 100 times the size of the sun.

STELLAR PROPERTIES 5. Stellar Composition - using spectral examinations. Detecting two or more lines of that element in the star’s spectrum indicates that element is present in that star. The brighter the spectral line, the greater amount of that element in the star.

STELLAR PROPERTIES 5. Stellar Composition - using spectral examinations. Hydrogen Helium Iron Calcium White light

STELLAR PROPERTIES 5. Stellar Composition - using spectral examinations. Most stars (98%) are made of hydrogen and helium. 1 or 2% of the star’s mass may contain iron, titanium, calcium, sodium...

STELLAR PROPERTIES 6. Stellar Mass - using direct observations through telescopes... This works only with visual binary stars, because the period of each star orbiting the other and the distances between each star must be known.

STELLAR PROPERTIES 6. Stellar Mass - using direct observations through telescopes... In combining Kepler’s Law of Harmony with Newton’s Law of Gravity: G T 2 = M 1 + M 2 4  2 R 3 And M 1 = D 1 2 M 2 = D 2 2 The masses can be calculated.

STELLAR PROPERTIES 6. Stellar Mass - using direct observations through telescopes... Stellar masses range from 0.5 to 50 times the mass of the sun. This technique works ONLY with multiple star systems; binaries, trinaries, etc…

H-R DIAGRAM A pattern of groups of stars emerge when plotting the Abs. Mag. of a star as a function of its temperature. 30K 10K 7.5K 6K 5K 3K TEMPERATURE (ºK) ABSOLUTE MAGNITUDE White Dwarfs Red Giants Super Giants Main Sequence Red Dwarfs Wolf-Rayet Stars Hypergiants Red Giants Super Giants Hypergiants White Dwarfs Red Dwarfs

H-R DIAGRAM A pattern of groups of stars emerge when plotting the Abs. Mag. of a star as a function of its temperature. RUSSELL-VOGT THEOREM – the equilibrium structure of an ordinary star is determined uniquely by its mass and chemical composition. 30K 10K 7.5K 6K 5K 3K TEMPERATURE (ºK) ABSOLUTE MAGNITUDE

ASTRONOMY Stars