Solar System & Sun

Solar System

The Solar System consists of the Sun and those celestial 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 retinue of 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, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far more massive than the terrestrials.

The Solar System is also home to two regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these regions, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognised to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. In addition to thousands of small bodies in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions.

The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
Six of the planets and three of the dwarf planets are orbited by natural satellites, usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.

Sun

The Sun is the most prominent feature in our solar system. It is the largest object and contains approximately 98% of the total solar system mass. One hundred and nine Earths would be required to fit across the Sun's disk, and its interior could hold over 1.3 million Earths. The Sun's outer visible layer is called the photosphere and has a temperature of 6,000°C (11,000°F). This layer has a mottled appearance due to the turbulent eruptions of energy at the surface.
Solar energy is created deep within the core of the Sun. It is here that the temperature (15,000,000° C; 27,000,000° F) and pressure (340 billion times Earth's air pressure at sea level) is so intense that nuclear reactions take place. This reaction causes four protons or hydrogen nuclei to fuse together to form one alpha particle or helium nucleus. The alpha particle is about .7 percent less massive than the four protons. The difference in mass is expelled as energy and is carried to the surface of the Sun, through a process known as convection, where it is released as light and heat. Energy generated in the Sun's core takes a million years to reach its surface. Every second 700 million tons of hydrogen are converted into helium ashes. In the process 5 million tons of pure energy is released; therefore, as time goes on the Sun is becoming lighter.

The chromosphere is above the photosphere. Solar energy passes through this region on its way out from the center of the Sun. Faculae and flares arise in the chromosphere. Faculae are bright luminous hydrogen clouds which form above regions where sunspots are about to form. Flares are bright filaments of hot gas emerging from sunspot regions. Sunspots are dark depressions on the photosphere with a typical temperature of 4,000°C (7,000°F).

The corona is the outer part of the Sun's atmosphere. It is in this region that prominences appears. Prominences are immense clouds of glowing gas that erupt from the upper chromosphere. The outer region of the corona stretches far into space and consists of particles traveling slowly away from the Sun. The corona can only be seen during total solar eclipses.

The Sun appears to have been active for 4.6 billion years and has enough fuel to go on for another five billion years or so. At the end of its life, the Sun will start to fuse helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow the Earth. After a billion years as a red giant, it will suddenly collapse into a white dwarf -- the final end product of a star like ours. It may take a trillion years to cool off completely.

Comet; Asteroid and Meteorites

COMET

A comet is a small solar system body bigger than a meteoroid[citation needed] that, when close enough to the Sun, exhibits a visible coma (fuzzy "atmosphere"), and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few hundred metres to tens of kilometres across.

Comet Swarm Delivered Earth's Oceans?

A barrage of comets may have delivered Earth's oceans around 3.85 billion years ago, a new study suggests.

Scientists have long suspected that Earth and its near neighbors were walloped by tens of thousands of impactors during an ancient event known as the Late Heavy Bombardment.
This pummeling disfigured the moon, leaving behind massive craters that are still visible, preserved for millennia in the moon's airless environment. But it's been unclear whether the impactors were icy comets or rocky asteroids.

Now, based on levels of a certain metal in ancient Earth rocks, a team led by Uffe Jorgensen of the Niels Bohr Institute in Denmark says comets were the culprits.
Whether Earth had oceans before any comets arrived has been intensely debated, Jorgensen noted.
Some experts say enough water could have existed from the moment Earth formed, while others argue that the young planet's heat would have vaporized any liquids.
"It's the kind of subject that can make scientists fight physically with one another," Jorgensen said.
His team thinks early Earth was just too hot to retain large bodies of water. But by the time of the Late Heavy Bombardment, things had cooled down, allowing meltwater from the flurry of comets to become the world's first seas.
"We may sip a piece of the impactors every time we drink a glass of water," the study authors write in their paper, which will be published in an upcoming issue of the journal Icarus.

