The outer limit of the Oort cloud defines the cosmographic boundary of the Solar System and the extent of the Sun's Hill sphere

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Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clustered groups of stars, generally containing fewer than a few hundred members, and are often very young.

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The rest mass of the neutrino is much smaller than that of the other known elementary particles excluding massless particles.The weak force has a very short range, the gravitational interaction is extremely weak due to the very small mass of the neutrino, and neutrinos do not participate in the strong interaction.Thus, neutrinos typically pass through normal matter unimpeded and undetected

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The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.The boundary of no escape is called the event horizon. Although it has a great effect on the fate and circumstances of an object crossing it, it has no locally detectable features according to general relativity.In many ways, a black hole acts like an ideal black body, as it reflects no light

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Some of the earliest cosmological models of the universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center.Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus's work as well as Johannes Kepler's laws of planetary motion and observations by Tycho Brahe

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It has a diameter of about 46.56 kiloparsecs (152,000 light-years)[8] and is approximately 765 kpc (2.5 million light-years) from Earth. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the princess who was the wife of Perseus in Greek mythology.

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The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικὸς κύκλος (galaktikòs kýklos), meaning "milky circle".[26][27] From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe.[28] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis,[29] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

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It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, and slightly more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. Being composed primarily of gases and liquids, it has no well-defined solid surface

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Uranus has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. The planet is similar in composition to Neptune, and both have bulk chemical compositions which differ from those of the other two giant planets, Jupiter and Saturn (the gas giants). For this reason, scientists often distinguish Uranus and Neptune as "ice giants

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It is a gas giant with an average radius of about nine and a half times that of Earth.It has only one-eighth the average density of Earth, but is over 95 times more massive.Saturn's interior is most likely composed of a rocky core, surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium

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