Stellar Observations
Goals of the Lab
To observe a number of bright stars.
To study through observations the relationship between a star's color and its spectral type.
Requirements: several (at least 4 pages of) observation templates generic observation template and a protractor
Introduction
Astronomers use the magnitude system to describe the brightness of stars. The apparent magnitude (m) of a star is how bright a star appears from Earth; this variable does not take into account how far or close the star is to us. The apparent magnitude scale isn't intuitive, so here are few familiar examples to give you a sense of scale. The Sun: m = -26.7; the full Moon m = -12.6; Mars m = -2.8; the star Sirius m = -1.5; the star Vega m = 0.03; the faintest stars visible with the naked eye (under a good, dark sky) m = +6; the brightest quasar m = +12.8; and the faintest objects visible with the Hubble Space Telescope m = +30.
The absolute magnitude (M) of a star takes into account how far away the star is from Earth. The absolute magnitude is a measure of a star's luminosity, which is the total amount of energy radiated by the star every second. If you know both the star's apparent magnitude and its absolute magnitude, you can calculate the distance from the Earth to the star.
The Hertzsprung-Russell Diagram (H-R Diagram), shown below, plots the luminosity (or absolute magnitude) of a star versus its temperature (measured by the color). Stars of different spectral types fall at different palces on the diagram. The spectral type of a star has a letter, an arabic number, and a roman numeral. The first letter is the broad classification (O, B, A, F, G, K, or M). The number following it is a subclass within the broad classification, and may be ignored for this lab. The Roman numeral represents the luminosity class (I and II = supergiant; III = giant; IV = subgiant ; V = main sequence).
![[H-R Diagram]](hr.png)
Part I: Bright Star Observations
Table of Stars
The word "type" in the table below means the spectral classification of the star. "Mag." is the observed (not absolute) visual magnitude.
(All data on this table comes from SIMBAD.)
| Common Name |
Name | RA | Dec | Mag. | Type | Notes |
|---|---|---|---|---|---|---|
| Arcturus | α Bootis | 14h 15m 39.7s | +19° 10' 57" | 0.0 | K1III | |
| ε Sagittarii | 18h 24m 10.3s | -34° 23' 03.5" | 1.79 | B9.5III | ||
| Procyon | α Canis Minoris | 07h 39m 18.1s | +05° 13' 29" | 0.34 | F5IV | Gomeisa | β Canis Minoris | 07h 27m 09s | +08° 17' 22" | 2.89 | B8Ve | Antares | α Scorpii | 16h 29m 24.5s | -26° 25' 55" | 1.1 | M1I |
| β Herculis | 16h 30m 13.2s | +21° 29' 23" | 2.8 | G7III | ||
| Vega | α Lyrae | 18h 36m 56.3s | +38° 47' 01" | 0.0 | A0V | |
| γ Cygni | 20h 22m 13.7s | +40° 15' 24" | 2.2 | F8I | ||
| Deneb | α Cygni | 20h 41m 25.9s | +45° 16' 49" | 1.3 | A2I | |
| ε Cygni | 20h 46m 12.7s | +33° 58' 13" | 2.5 | K0III | ||
| Sadalmelik | α Aquarii | 22h 05m 47.03s | -00°19' 11" | 3.0 | G2I | |
| Fomalhaut | α Piscis Austrini | 22h 57m 39.0s | -29° 37' 20" | 1.2 | A3V | |
| Markab | α Pegasi | 23h 04m 45.7 s | +15° 12' 19" | 2.5 | B9III | |
| Scheat | β Pegasi | 23h 03m 46.5 s | +28° 04' 58" | 2.44 | M2.5II-III | |
| Mirfak | α Persei | 03h 24m 19.4 s | +49° 51' 40" | 1.79 | F5Ib | |
| Algol | β Persei | 03h 08m 10.1 s | +40° 57' 20.3" | 1.58 | B8V | |
| β Tauri | 05h 26m 17.5 s | +28° 36' 27" | 1.65 | B7III | ||
| Alpheratz | α Andromedae | 00h 08m 23.3s | +29° 05' 26" | 2.0 | B8IV | |
| Mirach | β Andromedae | 01h 09m 43.9s | +35° 37' 14" | 2.0 | M0III | |
| Caph | β Cassiopeiae | 00h 09m 10.7s | +59° 08' 59" | 2.3 | F2IV | |
| Hamal | α Arietis | 02h 07m 10.4s | +23° 27' 45" | 2.0 | K2III | |
| Menkar | α Ceti | 03h 02m 16.8s | +04° 05' 23" | 2.6 | M1III | |
