Looking in all directions, how many stars are there brighter than a particular visual magnitude? Here’s an empirical formula that gives an approximation. It can be used over the range mv = +4.0 to +25.0.
where S is the approximate number of stars brighter than apparent visual magnitude m in the entire sky
| Apparent Visual Magnitude | # of Stars |
|---|---|
| 4.0 | 552 |
| 4.1 | 618 |
| 4.2 | 690 |
| 4.3 | 772 |
| 4.4 | 863 |
| 4.5 | 964 |
| 4.6 | 1,077 |
| 4.7 | 1,204 |
| 4.8 | 1,345 |
| 4.9 | 1,503 |
| 5.0 | 1,679 |
| 5.1 | 1,875 |
| 5.2 | 2,094 |
| 5.3 | 2,338 |
| 5.4 | 2,611 |
| 5.5 | 2,914 |
| 5.6 | 3,253 |
| 5.7 | 3,631 |
| 5.8 | 4,051 |
| 5.9 | 4,520 |
| 6.0 | 5,042 |
| 6.1 | 5,623 |
| 6.2 | 6,271 |
| 6.3 | 6,992 |
| 6.4 | 7,794 |
| 6.5 | 8,687 |
| 6.6 | 9,681 |
| 6.7 | 10,786 |
| 6.8 | 12,015 |
| 6.9 | 13,382 |
| 7.0 | 14,900 |
| 7.1 | 16,588 |
| 7.2 | 18,464 |
| 7.3 | 20,547 |
| 7.4 | 22,860 |
| 7.5 | 25,428 |
| 7.6 | 28,278 |
| 7.7 | 31,441 |
| 7.8 | 34,949 |
| 7.9 | 38,839 |
| 8.0 | 43,152 |
| 8.1 | 47,932 |
| 8.2 | 53,229 |
| 8.3 | 59,096 |
| 8.4 | 65,592 |
| 8.5 | 72,784 |
| 8.6 | 80,743 |
| 8.7 | 89,549 |
| 8.8 | 99,287 |
| 8.9 | 110,055 |
| 9.0 | 121,955 |
| 9.1 | 135,104 |
| 9.2 | 149,627 |
| 9.3 | 165,662 |
| 9.4 | 183,362 |
| 9.5 | 202,891 |
| 9.6 | 224,431 |
| 9.7 | 248,181 |
| 9.8 | 274,358 |
| 9.9 | 303,200 |
| 10.0 | 334,965 |
| 10.1 | 369,938 |
| 10.2 | 408,426 |
| 10.3 | 450,768 |
| 10.4 | 497,330 |
| 10.5 | 548,514 |
| 10.6 | 604,755 |
| 10.7 | 666,528 |
| 10.8 | 734,349 |
| 10.9 | 808,780 |
| 11.0 | 890,430 |
| 11.1 | 979,963 |
| 11.2 | 1,078,096 |
| 11.3 | 1,185,610 |
| 11.4 | 1,303,349 |
| 11.5 | 1,432,229 |
| 11.6 | 1,573,241 |
| 11.7 | 1,727,456 |
| 11.8 | 1,896,035 |
| 11.9 | 2,080,230 |
| 12.0 | 2,281,392 |
| 12.1 | 2,500,983 |
| 12.2 | 2,740,574 |
| 12.3 | 3,001,863 |
| 12.4 | 3,286,675 |
| 12.5 | 3,596,976 |
| 12.6 | 3,934,877 |
| 12.7 | 4,302,651 |
| 12.8 | 4,702,734 |
| 12.9 | 5,137,742 |
| 13.0 | 5,610,480 |
| 13.1 | 6,123,951 |
| 13.2 | 6,681,371 |
| 13.3 | 7,286,180 |
| 13.4 | 7,942,053 |
| 13.5 | 8,652,916 |
| 13.6 | 9,422,957 |
| 13.7 | 10,256,640 |
| 13.8 | 11,158,721 |
| 13.9 | 12,134,260 |
| 14.0 | 13,188,640 |
| 14.1 | 14,327,575 |
| 14.2 | 15,557,134 |
| 14.3 | 16,883,749 |
| 14.4 | 18,314,236 |
| 14.5 | 19,855,805 |
| 14.6 | 21,516,082 |
| 14.7 | 23,303,122 |
| 14.8 | 25,225,420 |
| 14.9 | 27,291,933 |
| 15.0 | 29,512,092 |
| 15.1 | 31,895,815 |
| 15.2 | 34,453,520 |
| 15.3 | 37,196,142 |
| 15.4 | 40,135,142 |
| 15.5 | 43,282,516 |
| 15.6 | 46,650,811 |
| 15.7 | 50,253,128 |
| 15.8 | 54,103,131 |
| 15.9 | 58,215,053 |
| 16.0 | 62,603,700 |
| 16.1 | 67,284,449 |
| 16.2 | 72,273,253 |
| 16.3 | 77,586,632 |
| 16.4 | 83,241,673 |
| 16.5 | 89,256,016 |
| 16.6 | 95,647,847 |
| 16.7 | 102,435,879 |
| 16.8 | 109,639,337 |
| 16.9 | 117,277,932 |
| 17.0 | 125,371,840 |
| 17.1 | 133,941,667 |
| 17.2 | 143,008,417 |
