# Astronomy Lab 17 Griggs University HR Diagram Assignment

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andythewxman
School: UT Austin

Here you go. Please let me know if there are any questions. All values were obtained using Stellarium and calculations were made using the provided equations.

Griggs University
Astronomy Lab 17
HR Diagram
To be completed after reading Arny, Chapter 13
Name: __________________
Objectives: To observe the relationship between a star’s intrinsic brightness (absolute
magnitude) and surface temperature. To organize the data into an HR diagram and interpret the
diagram. To determine the distance to the Pleiades star cluster by constructing its HR diagram.
Definitions:
Magnitude: a measure of the brightness of a star. The larger the magnitude, the fainter the star.
Intensity (I): the rate we receive light energy at the earth from a given star.
Apparent magnitude (m or V): the visual brightness of the star as observed from earth. The
apparent magnitude depends on both the luminosity of the star and the distance. Brightest stars
are 1st magnitude or less, faintest stars visible to naked eye are 6th magnitude. Each difference
in magnitude of 1 corresponds to a ratio of intensity of 2.5  . Each difference in magnitude of
5 corresponds to a ratio of intensity of 100  . This may be written mathematically as,

I 
m1 − m2 = 2.5 log  2 
 I1 

.

(1)

Absolute magnitude (M): the apparent magnitude a star would have if placed 10 pc away. It
depends only on the star’s luminosity. From Eq. (1), we see

 I 10 pc 

(2)
m − M = 2.5 log 
 I 
Where I10 pc is the brightness the star would have at 10 pc.
Luminosity (L): the total power output of the star in units of either watts or solar luminosity
units.
Distance (d): The distance in pc = 1/p, where p is the parallax angle in seconds of arc. It i...

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Tutor went the extra mile to help me with this essay. Citations were a bit shaky but I appreciated how well he handled APA styles and how ok he was to change them even though I didnt specify. Got a B+ which is believable and acceptable.

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