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8 March 2011 -- subst for Roberta
Stars
8 March 2011 -- subst for Roberta
8 March 2011 -- subst for Roberta
( PARENTHETICAL )
STAR NAMES
SIRIUS
a CMa ( Alpha Canis Majoris )
HR 2491
HD 48915
SAO 151881
... and other STAR CATALOGS
8 March 2011 -- subst for Roberta
DISTANCES OF STARS
8 March 2011 -- subst for Roberta
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 )
8 March 2011 -- subst for Roberta
8 March 2011 -- subst for Roberta
LET’S CHOOSE GOOD
MEASUREMENT UNITS …
8 March 2011 -- subst for Roberta
ANGLES
360 degrees ( 360° ) in a circle
60 arc-minutes ( 60  ) in 1°
60 arc-seconds ( 60  ) in 1 
8 March 2011 -- subst for Roberta
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;
8 March 2011 -- subst for Roberta
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.
8 March 2011 -- subst for Roberta
ANGLES
360 degrees ( 360° ) in a circle
60 arc-minutes ( 60  ) in 1°
60 arc-seconds ( 60  ) in 1 
we measure parallaxes in arcsec.
8 March 2011 -- subst for Roberta
UNITS FOR DISTANCES
ASTRONOMICAL UNIT ( AU )
= AVERAGE DISTANCE OF SUN
= RADIUS OF EARTH’S ORBIT
8 March 2011 -- subst for Roberta
UNITS FOR DISTANCES
ASTRONOMICAL UNIT ( AU )
=
=
=
=
AVERAGE DISTANCE OF SUN
RADIUS OF EARTH’S ORBIT
about 150 million kilometers
about 93 million miles
8 March 2011 -- subst for Roberta
8 March 2011 -- subst for Roberta
STAR’S DISTANCE =
206,265 AU
_____________________
PARALLAX IN ARCSEC
8 March 2011 -- subst for Roberta
UNITS FOR DISTANCES
ASTRONOMICAL UNIT ( AU )
= AVERAGE DISTANCE OF SUN
= RADIUS OF EARTH’S ORBIT
8 March 2011 -- subst for Roberta
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)
8 March 2011 -- subst for Roberta
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”
8 March 2011 -- subst for Roberta
PARALLAX DISTANCES SHOWED
THAT STARS ARE NOT ALIKE
8Ast
March
1001,
2011
29 -March
subst2010
for Roberta
-- KD
HIPPARCOS
SATELLITE
MEASURED
ULTRA - PRECISE
PARALLAXES
8 March 2011 -- subst for Roberta
8 March 2011 -- subst for Roberta
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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8 March 2011 -- subst for Roberta
“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” )
8 March 2011 -- subst for Roberta
8 March 2011 -- subst for Roberta
8 March 2011 -- subst for Roberta
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
Tau Ceti ...
3.50
HD 191849 ...
7.97
(fairly bright)
(faint)
8 March 2011 -- subst for Roberta
8 March 2011 -- subst for Roberta
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)
8 March 2011 -- subst for Roberta
THE MAGNITUDE SCALE IS LOGARITHMIC -1.0 magnitude = 100
2.0 =
39.8
3.0 =
15.8
(2.512 x fainter)
8 March 2011 -- subst for Roberta
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
8 March 2011 -- subst for Roberta
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.
8 March 2011 -- subst for Roberta
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.
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“APPARENT VISUAL MAGNITUDES”
Sun:
m =
Moon:
Venus:
Mars:
Sirius:
Vega:
Antares:
Polaris:
-27
-10
-4
-2.5
-1.4
0.0
1.0
2.3
8 March 2011 -- subst for Roberta
“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
8 March 2011 -- subst for Roberta
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
8 March 2011 -- subst for Roberta
LUMINOSITY OR ABSOLUTE MAGNITUDE
OF A STAR
8 March 2011 -- subst for Roberta
LUMINOSITY OR ABSOLUTE MAGNITUDE
OF A STAR
upper-case M
( m is apparent magnitude.)
8 March 2011 -- subst for Roberta
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
8 March 2011 -- subst for Roberta
LUMINOSITY OR ABSOLUTE MAGNITUDE
OF STARS
M FOR THE SUN = + 4.8
FAINTEST RED DWARF STARS: M = + 18
8 March 2011 -- subst for Roberta
LUMINOSITY OR ABSOLUTE MAGNITUDE
OF STARS
M FOR THE SUN = + 4.8
FAINTEST RED DWARF STARS: M = + 18
MOST LUMINOUS STARS: M = - 12
8 March 2011 -- subst for Roberta
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 )
8 March 2011 -- subst for Roberta