The dreams and work of a generation of astronomers headed for an orbit around the sun on Saturday in the form of the biggest and most expensive space-based observatory ever built. The James Webb Space Telescope, a joint effort of NASA, the European Space Agency and the Canadian Space Agency, lifted off from a spaceport near the Equator in Kourou, French Guiana, a teetering pillar of fire and smoke embarking on a million-mile trip to the morning of time.

“The world gave us this telescope and we’re handing it back to the world today,” said Gregory Robinson, the Webb telescope’s program director, during a post-launch news conference in French Guiana.

The telescope, named for the NASA administrator who led the space agency through the early years of the Apollo program, is designed to see farther in space and further back in time than the vaunted Hubble Space Telescope. Its primary light gathering mirror is 21 feet across, about three times bigger than Hubble, and seven times more sensitive.

The Webb’s mission is to seek out the earliest, most distant stars and galaxies, which appeared 13.7 billion years ago, burning their way out of a fog leftover from the Big Bang (which occurred 13.8 billion years ago).

Astronomers watching the launch remotely from all over the world, many Zooming together in their pajamas, were jubilant.

“What an incredible Christmas present,” said Garth Illingworth of the University of California, Santa Cruz.

Tod Lauer of the National Science Foundation’s NOIRLab, in an email exchange with other astronomers reported his feeling about the launch: “Just enjoying the most sacred of all space words, “Nominal!” he said, referring to the lingo used by launch teams to describe rockets operating as expected.

To which Alan Dressler, a Carnegie Observatory astronomer and one of the founders of the Webb telescope project, replied, “Hallelujah! — another sacred word for the moment, Tod.”

Priyamvada Natarajan, a cosmologist at Yale, emailed from India to describe herself as “Just utterly utterly elated! — wow! wow!”

In Baltimore at the Space Telescope Science Institute, the headquarters for Webb’s mission operations, a small group of scientists and NASA officials erupted in screams of joy and applauded during the launch.

The flight operations team in another part of the institute then watched as Webb deployed its solar array, then its communications antenna minutes later. Roughly 100 mission personnel will command the spacecraft’s deployments, alternating between 12 hour shifts 24 hours a day as it begins its journey to a point beyond the moon.

“They’ve got real work to do,” said Kenneth Sembach, the institute’s director. “Our teams have spent the last two years doing numerous rehearsals.”

Equipped with detectors sensitive to infrared or “heat radiation,” the telescope will paint the universe in colors no human eye has ever seen. The expansion of the universe shifts the visible light from the earliest, most distant galaxies into the longer infrared wavelengths.

Studying the heat from these infant galaxies, astronomers say, could provide important clues to when and how the supermassive black holes that squat in the centers of galaxies form. Closer to home in the present, the telescope will sniff at the atmospheres of planets orbiting nearby stars, looking for the infrared signatures of elements and molecules associated with life, like oxygen and water.

The Webb will examine all of cosmic history, billions of years of it, astronomers say — from the first stars to life in the solar system. This week, the NASA administrator Bill Nelson called the telescope a “keyhole into the past.”

“It is a shining example of what we can accomplish when we dream big,” he said. After the launch he said, “It’s a great day for planet Earth.”

The beginning of the telescope’s journey did not go unnoticed by the space agency’s paymasters in Congress, who have stuck with the project for decades now.

“Today’s successful launch of the James Webb Space Telescope marks a historic milestone in our advancement of astrophysics and space science,” said Representative Eddie Bernice Johnson, Democrat of Texas and chairwoman of the House Science, Space, and Technology Committee, in a news release.

Saturday’s successful launch caps an expensive effort that stretched over 25 years of uncertainty, mistakes and ingenuity. Webb’s 18 gold-plated hexagonal mirrors, advanced temperature controllers and ultrasensitive infrared sensors were pieced together in a development timeline filled with cost overruns and technical hurdles. Engineers had to invent 10 new technologies along the way to make the telescope far more sensitive than Hubble.

When NASA picked the Northrop Grumman company to lead Webb’s construction in 2002, mission managers estimated that it would cost $1 billion to $3.5 billion and launch to space in 2010. Over-optimistic schedule projections, occasional development accidents and disorganized cost reporting dragged out the timeline to 2021 and ballooned the overall cost to $10 billion.

Even its final lap to the launchpad seemed perilous as a mishap in the Kourou rocket bay, disconnected cables and worrisome weather reports moved the Webb’s departure date deeper into December, until a Christmas morning launch could not be avoided.

“I’m so happy today,” said Josef Aschbacher, director general of the European Space Agency. But he added, “It’s very nerve racking, I couldn’t do launches every single day, this would not be good for my life expectancy.”

For astronomers and engineers, the launch was also a suspenseful sight to take in.

“It was hard to sleep last night,” said Adam Riess, an astrophysicist and Nobel laureate who will use the Webb telescope to measure the expansion rate of the universe.

But the launch itself is only the first step in an even more treacherous journey that astronomers and rocket engineers have called “six months of anxiety.”

The solar panel deployment half an hour into the flight was the first in a monthlong series of maneuvers and deployments with what NASA calls “344 single points of failure.”

“I could finally start breathing again when the solar arrays came out,” said Pam Melroy, NASA’s deputy administrator. “We have so many hard days ahead of us, but you can’t even get started on any of that until this part goes perfectly.”

Among the most tense moments, astronomers say, will be the unfolding of a giant sunscreen, the size of a tennis court, designed to keep the telescope in the dark and cold enough so that its own heat doesn’t swamp the heat from distant stars. The screen is made of five layers of a plastic called Kapton, which is similar to mylar, and as flimsy as mylar. It has occasionally ripped during rehearsals of its deployment.

If all goes well, astronomers will start to see the universe in a new light next summer. They are most looking forward to what they didn’t expect. As Thomas Zurbuchen, NASA’s associate administrator for science, said recently: “Every time we launch a big bold telescope, we get a surprise. This one is the biggest and boldest yet.”

But if anything goes wrong in the coming weeks and months, the field of astronomy’s view of the origins of existence may be imperiled. When problems snarled the work of the Hubble in the 1990s, NASA sent astronauts in the space shuttles to perform repair work. The Webb telescope is headed to a point beyond the moon where no spacecraft has ever carried humans before (although Ms. Melroy says NASA has contemplated a robotic repair mission if one were needed).

“I tell friends of mine who are not astronomers, after the launch, you mostly want to hear 30 days of nothing,” Dr. Riess said. “And we’ll be really happy if we hear nothing.”

Dennis Overbye reported from New York, and Joey Roulette from Baltimore.