what is star in sky ? : Supernova

what is star in sky ? : Supernova

  • A STAR IS AN ENORMOUS BALL OF EXTREMELY HOT GAS THAT PRODUCES ENERGY IN ITS CORE AND EMITS THIS ENERGY AT ITS SURFACE.

All the individual stars we can see in the night sky are part of our own galaxy, the Milky Way. Although in cosmic terms these are all “local” stars, they are actually fantastically far away—the closest is nearly 25 trillion miles (40 trillion km) distant, and most are much farther off. Overall in our galaxy there are more than 200 billion stars, of which about 10,000 are visible to the naked eye.

Star appearance and variationWe see all stars in the night sky as just tiny pinpricks of light. Some look brighter than others, but with the unaided eye they don’t seem to differ much in color: all look rather white. In fact, stars are much more varied than might at first appear. They come in a vast range of sizes and temperatures, in an array of colors, and also differ greatly in age and life span. Many of these characteristics of stars are related. For example, a star’s surface temperature and color are closely linked—a star with a relatively low surface temperature glows red, whereas hotter stars appear (with increasing temperature) orange, yellow, white, or blue.

  • STAR BRIGHTNESS AND DISTANCESTARS DIFFER HUGELY IN THEIR BRIGHTNESS AND IN THEIR DISTANCE FROM EARTH, ALTHOUGH ALL, APART FROM THE SUN, ARE EXTREMELY REMOTE. HOW BRIGHT A STAR LOOKS FROM EARTH DEPENDS OF COURSE PARTLY ON HOW FAR AWAY IT IS.

Because stars are so far away, obtaining data about them is tricky. Most of the data about any star comes from studying the light and other radiation coming from it, while the distance to the least remote stars can be worked out by measuring tiny annual variations in their sky positions. BrightnessThere are two different ways of stating a star’s brightness: apparent magnitude, which indicates how bright a star looks from Earth, and absolute magnitude, which expresses how bright it would look from a set distance—a better indicator of how brilliant it truly is. On both scales, a change of +1 on the scale means a decrease, and a change of -1 means an increase, in brightness. So, on the apparent magnitude scale, stars just visible to the naked eye score +6 or +5, while very bright stars score about +1 to 0, and the four very brightest have negative scores. The absolute magnitude scale runs from around +20 for some exceptionally dim red dwarfs to around -8 for the brightest supergiant stars. A star’s absolute magnitude is related to a measurement called its visual luminosity. This is the amount of light energy that a star emits per unit of time. Luminosity is often stated relative to that of the Sun. ▽ Brightness comparisonsThe apparent and absolute magnitudes, and luminosities, of 11 different stars, including the Sun, are compared in the table below. The stars range from the relatively nearby red dwarf, Proxima Centauri, to distant but fantastically luminous supergiants, such as Rigel.DistanceStars other than the Sun are so far away that a special unit is needed to express the distance to them. This unit is the light-year and is the distance light travels through space in a year, which is about 5.9 trillion miles (9.5 trillion km). The 100 brightest stars we can see in the night sky vary from 4.4 to around 2,500 light-years away. The distances to stars can be measured in various ways. For relatively nearby stars, a method called parallax is used (see right). For more remote stars, astronomers have to use more complex indirect methods. Because these methods are less precise, the distances to many stars, even to some of the brightest in the sky, are known only approximately.

INSIDE A STARA

  • STAR IS EFFECTIVELY A MACHINE FOR TRANSFERRING FANTASTIC AMOUNTS OF ENERGY FROM ITS CENTRAL CORE, WHERE THE ENERGY IS PRODUCED, OUT TOWARD ITS FIERY SURFACE. THIS JOURNEY CAN TAKE 100,000 YEARS OR MORE.

In a star, there is continuous flow of this energy from core to surface, where it escapes into space. The flow creates an outward-acting pressure, without which the star would collapse. The source of energy in the core of a star is the joining together, or fusion, of atomic nuclei (the central parts of atoms) to make larger nuclei. Energy production and transferNuclear fusion involves a tiny loss of mass, which is converted into energy. In most stars the dominant process is one in which hydrogen nuclei combine to form helium nuclei. From the core of a star, energy moves outward by radiation and convection. Radiation is the transfer of energy in the form of light, radiant heat, X-rays, and so on, all of which can be thought of as consisting of tiny packets of energy, called photons. Within a typical star, the gaseous material is so tightly packed that photons cannot travel far before they are absorbed and then reemitted in a different direction. So, energy transferred in this way travels outward in a slow, zigzag fashion. Convection carries energy toward the surface through circular motions of hot gas outward and denser cooler gas inward. Many stars contain layers, with different densities, some transferring energy by radiation, others by convection.

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STARBIRTH

  • STARS FORM OUT OF VAST CLOUDS OF COOL GAS AND DUST, CALLED MOLECULAR CLOUDS, THAT OCCUPY PARTS OF INTERSTELLAR SPACE. THE PROCESS OF STAR FORMATION WITHIN THESE CLOUDS CAN TAKE MILLIONS OF YEARS.

