Download Astronomy Lecture Notes: Stellar Nomenclature I Introduction

Astronomy Lecture Notes: Stellar Nomenclature I
1. Introduction
a. Administer the quiz
b. Return previous quiz
c. Remind them that HW is due next class
d. Announce the next HW assigent due Tuesday, Apr 09 on Hot Tips
e. Point out the constellation handouts on Hot Tips
2. Stellar nomenclature
a. Goal: Are the stars in the night sky like the Sun? Is the Sun a common type of star? Where do
we fit in?
i. We will reach this goal by learning the vocabulary that astronomers use to describe stars
and then examine many stars in the sky to discover where the Sun fits in.
ii. We’ll start with the stars in the Big Dipper (Is the Big Dipper a constellation or an
asterism?)
iii. Examine the Ursa Major handout front and back.
b. Stellar Names
i. Only the brightest stars have formal names, but all stars have official designations.
ii. Many formal names of stars are middle eastern in origin to honor the middle eastern
astronomers that carried on work in astronomy after the fall of the Roman Empire.
iii. Examples: See the names of stars in the Big Dipper
c. Apparent Magnitudes
i. Apparent magnitude is a code for brightness
ii. Established visually by Hipparcos around 140 B.C.E.
iii. Now measured using solid state photometers
iv. A backward scale with the brightest star represented by the lowest number
v. The brightest star in the entire sky is Sirius in Canis Major at m = -1.4
vi. All seven stars in the Big Dipper are brighter than m = 2
vii. The limit of naked eye visibility is m = 6
viii. Magnitude Rules:
1. If one star is 1 magnitude brighter than another then that star is actually about 2.5
times brighter as measured in Watts/m2 by a photometer.
2. If one star is 5 magnitudes brighter than another then that star is actually exactly
100 times brighter as measured in Watts/m2 by a photometer.
3. Example: See the stars in the Big Dipper.
4. The apparent magnitude of the Sun is about -28.
5. We’ll see this same rule again in Absolute Magnitudes where it is more useful.
6. Apparent magnitude of a star depends on luminosity and distance.
7. Which star in the Big Dipper is brightest? Which is dimmest?
d. Absolute Magnitudes
i. Absolute magnitude is a code for luminosity
ii. It is also a backward scale
iii. All stars range from an absolute magnitude of -10 to +20.
iv. The Absolute Magnitude of the Sun is +4.8 which we will frequently approximate as +5.
v. What is the relationship between Absolute Magnitude and Luminosity? See the two
magnitude rules:
1. If one star is 1 magnitude more luminous than another then that star is actually
about 2.5 times more luminous as measured in Watts.
2. If one star is 5 magnitudes more luminous than another then that star is actually
exactly 100 times more luminous as measured in Watts.
3. Example: See the stars in the Big Dipper.
4. Which star in the Big Dipper is most luminous? Which is least luminous? How
many times more luminous is the most compared to the least luminous?
e. Stellar Distances
i. Stellar Parallax
1.
ii. Spectroscopic Parallax
f. Stellar motions
i. Radial Velocity from the Doppler Shift – easily measured with a spectrometer.
1. Example: Barnard’s Star, vRadial= ???
ii. Proper Motion from two photographs taken at different times – not easy to measure
because the shift of a star’s position across the sky is usually very small requiring a long
time interval (decades to centuries) between the photographs.
1.  has units of arcsec/year
2. Example: Barnard’s star has the largest proper motion of ???
3. Show the presentation of Barnard’s Star Proper Motion
4. Show the movie of the proper motion of the stars in Ursa Major
iii. Tangential Velocity is determined from the proper motion and the star’s distance.
km
 arcsec/year  d pc
1. vTangential  4.74
sec
2. Example: Barnard’s star
iv. Space Velocity is given using the Pythagorean Theorem
2
2
1. vTangential  vRadial
 vTangential
2. Example: Barnard’s Star