Title: Galileo Galilei
Content:  Galileo observed the Milky Way, previously believed to be nebulous, and found it to be a multitude of stars packed so densely that they appeared from Earth to be clouds. He located many other stars too distant to be visible with the naked eye. He observed the double star Mizar in Ursa Major in 1617. In the Starry Messenger, Galileo reported that stars appeared as mere blazes of light, essentially unaltered in appearance by the telescope, and contrasted them to planets, which the telescope revealed to be discs. But shortly thereafter, in his Letters on Sunspots, he reported that the telescope revealed the shapes of both stars and planets to be "quite round". From that point forward, he continued to report that telescopes showed the roundness of 
Title: Galileo Galilei
Content:   Using his refracting telescope, Galileo observed in late 1609 that the surface of the Moon is not smooth. Early the next year, he observed the four largest moons of Jupiter. Later in 1610, he observed the phases of Venus—a proof of heliocentrism—as well as Saturn, though he thought the planet's rings were two other planets. In 1612, he observed Neptune and noted its motion, but did not identify it as a planet. Galileo made studies of sunspots, the Milky Way, and made various observations about stars, including how to measure their apparent size without a telescope.
Title: Sidereus Nuncius
Content:  Galileo reported that he saw at least ten times more stars through the telescope than are visible to the naked eye, and he published star charts of the belt of Orion and the star cluster Pleiades showing some of the newly observed stars. With the naked eye observers could see only six stars in the Taurus cluster; through his telescope, however, Galileo was capable of seeing thirty-five – almost six times as many. When he turned his telescope on Orion, he was capable of seeing eighty stars, rather than the previously observed nine – almost nine times more. In Sidereus Nuncius, Galileo revised and reproduced these two star groups by distinguishing between the stars seen without the telescope and those seen with it. Also, when he observed some of the "nebulous" stars in the Ptolemaic star catalogue, he saw that rather than being cloudy, they were made of many small stars. From this he deduced that the nebulae and the Milky Way were "congeries of innumerable stars grouped together in clusters" too small and distant to be resolved into individual stars by the naked eye. 
Title: History of the telescope
Content:  with a 37mm objective (which he would stop down to 16mm or 12mm) and a 23x magnification. With this last instrument he began a series of astronomical observations in October or November 1609, discovering the satellites of Jupiter, hills and valleys on the Moon, the phases of Venus and observed spots on the sun (using the projection method rather than direct observation). Galileo noted that the revolution of the satellites of Jupiter, the phases of Venus, rotation of the Sun and the tilted path its spots followed for part of the year pointed to the validity of the sun-centered Copernican system over other Earth-centered 
Title: Visible-light astronomy
Content:  with up to 30× magnification. With a Galilean telescope, the observer could see magnified, upright images on Earth; it was what is commonly known as a terrestrial telescope or a spyglass. Galileo could also use it to observe the sky, and for a time was one of those who could construct telescopes good enough for that purpose. On 25 August 1609, Galileo demonstrated one of his early telescopes, with a magnification of up to 8 or 9, to Venetian lawmakers. Galileo's telescopes were also a profitable sideline, selling them to merchants who found them useful both at sea and as items of trade. He published his initial telescopic astronomical observations in March 1610 in a brief treatise titled Sidereus Nuncius (Starry Messenger).
Title: Galileo Galilei
Content:   Based only on uncertain descriptions of the first practical telescope which Hans Lippershey tried to patent in the Netherlands in 1608, Galileo, in the following year, made a telescope with about 3x magnification. He later made improved versions with up to about 30x magnification. With a Galilean telescope, the observer could see magnified, upright images on the Earth—it was what is commonly known as a terrestrial telescope or a spyglass. He could also use it to observe the sky; for a time he was one of those who could construct telescopes good enough for that purpose. On 25 August 1609, he demonstrated one of his early telescopes, with a magnification of about 8 or 9, to Venetian lawmakers. His telescopes were also a profitable sideline for Galileo, who sold them to merchants who found them useful both at sea and as items of trade. He published his initial telescopic astronomical observations in March 1610 in a brief treatise entitled Sidereus Nuncius (Starry Messenger). 
Title: Galilean moons
Content:  body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter. Galileo's discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than Earth dealt a blow to the then-accepted Ptolemaic world system, which held that Earth was at the center of the universe and all other celestial bodies revolved around it. Galileo's March 13, 1610, Sidereus Nuncius (Starry Messenger), which announced celestial observations through his 
Title: Mass
Content:  values is constant for all planets in the Solar System. On 25 August 1609, Galileo Galilei demonstrated his first telescope to a group of Venetian merchants, and in early January 1610, Galileo observed four dim objects near Jupiter, which he mistook for stars. However, after a few days of observation, Galileo realized that these "stars" were in fact orbiting Jupiter. These four objects (later named the Galilean moons in honor of their discoverer) were the first celestial bodies observed to orbit something other than the Earth or Sun. Galileo continued to observe these moons over the next eighteen months, and by the middle of 1611, he had obtained remarkably accurate estimates for their periods.
Title: Galileo affair
Content:  Galileo began his telescopic observations in the later part of 1609, and by March 1610 was able to publish a small book, The Starry Messenger (Sidereus Nuncius), describing some of his discoveries: mountains on the Moon, lesser moons in orbit around Jupiter, and the resolution of what had been thought to be very cloudy masses in the sky (nebulae) into collections of stars too faint to see individually without a telescope. Other observations followed, including the phases of Venus and the existence of sunspots. Galileo's contributions caused difficulties for theologians and natural philosophers of the time, as they contradicted scientific and 
Title: Galilean moons
Content:  As a result of improvements Galileo Galilei made to the telescope, with a magnifying capability of 20×, he was able to see celestial bodies more distinctly than was previously possible. This allowed Galilei to observe in either December 1609 or January 1610 what came to be known as the Galilean moons. On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter's moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from January 8 to March 2, 1610. In these observations, he discovered a 

Question: what objects did galileo see with his telescope?
Answer: Ganymede.