A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants. Typical galaxies range from dwarfs with as few as ten million stars up to giants with a hundred trillion stars, all orbiting the galaxy's center of mass. Galaxies may contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.
Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.
There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe
A star is a massive, luminous ball of plasma held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun.
Our Sun is the best known (and most visible) example of a yellow dwarf star. Each second, it fuses approximately 600 million tons of hydrogen to helium, converting about 4 million tons of matter to energy
A yellow dwarf star will fuse hydrogen for approximately 10 billion years, until it is exhausted at the center of the star. When this happens, the star expands to many times its previous size and becomes a red giant. Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core cools and contracts into a compact, dense white dwarf.
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants. A hot, luminous main sequence star may also be referred to as a giant. Apart from this, because of their large radii and luminosities, giant stars lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung-Russell diagram and correspond to luminosity classes II or III
A star becomes a giant star after all the hydrogen available for fusion at its core has been depleted and, as a result, it has left the main sequence. A star whose initial mass is less than approximately 0.25 solar masses will not become a giant star. For most of their lifetimes, such stars have their interior thoroughly mixed by convection and so they can continue fusing hydrogen for a time in excess of [10 to the 12th power] years, much longer than the current age of the Universe. Eventually, however, they will develop a radiative core, subsequently exhausting hydrogen in the core and burning hydrogen in a shell surrounding the core. (Stars with mass in excess of 0.16 solar masses may expand at this point, but will never become very large.) Shortly thereafter the star's supply of hydrogen will be completely exhausted and it will become a helium white dwarf.
A blue giant is a massive star that has exhausted the hydrogen fuel in its core and left the main sequence. Blue giants have a surface temperature of around 30 000 K and a luminosity some 10 000 times that of the Sun. As they grow older they expand and cool, eventually becoming red giants, or continuing fusion into a more luminous or massive star.
 
The Solar System consists of the Sun and the astronomical objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants".
 
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune.
Planets above 10 Earth masses are termed giant planets. Below 10 Earth masses they are called super earths or, sometimes probably more accurately for the higher mass examples, "Gas Dwarfs" using a model where that exoplanet was mostly composed of hydrogen and helium.
Objects above 13 Jupiter masses are called brown dwarfs and these occupy the mass range between that of large gas giant planets and the lowest mass stars.
A gas giant is a massive planet with a thick atmosphere and a solid core. The "traditional" gas giants, Jupiter and Saturn, are composed primarily of hydrogen and helium. Uranus and Neptune are sometimes called ice giants, as they are mostly composed of water, ammonia, and methane ices. Among extrasolar planets, Hot Jupiter's are gas giants that orbit very close to their stars and thus have a very high surface temperature. Hot Jupiter's are currently the most common form of extrasolar planet known, perhaps due to the relative ease of detecting them.
Gas giants are commonly said to lack solid surfaces, but it is closer to the truth to say that they lack surfaces altogether since the gases that make them up simply become thinner and thinner with increasing distance from the planets' centers, eventually becoming indistinguishable from the interstellar medium. Therefore landing on a gas giant may or may not be possible, depending on the size and composition of its core.
Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass. Their structures are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable rocky core. The outermost portion of the hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The metallic hydrogen layer makes up the bulk of each planet, and is described as "metallic" because the great pressure turns hydrogen into an electrical conductor. The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood
Uranus and Neptune have distinctly different interior compositions from Jupiter and Saturn. Models of their interior begin with a hydrogen-rich atmosphere that extends from the cloud-tops down to about 85% of Neptune's radius and 80% of Uranus'. Below this point is predominantly "icy", composed of water, methane and ammonia. There is also some rock and gas but various proportions of ice/rock/gas could mimic pure ice so the exact proportions are unknown. Very hazy atmosphere layers with a small amount of methane gives them aquamarine colors such as baby blue and ultramarine colors respectively. Both have magnetic fields that are sharply inclined to their axes of rotation. Unlike the other gas giants, Uranus has an extreme tilt that causes its seasons to be severely pronounced.
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets.
The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar radiation, permitting life on land.[19] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life for at least another 500 million years.
Earth's magnetic field (and the surface magnetic field) is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole (see Magnetic North Pole) and the other magnetic field N pole near the Earth's geographic south pole (see Magnetic South Pole). This makes the compass usable for navigation. The cause of the field can be explained by dynamo theory. A magnetic field extends infinitely, though it weakens with distance from its source. The Earth's magnetic field, also called the geomagnetic field, which effectively extends several tens of thousands of kilometers into space, forms the Earth's magnetosphere. A paleomagnetic study of Australian red dacite and pillow basalt has estimated the magnetic field to be at least 3.5 billion years old
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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