“…if our Sun were near the center of NGC 362, the night sky would glow like a jewel box of bright stars. Hundreds of stars would glow brighter than Sirius, and in many different colors…”
In 1909, Ejnar Hertzsprung was the first to suggest that Sirius was a member of the Ursa Major Moving Group, based on his observations of the system’s movements across the sky. The Ursa Major Group is a set of 220 stars that share a common motion through space and were once formed as members of an open cluster, which has since become gravitationally unbound.
Analyses in 2003 and 2005 found Sirius’s membership in the group to be questionable: the Ursa Major Group has an estimated age of 500±100 million years, whereas Sirius, with metallicity similar to the Sun’s, has an age that is only half this, making it too young to belong to the group.
Sirius may instead be a member of the proposed Sirius Supercluster, along with other scattered stars such as Beta Aurigae, Alpha Coronae Borealis, Beta Crateris, Beta Eridani and Beta Serpentis. This is one of three large clusters located within 500 light-years (150 pc) of the Sun. The other two are the Hyades and the Pleiades, and each of these clusters consists of hundreds of stars.
Deneb lies at one vertex of a widely spaced asterism called the Summer Triangle, the other two members of which are the zero-magnitude stars Vega in the constellation. Lyra and Altair in Aquila. This formation is the approximate shape of a right triangle, with Deneb located at one of the acute angles.
The Summer Triangle is recognizable in the northern skies for there are few other bright stars in its vicinity. Deneb is also easily spotted as the tip of the Northern Cross asterism made up of the brightest stars in Cygnus, the others being Beta (Albireo), Gamma, Delta, and Epsilon Cygni. It never dips below the horizon at or above 45° north latitude, just grazing the northern horizon at its lowest point at such locations as Minneapolis Montréal and Turin.
In the northern hemisphere Deneb is high in the sky during summer evenings. In the southern hemisphere, Deneb is not at all visible south of 45° south parallel, so it just barely rises above the horizon in Tasmania and southern New Zealand during the southern winter (which corresponds to the northern summer).
Reading it, learning it and keeping it by your side.
Newton’s laws of motion are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces. They have been expressed in several different ways over nearly three centuries, and can be summarized as follows:
1.First law: The velocity of a body remains constant unless the body is acted upon by an external force. 2.Second law: The acceleration a of a body is parallel and directly proportional to the net force F and inversely proportional to the mass m, i.e., F = ma. 3.Third law: The mutual forces of action and reaction between two bodies are equal, opposite and collinear.
The three laws of motion were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis Principia Mathematica, first published on July 5, 1687. Newton used them to explain and investigate the motion of many physical objects and systems. For example, in the third volume of the text, Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler’s laws of planetary motion.
Newton’s laws are applied to bodies (objects) which are considered or idealized as a particle, in the sense that the extent of the body is neglected in the evaluation of its motion, i.e., the object is small compared to the distances involved in the analysis, or the deformation and rotation of the body is of no importance in the analysis. Therefore, a planet can be idealized as a particle for analysis of its orbital motion around a star.
In their original form, Newton’s laws of motion are not adequate to characterize the motion of rigid bodies and deformable bodies. Leonard Euler in 1750 introduced a generalization of Newton’s laws of motion for rigid bodies called the Euler’s laws of motion, later applied as well for deformable bodies assumed as a continuum. If a body is represented as an assemblage of discrete particles, each governed by Newton’s laws of motion, then Euler’s laws can be derived from Newton’s laws. Euler’s laws can, however, be taken as axioms describing the laws of motion for extended bodies, independently of any particle structure.
Isaac Newton
Newton’s Laws hold only with respect to a certain set of frames of reference called Newtonian or inertial reference frames. Some authors interpret the first law as defining what an inertial reference frame is; from this point of view, the second law only holds when the observation is made from an inertial reference frame, and therefore the first law cannot be proved as a special case of the second. Other authors do treat the first law as a corollary of the second. The explicit concept of an inertial frame of reference was not developed until long after Newton’s death.
In the given interpretation mass, acceleration, momentum, and (most importantly) force are assumed to be externally defined quantities. This is the most common, but not the only interpretation: one can consider the laws to be a definition of these quantities.
At speeds approaching the speed of light the effects of special relativity must be taken into account.
