The most distant thing some people can see without a telescope
On a truly dark night, a few sharp-eyed observers can make out a faint smudge in the constellation Triangulum. It is a spiral galaxy almost 2.9 million light-years away, about half again as far as Andromeda, and for those who can see it, the farthest permanent object visible to the unaided eye.
Catalogued as Messier 33, the Triangulum Galaxy is the third-largest member of our Local Group after Andromeda and the Milky Way, about 61,100 light-years across or roughly 70 percent of our galaxy's size. It may hold some 40 billion stars, against 400 billion for the Milky Way and a trillion for Andromeda. Seeing it unaided is hard: its total brightness is magnitude 5.72, but because the light is spread out, its effective magnitude is about 6.6, fainter than many people can manage even far from city lights. It serves as a benchmark on the Bortle scale of sky darkness.
The Italian astronomer Giovanni Battista Hodierna probably noted it before 1654 as a cloudy patch near the Triangle. Charles Messier found it independently on the night of 25 to 26 August 1764, and William Herschel, who mostly avoided Messier's objects, made an exception in 1784. Herschel also listed its biggest star-forming cloud separately; now called NGC 604, it spans nearly 1,500 light-years, among the largest such regions known.
It helped settle a great question. Lord Rosse identified it as a spiral nebula in 1850, and in 1926 Edwin Hubble showed that 35 of its variable stars were Cepheids, whose distances marked spiral nebulae as separate galaxies rather than clouds inside our own. Distance estimates still vary by method, from about 2.38 to 3.07 million light-years.
Its relationship with Andromeda, about 0.75 million light-years away, is intriguing. Water masers on opposite sides revealed in 2005 that it moves at around 190 kilometres per second relative to the Milky Way, heading toward Andromeda, and a clumpy hydrogen stream linking the two, confirmed in 2011, hints at past tidal contact. It may be Andromeda's satellite. Its disc, tilted 54 degrees to our view, has no central bulge.
Source: Triangulum Galaxy
