8 march 2011 -- subst for roberta stars. 8 march 2011 -- subst for roberta
TRANSCRIPT
8 March 2011 -- subst for Roberta
Stars
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8 March 2011 -- subst for Roberta
( PARENTHETICAL )
STAR NAMES
SIRIUS CMa ( Alpha Canis Majoris ) HR 2491 HD 48915 SAO 151881
... and other STAR CATALOGS
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DISTANCES OF STARS
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DISTANCES OF STARS
EARLIEST IDEA: PARALLAX
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TRIGONOMETRIC PARALLAX
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STELLAR PARALLAX
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( SOMETIMES IT’S CALLED ANNUAL PARALLAX )
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PARALLAX GETS SMALLER AS
DISTANCE GROWS LARGER
( and vice-versa )
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8 March 2011 -- subst for Roberta
LET’S CHOOSE GOOD
MEASUREMENT UNITS …
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ANGLES
360 degrees ( 360° ) in a circle
60 arc-minutes ( 60 ) in 1°
60 arc-seconds ( 60 ) in 1
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ANGLES
360 degrees ( 360° ) in a circle
60 arc-minutes ( 60 ) in 1°
60 arc-seconds ( 60 ) in 1 so …
1,296,000 arc-seconds in a circle!
An arc-second ( arcsec ) is small;
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ANGLES
360 degrees ( 360° ) in a circle
60 arc-minutes ( 60 ) in 1°
60 arc-seconds ( 60 ) in 1
1 inch subtends 1 arcsec at a
distance of 3.3 miles.
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ANGLES
360 degrees ( 360° ) in a circle
60 arc-minutes ( 60 ) in 1°
60 arc-seconds ( 60 ) in 1 we measure parallaxes in arcsec.
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UNITS FOR DISTANCES
ASTRONOMICAL UNIT ( AU )
= AVERAGE DISTANCE OF SUN
= RADIUS OF EARTH’S ORBIT
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UNITS FOR DISTANCES
ASTRONOMICAL UNIT ( AU )
= AVERAGE DISTANCE OF SUN
= RADIUS OF EARTH’S ORBIT
= about 150 million kilometers
= about 93 million miles
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8 March 2011 -- subst for Roberta
STAR’S DISTANCE =
_____________________206,265 AU
PARALLAX IN ARCSEC
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UNITS FOR DISTANCES
ASTRONOMICAL UNIT ( AU )
= AVERAGE DISTANCE OF SUN
= RADIUS OF EARTH’S ORBIT
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UNITS FOR DISTANCES
1 PARSEC = 206,265 AU
(explain)
usually abbrev. “PC”
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STAR’S DISTANCE =
_____________________1 PARSEC
PARALLAX IN ARCSEC
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UNITS FOR DISTANCES
1 PARSEC = 206,265 AU
= almost 31 trillion kilometers, urggh.
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By the way …
1 parsec = 3.26 LIGHTYEARS.
Nearest star is 1.3 pc or 4.3 ly.
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c. 1590: TYCHO BRAHE TRIED
TO DETECT PARALLAX AND FAILED.
(explain)
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c. 1680: NEWTON PREDICTED PARALLAX
WOULD BE VERY SMALL,
D ( nearest star ) = maybe 300,000 AU
(explain how he guessed)
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8 March 2011 -- subst for Roberta
SUCCESSFUL PARALLAXES, AROUND 1839:
61 CYGNI (BESSEL, GERMANY)
VEGA (STRUVE, RUSSIA)
ALPHA CENTAURI (HENDERSON, S.AFRICA)
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ALPHA CENTAURI A, B, C
DISTANCE =
270,000 AU
1.3 PC ( parsec )
4.3 LY ( lightyear )
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ALPHA CENTAURI
9 MILES
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THE THREE STARS OF ALPHA CENTAURI
A : YELLOW, SOMEWHAT
BRIGHTER THAN THE SUN;
B : ORANGE, 40% AS BRIGHT
AS THE SUN;
C : RED, 1 / 17000 AS BRIGHT
AS THE SUN.
“PROXIMA CENTAURI”
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PARALLAX DISTANCES SHOWED
THAT STARS ARE NOT ALIKE
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HIPPARCOS
SATELLITE
MEASURED
ULTRA - PRECISE
PARALLAXES
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NEXT: MOTIONS OF THE STARS IN SPACE
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PROPER MOTION OF BARNARD’S STAR
( 6 L.Y. AWAY )
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THE SUN, FOR EXAMPLE, MOVES
ABOUT 15 KILOMETERS PER SECOND
= 33,000 M.P.H.
= 3 A.U. PER YEAR.
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“STATISTICAL PARALLAX”
BASED ON PROPER MOTIONS
( explain )
-- GOOD ONLY ON AVERAGE,
BUT OK OUT TO ABOUT 5000 L.Y. --
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REMEMBER THE DOPPLER EFFECT?
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RADIAL VELOCITY measured with
the Doppler effect …
Examples …
20 km / s means the star is moving
toward us ( “blueshift” )
+ 15 km / s means it’s moving away
( “redshift” )
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WE QUOTE THE BRIGHTNESS OF EACH STAR AS A “MAGNITUDE”.
