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Astronomy

Neutron Stars

Supernova with a neutron star in the middle

A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Neutron stars are the smallest and densest stellar objects, excluding black holes and hypothetical white holes, quark stars, and strange stars. Neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass of about 1.4 solar masses. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

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Astronomy

Exoplanets

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such. The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 December 2020, there are 4,379 confirmed exoplanets in 3,237 systems, with 717 systems having more than one planet.

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Astronomy

Quasars

A quasar (/ˈkweɪzɑːr/; also known as a quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk. As gas in the disk falls towards the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum. The power radiated by quasars is enormous; the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way. Usually, quasars are categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.

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Astrophysics

Dark Energy

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from supernovae measurements, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating.

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Physics

Higgs Boson

The Higgs boson is an elementary particle in the Standard Model of particle physics, produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. It is named after physicist Peter Higgs, who in 1964, along with five other scientists, proposed the Higgs mechanism to explain why some particles have mass. (Particles acquire mass in several ways, but a full explanation for all particles had been extremely difficult).

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Physics

String Theory

In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus string theory is a theory of quantum gravity.

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Definitions

Asteroid
A solid body orbiting the Sun that consists of metal and rock. Most are only a few miles in diameter and are found between the orbits of Mars and Jupiter, too small and far away to be seen easily in a small telescope. A few venture closer to the Sun and cross Earth’s orbit.
Comet
A comet is a “dirty snowball” of ice and rocky debris, typically a few miles across, that orbits the Sun in a long ellipse. When close to the Sun, the warmth evaporates the ice in the nucleus to form a coma (cloud of gas) and a tail. Named for their discoverers, comets sometimes make return visits after as little as a few years or as long as tens of thousands of years.
Eccentricity
The measure of how much an orbit deviates from being circular.
Eclipse
An event that occurs when the shadow of a planet or moon falls upon a second body. A solar eclipse occurs when the Moon’s shadow falls upon Earth, which we see as the Moon blocking the Sun. When Earth’s shadow falls upon the Moon, it causes a lunar eclipse.
Light-year
The distance that light (moving at about 186,000 miles per second) travels in one year, or about 6 trillion miles.

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Important Numbers

Be aware of the 13 most important numbers of our universe!
Number Value
Universal Gravitational Constant 6.67408 × 10-11 m3 kg-1 s-2
Speed of Light 299,792,458 m s-1
Ideal Gas Constant 8.314 J mol-1 K-1
Absolute Zero −273.15 °C
Avogadro's Number 6.02214076 × 1023 mol-1
Relative Strength of Electricity and Gravity Gravity is 137 times stronger than Electromagnetism
Boltzmann's Constant 1.3807 x 10-23 J K-1
Planck's Constant 6.62607004 × 10-34 m2 kg s-1
The Schwarzschild Radius 2 GM/c2
Efficiency of Hydrogen Fusion 0.7%
Chandrasekhar Limit 1.4 Solar Masses
Hubble Constant 73.8 km sec-1 Mpc-1
Omega 0.6889 ± 0.0056

