"Mountains Of Creation"

This majestic false color image from NASA's Spitzer Space Telescope shows the "mountains" where stars are born. Dubbed "Mountains of Creation" by Spitzer scientists, these towering pillars of cool gas and dust are illuminated at their tips with light from warm embryonic stars. The pillars in this Spitzer image are part of the Cassiopeia constellation 7,000 light-years away and 50 light-years across. The red color in the image represents minute organic hydrocarbons. These building blocks of life are often found in star forming clouds of gas and dust. Like small dust grains, they are heated by the light from the young stars, then emit energy at infrared wavelengths. For a full image description, visit http://photojournal.jpl.nasa.gov/catalog/PIA03096.

Pre-star Nebula (Dust Clouds)

Dark Nebula are interstellar clouds that are so dense that they block all light from stars and other sources of visible radiation that are behind them. The blocking of all visible light is caused by a super large, dense collection of dust grains located in the coldest parts of interstellar clouds. However, they "can" be studied using infrared and radio frequency techniques.

Dark Nebula are a collection of sub-micrometer (less than a a millionth of a meter) sized dust particles, coated with frozen carbon monoxide and nitrogen. They block all light at visible wavelengths. The shape of Dark Nebula is very irregular, they have no clearly defined boundaries. Dark Nebula are the spawning grounds of stars and planets.

Bok Globules are small interstellar clouds of very cold gas and dust, i.e. small Dark Nebula. See the image at the left of Thackeray's Globules, a set of Bok Globules in IC 2944. Bok Globules were originally described as black splotches in front of dense fields of stars by Bart Bok of the University of Arizona in the 1940s. Bok Globules typically have a mass of about 2 to 50 solar masses within a light year or so end to end.

Bok globules are relatively isolated and often contain cores of new stars. An analysis of infrared observations published in 1990 confirmed that stars were being born inside Bok Globules. The nearest Bok Globules are about four times closer than the closest giant Dark Nebula complexes. This allows more detailed observation of the forming of new stars.  Top

Star Formation

In Cosmology, the "Solar Nebular Disk Model" (SNDM) is the most widely accepted model explaining the formation and evolution of our sun and solar system. This model is now being applied to star and planet formation across the universe. According to the Nebula Model, stars form in massive, dense clouds of hydrogen. See the actual Orion Nebula proto-planetary dense cloud system to the left.

Stars emerge from clouds of gas and fine dust, in regions which have collapsed under the effects of their own gravity. Eventually, dense hot cores form and ignite into stars. At first, these young stars are enveloped in the remaining gas and dust, which eventually settles into a disc, or proto-planetary disc. Over time, the dust particles will stick together, growing into clumps the size of sand grains and pebbles. They eventually grow into asteroids, comets and planets.

The center of the proto-star keeps growing denser and hotter until thermonuclear fusion begins. A new star is now born. Hydrogen atoms then begin to combine to form helium atoms, which releases the energy that makes the star begin to shine.

 

Here is a more detailed star formulation explanation:

• An interstellar cloud (nebula) is normally several light years across with a net rotation direction. A small random clump in the cloud causes a contraction to begin. The clump begins to grow and eventually produces a run-away process.

• Initially, most of the motions of the cloud particles are random, but the nebula has a net overall rotation direction. As the collapse proceeds, the rotational speed of the smaller cloud gradually increases due to the conservation of angular momentum which has to be maintained. (The net rotational energy of the dust and other materials is transferred to the newly formed small cloud causing it to spin faster and faster.)

• The gravitational collapse is much more efficient along the spin axis, so the rotating ball collapses into a thin disk with a diameter of about 200 AU (one astronomical unit is the distance from the earth to the sun) with most of the mass concentrated near the center. As the cloud contracts, its gravitational potential energy is converted into kinetic energy of the individual gas particles. Collisions between particles converts this energy into heat. The solar nebula becomes hottest at the center where most of the mass is condensing to form the proto-sun (the cloud of hot gas that eventually becomes a sun). At some point the central temperature rises to about 10 million degrees K. The collisions among the atoms are so violent that nuclear reactions begin. At this point a sun is born as a new star.