Comets' Metal

Jorgensen and colleagues arrived at this conclusion after measuring the levels of iridium in surface and near-surface rocks from Greenland—some of the oldest known rocks in the world, dating back to the time of the bombardment.
Iridium is a scarce metal on Earth, but it's relatively common in comets and asteroids.
According to the team's calculations, iridium levels in the rocks around an asteroid impact should be about 18,000 parts per trillion. A comet impact, meanwhile, should leave behind only about 130 parts per trillion. That's because comets would carry less metal, since they're mostly made of loosely packed water ice with some rocky debris.
Comets also strike Earth at higher speeds, because of their longer orbits around the sun.
As a result, "the explosion formed by a comet is more violent than from an asteroid, and the amount of material—including iridium—thrown back into space is larger," Jorgensen said.
The team found that the Greenland rocks contained about 150 parts per trillion of iridium, supporting the idea that comets were the main players in the Late Heavy Bombardment.
All that ice from the comet swarm then thawed to create a global ocean more than half a mile (about a kilometer) deep, the team calculates.
The moon, meanwhile, lacks an ocean because its gravity is much weaker than Earth's, so most if not all of the debris from a comet strike would be thrown back into space, Jorgensen said.
But Nicolas Dauphas, a geophysicist at the University of Chicago, isn't yet convinced that the bombardment featured comets, not asteroids.
The new study, he said, relies on too many estimates—such as the predicted amount of iridium deposited following an impact.
"I am afraid [they have] stretched their conclusions too far," Dauphas said.

Accidental Life?

Chandra Wickramasinghe, an astrobiologist at Cardiff University in the U.K. not involved in the new study, also supports the theory of an ancient comet bombardment.
And he thinks it's possible that comets seeded Earth not only with water but with life.
According to some controversial studies, the oldest evidence for life on Earth dates back to about 3.85 billion years ago, around the time of the Late Heavy Bombardment, he noted.

Asteroid

Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are smaller than planets but larger than meteoroids. The term "asteroid" has historically been applied primarily to minor planets of the inner Solar System, as the outer Solar System was poorly known when it came into common usage. The distinction between asteroids and comets is made on visual appearance: Comets show a perceptible coma while asteroids do not.

Asteroid Impact

There have been many Asteroid Impact videos but not one is as beautiful and scary as this one. If you realise this could indeed happen some day it makes you wonder will mankind be on outer space in time or will we all die when this happens? Check out this HD CGI footage by Discovery Channel about an Asteroid the size of the moon hitting planet earth.

Meteorites

A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids. When it enters the atmosphere, impact pressure causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting/falling star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.

More generally, a meteorite on the surface of any celestial body is a natural object that has come from elsewhere in space. Meteorites have been found on the Moon and Mars.

It's amazing what a rover can find laying by the side of the road. The Mars Exploration Rover Opportunity has found a rock that apparently is another meteorite. Less than three weeks ago, Opportunity drove away from a larger meteorite called "Block Island" that the rover examined for six weeks. Now, this new meteorite, dubbed "Shelter Island," is another fairly big rock, about 47 centimeters (18.8 inches) long, that fell from the skies. Block Island is about 60 centimeters (2 feet) across and was just 700 meters (about 2,300 feet) away from this latest meteorite find. At first look, the two meteorites look to be of a similar makeup; Opportunity found that Block Island was is made of nickel and iron.

Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called falls. All other meteorites are known as finds. As of mid-2006, there are approximately 1,050 witnessed falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite finds.

Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites are largely composed of metallic iron-nickel; and, stony-iron meteorites contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy.

Meteor Crater

 

Galaxies

Galaxies

How Many Galaxies Have We Discovered?

Astronomers think that there are hundreds of billions galaxies in the universe, however the exact number is not known. But astronomers should know how many galaxies we've actually seen and discovered, right?

Well, not necessarily.

“We don’t know,” says Ed Churchwell, professor of astronomy at the University of Wisconsin-Madison. “We know it’s a very large number.”

In just one image for example, the Hubble Ultra Deep Field, above, there are about 10,000 galaxies visible.

In our own galaxy, There are between 4 billion 100-300 billion stars in the Milky Way. At most, 8,479 of them are visible from Earth. Roughly 2,500 stars are available to the unaided eye in ideal conditions from a single spot at a given time.

But the number of galaxies will keep growing as our telescopes get better and can look out and back farther in time.


“To count them all, you have to be able to look far enough back in time or deep enough in space to see when galaxies were formed,” Churchwell says. “We haven’t reached that point yet. It’s not a well-determined number, but at some point we’re going to reach it.”