| Capella | α Aurigae | 05h 16m 41.4s | +45° 59' 53" | 0.1 | G5III | |
| θ Aurigae | 05h 59m 43.3s | +37° 12' 45" | 2.65 | A0p | ||
| Betelgeuse | α Orionis | 05h 55m 10.3s | +07° 24' 25" | 0.6 | M1I | |
| Bellatrix | γ Orionis | 05h 25m 07.9s | +06° 20' 59" | 1.64 | B2III | |
| Sirius | α Canis Majoris | 06h 45m 08.9s | -16° 42' 58" | -1.5 | A1V | |
| β Trianguli | 02h 09m 32.6s | +34° 59' 14" | 3.0 | A5III | ||
| Arneb | α Leporis | 05h 32m 43.8s | -17° 49' 20.3" | 2.58 | F0Ib |
Note:All observations should include N/S and E/W, the time, date, and weather conditions.
Choose at least eight stars on the Table of Stars above to observe. Use your star wheel to determine which stars are up or will be up during lab. Note that, some of these stars will not rise until later at night and/or later in the semester. Similarly, some will set later in the night and later in the semester.
Of the eight stars you choose, you need at least one of each type: B, A, F, G, K, and M.
Perform the following steps in this section on each star you've chosen.
Center the star in the 25mm eyepiece.
If you are sharing the telescope with a lab partner, each partner needs to find at least four of the stars you're observing; don't depend on one partner to find all of the stars. Each person needs to make a sketch of every star observed. Even if the person you are sharing the telescope with found the star, you cannot copy his/her drawing!
Indicate who found each star on your drawing.
Sketch the field of view of the 25mm eyepiece. Stare for a couple of minutes; you should be able to see some other stars in the field of view. Use larger dots to represent brighter stars. Label the sketch with the name of your target star, and draw a little arrow on your sketch to indicate which is the target star. Determine which way is N/S and E/W using the method you learned in the Telescope Basics lab. Label your sketch with these directions.
For each star, note the appearance of the star. Describe its brightness (relative to other stars) and its color -in the eyepiece and with your naked eye. Note that slightly de-focusing the telescope may help you see a star's color.
Part II: Analysis and Contemplation
From your observations, do you notice a correlation between the types of the stars and their colors? Given what you've learned about stellar classifications in class, are your observations what you expected?
Create a table as follows for each star you observed:
Star Apparent Magnitude (m) Absolute Magnitude (M) distance (d) in pc Star Type Procyon 0.34 2.65 3.5 pc subgiant Fill in the table with values for each star you observed. The apparent magnitude (m) and the star type for each star are listed in the table above. The absolute magnitude (M) can be found using the H-R diagram from the introduction (taking into account the spectral classification (OBAFGKM) and the luminosity class (I, II, III, IV, or IV). Once m and M are known, you can calculate the distance using the following formula:
m = M + 5log(d/10pc)You will need to solve for d. The distance you calculate will be in parsecs (pc).
Looking at your table, do you notice any correlation between apparent magnitude, and how bright the star appeared to you when you observed it in the telescope? Do certain apparent magnitudes correspond to words or descriptions you used in your observations of the stars?
Looking at your table, do you notice any correlation between distance and apparent magnitude, i.e. how bright it appears to us? Do you notice any relationship between the type of star and its apparent magnitude? Why do some stars appear brighter than others?
How bright would each star appear to us on Earth if it replaced the Sun? Do you think that life on Earth would be the same with this new star? Explain.