[Science] guide on stellar objects

[vincent]

post facts/theories here relating to objects in the cosmos

Technical aspects of information in this post has been copied from sources given the fact that i don't know much, but i like to share knowledge hence the reason for this thread

I decided to post this for some members here who might be curious about Astonomy and stellar objects and how they work (at least what we know now) Please note that i don't deem myself an expert, i'm very very far from that, i'm a noob, but a curious noob. And with that let us begin :)

I'll start off with what everyone sees the most in the sky. and that is none other than:

Stars - The cosmic furnaces of the universe

Our sun is a star, of course. There are more stars in space than there are grains of sands on the beaches of Earth. And like the citizens of Earth some are big, some are small, some younger than others, some older than others, and some in there middle of there life (like our Sun).

Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. Some stars are relatively close (the closest 30 stars are within 40 parsecs-parsec - distance measurement, 3.3 light-years, 19.8 trillion miles, 33 trillion kilometers) and others are far, far away. Astronomers can measure the distance by using a method called parallax in which the change in a star's position in the sky is measured at different times during the year. Some stars are alone in the sky, others have companions (binary stars) and some are part of large clusters containing thousands to millions of stars. Not all stars are the same. Stars come in all sizes, brightnesses, temperatures and colors. Let's take a closer look at the features of stars.

Stars have many features that can be measured by studying the light they emit.

- temperature

- spectrum or wavelengths of light emitted

- brightness

- luminosity

- size (radius)

- mass

- movement (toward or away from us, rate of spin)

now some stars are cooler while others are hotter, blue stars are hotter where more red stars are cooler. a stars temperature is measured in a unit called a Kelvin (The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined by two facts: zero kelvins is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The Celsius temperature scale is now defined in terms of the kelvin, with 0 ?C corresponding to 273.15 kelvins, approximately the melting point of water under ordinary conditions.

The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin; his barony was in turn named after the River Kelvin, which runs through the grounds of the University of Glasgow.)

Like i stated at the beginning, Stars are furnaces (huge balls of gas)

they are born in large cold (10 kelvins) of dust and gas usually called nebulae (more on that later) thesestellar nurseries> between existing stars of a galaxy

( in our galaxy they are in thearms> (more on that later too)

Steps on how a star is born: ( at least what astonomers know so far)

1. Usually, some type of gravity disturbance happens to the cloud such as the

2. passage of a nearby star or the shock wave from an exploding supernova.

3. The disturbance causes clumps to form inside the cloud.

4. The clumps collapse inward drawing gas inward by gravity.

5. The collapsing clump compresses and heats up.

6. The collapsing clump begins to rotate and flatten out into a disc.

7. The disc continues to rotate faster, draw more gas and dust inward, and heat up.

8. After about a million years or so, a small, hot (1500 degrees Kelvin), dense core forms in the disc's center called a protostar.

9. As gas and dust continue to fall inward in the disc, they give up energy to the protostar, which heats up more

10. When the temperature of the protostar reaches about 7 million degrees Kelvin, hydrogen begins to fuse to make helium and release energy.

11. Material continues to fall into the young star for millions of years because the collapse due to gravity is greater than the outward pressure exerted by nuclear fusion. Therefore, the protostar's internal temperature increases.

If sufficient mass (0.1 solar mass or greater) collapses into the protostar and the temperature gets hot enough for sustained fusion, then the protostar has a massive release of gas in the form of a jet called a bipolar flow. If the mass is not sufficient, the star will not form, but instead become a brown dwarf.

12. The bipolar flow clears away gas and dust from the young star. Some of this gas and dust may later collect to form planets.

Theyoung star> is now stable in that the outward pressure from hydrogen fusion balances the inward pull of gravity. The star enters the main sequence; where it lies on the main sequence depends upon its mass. Now that the star is stable, it has the same parts as our Sun:

core - where thenuclear fusion> reactions occur

radiative zone - where photons carry energy away from the core

convective zone - where convection currents carry energy toward the surface

655 million tons of hydrogen are fused into 650 million tons of helium every second! at a temperature of 15 million degrees C. the missing 5 million tons of matter are converted into 400 trillion trillion watts of energy in the process.

Death of a star> (how, depending on the size/mass of a star) After a tortuous trek ( wrong trek LOC:pp ) the core-generated energy works it's way to the surface and is radiated into spacemosly as light. Midlife crisis will be triggered by a depletion of hypdrogen at the core. Starved for fuel to stoke it's nuclear furnace, it will face an energy crunch. the thermonuclear reactions wil then be transferred to a shell around the core, where hydrogen will still exist, the core will contract, which heat up the surrounding layer of burning hydrogen, accelerating the reactions and producing more energy. It swells in size, this will mark the beggining of the end for our sun and Earth itself as it is consumed by thisRed Giant> (remember the redder a star, the cooler it is. right now our sun is considered yellow) It is estimated that our sun is halfway through it's nuclear fuel.

