HOUSTON-The Houston Astros wrapped up an improbable season with an improbable ending today as Mike Scott pitched a no-hitter that clinched their first division championship in six years. The Astros, who have been in first place since July 19 after being picked by many to finish at the bottom of the division, had maintained all along that they wanted to earn the title by winning the clinching game, rather than depending on the misfortunes of their closest competitors. But no one envisioned that they would complete their quest in such an emotional and dramatic fashion.

Scott, who was traded to the Astros by the Mets for Danny Heep four seasons ago, struck out 13 batters and allowed only three base-runners in a masterly 2‚0 triumph over the San Francisco Giants. Scott, the league leader in strikeouts (298) and earned run average (2.25), won his 18th game of the season against 10 losses as Houston reached the League Championship Series for the first time since 1980 and only the second time in the 25-year history of the franchise.

The Astros began the day knowing that a victory by the Atlanta Braves over the Cincinnati Reds earlier this afternoon was all they needed to clinch the title. But after they learned during batting practice that the Reds had gained a 6‚4 victory, the Astros knew they could clinch by themselves. Well into the game, however, their quest for the title seemed secondary to Scott’s performance. Going into the ninth inning, he said he was “more pumped up than nervous” as the crowd of 32,808 gave him a standing ovation. “I just wanted to keep us in the game,” Scott said. “We would have been real disappointed if we didn’t win and had to go to Atlanta and do it in front of 5,000 people.”

Scott, a 6-foot-3-inch right-hander who is 31 years old, had never pitched a no-hitter at any level, and after the first pitch of the game it didn’t seem as if this would be his first one. His elusive split-fingered fastball hit Dan Gladden, the Giants’ leadoff hitter, squarely in the back. “For the first two innings, he had what I would call mediocre stuff,” said Alan Ashby, the Astros’ catcher. “But after that, it was Katy, bar the door.” More than two hours after that first errant pitch, Scott entered the ninth by striking out Gladden and Rob Thompson and gaining the final out on a ground ball by Will Clark to Glenn Davis, the first baseman, who fielded the ball cleanly and completed the play unassisted.

“I told him to take it himself because I didn’t want to bobble it,” said Scott, who was running toward first in a not-sowholehearted effort to cover the bag. The Astros mobbed their pitcher. In contrast to the destructive scene that marred the Mets’ division-clinching victory last week, not one fan came onto the Astrodome field. Fourteen mounted policemen seemed to be enough of a deterrent.

In the best-of-seven-game league championship series, Mike Scott won both games he started against the Mets, allowing one earned run in 18 innings. The Mets won the pennant, however, by 7‚6 in Game 6 as Jesse Orosco struck out Kevin Bass with the tying run on third base in the last of the 16th inning.