GRAVITY
Newton’s law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. (Separately it was shown that large spherically symmetrical masses attract and are attracted as if all their mass were concentrated at their centers.)
This is a general physical law derived from empirical observations by what Newton called induction. It is a part of classical mechanics and was formulated in Newton’s work Philosophiae Naturalis Principia Mathematica (“the Principia”), first published on 5 July 1687. (When Newton’s book was presented in 1686 to the Royal Society, Robert Hooke made a claim that Newton had obtained the inverse square law from him .) In modern language, the law states the following:
Every point mass attracts every single other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them:
where: F is the force between the masses, G is the gravitational constant, m1 is the first mass, m2 is the second mass, and r is the distance between the masses.
Assuming SI units, F is measured in newtons (N), m1 and m2 in kilograms (kg), r in meters (m), and the constant G is approximately equal to 6.674×10-11 N m2 kg-2.The value of the constant G was first accurately determined from the results of the Cavendish experiment conducted by the British scientist Henry Cavendish in 1798, although Cavendish did not himself calculate a numerical value for G.
This experiment was also the first test of Newton’s theory of gravitation between masses in the laboratory. It took place 111 years after the publication of Newton’s Principia and 71 years after Newton’s death, so none of Newton’s calculations could use the value of G; instead he could only calculate a force relative to another force.
Newton’s law of gravitation resembles Coulomb’s law of electrical forces, which is used to calculate the magnitude of electrical force between two charged bodies. Both are inverse-square laws, in which force is inversely proportional to the square of the distance between the bodies. Coulomb’s Law has the product of two charges in place of the product of the masses, and the electrostatic constant in place of the gravitational constant.
Newton’s law has since been superseded by Einstein’s theory of general relativity, but it continues to be used as an excellent approximation of the effects of gravity. Relativity is required only when there is a need for extreme precision, or when dealing with gravitation for extremely massive and dense objects.
[Deneb]…it is estimated to be somewhere between 55,000 and 196,000 times as luminous as the Sun…’
Deneb is the brightest star in the constellation of Cygnus. It is one of the vertices of the asterism known as the Summer Triangle and forms the ‘head’ of the Northern Cross.
It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous stars.
However, its exact distance (and hence luminosity) has been difficult to calculate; it is estimated to be somewhere between 55,000 and 196,000 times as luminous as the Sun.
Matter commonly exists in four states (or phases): solid, liquid and gas, and plasma. However, advances in experimental techniques have revealed other previously theoretical phases, such as Bose–Einstein condensates and fermionic condensates. A focus on an elementary-particle view of matter also leads to new phases of matter, such as the quark–gluon plasma.
For much of the history of the natural sciences people have contemplated the exact nature of matter. The idea that matter was built of discrete building blocks, the so-called particulate theory of matter, was first put forward by the Greek philosophers Leucippus (~490 BC) and Democritus (~470–380 BC).
Albert Einstein showed that ultimately all matter is capable of being converted to energy (known as mass-energy equivalence) by the famous formula E = mc2, where E is the energy of a piece of matter of mass m, times c2 the speed of light squared.
Custodite interrogans ænigmatibus.
As the speed of light is 299,792,458 metres per second (186,282 mi/s), a relatively small amount of matter may be converted to a large amount of energy. An example is that positrons and electrons (matter) may transform into photons (non-matter). However, although matter may be created or destroyed in such processes, neither the quantity of mass or energy change during the process.
If the known laws of physics are extrapolated beyond where they have been verified, there is a singularity. Some estimates place this moment at approximately 13.8billion years ago, which is thus considered the age of the universe. After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies.
Since Georges Lemaître first noted, in 1927, that an expanding universe might be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion. While the scientific community was once divided between supporters of two different expanding universe theories, the Big Bang and the Steady State theory, accumulated empirical evidence provides strong support for the former.
In 1929, from analysis of galactic redshifts, Edwin Hubble concluded that galaxies are drifting apart; this is important observational evidence consistent with the hypothesis of an expanding universe.
In 1965, the cosmic microwave background radiation was discovered, which was crucial evidence in favour of the Big Bang model, since that theory predicted the existence of background radiation throughout the universe before it was discovered. More recently, measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy‘s existence. The known physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature.
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