HOUSTON -- As much as the 2012 season marked the dawn of new guidance for the Astros under owner Jim Crane and general manager Jeff Luhnow, more than anything else it marked the end of an era for baseball fans in Houston.

After 51 seasons as members of the National League, the Astros will embark on new territory in 2013 when they make an unprecedented move to the American League. Houston will compete in the AL West, giving each division five teams and each league 15 teams.

Houston's final season in the NL saw it produce a club-record 107 losses, the product of a team relying on young players at many spots around the diamond while it tries to rebuild. The Astros had the most losses in baseball for the second straight year, but they weren't without some bright spots.

Year in Review
Looking back at 2012
MLB statistical leaders
Final standings

The emergence of young players like Jose Altuve, Lucas Harrell and Justin Maxwell gave the Astros reasons for optimism, and a flurry of midseason trades brought a handful of promising young players to Houston who will be given an opportunity in the coming years.

The Astros actually got off to a pretty good start in the NL Central and were one game under .500 in late May before things fell apart. A bullpen which had been among the best in baseball the first six weeks of the season suddenly collapsed thanks to a large workload.

Despite the contributions of Altuve, who made the All-Star team at 22 years old, Maxwell, who slugged 18 homers and tallied 53 RBIs, and Jed Lowrie, the Astros struggled to score runs and ranked last in the NL in several run-producing categories.

As the team plodded through July by winning only three games, change was inevitable. Brad Mills was let go as manager on Aug. 18 and replaced by Triple-A manager Tony DeFrancesco, who guided the club to a 15-15 mark in the final 30 games.

With young players getting more playing time and contributing in bigger roles, the Astros closed out the season with some belief they'll able to compete in the AL sooner than expected.

Here were the Astros' top five storylines in a roller-coaster 2012:

5. The emergence of Altuve and Harrell

Not much was known about Altuve and Harrell entering last season, except that Altuve was going to be a given a shot to see what he can do in a full season at second base and Harrell had done just enough in Spring Training to win a spot in the rotation.

As it turned out, Harrell was the team's most consistent starting pitcher, going 11-11 with a 3.76 ERA as a rookie. He ranked second among all rookies in innings pitched with 193 2/3 and was second among NL rookies in wins. As much as the Astros' pitching staff struggled in '12, the emergence of Harrell proved to be crucial.

Altuve, meanwhile, made the NL All-Star team in a season in which he finished with a .290 batting average, seven homers, 37 RBIs and 33 stolen bases. The Astros may have found their second baseman of the future.

4. Early-season success gives way to struggles

The Astros defied the odds early in the season, hanging around in the NL Central for the first six weeks. They were 22-23 after Harrell outdueled Clayton Kershaw on May 25 in Los Angeles and were just three games under .500 when they went to Colorado for a four-game series against the Rockies a few days later.

The thin air of Denver and the free-swinging Rockies roughed up the Houston pitching staff en route to a four-game sweep in which Colorado outscored the Astros, 40-23. The pitching staff, especially the bullpen, never recovered. Houston won a combined eight games in July and August en route to a club record 107 losses.

3. Astros call upon Porter for the future

With new management and ownership in place and with Mills in the final year of his contract, it certainly wasn't surprising the team decided to make a change. Mills was let go on Aug. 18, with DeFrancesco guiding the team the final six weeks.

The Astros named Bo Porter as their 17th manager on Sept. 27, but he didn't take control of the club until the Washington Nationals' playoff run ended. Porter was Washington's third-base coach, but at 40 became the youngest manager in the Major Leagues.

Porter, who promised the Astros will play fundamentally solid baseball and play the game hard, is taking over a difficult situation. The Astros are rebuilding and heading to perhaps the toughest division in baseball in the AL West. There's nowhere to go but up.

2. A flurry of trades brings in more prospects

Luhnow made five trades in July, each of which jettisoned a veteran player(s) in exchange for mostly prospects.

The Astros traded away first baseman Carlos Lee, starting pitchers Wandy Rodriguez and J.A. Happ, relievers Brett Myers and Brandon Lyon and third baseman Chris Johnson in a span of less than four weeks. Houston got 17 players in return in those deals, mostly Minor Leaguers.

Among the players the Astros received were third baseman Matt Dominguez, who could start in 2013, and promising young outfielders Robbie Grossman and Marc Krauss. The loss of the veteran players led to some growing pains at the Major League level, but the Astros shaved some payroll and acquired players they believe will play big roles in the future.

1. Goodbye to you, National League

After 51 seasons in the league, the Astros played their final game as a member of the NL on the final day of the season. Houston will be the first franchise in baseball history to move to the AL from the NL.

The Astros went 3,999-4,134 during their time as an NL club, making the playoffs nine times in that span, including winning the NL pennant in 2005. Houston will open play in 2013 as a member of the AL West with Texas, Oakland, Seattle and the Los Angeles Angels. A new era will begin in Houston.