What prevented further star collapse? As the central temperature and density increased, so did the internal pressure resulting in a force pushing outward. After about 50 million years, the new star reached a balance between the gravitational forces pulling inward and the internal pressure pushing outward. This equilibrium of forces determined the diameter of the star. A sun-like star usually takes about a 100 million years to form.   Top

Oldest Stars In The Universe

In February, 2014 the international media trumpeted the discovery by Australian scientists of the oldest star in the universe - SM 0313. SM 0313, shown at he left, was born about 13.6 billion years ago - just 100 to 200 million years after the Big Bang (13.8 billion years ago). Remarkably, it is located right in our own Milky Way just 6,000 light years away. Scientists suggest that SM 0313 is an elusive Population II star - a star that was formed in pristine gas from the remnants of one of the universe’s very first supernova explosions.

However, only six months earlier, HD 140283 the "Methuselah Star", whose estimated age was 14.5 billion years, was touted as the oldest star in the universe. That age would make HD 140283 older than the universe, but the uncertainty in the age calculation of plus or minus 800,000 years, would bring it back into line with current cosmological measurements of the universe's age of 13.8 billion years.

The Methuselah star has lived through many changes over its long life. It was likely also born in a very early dwarf galaxy. The dwarf galaxy eventually was gravitationally shredded and sucked in by our emerging Milky Way about 12 billion years ago. The star retains its very elongated orbit from that cannibalistic merger. It is passing through our local solar neighborhood at a rocket speed of 800,000 miles per hour. The Methuselah star has been known for more than a century because of its fast motion across the sky. The high rate of speed is evidence that the star is only a visitor to our neighborhood. Its orbit carries it down through the plane of our galaxy from the ancient halo of stars that encircle the Milky Way. It will eventually return to that halo.

The discovery of SM 0313 may be the oldest star known in the universe, but given the uncertainties involved, maybe it isn't. To understand the age of stars, we have to understand how they were formed and how they have evolved. Realizing the complexities of the physics involved, the uncertainties of the early universe, and other factors in the calculations, precise dating is very difficult and should be taken as an educated estimate. So, the best that can be said at this time is that "SM 0313 and HD 140283 are the two oldest known stars". Older stars may be discovered going forward and estimates can change with new technology and more early Cosmos knowledge.  Top

Star Birth Is Past Its Peak

The first stars flickered to life some 500 million years after the Big Bang.  Stars burst into being slowly at first, but the birth rate picked up as the earliest stars exploded and as gravity brought more primordial gas together.  The rate of star formation peaked about 12 billion billion years ago.  See the graph to the left from the University of Virginia.

In the last few billion years, star births have tailed off drastically.  A recent sky survey shows the rate of new star formation in the universe is just 3% of its peak and still declining.  The reason for this fall off is that the primordial gas in the early universe has largely been consumed by new stars over the years.

Our Milky Way is quite unique in the universe because it is still relatively rich in gas and dust.  Most star formation now occurs in the spiral arms of spiral galaxies like our Milky Way. But over the coming billions of years, most spirals will eventually merge into elliptical galaxies, a process which will strip away most gas and dust and result in very low rates of star formation.

But stars will not totally disappear.  Massive bright blue and white stars will die off rather quickly.  But low-mass red stars, which make up the majority of the 10^21 stars in the universe, use up their internal fuel very slowly.  Astronomers calculate these low-mass stars will burn for hundreds of billions of years.  So the universe is in no danger of going dark any time soon. 

Also, consider that when the universe went through intense periods of star formation in the distant past, only about 10% of the gas that made up star forming regions actually got locked up in stars. The remaining 90% got blown back into the interstellar medium where it will someday form stars again in the future. Furthermore, most of the stars that form will eventually die in either a supernova or a planetary nebula, returning a huge fraction (perhaps half of a star’s worth) of unburned fuel back into the interstellar medium. This unburned fuel will be on top of the large gas fraction that never formed stars during the initial starburst.

So, star formation is not going to drop to zero in the near future, it will continue on a low rate compared to the past. But, if you sum up the total number of new stars in the universe for many hundreds of billions of years into the future (not just 14 billion in the past), it will be far greater than the number of stars that already exist up until this point in time.  Top

How Do Stars Die?

