or discovering Uranus) also observed Triangulum in the late 1700s, and also noted a large region of gas within the galaxy that is today noted as NGC 604. M33 itself was considered a "spiral nebula," and it was unclear if M33 was part of the Milky Way Galaxy.
By the 1920s, observations by Edwin Hubble noted that M33 is a separate "stellar system" based upon observations of Cepheid variable stars. These stars are often used to measure cosmic distances in space because their luminosity is the same no matter where they are located.
Later observations of the galaxy revealed that it is approaching the Milky Way Galaxy at about 62,000 mph (100,000 kph). Some astronomers believe that Triangulum is "gravitationally trapped" by the massive Andromeda Galaxy that is also hurtling toward our galaxy, the European Southern Observatory stated.
Among Triangulum's most distinctive features is NGC 604, a region of starbirth so big that the Space Telescope Science Institute once described it as "monstrous." Their 2003 estimate says that the gas cloud has more than 200 blue stars and that it is more than 1,300 light years across — or about 100 times bigger than the Orion Nebula. (More recent estimates peg it at 1,500 light-years.)
"If NGC 604 were at the same distance from Earth as the Orion Nebula, it would be the brightest object in the night sky (besides the moon)," NASA stated.
The young stars are extremely hot, at 72,000 degrees F (40,000 C), and the biggest ones are 120 times the mass of the sun. Radiation pumping out from the young stars floods into the gas in the region, making it fluoresce or glow, STScI added.
"Within our Local Group, only the Tarantula Nebula in the Large Magellanic Cloud exceeds NGC 604 in the number of young stars even though the Tarantula Nebula is slightly smaller in size," STScI stated.
Studying M33 in infrared light, NASA's Wide-field Infrared Survey Explorer (WISE) revealed hotspots of activity within the galaxy in 2011 while also showing that the center of the galaxy doesn't have much going on within it.
"Areas in the spiral arms that are hidden behind dust in visible light shine through brightly in infrared light, showing where clouds of cool gas are concentrated," NASA wrote at the time.
"There isn't a lot of star formation occurring near the center of M33. It would be difficult to deduce this lack of activity in the core by only looking a visible-light image, where the core appears to be the brightest feature."
NASA added that the galaxy looks "surprisingly bigger" than an optical image would make it appear, because cold dust is visible further in space than what astronomers initially expected.