4. Ariana Grande: 21 Under 21 (2013)

Last Year's Rank: N/A

Birthdate: June 26, 1993 (age 20)

Why She's Hot: A huge number of people knew the name Ariana Grande before her single "The Way" debuted at No. 10 on the Hot 100 chart in March; at that point, the co-star of Nickelodeon's hit series "Victorious" had 5.4 million Twitter followers and 2.6 million Facebook fans. But the success of "The Way," featuring Mac Miller, helped turn Grande into a musical star capable of commanding all-ages crowd and scoring a No. 1 album debut. "Yours Truly," which sold 138,000 copies in its first week of release (according to Nielsen SoundScan), was a critically acclaimed R&B collection with cameos from Big Sean, MIKA and the Wanted's Nathan Sykes. With showy touring opportunities coming (she's already opened for Justin Bieber on a few dates), Grande seems more than capable to sustain her stardom following a very quick ascent.