8 march 2011 -- subst for roberta stars. 8 march 2011 -- subst for roberta

8 March 2011 -- subst for Roberta

( PARENTHETICAL )

STAR NAMES

SIRIUS CMa ( Alpha Canis Majoris ) HR 2491 HD 48915 SAO 151881

... and other STAR CATALOGS

DISTANCES OF STARS

EARLIEST IDEA: PARALLAX

TRIGONOMETRIC PARALLAX

STELLAR PARALLAX

( SOMETIMES IT’S CALLED ANNUAL PARALLAX )

PARALLAX GETS SMALLER AS

DISTANCE GROWS LARGER

( and vice-versa )

LET’S CHOOSE GOOD

MEASUREMENT UNITS …

ANGLES

360 degrees ( 360° ) in a circle

60 arc-minutes ( 60 ) in 1°

60 arc-seconds ( 60 ) in 1

ANGLES

60 arc-seconds ( 60 ) in 1 so …

1,296,000 arc-seconds in a circle!

An arc-second ( arcsec ) is small;

ANGLES

60 arc-seconds ( 60 ) in 1

1 inch subtends 1 arcsec at a

distance of 3.3 miles.

ANGLES

60 arc-seconds ( 60 ) in 1 we measure parallaxes in arcsec.

UNITS FOR DISTANCES

ASTRONOMICAL UNIT ( AU )

= AVERAGE DISTANCE OF SUN

= RADIUS OF EARTH’S ORBIT

UNITS FOR DISTANCES

= about 150 million kilometers

= about 93 million miles

STAR’S DISTANCE =

_____________________206,265 AU

PARALLAX IN ARCSEC

UNITS FOR DISTANCES

1 PARSEC = 206,265 AU

(explain)

usually abbrev. “PC”

STAR’S DISTANCE =

_____________________1 PARSEC

PARALLAX IN ARCSEC

UNITS FOR DISTANCES

1 PARSEC = 206,265 AU

= almost 31 trillion kilometers, urggh.

By the way …

1 parsec = 3.26 LIGHTYEARS.

Nearest star is 1.3 pc or 4.3 ly.

c. 1590: TYCHO BRAHE TRIED

TO DETECT PARALLAX AND FAILED.

(explain)

c. 1680: NEWTON PREDICTED PARALLAX

WOULD BE VERY SMALL,

D ( nearest star ) = maybe 300,000 AU

(explain how he guessed)

SUCCESSFUL PARALLAXES, AROUND 1839:

61 CYGNI (BESSEL, GERMANY)

VEGA (STRUVE, RUSSIA)

ALPHA CENTAURI (HENDERSON, S.AFRICA)

ALPHA CENTAURI A, B, C

DISTANCE =

270,000 AU

1.3 PC ( parsec )

4.3 LY ( lightyear )

ALPHA CENTAURI

9 MILES

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”

PARALLAX DISTANCES SHOWED

THAT STARS ARE NOT ALIKE

8 March 2011 -- subst for Roberta Ast 1001, 29 March 2010 -- KD

HIPPARCOS

SATELLITE

MEASURED

ULTRA - PRECISE

PARALLAXES

NEXT: MOTIONS OF THE STARS IN SPACE

PROPER MOTION OF BARNARD’S STAR

( 6 L.Y. AWAY )

THE SUN, FOR EXAMPLE, MOVES

ABOUT 15 KILOMETERS PER SECOND

= 33,000 M.P.H.

= 3 A.U. PER YEAR.

“STATISTICAL PARALLAX”

BASED ON PROPER MOTIONS

( explain )

-- GOOD ONLY ON AVERAGE,

BUT OK OUT TO ABOUT 5000 L.Y. --

REMEMBER THE DOPPLER EFFECT?

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” )

WE QUOTE THE BRIGHTNESS OF EACH STAR AS A “MAGNITUDE”.

ABOUT 2100 YEARS AGO, HIPPARCHUS

(= HIPPARCOS) MADE A LIST OF 1000 STARS...

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”.

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)

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.

THE MAGNITUDE SCALE IS LOGARITHMIC --

1.0 magnitude = 100 2.0 = 39.8 (2.512 x fainter)

1.0 magnitude = 100 2.0 = 39.8 (2.512 x fainter) 3.0 = 15.8

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

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.

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

“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

MAGNITUDES ALSO GIVE COLORS. WE USE FILTERS, FOR INSTANCE ‘V’ = “visual”, yellow-green, ‘B’ = “blue”

U B V R I J K L ... filters

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

LUMINOSITY OR ABSOLUTE MAGNITUDE

OF A STAR

upper-case M

( m is apparent magnitude.)

OF A STAR

M = THE APPARENT MAGNITUDE m

THAT THE STAR WOULD HAVE

AT A STANDARD DISTANCE OF

10 PARSECS = 33 LIGHTYEARS

OF A STAR

TO CALCULATE M , WE NEED TO

KNOW BOTH m AND D .

OF STARS

M FOR THE SUN = + 4.8

OF STARS

FAINTEST RED DWARF STARS: M = + 18

OF STARS

MOST LUMINOUS STARS: M = - 12

OF STARS

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 stars. 8 march 2011 -- subst for roberta

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