The estimate of how many galaxies there are in the universe is done by counting how many galaxies we can see in a small area of the sky. This number is then used to guess how many galaxies there are in the entire sky.


For the time being, the hundreds of billions in the tally are extrapolated from the Hubble Ultra Deep Field, taken over a time period in 2003 and 2004. Pointed at a single piece of space for several months — a spot covering less than one-tenth of one-millionth of the sky — Hubble returned an image of galaxies 13 billion light years away.

"Você olha para isso e dizer, 'Como muitas galáxias posso ver?" Churchwell explica. “And that turns out to be a very large number.” "E isso acaba por ser um número muito grande."



“Then you take that number of galaxies from that postage-stamp-sized piece of the sky and multiply it by the number of postage-stamp-sized pieces of sky,” Churchwell says. "Então você pega o número de galáxias do que um selo de tamanho pedaço do céu e multiplicá-lo pelo número de um selo pedaços do céu", Churchwell diz. “And that turns out to be a much larger number.” "E isso acaba por ser um número muito maior."

In the first Hubble Deep Field image , taken in 1995, there are about 3,000 galaxies visible in the image. Na primeira imagem do Hubble Deep Field, tomada em 1995, existem cerca de 3.000 galáxias visíveis na imagem.

Source: UW-M






Cosmic mystery



Researchers propose a new explanation for why some tiny galaxies have more than their fair share of dark matter.

Literally cloaked in darkness, the faintest galaxies in the universe have remained a mystery since their discovery more than two decades ago. Now a team of theorists has come up with a new explanation for the origin of these dim bodies. Known as dwarf spheroidal galaxies, these ancient stellar groupings not only serve as fossil remains of the early universe but have the highest known ratio of dark matter to ordinary, visible matter.

In the most widely accepted model of galaxy formation, an exotic type of invisible material, known as cold dark matter, provides the gravitational glue that draws together stars and gas and keeps galaxies, along with galaxy clusters, from flying apart. It would seem that all galaxies ought to have about the same ratio of dark matter to visible matter, because gravity builds all galaxies in the same way. Yet dwarf spheroidals are the most dark matter–dominated galaxies known, with 10 to 30 times the ratio of dark to visible matter as large galaxies including the Milky Way.

That’s the puzzle that Elena D’Onghia of the University of Zurich and the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and her colleagues set out to solve in a study posted online July 16 (http://arxiv.org/abs/0907.2442) and in an upcoming Nature.

Like other researchers, she and her collaborators assume that dwarf spheroidals were born with a lower, more typical ratio of dark to visible matter, but that much of the visible stuff somehow got pulled out.

Previous models suggest a complex, two-step process to explain the high ratio. But these models require a dwarf spheroidal to lie close to a galaxy as large as the Milky Way. In reality, some spheroidals lie far from such galaxies. Also, these models don’t easily explain the spherical shape of these galaxies or the diversity of their dark matter ratios.

In contrast, the new model proposed by D’Onghia’s team relies on the assumption that stars and gas rotate in fledgling galaxies, a property which the underlying dark matter model of formation requires.

If the rotation and orbit of stars in a dwarf spheroidal are in sync with the rotation of a slightly larger, nearby galaxy — possibly even just another dwarf spheroidal — the gravitational influences of the two galaxies on each other are enhanced, D’Onghia says.

Within 2 to 3 billion years, the gravitational pull would remove many stars from the lower-mass dwarf, D’Onghia says. Because dark matter does not rotate, it would be left behind in the dwarf galaxy. Depending on how closely the rotation of stars and gas aligns in neighboring galaxies, the dwarf spheroidals would end up with varying, but always high, ratios of dark to visible matter.

The proposed interaction could account for dwarf spheroidals, such as the recently discovered galactic duo Leo IV and Leo V, that don’t reside close to a large galaxy like the Milky Way, D’Onghia asserts.

“Certainly this is an idea that needs to be taken very seriously,” comments theorist James Bullock of the University of California, Irvine. “I bet some of the [dwarf spheroidals] formed this way, but I’m not sure if the numbers work out to explain all of them,” he adds.

D’Onghia and her collaborators simulate only the interaction of stars, not gas, cautions Rosemary Wyse of Johns Hopkins University in Baltimore, Md. But D’Onghia says that the rotating gas in a dwarf spheroidal, although more difficult to model than the stars, ought to be removed in a similar manner.

