During the 1950s astronomers used data from radio telescopes and determined that our spiral galaxy has four arms. In 2008, images from NASA's Spitzer Space Telescope showed that the Milky Way only had two arms. However, the conclusion of a 12-year-long study has shown that there are in fact four arms on our galaxy. The results were published in the Monthly Notices of the Royal Astronomical Society.
During the 12 year study, a team of researchers studied around 1650 massive stars with radio telescopes and reconfirmed that based on the distribution and luminosity, there are four arms on the spiral of our galaxy. This contradicts findings from the Spitzer Space Telescope that showed that there are 110 million stars, and those two were confined to two spiral arms, not four.
So how was Spitzer so wrong? Well, the images weren’t exactly wrong, but they weren’t able to see the entire picture. Spitzer works on infrared, and it is able to capture information about stars much like our sun, which are relatively low mass and are cooler. Because this recent data focused on hot, massive stars, Spitzer was blind to them and was unable to factor them in.
Massive stars are somewhat rare and can live for about 10 million years, which is only a blink of an eye on the cosmic time scale. They are born, live, and die within the same arm. Stars that are more like our sun have more time to spin about in the galaxy and spread out. Gravitational pull is only strong enough to collect stars in two of the arms, which Spitzer was able to detect. However, the other two arms have enough compressed gas to allow for massive stars to form. The better astronomers understand the structure of our galaxy, the easier it becomes to understand how and why massive stars form.
Because we are not able to directly observe the entire structure of the Milky Way (on account of being inside it and all), we must rely on observations from instruments like radio telescopes and the Spitzer Space Telescope. Unfortunately, there are times like these when two data sets conflict. This is why it is important for astronomers to factor in all of the evidence before making claims about absolutes, and also to keep revisiting ideas when new evidence is introduced.