Influential Figures

Nicolaus Copernicus
In 16th century Poland, astronomer Nicolaus Copernicus (1473–1543) proposed a model of the solar system that involved the Earth revolving around the sun. The model wasn't completely correct, as astronomers of the time struggled with the backwards path Mars sometimes took, but it eventually changed the way many scientists viewed the solar system. Copernicus finished the first manuscript of his book, "De Revolutionibus Orbium Coelestium" ("On the Revolutions of the Heavenly Spheres") in 1532. In it, Copernicus established that the planets orbited the sun rather than the Earth. He laid out his model of the solar system and the path of the planets. He didn't publish the book, however, until 1543, just two months before he died. He diplomatically dedicated the book to Pope Paul III. The church did not immediately condemn the book as heretical, perhaps because the printer added a note that said even though the book's theory was unusual, if it helped astronomers with their calculations, it didn't matter if it wasn't really true, according to Famous Scientists. It probably also helped that the subject was so difficult that only highly educated people could understand it. The Church did eventually ban the book in 1616.
Johannes Kepler
Using detailed measurements of the path of planets kept by Danish astronomer Tycho Brahe, Johannes Kepler (1571–1630) determined that planets traveled around the sun not in circles but in ellipses. In so doing, he calculated three laws involving the motions of planets that astronomers still use in calculations today However, closed minds put Kepler's work at risk. Born in December 1571, in Weil der Stadt in Swabia, in southwest Germany, young Johannes Kepler was a sickly child of poor parents. He was awarded a scholarship to the University of Tübingen, where he studied to become a Lutheran minister. While there, he was introduced to the work of Nicolaus Copernicus, who had written that the planets orbited the sun rather than Earth. In 1594, Kepler became a professor of mathematics at a seminary in Graz, Austria, as well as district mathematician and calendar maker. In search of the most detailed notes about the paths of the planets, Kepler contacted astronomer Tycho Brahe. A wealthy Danish nobleman, Brahe built an observatory in Prague where he tracked the motions of the planets and maintained the most accurate observations of the solar system at the time. In 1600, Brahe invited Kepler to come work with him.
Galileo Galilei
Born in Italy, Galileo Galilei (1564–1642) is often credited with the creation of the optical telescope, though in truth he improved on existing models. The astronomer (also mathematician, physicist and philosopher) turned the new observational tool toward the heavens, where he discovered the four primary moons of Jupiter (now known as the Galilean moons), as well as the rings of Saturn. Galileo is often incorrectly credited with the creation of a telescope. (Hans Lippershey applied for the first patent in 1608, but others may have beaten him to the actual invention.) Instead, he significantly improved upon them. In 1609, he first learned of the existence of the spyglass, which excited him. He began to experiment with telescope-making, going so far as to grind and polish his own lenses. His telescope allowed him to see with a magnification of eight or nine times. In comparison, spyglasses of the day only provided a magnification of three. It wasn't long before Galileo turned his telescope to the heavens. He was the first to see craters on the moon, he discovered sunspots, and he tracked the phases of Venus. The rings of Saturn puzzled him, appearing as lobes and vanishing when they were edge-on — but he saw them, which was more than can be said of his contemporaries.
Isaac Newton
Sir Isaac Newton contributed significantly to the field of science over his lifetime. He invented calculus and provided a clear understanding of optics. But his most significant work had to do with forces, and specifically with the development of a universal law of gravity. He calculated three laws describing the motion of forces between objects, known today as Newton's laws. As a student, Newton studied the most advanced mathematical texts of his time. While on hiatus, he continued to study mathematics, laying the ground for differential and integral calculus. He united many techniques that had previously been considered seperately, such as finding areas, tangents, and the lengths of curves. He wrote De Methodis Serierum et Fluxionum in 1671, but was unable to find a publisher. Newton also established a cohesive scientific method, to be used across disciplines. Previous explorations of science varied depending on the field. Newton established a set format for experimentation still used today.
Albert Einstein
In the early 20th century, German physicist Albert Einstein (1879–1955) became one of the most famous scientists ever after proposing a new way of looking at the universe that went beyond current understanding Einstein suggested that the laws of physics are the same throughout the universe, that the speed of light in vacuum is constant, and that space and time are linked in an entity known as space-time, which is distorted by gravity. A major validation of Einstein's work came in 1919, when Sir Arthur Eddington, secretary of the Royal Astronomical Society, led an expedition to Africa that measured the position of stars during a total solar eclipse. The group found that the position of stars was shifted due to the bending of light around the sun. (In 2008, a BBC/HBO production dramatized the story in "Einstein and Eddington."). Einstein remained in Germany until 1933, when dictator Adolf Hitler rose to power. The physicist then renounced his German citizenship and moved to the United States to become a professor of theoretical physics at Princeton. He became a U.S. citizen in 1940 and retired in 1945.
Edwin Hubble
American astronomer Edwin Hubble (1899–1953) calculated that a small blob in the sky existed outside of the Milky Way. Prior to his observations, the discussion over the size of the universe was divided as to whether or not only a single galaxy existed. Hubble went on to determine that the universe itself was expanding, a calculation which later came to be known as Hubble's law. Hubble's observations of the various galaxies allowed him to create a standard system of classification still used today. In the 1920s, the small, diffuse patches in the sky were termed nebulae, and were thought to exist within the Milky Way. While examining images of NGC 6822, M33 and M31 individually, Hubble noticed a pulsating star known as a Cepheid variable inside each one. Cepheids are special because their pulsation allows for precise measurements of distance. Hubble calculated how far away each Cepheid lay — and thus how far to each nebula — and realized they were too distant to be inside of the Milky Way. Astronomers realized that these nebulae were in fact galaxies like the Milky Way, each containing billions of stars. The universe, once thought to be contained by the Milky Way, expanded significantly in the eyes of astronomers.

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