How many stars are in our galaxy?
Milky Way
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Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum-Centaurus Arm contains approximately 30% more red giants than would be expected in the absence of a spiral arm.[117][120] This observation suggests that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars.[80] In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way.[121][122][123] Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.[123]