Stars Like the Sun.  When the core of a star runs out of hydrogen fuel, it will contract under the weight of gravity. However, some hydrogen fusion will still occur in the upper layers. As the core contracts, it heats up. This heats the upper layers, causing them to expand. As the outer layers expand, the radius of the star will increase and it will become a red giant. The radius of the red giant sun will be just beyond the Earth's orbit.

At some point after this, the core will become hot enough to cause the helium to fuse into carbon. When the helium fuel runs out, the core will expand and cool. The upper layers will expand and eject material that will collect around the dying star to form a planetary nebula. Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire process will take a few billion years.

Stars More Massive Than the Sun.  When the core runs out of hydrogen, these stars fuse helium into carbon just like the Sun. However, after the helium is gone, their mass is enough to fuse carbon into heavier elements such as oxygen, neon, silicon, magnesium, sulfur and iron. Once the core has turned to iron, it can burn no longer. The star collapses by its own gravity and the iron core heats up.

The core becomes so tightly packed that protons and electrons merge to form neutrons. In less than a second, the iron core, which is about the size of the earth, shrinks to a neutron core with a radius of about 6 miles. The outer layers of the star fall inward on the neutron core, thereby crushing it further. The core heats to billions of degrees and explodes into a supernova (see the death of star NG 1281903 in the Butterfly Nebula above). The explosion releases large amounts of energy and material into space. The remains of the core can form a neutron star or a black hole depending upon the mass of the original star. See the Supernova Page for a lot more information on Supernovas.  Top

Planet Formation

Star formation is a complex process, which is believed to always produce a gaseous proto-planetary disk around a young star. The proto-planetary disk is a thin accretion disk which proceeds to feed a lot of mass to the young central star. Initially very hot, the disk later cools forming small dust grains of tiny rocks and ice. These grains eventually coagulate into kilometer-sized planetesimals.

The process of an accretion disk forming planets is much different from an accretion disk surrounding a black hole. In the planet context, accretion refers to the process of cooled, solidified grains of dust and ice orbiting the proto-star, colliding and sticking together and gradually growing.

If the disk is massive enough runaway accretion begins, resulting in the rapid (100,000 to 300,000 years) formation of moon-sized planetary embryos. This includes the high energy collisions between sizable planetesimals that combine to ultimately form full size planets. See the fantastic photo of an actual early planet formation in the accretion disk around the young star HL Tauri at the left.

Careful observation of accretion disks shows that dust grains grow to about one centimeter in a thousand years. The accretion process continues to grow the embryos into one kilometer (km) planetesimals and then into 1,000 km sized bodies. Near the star, the planetary embryos go through a stage of violent mergers, producing a few "rocky" planets like earth. This last stage takes around 100 million to a billion years. Some planets beyond the "frost line" (where planetary embryos are mainly made of ice) experience runaway accretion which results in very large "icy" planets like Jupiter and Saturn. Accretion stops when the surrounding gas clouds are completely exhausted.

The star formation process results in planet type gaseous accretion disks around all young stellar embryos. About one million years old, 100% of all stars have accretion disks. This conclusion is supported by the gaseous, dusty disks observed around early proto-stars as well as by theoretical analysis.  Top

Why Is The "Solar Nebular Disk Model" Believable?

A model of solar system formation must take into account the following confirmed facts:

• All the orbits of the planets revolve in a counter-clockwise direction and all the planets (except Pluto) have orbital planes that are in the same central plane within six degrees.

• The inner planets are dense, rocky and small, while the outer planets are gaseous and large.

Evidence supporting the Solar Nebular Disk Model (SNDM) is as follows:

• All the planets orbit the sun in the same direction and in the same plane. Most of their moons also orbit in the same direction. This would be as expected if they all formed from a thin disk of debris rotating rapidly around a proto-sun as the SNDM specifies.

• The planets also have the right characteristics to have formed from a disk of mainly hydrogen around a young, hot sun. The inner planets (Mercury, Venus, Earth, and Mars) have very little hydrogen in them as the disk would have been far too hot for hydrogen to condense when they formed. Even if the inner planets had hydrogen and helium, their proximity to the sun would have heated up the gases and caused them to escape.