Jorge Peñarrubia of the University of Cambridge in England takes a contrarian view. “In my opinion, the whole problem may be a theoretical misconception,” due to uncertainties about star formation in galaxies, he says. Although dark matter models require that stars form in rotating disks, star-forming regions in the Milky Way indicate that most stars form in clusters instead. If stars in dwarf spheroidals don’t form in rotating disks, the scenario proposed by D’Onghia and her collaborators wouldn’t provide an explanation, he says.

Celestial Objects

Celestial Objects Anyone Can See With a Small Telescope

Orion Nebula

Granted, with small telescopes, it won't look like this Hubble Space Telescope image, but The Great Nebula is even visible with the naked eye in the northern hemisphere, and looks pretty impressive in small telescope, too.

To find it, those in the northern hemisphere will have to wait until cooler weather approaches.

But look for Orion's belt, three bright stars in a row. Hanging south from the belt is Orion's sword, composed of three bright dots; the center dot is the great nebula.

Andromeda Galaxy

A.K.A M31, this beautiful galaxy is another naked eye object that shows up well in small telescopes. To find it, locate the North Star, then the constellation Cassiopeia, which looks like a giant "W" and is directly across the Big Dipper, with the North Star in between the two. Look at the right "V" shape within the larger "W" of Cassiopeia; 15 degrees down from the tip of the 'V' is M31.

Hercules Globular Cluster

It is relatively close, only about 25,000 light-years away and it pretty big –about 150 light-years wide, making it an easy target. Hercules is best viewed from the northern hemisphere in the summer months during a new moon. Locate Hercules by looking for the trademark trapezoidal keystone within the constellation. M13 is the brightest spot on the western side of the shape, about 20 degrees due west of the constellation Lyra.

Crab Nebula

This is the left-overs from a supernova that occurred in the year 1045. Back then it was bright enough to see in the daytime, and now it makes for a great sight at night, but a telescope is required. M1 is located on the southern horn of Taurus, the bull shaped constellation southeast of Orion. The object is best seen using a 200x zoom from the northern hemisphere around midnight.

Whirlpool Galaxy

A.K.A. M51, this is one of the largest galaxies visible without using professional telescope. Millions of years ago two galaxies collided to create this colorful and dramatic object. To find it, look about 3.5 degrees southeast of the last star in the Big Dipper's handle.

Black Holes

Black Holes

Black holes are the cold remnants of former stars, so dense that no matter—not even light—is able to escape their powerful gravitational pull.

While most stars end up as white dwarfs or neutron stars, black holes are the last evolutionary stage in the lifetimes of enormous stars that had been at least 10 or 15 times as massive as our own sun.

When giant stars reach the final stages of their lives they often detonate in cataclysms known as supernovae. Such an explosion scatters most of a star into the void of space but leaves behind a large "cold" remnant on which fusion no longer takes place.

In younger stars, nuclear fusion creates energy and a constant outward pressure that exists in balance with the inward pull of gravity caused by the star's own mass. But in the dead remnants of a massive supernova, no force opposes gravity—so the star begins to collapse in upon itself.

With no force to check gravity, a budding black hole shrinks to zero volume—at which point it is infinitely dense. Even the light from such a star is unable to escape its immense gravitational pull. The star's own light becomes trapped in orbit, and the dark star becomes known as a black hole.

Black holes pull matter and even energy into themselves—but no more so than other stars or cosmic objects of similar mass. That means that a black hole with the mass of our own sun would not "suck" objects into it any more than our own sun does with its own gravitational pull.

Planets, light, and other matter must pass close to a black hole in order to be pulled into its grasp. When they reach a point of no return they are said to have entered the event horizon—the point from which any escape is impossible because it requires moving faster than the speed of light.

Small But Powerful

Black holes are small in size. A million-solar-mass hole, like that believed to be at the center of some galaxies, would have a radius of just about two million miles (three million kilometers)—only about four times the size of the sun. A black hole with a mass equal to that of the sun would have a two-mile (three-kilometer) radius.

Because they are so small, distant, and dark, black holes cannot be directly observed. Yet scientists have confirmed their long-held suspicions that they exist. This is typically done by measuring mass in a region of the sky and looking for areas of large, dark mass.

