One of the special joys of getting out under a dark rural sky this time of year is seeing the gossamer beauty of the surprisingly expansive star cluster called Melotte 111, also known as the Coma star cluster. Mel 111 makes up a large part of the constellation Coma Berenices, “Berenice’s Hair”. This constellation, which entertains the North Galactic Pole as well as a gaggle of galaxies, can be found about midway between Denebola (some call the Coma star cluster the end of the “tail” of Leo the Lion) and Arcturus, as well as midway between Spica and the Big Dipper. Coma Berenices is transiting the meridian this week as evening twilight ends. At a distance of just 284 light years, the Coma star cluster is the third nearest star cluster to us, surpassed only by the open cluster remnant Collinder 285—the Ursa Major association (80 ly)—and the Hyades (153 ly).
Tag: Big Dipper
M81 and M82 from HST
The galaxy pair M81 and M82 in Ursa Major must rank near the top of the list of best-loved objects for any Northern Hemisphere amateur astronomer. So, to see such a familiar object as these in breathtaking Hubble Space Telescope detail is thrilling indeed:
M81 and M82 lie little more than a moon-width apart in the constellation Ursa Major, 11.8 million and 11.5 million light years, respectively, from Earth. Check out this pretty pair with either binoculars or a telescope any clear evening during the next few days. Both galaxies transit the meridian on April 14 at the end of evening twilight, so this is the perfect time to observe them at their highest in the sky. You can find Bode’s Galaxy (M81) and the “Silver Sliver” (M82) by drawing an imaginary diagonal across the bowl of the Big Dipper, opposite (rather than along) the handle, and extending the diagonal beyond the bowl almost as far as the two bowl stars are apart. Or, using the chart I created below, draw an imaginary line between Dubhe and 24 UMa, then go about four-fifths of the way to 24 UMa. M81 & M82 lie about 0.4° (a little less than a moon-width) perpendicular to that line on the Polaris side. Bingo, you’ve got ’em!
Why are the Pleiades called the Seven Sisters?
The famous and beautiful Pleiades star cluster, which lies between 429 and 448 light years from us in the constellation Taurus the Bull, contains at least 2,109 stars that were formed around 125 million years ago—relatively recently on an astronomical timescale. But when you look at the Pleiades with the unaided eye, unless you have unusually good vision and excellent sky conditions, you’ll see only six Pleiads. If you see more than that, you’ll probably be able to see 8 or 9 Pleiads, maybe more. But not seven. So, why are the Pleiades called the Seven Sisters?
Here’s my conjecture. Take a look at the Pleiades on a dark, moonless night. What do you see? I think you’ll see a group of stars forming a tiny dipper shape, reminiscent of the much larger Little Dipper.
How many stars make up the Little Dipper shape? Seven. How many stars make up the Big Dipper shape? Seven. How many bright stars does nearby Orion have? Seven. Given this, and the fact that seven has long been considered a mystical number, it comes as no surprise, perhaps, that the Pleiades are called the Seven Sisters and not the Six Sisters or the Eight Sisters. How many do you see?
The Pleiades will culminate1 at midnight for SW Wisconsinites on Friday night / Saturday morning, November 17/18.
1cross the celestial meridian; reach their highest point in the sky, due south
References
Bouy H., et al., 2015, A&A, 577, A148
Galli P. A. B., Moraux E., Bouy H., Bouvier J., et al., 2017, A&A, 598, A48
Stauffer J. R., Schultz G., Kirkpatrick J. D., 1998, ApJ, 499, L19