Clogged with helium the gravitional compression escalates the internal temperature to burn the less efficient helium, rather than hydrogen. The star then gets bigger and the stellar furnace gets is cranked up to higher and higher temperatures. Heavier elements are created and promptly burned: carbon, nitrogen, oxygen, magnesium, and finally

iron. But iron cannot be used as stellar fuel. Iron is as inefficient as stone in a fireplace. the star becomes rotten at the core, choking on waste products of it's fuel consumption. gravity takes over and the core implodes, while the outer layers are blown off in a supernova (more on these later) .

Sun with masses much larger than our sun meet a different fate, they form Black holes. a black hole forms in the same type of supernova fireball that creates neutron stars. If the exploding star exceeds about 10-15 time the mass of the sun

( the exact figure remains uncertain), the remnant imploding core can be more than four solar masses. gravity takes ultimate control, crushing atomic particles into each other with such fury that nothing can stop the infall (not even light).

the matter creates the black hole leaving only the gravitational field (like the Cheshire cat in Alice in Wonderland)

Galaxies> - cities of stars

We live in a galaxy which is a collection of billions to trillions of stars along with dust and gas, all surrounding a common center of gravity. ( it is believed that some galaxies havemassive>black holes in there centers including our very own Milky Way)

Galaxies come in 3 main types or classes Ellipticals. spirals, irregulars( you can refer to the Hubble Sequence for more info on that) Galaxies are the cells of the immense cosmic body we percieve as the universe. Stars, planets, comets, dust, gas and nebulas in profusion are the galactic ingredients. our Milky way Galaxy is a typical spiral galaxy: an enormous disk about 90,000 light years from edge to edge, with a buldge at the center that is believed to be 10,000 light years thick. In our region, 27,000 light years from the center, the galaxy is less than 2,00 light years thick. our sun takes 200 million light years to orbit once around the Galaxy. the glue represents the 200-light year thick region where nebulas and newborn stars are concentrated in the central zone. The sun stays in this zone oscillating above or below it.

Supernova> - a star's corpse

A giant star going supernova releases as much energy as the radiation of 10 billion ordinary stars. There are two possible routes to this end: either a massive star may cease to generate fusion energy in its core and collapse inward under the force of its own gravity, or a white dwarf star may accumulate material from a companion star until it reaches a critical mass and undergoes a thermonuclear explosion. The explosion drives a blast wave into the surrounding space, forming a supernova remnant. One famous example of this process is the remnant of SN1604. Supernova explosions are the main source of all the elements heavier than oxygen, and they are the only source of many important elements.

Planets> - "Wanderers"

Are objects in space that orbit around a star or stars (as recently discovered, planets can orbit binary star systems) they are belived to be formed from collapsing Nebulas from which stars are born. our solar system contains 9 known planets, recently discovered a 10th-ish planet found dubbed 90377 Sedna two main types of planets are classifiedGas Giants> androcky planets> ours is currently in a class of it's own dubbed terrestial planet> for the fact that it harbors life. Planets are believed to be grouped into a solar system which consists of a group of planets orbiting a star(s) these orbits are held by the gravitational pull from a star. The word planet isn't bound to only worlds. Asteroids, Comets and Meteors are considered planets ( A planet- from the Greek πλανήτης, planētēs which means "wanderer" or more forcefully "vagrant, tramp)

comets>Asteroids>Meteor>

More to follow....

Edited July 13, 2005 by ripgut

[j.nudd]

Very interesting, I learned some cool stuff. Thanks :)

[zhangm Supervisor]

I actually know quite a bit about astronomy, considering I've spent several months studying it. Its a very interesting subject, though confusing and intimidating for some.

Just adding some stuff...

Stellar Mass Limits:

Becoming a star is rather difficult. "If we didn't have advanced knowledge that stars exist, science can offer plenty of convincing reasons why stars couldn't possibly form." Molecular clouds are the birthplace of stars. These giant, massive conglomerations of gas may contain enough material to make thousands of stars. Most stars, in fact, are born in clusters, which eventually drift apart as the parent cloud is dispersed. Magnetic fields, and the fact that molecules in the molecular cloud's own internal pressure normally prevent the cloud from collapsing, but occasionally, some force throws the cloud's equilibrium off balance, and small pockets of denser gas form. If these pockets are dense enough, material will begin to collapse into them by virtue of gravitational attraction.

