1. Stellar Parallax Magnitude and H-R Diagram

2. Distance to Stars is Measured in:
Km – kilometers (#’s become too large to write)
AU – Astronomical Unit (the average distance of Earth-Sun 1.5 x 108
(#’s can still become too large)
L.Y. – Light Year, the distance light travels in 1 yr. at the speed of 186,000 mi/sec OR 300,000 km/sec
Parsec L.Y. ( 1 sec or arc)

3. Stellar Parallax
The shift in the direction of a star caused by the change in the
position of the Earth as it moves about the Sun
A technique that measures the slight shift in nearby stars
to determine
their distances.

4. We use trigonometry to calculate the distance to the star
(Hypotenuse)
Right angle
Use the Formula:
Sino = opposite
hypotenuse
Opposite = Earth/Sun
Average distance (1 AU)
1.5x 108 km
Why is stellar parallax effective for measuring the distance to only the closest stars?

5. Sample Calculation:
Bernard’s Star
Sino = 2.69 x 10-6
Formula: Sino = opposite/hypotenuse
2.69 x 10-6 = 1.5 x 108 km X
X = 1.5 x 108
2.69 x x 10-6
X = x Km
(5.5 x 1013)

6. The farther away the star the smaller the angle of parallax becomes
The farther away the star the smaller the angle of parallax becomes. Eventually the angle will become so small we are unable to measure it

7. Absolute Magnitude and Apparent Magnitude

8. Magnitude – The brightness of a star
Apparent Magnitude
The brightness as we see it from Earth
Absolute Magnitude
The actual brightness of a star when compared to other stars at a standard distance of 10 parsecs. (32.6 L.Y.s)
Luminosity - the total amt. of energy a star is sending out. This is independent of how far away the star is.
The greater the luminosity the greater the absolute magnitude.

10. Stars must be compared at 10 parsecs
Compare the two stars below.
If Sirius appears very bright to us
What is the distance for Sirius,
2.6 pc or 5 pc?

13. The scale for measuring magnitude is based on the larger the # the less bright!!
The Sun - very bright at -26
Polaris – less bright at +2

14. What factors determine the Magnitude?
Size
Temperature
Distance

15. Comparing Luminosity to Absolute Magnitude
Luminosity – the amt. of energy released by star
Magnitude – the brightness of a star
SUN ->
The Sun has a luminosity of 1.
A luminosity of 10,000 means
The star is 10,000 times more
Energy than the Sun.

16. Compare the magnitudes. Is star’s distance close? Far away? Star
Apparent Magnitude
Absolute Magnitude
distance
The Sun
-26.93
4.8 ?
Sirius
-1.44
1.41
Vega
0.03
0.58
Betelgeus
0.45
-5.14
GJ 75
5.63
Star
Apparent Magnitude
Absolute Magnitude
distance
The Sun
-26.93
4.8 ?
Sirius
-1.44
1.41
Vega
0.03
0.58
Betelgeus
0.45
-5.14
GJ 75
5.63
Complete the practice sheet **Magnitude Questions