The molecular clouds where stars are born can be hundreds of light-years across. Most sites of star formation are hidden inside these dense dusty clouds. However, there are places where the radiation from brilliant newly formed stars is clearing the dust away and is lighting up the surrounding gas. We see these star-forming regions as bright nebulae. They include the Eagle Nebula (see opposite) in Serpens, the Orion Nebula , and many others. Some specific dark concentrations of dust and gas sometimes seen within molecular clouds are known as Bok globules. These frequently result in the formation of double or multiple star systems.Star formationFor star formation to start within a molecular cloud, a triggering event is needed. This could be a nearby supernova explosion, the passage of the cloud through a more crowded region of space, or an encounter with a passing star. The tidal forces and pressure waves that come into action during these situations push and pull at the cloud, compressing parts until some regions become dense enough for stars to form. Gravity then does the rest of the work of forming each star, pulling more and more material onto the developing knot of matter and concentrating most of it at the center. As the material grows denser, random motions are transformed into a uniform spin around a single axis. Collisions between particles jostling within the cloud raise its temperature, notably in the center, and the newly forming star begins to glow with infrared (heat) radiation. At this stage, the protostar (newly forming star) is quite unstable.It loses mass by expelling gas and dust, directed in two opposingjets from its poles. At its center, it eventually becomes so hot that nuclear fusion starts, and as the balance between gravity and outward-acting pressure begins to equalize, the protostar settles down to become a main-sequence star.

SUPERNOVAEA

  • SUPERNOVA IS THE CATACLYSMIC EXPLOSION OF, INMOST CASES, A HIGH MASS STAR AT THE END OF ITS LIFE.A SUPERNOVA BLASTS OUT SO MUCH LIGHT AND OTHERENERGY THAT IT CAN BRIEFLY OUTSHINE A GALAXY.

Supernovae are quite rare astronomical events in individual galaxies.None has been clearly observed in our galaxy since 1604, whena supernova some 20,000 light-years away was visible to the nakedeye. However, a growing number of supernovae have been spottedin other galaxies, including one in the Large Magellanic Cloud (asatellite galaxy of the Milky Way) in 1987. A new, bright, supernova might occur in our galaxy at any time.Types and causesSupernovae are classified according to their spectra into various types,such as 1a, 1b, and II. Types II and Ib are the main varieties in whichvery high mass stars explode. As they reach the end of their life, these stars swell into supergiants and obtain their energy from nuclear fusion reactions going on in their cores and in a series of shells or layers surrounding their cores. Eventually they start making iron in their cores,but fuel for this process soon runs out. As iron itself cannot be fused to supply energy, energy output in the core suddenly ceases, and thist a massive explosion.Some chemical elements can be forged only in the extreme high-energy conditions of a supernova.

BLACK HOLES

  • A BLACK HOLE IS ONE OF THE STRANGEST OBJECTS IN THE UNIVERSE—A REGION OF SPACE WHERE MATTER HAS BEEN SQUEEZED INTO A MINUSCULE POINT OR RING OF INFINITE DENSITY, CALLED A SINGULARITY.

In a spherical region around the singularity, the gravitational pull toward the center is so strong that nothing, not even light,can escape. The boundary of the region of no escape is called the event horizon, and anything passing inward through this boundary can never return. There are two main types of black hole. Stellar black holes form from the collapse of the cores of supergiant stars that have exploded as supernovas at the ends of their lives. Supermassive black holes are much bigger and are thought to exist at the centers of most galaxies.Detecting black holesBecause it emits no light, a black hole cannot be observed or imaged directly However, some black holes can be detected from their strong gravity, which attracts other matter. These black holes may have disk-shaped collections of gas and dust around them that are spiraling into the black hole, at the same time throwing off vast amounts of X-rays or other radiation. The easiest ones to detect are those that produce jets of high-energy particles from their poles.

MULTIPLE

  • STARSOUR SUN IS A LONE STAR WITH NO COMPANIONS. HOWEVER, MOST OF THE STARS WE CAN SEE IN THE SKY BELONG TO MULTIPLE-STAR SYSTEMS—THAT IS, TWO OR MORE STARS ORBITING EACH OTHER,BOUND BY GRAVITY.

The stars in a multiple-star system can orbit one another in various different ways.A pair of stars circling around a common center of gravity is called a binary system.If the two stars have the same mass, the center of gravity is halfway between them.More commonly, one star is heavier than the other, and the two stars have orbits of different sizes. In systems of three or more stars, various more complicated orbits are possible. For example, two stars may orbit each other closely, with the third circling the closely orbiting pair at a great distance. Overall, more than half the stars in the Milky Way Galaxy are part of multiple-star systems. These systems are different from star clusters which are large collections of stars only loosely bound by gravity.True and optical binariesA star that looks like a single point of light may actually consist of two stars located very close together in the sky. Where these stars are also close together in space and gravitationally bound—they orbit each other—they are known as “true” binaries.An example is Albireo in the constellation Cygnus . In contrast,some star pairs that happen to be close in the sky are not close in space, and are not gravitationally bound—they just happen to be in the same direction as seen from Earth. Doubles of this sort are called optical doubles. An example is a star pair called Algedi, or Alpha Capricorni, in the constellation Capricornus (see Its two components are more than 600 light-years apart.

