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Brightness of stars is assigned a number starting with the brightest star starting at about -1 magnitude. Dimmer stars are zero or positive numbers. The larger the number means the dimmer the star is. For example, a star -1 magnitude is brighter than a star 0 magnitude. A star 0 magnitude is brighter than a star 1 magnitude. A star 1 magnitude is brighter than a star 2 magnitude. A star 4 magnitude is brighter than a star 5 magnitude. Magnitude sequence for stars starting with the brightest is -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 magnitude, ... etc.
The decimal point is not used when star magnitudes are used on a star map. The decimal point could be confused for a star on the map. At the top of this page is the constellation Ursa Minor with star magnitudes for some of its stars. For example, magnitude 31 on the star map mean 3.1 and magnitude 55 on the star map mean 5.5.
Historicaly the magnitude system started with Hipparcus and Ptolemy when they divided the stars into six magnitudes. About 20 of the brightest stars that they could observe from their location were assigned to the first magnitude. The next set of bright stars were assigned to second magnitude and so forth. Sixth magnitude stars were assigned to stars that were barely visible to the unaided eye under favorable conditions. It was empirically determined that the ratio of first magnitude to sixth magnitude was 100 to 1. A logarithmic scale of 2.512 between magnitude levels is implemented. For example, a first magnitude star is 100 brighter than a sixth magnitude star or the sixth magnitude star is 1/100 or .01 dimmer that a first magnitude star. Second example, a fifth magnitude star is 2.512 times brighter than a sixth magnitude star or the sixth magnitude star is 1/2.512 or .40 dimmer that a fifth magnitude star. A star is 2.512 times brighter than a star one magnitude less.
Six Star Magnitude Table
| Star Magnitude | How Much Brighter than a Sixth Magnitude Star |
Logarithmic scale of 2.512 X between magnitude levels Starting at Sixth Magnitude |
|---|---|---|
| 1 | 100 Times | 2.51 x 2.51 x 2.51 x 2.51 x 2.51 |
| 2 | 39.8 Times | 2.51 x 2.51 x 2.51 x 2.51 |
| 3 | 15.8 Times | 2.51 x 2.51 x 2.51 |
| 4 | 6.3 Times | 2.51 x 2.51 |
| 5 | 2.51 Times | 2.51 x |
| 6 |
With the invention of the telescope and modern equipment to measure star magnitudes the scale has been extended in both directions. Dimmer stars are assigned magnitudes larger than 6 ( 6, 7, 8, 9, ... 30th ... etc.) The Hubble Space Telescope Deep Field image contains some galaxies as faint as 30th magnitude. First magnitude stars are corrected across the scale of 1, 0, -1 with the brightest star Sirius at -1.44. The scale increases in brightness with negative numbers. For example, the brightest planet Venus varies in brightness and is about -4.4 magnitude at maximum brightness. The Moon is -12.7 magnitude at maximum brightness and the Sun is -26.75 magnitude.
The below Star Magnitude Table Based on -1 Magnitude Star shows how much dimmer than a -1 magnitude star are stars to 19th magnitude. For example, most 10 x 50 or 7 x 50 binoculars can detect a 9 magnitude star. A 9 magnitude star is one tenth thousand (1/10,000 or .0001) dimmer than -1 magnitude star.
Star Magnitude Table Showing How Much Dimmer
Other Magnitudes are as
Compared to a -1 Magnitude Star
| Star Magnitude | How Much Dimmer than a -1 Magnitude Star |
How Much Dimmer than a -1 Magnitude Star |
|---|---|---|
-1 |
|
|
0 |
1/2.51 |
0.398 |
1 |
1/6.31 |
0.158 |
2 |
1/15 |
0.063 |
3 |
1/39 |
0.0251 |
4 |
1/100 |
0.0100 |
5 |
1/251 |
0.00398 |
|
6 |
1/630 |
0.00158 |
7 |
1/1,584 |
0.000630 |
8 |
1/3,981 |
0.000251 |
9 |
1/10,000 |
0.000100 |
10 |
1/25,118 |
0.0000398 |
11 |
1/63,095 |
0.0000158 |
12 |
1/158,489 |
0.00000631 |
13 |
1/398,107 |
0.00000251 |
14 |
1/1,000,000 |
0.00000100 |
15 |
1/2,511,886 |
0.000000398 |
16 |
1/6,309,573 |
0.000000158 |
17 |
1/15,848,931 |
0.000000063 |
18 |
1/39,810,717 |
0.000000025 |
19 |
1/100,000,000 |
0.000000010 |
The stars of Ursa Minor is a good constellation to determine how faint of a star can be observed. On star maps bright stars are represented with large dots while dimmer stars are represented with smaller dots. The brightness of the stars of Ursa Minor get fainter starting with Polaris at 2.0 magnitude which is located on the right side of the below star maps. The rest of the stars starting from bright to dim are 2.1, 3.1, 4.2, 4.3, 4.4, 5.0, 5.2 and 5.5 magnitude. Also note that Polaris is located in the same place in the sky throughout the year for each observing location. Because Portland, Oregon, U.S.A. is at north 45 degrees latitude Polaris is always located half way between the directly overhead and due north.
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Ursa Minor |
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Ursa Minor |
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