Beginner’s guide to estimating the magnitude of Algol:
This article deals with naked eye estimating the magnitude of Algol. The Algol can be estimated by comparing its brightness with two non-variable stars, one which is slightly brighter and the other one is slightly fainter than the variable star. Then mentally dividing the brightness level between the two stars into 10 equal parts and placing the variable on this line as explained below.
Use this finding chart for identification of Algol and comparison stars.
Before you start estimating the magnitude of Algol first take a good look at all the stars shown on the map above. Identify Algol = Beta (β ) Persei, Gamma (γ) Andromedae which lies to its west, Epsilon (ε) Persei, is to the east of Algol and Kappa (κ) Persei, which is to the north of Algol.
Now mentally divide the brightness difference between γ Andromedae (2.1) and κ Persei (3.8) into 10 equal parts. Each part is then (3.8 – 2.1)/10 = 0.17 per part.
Now take look at Algol and estimate where you will put its brightness between the two stars and then estimate the magnitude.
Note: shift your gaze quickly from one star to the other. Do not stare or look at the stars for long, it tends to make you feel slightly brighter
Let us take two examples.
1) If you think the brightness of it is halfway between γ And and κ Per
2.1 3.8
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Algol
then the magnitude of Algol will be
2.1 + 0.17 x 5 = 2.1 + 0.85 = 2.95
0r 3.8 – 0.17 x 5 = 3.8 – 0.85 = 2.95
Now compare Algol with ε Persei. Is Algol brighter or fainter than ε Persei, if Algol is brighter then check your estimate again. And then decide if you would estimate the magnitude of Algol to be 2.8, 2.9 or 3.0.
2) If you think that brightness of Algol somewhere close to marking 4
2.1 3.8
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Algol
then the magnitude of Algol will be
2.1 + 0.17 x 4 = 2.1 + 0.68 = 2.78
0r 3.8 – 0.17 x 6 = 3.8 – 1.02 = 2.78
You then round the figure as the magnitude of Algol to 2.8
Again compare Algol with ε Persei as described above.
Repeat this exercise a few times to be sure of your exercise and take the average value as the magnitude of the star.
Initially, it might take longer for you to estimate the magnitude of Algol. With practice, you will be estimating magnitude in just 10 minutes. Seasoned amateur astronomers have been able to estimate magnitudes to 0.2 or even up to 0.1.
Recording your observations:
You may record your observations in a table as shown below:
The naked eye estimate of the magnitude of variable star Algol
Comparison stars γ Andromedae (2.1), ε Persei (2.9) and κ Persei (3.8)
Observations taken by: ___________________
Location: Longitude _____________ Latitude ____________
Date time (IST or UT) magnitude estimated ..... ..... ..... ..... ..... .....
Notes: Date is always entered as a double date; the date of the sunset and then the date of the next sunrise like 7/8 Nov 2020. This avoids confusion. Time could be in Indian Standard Time or in Universel time, but be sure to mention this.
Page contributed by Arvind Paranjpye (paranjpye.arvind@gmail.com)