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ABOUT 2100 YEARS AGO, HIPPARCHUS
(= HIPPARCOS) MADE A LIST OF 1000 STARS...
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ABOUT 2100 YEARS AGO, HIPPARCHUS
(= HIPPARCOS) MADE A LIST OF 1000 STARS...
BRIGHTEST: “STARS OF THE FIRST MAGNITUDE”.
THEN “SECOND MAGNITUDE”, THIRD, ETC.
FAINTEST WE CAN SEE: “SIXTH MAGNITUDE”.
... simple “eye estimates”.
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MANY CENTURIES LATER, ASTRONOMERS LEARNED HOW TO MEASURE THE STARS’ MAGNITUDES...
for example -- Aldebaran ... m = 0.85 (fairly bright) Tau Ceti ... 3.50 HD 191849 ... 7.97 (faint)
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THE MAGNITUDE SCALE TURNED OUT TO BE LOGARITHMIC -- LIKE DECIBELS USED TO MEASURE SOUND.
80 DECIBELS = 1 90 DB = 10 100 DB = 100 ... and so on.
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THE MAGNITUDE SCALE IS LOGARITHMIC --
1.0 magnitude = 100 2.0 = 39.8 (2.512 x fainter)
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THE MAGNITUDE SCALE IS LOGARITHMIC --
1.0 magnitude = 100 2.0 = 39.8 (2.512 x fainter) 3.0 = 15.8
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THE MAGNITUDE SCALE IS LOGARITHMIC --
1.0 magnitude = 100 2.0 = 39.8 (2.512 x fainter) 3.0 = 15.8 4.0 = 6.31 5.0 = 2.512 6.0 = 1.0
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THE MAGNITUDE SCALE IS LOGARITHMIC --
1.0 magnitude = 100 2.0 = 39.8 (2.512 x fainter) 3.0 = 15.8 4.0 = 6.31 5.0 = 2.512 6.0 = 1.0 7.0 = 0.398 ... etc.
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THE MAGNITUDE SCALE IS LOGARITHMIC --
THE DIFFERENCE BETWEEN TWO STARS’ MAGNITUDES CORRESPONDS TO THE RATIO OF THEIR BRIGHTNESS.
Example: star A has m = 2.3, star B has m = 7.3; Difference ... 5.0 magnitudes.
So star A is 100 x as bright as star B.
8 March 2011 -- subst for Roberta “APPARENT VISUAL MAGNITUDES”
Sun: m = -27 Moon: -10 Venus: -4 Mars: -2.5 Sirius: -1.4 Vega: 0.0 Antares: 1.0 Polaris: 2.3
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“APPARENT VISUAL MAGNITUDES”
BRIGHTEST STAR (SIRIUS): m = -1.4
20th –BRIGHTEST STAR: +1.5 FAINTEST VISIBLE: ... about 6.5 WITH BINOCULARS: about 8 6-INCH TELESCOPE: about 12 PLUTO: 14 LIMIT WITH MODERN DETECTORS ON BIG TELESCOPES: 20 -- 28
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MAGNITUDES ALSO GIVE COLORS. WE USE FILTERS, FOR INSTANCE ‘V’ = “visual”, yellow-green, ‘B’ = “blue”
U B V R I J K L ... filters
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MAGNITUDES ALSO GIVE COLORS. WE USE FILTERS, FOR INSTANCE ‘V’ = “visual”, yellow-green, ‘B’ = “blue”
( B magnitude ) - ( V magnitude ) is a measure of the star’s color.
B – V = -0.3 ... blue, very hot = +0.6 ... like the Sun
= +2 ... red, cool star
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LUMINOSITY OR ABSOLUTE MAGNITUDE
OF A STAR
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LUMINOSITY OR ABSOLUTE MAGNITUDE
OF A STAR
upper-case M
( m is apparent magnitude.)
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LUMINOSITY OR ABSOLUTE MAGNITUDE
OF A STAR
M = THE APPARENT MAGNITUDE m
THAT THE STAR WOULD HAVE
AT A STANDARD DISTANCE OF
10 PARSECS = 33 LIGHTYEARS
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LUMINOSITY OR ABSOLUTE MAGNITUDE
OF A STAR
TO CALCULATE M , WE NEED TO
KNOW BOTH m AND D .
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LUMINOSITY OR ABSOLUTE MAGNITUDE
OF STARS
M FOR THE SUN = + 4.8
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LUMINOSITY OR ABSOLUTE MAGNITUDE
OF STARS
M FOR THE SUN = + 4.8
FAINTEST RED DWARF STARS: M = + 18
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LUMINOSITY OR ABSOLUTE MAGNITUDE
OF STARS
M FOR THE SUN = + 4.8
FAINTEST RED DWARF STARS: M = + 18
MOST LUMINOUS STARS: M = - 12
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LUMINOSITY OR ABSOLUTE MAGNITUDE
OF STARS
M FOR THE SUN = + 4.8
FAINTEST RED DWARF STARS: M = + 18
MOST LUMINOUS STARS: M = - 12
OVERALL RANGE: 30 MAGNITUDES,
= A LUMINOSITY FACTOR OF 1,000,000,000,000
( A TRILLION )
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