Many black holes exist in binary star systems. These holes may continually pull mass from their neighboring star, growing the black hole and shrinking the other star, until the black hole is large and the companion star has completely vanished.

Extremely large black holes may exist at the center of some galaxies—including our own Milky Way. These massive features may have the mass of 10 to 100 billion suns. They are similar to smaller black holes but grow to enormous size because there is so much matter in the center of the galaxy for them to add. Black holes can accrue limitless amounts of matter; they simply become even denser as their mass increases.

Black holes capture the public's imagination and feature prominently in extremely theoretical concepts like wormholes. These "tunnels" could allow rapid travel through space and time—but there is no evidence that they exist.

Beyond Any Reasonable Doubt: A Supermassive Black Hole Lives in Centre of Our Galaxy

One the one hand, this might not be surprising news, but on the other, the implications are startling. A supermassive black hole (called Sagittarius A*) lives at the centre of the Milky Way. This is the conclusion of a 16 year observation campaign of a region right in the centre of our galaxy where 28 stars have been tracked, orbiting a common, invisible point.

Usually these stars would be obscured by the gas and dust in that region, but the European Southern Observatory (ESO) in Chile has used its infrared telescopes to peer deep into the black hole's lair. Judging by the orbital trajectories of these 28 stars, astronomers have not only been able to pinpoint the black hole's location, they have also deduced its mass…

It has been long recognised that supermassive black holes probably occupy the centres of most galaxies, from dwarf galaxies to thin galactic disks to large spiral galaxies; the majority of galaxies appear to have them. But actually seeing a black hole is no easy task; astronomers depend on observing the effect a supermassive black hole has on the surrounding gas, dust and stars rather than seeing the object itself (after all, by definition, a black hole is black).

In 1992, astronomers using the ESO's 3.5-metre New Technology Telescope in Chile turned their attentions on our very own galactic core to begin an unprecedented observation campaign. Since 2002, the 8.2-metre Very Large Telescope (VLT) was also put to use. 16 years later, with over 50 nights of total observation time, the results are in.

By tracking individual stars orbiting a common point, ESO researchers have derived the best empirical evidence yet for the existence of a 4 million solar mass black hole. All the stars are moving rapidly, one star even completed a full orbit within those 16 years, allowing astronomers to indirectly study the mysterious beast driving our galaxy.

Apart from being the most detailed study of Sagittarius A*'s neighbourhood (the techniques used in this study are six-times more precise than any study before it), the ESO astronomers also deduced the most precise measurement of the distance from the galactic centre to the Solar System; our supermassive black hole lies a safe 27,000 light years away.

Quite simply, the object influencing these stars must be a supermassive black hole, there is no other explanation out there. Does this mean black holes have an even firmer standing as a cosmological "fact" rather than "theory"? It would appear so…


Sources: ESO, BBC

Invading black holes explain cosmic flashes

Black holes are invading stars, providing a radical explanation to bright flashes in the universe that are one of the biggest mysteries in astronomy today.


The flashes, known as gamma ray bursts, are beams of high energy radiation – similar to the radiation emitted by explosions of nuclear weapons – produced by jets of plasma from massive dying stars.

The orthodox model for this cosmic jet engine involves plasma being heated by neutrinos in a disk of matter that forms around a black hole, which is created when a star collapses.

But mathematicians at the University of Leeds have come up with a different explanation: the jets come directly from black holes, which can dive into nearby massive stars and devour them.

Their theory is based on recent observations by the Swift satellite which indicates that the central jet engine operates for up to 10,000 seconds - much longer than the neutrino model can explain.

Mathematicians believe that this is evidence for an electromagnetic origin of the jets, i.e. that the jets come directly from a rotating black hole, and that it is the magnetic stresses caused by the rotation that focus and accelerate the jet's flow.

For the mechanism to operate the collapsing star has to be rotating extremely rapidly. This increases the duration of the star's collapse as the gravity is opposed by strong centrifugal forces.

One particularly peculiar way of creating the right conditions involves not a collapsing star but a star invaded by its black hole companion in a binary system. The black hole acts like a parasite, diving into the normal star, spinning it with gravitational forces on its way to the star's centre, and finally eating it from the inside.