| 17.3 | 152,593,453 |
| 17.4 | 162,718,451 |
| 17.5 | 173,405,353 |
| 17.6 | 184,676,315 |
| 17.7 | 196,553,644 |
| 17.8 | 209,059,737 |
| 17.9 | 222,217,010 |
| 18.0 | 236,047,823 |
| 18.1 | 250,574,401 |
| 18.2 | 265,818,743 |
| 18.3 | 281,802,538 |
| 18.4 | 298,547,061 |
| 18.5 | 316,073,074 |
| 18.6 | 334,400,717 |
| 18.7 | 353,549,396 |
| 18.8 | 373,537,665 |
| 18.9 | 394,383,103 |
| 19.0 | 416,102,189 |
| 19.1 | 438,710,168 |
| 19.2 | 462,220,923 |
| 19.3 | 486,646,831 |
| 19.4 | 511,998,631 |
| 19.5 | 538,285,275 |
| 19.6 | 565,513,790 |
| 19.7 | 593,689,134 |
| 19.8 | 622,814,048 |
| 19.9 | 652,888,922 |
| 20.0 | 683,911,647 |
| 20.1 | 715,877,479 |
| 20.2 | 748,778,904 |
| 20.3 | 782,605,508 |
| 20.4 | 817,343,852 |
| 20.5 | 852,977,352 |
| 20.6 | 889,486,170 |
| 20.7 | 926,847,110 |
| 20.8 | 965,033,523 |
| 20.9 | 1,004,015,228 |
| 21.0 | 1,043,758,439 |
| 21.1 | 1,084,225,707 |
| 21.2 | 1,125,375,873 |
| 21.3 | 1,167,164,044 |
| 21.4 | 1,209,541,573 |
| 21.5 | 1,252,456,065 |
| 21.6 | 1,295,851,393 |
| 21.7 | 1,339,667,742 |
| 21.8 | 1,383,841,658 |
| 21.9 | 1,428,306,130 |
| 22.0 | 1,472,990,684 |
| 22.1 | 1,517,821,499 |
| 22.2 | 1,562,721,546 |
| 22.3 | 1,607,610,744 |
| 22.4 | 1,652,406,140 |
| 22.5 | 1,697,022,107 |
| 22.6 | 1,741,370,568 |
| 22.7 | 1,785,361,232 |
| 22.8 | 1,828,901,853 |
| 22.9 | 1,871,898,516 |
| 23.0 | 1,914,255,925 |
| 23.1 | 1,955,877,722 |
| 23.2 | 1,996,666,815 |
| 23.3 | 2,036,525,723 |
| 23.4 | 2,075,356,932 |
| 23.5 | 2,113,063,265 |
| 23.6 | 2,149,548,260 |
| 23.7 | 2,184,716,557 |
| 23.8 | 2,218,474,290 |
| 23.9 | 2,250,729,483 |
| 24.0 | 2,281,392,450 |
| 24.1 | 2,310,376,189 |
| 24.2 | 2,337,596,778 |
| 24.3 | 2,362,973,766 |
| 24.4 | 2,386,430,550 |
| 24.5 | 2,407,894,751 |
| 24.6 | 2,427,298,570 |
| 24.7 | 2,444,579,131 |
| 24.8 | 2,459,678,812 |
| 24.9 | 2,472,545,544 |
| 25.0 | 2,483,133,105 |
How many stars are there in our Milky Way galaxy? Between 100 and 400 billion stars. Many stars are not very luminous, and can only be seen in the immediate solar neighborhood. That is one source of uncertainty.
How many galaxies are there in the observable universe? Something like two trillion (2 × 1012).
How many stars are in the observable universe? Something like a septillion (1024). A trillion trillion!
And, just so you know, our universe is probably much larger than the volume that we can observe.
How does the Universe love thee? Let us count the stars…
References
“How many stars are in the sky?”, Space Math, NASA Goddard Space Flight Center, accessed February 29, 2020, https://spacemath.gsfc.nasa.gov/weekly/6Page103.pdf.
Wikipedia contributors, “Galaxy,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Galaxy&oldid=942479372 (accessed February 29, 2020).
Wikipedia contributors, “Milky Way,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Milky_Way&oldid=942977760 (accessed February 29, 2020).