  The outer planets (Jupiter, Saturn, Uranus, and Neptune) are mainly hydrogen, since hydrogen was the main component of the disk. The outer planets are much more massive because there was so much more material in the outer rings of the disk. The larger outer planets are beyond the "frost line" which made them icy, cold and gaseous as opposed to rocky and small like the inner planets.

  There are also five dwarf planets orbiting the Sun ( Pluto, Ceres, Eris, Haumea, and Makemake). Pluto was added to the list of dwarf planets in 2006, based on the fact that its mass is smaller than Eris, one of the dwarf planets. (Some astronomers believe that Pluto should belong to the comet family because it is so small.)  Top

The Asteroid Belt

Between the orbits of Mars and Jupiter, is a region called the Asteroid Belt (the white area in the image to the left). The Asteroid Belt is a ring of millions of relatively small rocky objects. The smallest are the size of small pebbles, while the largest known asteroid, Ceres, is about 600 miles in diameter. But, even Ceres is small compared to the planets of our solar system (about 500 Ceres sized objects could fit inside the earth). Most asteroids in the Asteroid Belt have an orbital period of 3 to 6 earth years, which means that it takes these asteroids 3 to 6 times longer than earth to make a trip around the sun.

What is the difference between a comet, an asteroid, and a meteoroid? A comet is a chunk of ice and rock from the outer Solar System that has a long gaseous tail. An asteroid is a rock that comes from between Jupiter and Mars and can vary greatly in size. (See the asteroid Itokawa at the left below photographed by a Japanese spacecraft. It is about 2,000 feet long and about 800 feet wide.) A meteoroid is bigger than a dust grain but smaller than a rocky asteroid. If the meteoroid strikes the earth, it is called a meteorite. A meteor is the gas trail of a space rock as it burns up in the atmosphere (some call it a falling star).

It's important to note that even though there are millions of asteroids, they are spread out over an extremely large area. If you were on an asteroid in our Solar System, it would be very difficult to see another one without a telescope. Today they rarely collide with each other.

Why does the Asteroid Belt exist? The main theory that astronomers suggest is that 4.6 billion years ago, when our solar system was forming, a planet tried to form between Mars and Jupiter. However, Jupiter’s gravitational force was too strong, so the material was unable to coalesce and form a planet. Even if a planet had formed, it would not have been anything to write home about. It is estimated that if you put all the asteroids in the asteroid belt together into one body, they would form an object less than half the size of our moon.

With all these asteroids orbiting around the sun not too far from the earth, it is reasonable that from time to time, the path of our planet and that of an asteroid will cross. In fact, scientists believe that about 65 million years ago, an asteroid about 6 miles across collided with earth and caused the extinction of the dinosaurs. More recently, in 1989 an asteroid that was a quarter-mile in diameter came within 400,000 miles of earth. The asteroid weighed 50 million tons and was traveling at 46,000 miles per hour. Astronomers estimate that the asteroid and the earth passed through the same point in space only six hours apart!  Top

NASA Wants To Mine Asteroids

In the wake of the European Space Agency's successful Philae landing on Comet 67P/C-G (see the photo to the left), NASA is also planning to land on and mine asteroids. NASA has contracted two private space firms, Deep Space Industries and Planetary Resources, to prepare for and ultimately execute missions to land on and mine asteroids for valuable resources.

Although these expeditions could theoretically return to earth with valuable minerals, the financial viability of the concept relies on the prospects of supplying other space missions with assets. If asteroid mining is successful, NASA, with many deep space missions planned in the coming century, would be able to save a lot of time and money.

NASA would benefit by supplying some of those missions (including International Space Station expeditions) with space-stored vital resources such as water and carbon minerals. Water, oxygen, nitrogen, and other materials with low boiling points are hard to come by in space, and are necessary for future space exploration. Water in particular is expensive because it is incompressible and heavy.

The fuel it takes to rocket out of earth's gravity makes launching anything into space exceptionally expensive. By contrast, asteroids have minuscule centers of gravity, making coming and going from them inexpensive. "Right now it costs $17 million per ton to get anything up in orbit," David Gump, vice chairman of Deep Space, told The Boston Globe. "If we can beat the price in 2022, we'll have a big market." Despite sounding like a science-fiction novel, the physics of asteroid mining are not overwhelming. There is a lot of engineering to be done, but many experts believe it is indeed possible.

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