The Largest Black Holes in the Universe

White Dwarfs

White Dwarfs

These ancient stars are incredibly dense. A teaspoonful of their matter would weigh as much on Earth as an elephant—5.5 tons. White dwarfs typically have a radius just .01 times that of our own sun, but their mass is about the same.


Stars like our sun fuse hydrogen in their cores into helium. White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel.

Fusion in a star's core produces heat and outward pressure, but this pressure is kept in balance by the inward push of gravity generated by a star's mass. When the hydrogen used as fuel vanishes, and fusion slows, gravity causes the star to collapse in on itself.

As the star condenses and compacts it heats up even further, burning the last of its hydrogen and causing the star's outer layers to expand outward. At this stage, the star becomes a large red giant.



Because a red giant is so large, its heat spreads out and the surface temperatures are predominantly cool, but its core remains red-hot. Red giants exist for only a short time—perhaps just a billion years compared with the ten billion the same star may already have spent burning hydrogen like our own sun.




Red giants are hot enough to turn the helium at their core, which was made by fusing hydrogen, into heavy elements like carbon. But most stars are not massive enough to create the pressures and heat necessary to burn heavy elements, so fusion and heat production stop.

White Dwarf Types

Further Incarnations



Such stars eventually blow off the material of their outer layers, which creates an expanding shell of gas called a planetary nebula. Within this nebula, the hot core of the star remains—crushed to high density by gravity—as a white dwarf with temperatures over 180,000 degrees Fahrenheit (100,000 degrees Celsius).



Eventually—over tens or even hundreds of billions of years—a white dwarf cools until it becomes a black dwarf, which emits no energy. Because the universe's oldest stars are only 10 billion to 20 billion years old there are no known black dwarfs—yet.



Estimating how long white dwarfs have been cooling can help astronomers learn much about the age of the universe.




But not all white dwarfs will spend many millennia cooling their heels. Those in a binary star system may have a strong enough gravitational pull to gather in material from a neighboring star. When a white dwarf takes on enough mass in this manner it reaches a level called the chandrasekhar limit. At this point the pressure at its center will become so great that runaway fusion occurs and the star will detonate in a thermonuclear supernova.




The life of star

Star

Size Of Planets and Stars to Scale




What is a star?
A star is a massive, luminous ball of plasma that is 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. Historically, the most prominent stars on the celestial sphere were grouped together into constellations, and the brightest stars gained proper names.

How a star emits light?

For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.

The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.

How a star is formed?

The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shock waves from supernovae (massive stellar explosions) or the collision of two galaxies (as in a starburst galaxy). Once a region reaches a sufficient density and it begins to collapse under its own gravitational force.

Diameter

Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.

The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required in order to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.

Stars range in size from neutron stars, which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun about 0.9 billion kilometres. However, Betelgeuse has a much lower density than the Sun.

Star system

The most current estimates guess that there are 125 to 200 billion galaxies in the Universe, each of which has hundreds of billions of stars. A recent German supercomputer simulation put that numbe...

A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems. Star system may also be used to refer to a system of a single star together with a planetary system of orbiting smaller bodies.

Binary star systems

A stellar system of two stars is known as a binary star, binary star system or physical double star. If there are no tidal effects, no perturbation from other forces, and no transfer of mass from one star to the other, such a system is stable, and both stars will trace out an elliptical orbit around the center of mass of the system indefinitely.

Multiple star systems

Multiple star systems or physical multiple stars are systems of more than two stars. Multiple star systems are called triple, trinary or ternary if they contain three stars; quadruple or quaternary if they contain four stars; quintuple with five stars; sextuple with six stars; septuple with seven stars; and so on. These systems are smaller than open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars.

Observation:

Most multiple star systems known are triple; for higher multiplicities, the number of known systems with a given multiplicity decreases exponentially with multiplicity. Because of the dynamical instabilities mentioned earlier, triple systems are generally hierarchical: they contain a close binary pair which has a more distant companion. Systems with higher multiplicities are also generally hierarchical. Systems with up to six stars are known; for example, Castor (Alpha Geminorum), which consists of a binary pair in a distant orbit of two closer binary pairs. Another system known with six stars is ADS 9731, which consists of a pair of two triple systems, each of which is a spectroscopic binary in orbit together with a single star.

