Vixen VC200L + EOS KISS X3 (Rebel T1i) ISO 12800, Exposure 1×120, 2×180 seconds, 1 Dark Frame Orion SkyGlow Filter Photoshop Elements 7, Noise Ninja M101 (NGC5457) はおおぐま座にある渦巻銀河。距離約2700万光年。1909年には超新星が出現し、M・ウォルフによって発見されている。その後1950年に17.5等、1970年8月には11.7等の合計3個の超新星が出現している。 双眼鏡では条件が良ければ大きく広がった円形で濃淡のない光斑として見えるという。口径8cmの望遠鏡の低倍率で濃淡が見えるが、双眼鏡の方がかえってよく見えるとする意見もある。 The Pinwheel Galaxy (also known as Messier 101 or NGC 5457) is a face-on spiral galaxy about 27 million light-years away in the constellation Ursa Major. M101 is a relatively large galaxy compared to the Milky Way. With a diameter of 170,000 light-years it is nearly twice the size of the Milky Way. It has a disk mass on the order of 100 billion solar masses, along with a small bulge of about 3 billion solar masses[5] Another remarkable property of this galaxy is its huge and extremely bright H II regions , of which a total of about 3000 can be seen on photographs. HII regions usually accompany the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where stars form. HII regions are ionized by large numbers of extremely bright and hot young stars. On photographs M101 can be seen to be asymmetrical on one side. It is thought that in the recent past (speaking in galactic terms) M101 underwent a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this encounter also amplified the density waves in the spiral arms of M101. The amplification of these waves leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.