These pockets eventually begin to glow as more and more material falls into the central mass, creating pressure, in turn, creating heat. If the central mass gains enough mass, and passes over 1/10th the mass of our sun, there is sufficient pressure and temperature to begin nuclear fusion, and a star is born. If the central mass cannot pass the threshold of 1/10th solar mass, the result is a brown dwarf.

On the other end of the scale, theoretically, a star cannot go over the mass of 100 suns, as at that point, the starlight alone would literally push away any gas that tried to join the star. Once stars begin undergoing nuclear fusion and generating light, the starlight pushes the surrounding gas cloud away from the stars, revealing them to observers like us. The Orion Nebula (M42) is an example of young stars, pushing their parent cloud away from them.

A star's existence is dependent on its ability to maintain hydrostatic equilibrium. Two primary forces are at work on a star; the collapsing force of gravity, and the explosive force of fusion. As long as these two forces balance each other perfectly, the star continues to shine normally. Near the end of its life, the star gradually loses its struggle to maintain equilibrium. As its source of fuel travels up the Periodic Table, from hydrogen, to helium, to carbon and onwards, the radiative force of fusion weakens. As a star switches from burning helium to burning carbon as its fuel, it swells, as it must burn so much more carbon to obtain energy to maintain equilibrium. Eventually, the star can't produce the energy to overcome gravity. In small to mid-mass stars, the core collapses and the outer layers of the star are expelled out into space, forming a planetary nebula. The hot carbon core remaining is known as a Wolf-Rayet star. For stars with a mass several times greater than our sun's, outer layers collapse far more forcefully onto the dense central core. As this wall of matter falls onto the core, it bounces, creating a shockwave of intense energy, literally blowing the star apart in what we know as a supernova. The remnant can either be a neutron star, or a black hole, depending on the original star's mass.

Without supernovae, we would not exist. Aside from releasing shockwaves that cause molecular clouds to collapse and create new stars, supernovae are responsible for spreading heavy elements out into space. If it weren't for supernovae, the Periodic Table would simply cease at carbon, and life as we know it would not exist.

[vincent]

White dwarfs

The star continues to collapse until the matter becomes so tightly packed that it becomes degenerate matter. A fundamental principle of quantum theory, the uncertainty principle, implies that individual paricles have uncertain locations. Only the region of space that they occupy can ever be ascertained. In a very crude analogy, we can concieve of an underlying motion, and therefore a pressure, possessed by elemantary paricles. A more precise definition requires us to consider the exclusion principle of quantum mechanics, which limits the number of electrons ina particular atomic level. At sufficient high densities, this limit is approached, which results ina pressure that is purely a quantum. mechanical concept. Unlike ordinary gas pressure, it depends not on temperature but on positional uncertainties of the atoms. Under normal circumstances, thisdegeneracy pressure is negligible, compared with ordinary pressure. When the matter is compressed to enormous densities, however, degeneracy pressure becomes increasingly important. the first particles to be affected are the electrons, which provide additional outward pressure when they become degenerate; they therefore resist being squeezed any further. when the stellar core reaches a state in which it is supported by degenerate electron pressure, the starhas to become a white dwarf. A star of 1 solar mass shrinks 100 times in size to become a white dwarf.

Occasionally, a white dwarf in a binary system can accrete mass from a companion star that has evolved into a giant star. The addition of hydrogen- rich fuel results in an explosive mixture, which astronomers observe as a nova. After the explosion, the process can repeat; novae are recurrent objects that increase in luminosity by a factor of 100 or more in the course of a month of so and may repeat every 10 to 20 years.

[vincent]

Neutron stars

More massive stars face a far more catastrophic fate. The initial mass of such a star was sufficiently high to ensure the strong gravitational forces and high central temperatures that result in the formation of an iron core of mass slightly greater than 1.4 solar masses. Once it exhausts the nuclear ful supply, the gravitional force is too strong to be supported by Electron degenerate pressure . the star collapses violently until, at far higher densities, the electrons and protons combine to form neutrons. These neutrons in turn eventually become degenerate. If degenerate neutron pressure can provide sufficient support to halt the collapse, a neutron star is formed A neutron star is typically only a few miles across, yet such a star may contain more mass than the entire sun (extremely heavy!) The neutron star forms from the core of the collapsing star, which must have been quite massive. As the star collapses, and enormous amount of energy in the form of x-rays, gamma rays, and neutrinos is released suddenly; this energy helps to blow off the out envelope of the star already greatly enriched by nucleosynthesis, ina viloent supernova explosion. A supernova resulting from a massive star death is said to of Type II: it's spectrum is characteristically hydrogen rich and it is associated with regions of star formation.

Edited August 26, 2005 by ripgut

[vincent]

Maybe this thread can be stickied? :shifty: along with the maths help and Software thread that ev0| posted?