Size Of The Universe

The Universe is really big. Take a tour to see how big is the Universe



The Universe comprises everything that physically exists, the entirety of space and time, all forms of matter and energy, and the physical laws and constants that govern them. However, the term Universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or Nature.

It may seem impossible that two galaxies on opposite sides can be separated by 93 billion light years after only 13 billion years, since special relativity states that matter cannot be accelerated to exceed the speed of light in a localized region of space-time. However, according to general relativity, space can expand with no intrinsic limit on its rate; thus, two galaxies can separate more quickly than the speed of light if the space between them grows. It is uncertain whether the size of the Universe is finite or infinite.

This high-resolution image of the Hubble ultra deep field, from the Hubble space telescope includes galaxies of various ages, sizes, shapes, and colors. The smallest, reddest galaxies, about 100, are some of the most distant galaxies to have been imaged by an optical telescope, existing at the time shortly after the Big Bang.

How Big is Our Universe?

The universe is a big, big place. But how big? And how do we know?


Throughout history, humans have used a variety of techniques and methods to help them answer the questions 'How far?' and 'How big?' Generations of explorers have looked deeper and deeper into the vast expanse of the universe. And the journey continues today, as new methods are used, and new discoveries are made.

Image the sun: Our sun, the nearest star, is 93 million miles away. That's why the sun, which is a million times the size of the Earth, looks so small. It would take the Space Shuttle seven months to fly there.

When we leave the solar system, we find our star and its planets are just one small part of the Milky Way galaxy. The Milky Way is a huge city of stars, so big that even at the speed of light, it would take 100,000 years to travel across it. All the stars in the night sky, including our Sun, are just some of the residents of this galaxy, along with millions of other stars too faint to be seen.

The further away a star is, the fainter it looks. Astronomers use this as a clue to figure out the distance to stars that are very far away. But how do you know if the star really is far away, or just not very bright to begin with? This problem was solved in 1908 when Henrietta Leavitt discovered a way to tell the 'wattage' of certain stars that changed their pulse rate linked to their wattage. This allowed their distances to be measured all the way across the Milky Way.

Image above: How Big is the Milky Way? Imagine that our entire Solar System were the size of a quarter. The Sun is now a microscopic speck of dust, as are its nine planets, whose orbits are represented by the flat disc of the coin. How far away is the nearest star to our sun? In our model, Proxima Centauri (and any planets that might be around it) would be another quarter, two soccer fields away. This is the typical separation of stars in our part of the galaxy.

Beyond our own galaxy lies a vast expanse of galaxies. The deeper we see into space, the more galaxies we discover. There are billions of galaxies, the most distant of which are so far away that the light arriving from them on Earth today set out from the galaxies billions of years ago. So we see them not as they are today, but as they looked long before there was any life on Earth.

Finding the distance to these very distant galaxies is challenging, but astronomers can do so by watching for incredibly bright exploding stars called supernovae. Some types of exploding stars have a known brightness - wattage - so we can figure out how far they are by measuring how bright they appear to us, and therefore how far away it is to their home galaxy.

The picture on the above was taken three weeks after the one on the left. In that time, a star at the edge of one of these distant galaxies has exploded -- "gone supernova." Can you spot the supernova in the picture at right? Even though the explosion is as bright as a billion suns, it is so far away that it is just a speck of light.

The image below is both the oldest and youngest picture ever taken. It is the oldest because it has taken the light nearly 14 billion years to reach us. And it is the youngest because it is a snapshot of our newborn universe, long before the first stars and galaxies formed. The bright patterns show clumps of simple matter that will eventually form stars and galaxies. This is as far as we can see into the universe. It is time, not space, which limits our view. Beyond a certain distance, light hasn't had time to reach us yet.

Image above: What is the furthest we can see? In 2003, NASA's WMAP satellite took images of the most distant part of the universe observable from Earth. The image shows the furthest we can see using any form of light. The patterns show clumps of matter that eventually formed into galaxies of stars.

So how big is the universe? No one knows if the universe is infinitely large, or even if ours is the only universe that exists. And other parts of the universe, very far away, might be quite different from the universe closer to home. Future NASA missions will continue to search for clues to the ultimate size and scale of our cosmic home.

Origins of the Universe

About a half a million years after the big bang, the cosmos began to cool, as it cooled, the hot subatomic particles that constituted the early universe began to slow down and stop colliding with