STAR CLUSTERSA

  • LARGE GROUP OF STARS BOUND TOGETHER BY GRAVITY—ANYTHING FROM A DOZEN TO SEVERAL MILLION STARS—IS CALLED A STAR CLUSTER. THE MILKY WAY GALAXY CONTAINS THOUSANDS OF THESE SPECTACULAR STAR AGGREGATIONS.

Star clusters fall into two types: globular and open.Globular clusters are ancient, dense cities of stars, some containing more stars than a small galaxy. Open clusters,in contrast, are young, contain far fewer stars, and are often the site of new star creation. Many open clusters, and a few globular ones, can be seen in the night sky with the naked eye. Both types can be a magnificent sight when viewed through binoculars or a telescope.Globular ClustersGlobular clusters are groups of between 10,000 and several million mostly very old stars arranged roughly in a sphere.More than 150 clusters like this exist in the Milky Way —each can last for 10 billion years. The stars in a cluster tend to be concentrated toward its center and move in random circular orbits around the center.Many globular clusters consist of a single population of stars that all have the same origin, similar ages, and chemical composition. However, some contain two or more populations that formed at different times—through some of the more massive stars in the initial population dying and materials from them being recycled into a second star generation.Open ClustersOpen clusters are groups of up to a few thousand stars that were formed roughly at the same time from the same cloud of gas and dust. They are more loosely bound by gravity than globular clusters, and they survive for a shorter time—from a few hundred million up to a few billion years.Unlike globular clusters, which occur in all types of galaxies,open clusters are found only in spiral and irregular ones,where stars are actively being created. Around 1,100 clusters of this type have been identified so far in the Milky Way.

MULTIPLANETARY

  • SYSTEMSMANY EXOPLANETS RESIDE WITHIN MULTIPLANET SYSTEMS—GROUPS OF TWO OR MORE PLANETS ALL ORBITING THE SAME DISTANT STAR OR EVEN, IN SOME CASES, A PAIR OF STARS THAT ARE THEMSELVES CIRCLING EACH OTHER.

These intriguing multiplanet systems are quite diverse in terms of the mix of different sizes of planets they contain, the types of host star,and the number of planets that orbit within the host star’s (or stars’)habitable zone. Hundreds have been found already, at distances ranging from a few light-years to thousands of light-years from Earth.Only a few of the systems that have been discovered bear much resemblance to our Solar System, although a handful contain one or more roughly Earth-sized planets within a star’s habitable zone,and so hold out the possibility of harboring life. But new systems are regularly detected, and data about the planets already found is frequently being updated, so this situation is constantly changing.

GALAXIES

  • GALAXIES ARE FOUND IN A HUGE VARIETY OF SHAPES AND SIZES, FROM COMPLEX SPIRALS LIKE OUR MILKY WAY TO HUGE BALLS OF ANCIENT RED AND YELLOW STARS, AND SHAPELESS CLOUDS OF GAS, DUST, AND NEWBORN STARS.

Galaxies are the only places in the Universe where matter is densely packed enough for stars to form, and most stars spend their whole lives within them. Held together by gravity, most galaxies are thought to have a supermassive black hole at their center.Types of galaxiesAmerican astornomer Edwin Hubble confirmed the existence of galaxies beyond the Milky Way in the 1920s. He subdivided them into several distinct types distinguished by a code of letters and numbers.Elliptical galaxies are all roughly ball-shaped, but range from rounded spheres to elongated cigars. Today we know that they are dominated by old red and yellow stars. Spirals (types S and SB) are flattened disks with dense areas of star formation in the spiral arms,and older red and yellow stars in the center. Lenticulars (type S0) have a central hub surrounded by a disk, but no spiral arms, while Irregulars (type Irr I and II) are fairly shapeless clouds rich in star-forming material.

  • ALL THE STARS WE CAN SEE IN THE SKY LIE WITHIN THE CONFINES OF OUR HOME GALAXY, THE MILKY WAY. THIS VAST STAR SYSTEM, CONTAINING HUNDREDS OF BILLIONS OF STARS, IS A BARRED SPIRAL WITH A COMPLEX STRUCTURE AND IS ABOUT 120,000 LIGHT-YEARS ACROSS.

The Milky Way’s visible stars form a disk centered on a bulging hub. Despite its vast diameter, the disk averages only a thousand light-years deep. From our point of view on Earth, we see many more stars looking across the plane of the disk than when we look “up” or “down” from the plane and out into intergalactic space. This is why we see our galaxy as a broad band whose countless faint and distant stars merge together in a milky band of light.The central bulge of the Milky Way is dominated by low-mass, red and yellow stars with a high metallicity but the surrounding disk is filled with gas, dust, and younger stars. As with all spirals, stars are scattered across the disk, but the brightest are concentrated into the spiral arms. Stars Cross section of the Milky Way Seen from the side, the Milky Way consists of a disk of stars around a bulging hub some 8,000 light-years across. A broad halo region above and below the galaxy appears largely empty,but is home to gloular

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