A star is an enormous astronomical body in space that produces a tremendous amount of light and other forms of energy (i.e. heat in the form of infrared radiation). The sun and most other stars are made of gas and a hot, gas-like substance known as plasma. But some stars, called white dwarfs and neutron stars, consist of tightly packed atoms or subatomic particles. These stars are therefore much more dense than anything on Earth.
A star has five main characteristics: (1) brightness, which astronomers describe in terms of magnitude or luminosity; (2) color; (3) surface temperature; (4) size; and (5) mass (amount of matter). Stars are largely classified by their size and intensity, of which there are many. The sun's radius (distance from its center to its surface) is about 432,000 miles (695,500 kilometers). For that reason, and its specific spectrum, astronomers class the Sun as a Main Sequence Star. Some of the stars known as supergiants have a radius about 1,000 times that of the sun. The smallest stars are the neutron stars, some of which have a radius of only about 6 miles (10 kilometers).
About 75 percent of all stars are members of a binary system, a pair of closely spaced stars that orbit each other. The sun is not a member of a binary system. However, its nearest known stellar neighbor, Proxima Centauri, is part of a multiple-star system that also includes Alpha Centauri A and Alpha Centauri B.
The distance from the sun to Proxima Centauri is more than 25 trillion miles (40 trillion kilometers). This distance is so great that light takes 4.2 years to travel between the two stars. Scientists say that Proxima Centauri is 4.2 light-years from the sun. One light-year, the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46 trillion kilometers).
Stars are grouped in huge structures called galaxies. Telescopes have revealed galaxies throughout the universe at distances of 12 billion to 16 billion light-years. The sun is in a galaxy called the Milky Way that contains more than 100 billion stars. There are more than 100 billion galaxies in the universe, and the average number of stars per galaxy may be 100 billion. Thus, more than 10 billion trillion stars may exist. But if you look at the night sky far from city lights, you can see only about 3,000 of them without using binoculars or a telescope.
Stars, like people, have life cycles -- they are born, pass through several phases, and eventually die. Stars like the sun will grow to become a red giant. Late in the sun's lifetime, it will cast off its outer layers. The remaining core, called a white dwarf, will slowly fade to become a black dwarf. Other stars will end their lives in different ways. Some will not go through a red giant stage. Instead, they will merely cool to become white dwarfs, then black dwarfs. A small percentage of stars will die in spectacular explosions called supernovae.
- Main article: Star formation
The sun is conventionally considered a star, although the creation account of Genesis mentions the stars secondarily after recording the creation of the sun on day four. Evolutionists assert that star formation or change must take millions of years to take place, but star alteration has been observed to take place in a few years' time.These still widely held views of modern science are actually founded in theories of stellar evolution that are primarily based on clues obtained from studies of the stellar spectra related to luminosity.
God made two great lights--the greater light to govern the day and the lesser light to govern the night. He also made the stars. God set them in the expanse of the sky to give light on the earth, to govern the day and the night, and to separate light from darkness. And God saw that it was good. Genesis 1:16-18
Stars are generally associated with the night, since the light of the sun usually supersedes that of the far more distant stars. The closest star system outside our own, Proxima Centauri (or Alpha Centauri C), is believed to be a distance of over four light years from earth’s solar system. (Uniformitarian astronomers often hypothesize an even closer star to our solar system. They call this star Nemesis and credit it with occasionally stirring comets to leave the Oort cloud and start their long-period elliptical orbits.)
- Main Article: Stellar measurement
The oldest method of measuring the distance from our solar system to a distant star is the parallax method. To use this method, astronomers measure the right ascension on the sky of the star at two times of the year, half a year apart. The two measurements will differ by a small angle with respect to the most distant stars in that region of the sky. Exactly half this angle is the parallax angle. This is the angle that the star makes with the sun and the position of the earth at a right angle with that star.
However, the error of measurement of parallax angle is 0.005 arc seconds, and beyond a distance of 100 parsecs (about 330 light years), this error becomes significant. 700 stars are near enough to measure their distances directly by using parallax. To measure distances further out than this, astronomers typically use absolute and relative magnitudes, or they apply Hubble's Law to the star's estimated redshift.
Positions and movements
The most common system for describing the position of a star in the sky is the equatorial system. This system uses two coordinates:
- Right ascension on the sky, or the number of hours required for the earth to rotate before an observer can see the star at its highest point in the sky. The zero for right ascension is midnight on the day of the vernal equinox.
- Declination, or the north-south angle between the star and the celestial equator.
All stars move, but the most distant stars are considered "fixed" because their motion would be undetectable. The proper motion (symbol m) of any star is the angular velocity of its position across the sky. This describes the motion at right angles to the line of sight of the observer. To convert this to actual tangential velocity, multiply the tangent of this angular velocity by the star's distance.
The motion in line of sight, or radial velocity, is currently determined from spectral shift.
The visual magnitude system is defined as follows: a star of any given magnitude is about 2.512 times as bright as is a star of the next magnitude. Hipparchus devised the magnitude system, and Ptolemy refined it further. By convention, an arbitrary sample of the twenty brightest stars that they could observe were assigned to the first magnitude, and the stars that they could barely observe were assigned to the sixth.
Colors and spectra
The color of a star is objectively quantifiable. To determine color, astronomers view the star through a variety of colored filters and compute color indices as the differences in apparent magnitudes through the various filters. Stellar colors vary, in order from the coolest to the hottest, from red to yellow to white to blue-white to blue or violet. This is the same gamut of colors that a black body shows as its temperature rises.
In addition, each star has a unique spectrum, which depends on the gases and other elements that it contains, and their distribution. A spectrum can serve two purposes:
- It can serve as a unique signature for the star, to distinguish it from other stars.
- It can provide information on the star's radial velocity vis-à-vis the earth.
To accomplish the latter, astronomers note the placement of various lines in the spectrum and then determine the star's likely constituent elements from the spacing of those lines. Lines that are out of place are shifted, either toward the blue or toward the red. Nearly all stellar spectra are shifted toward the red; this redshift indicates a recession, either of the star or of the part of space where the star resides. 
- Main Article: Stellar classification
In the late nineteenth century, astronomers at the Harvard University observatory developed the first classification scheme for stellar spectra that would become known as the Harvard spectral classification. In 1924, Annie Jump Cannon refined the classification from the original A-Q gamut to the familiar "OBAFGKM" gamut. Astronomers have since added classes to this range at the high end and the low.
The classic Harvard spectral classes are O, B, A, F, G, K, and M. Each of these has ten subclasses, varying from 0 to 9 in order of decreasing stellar temperature. Thus, for example, the next class after an F9 star is a G0 star. Recently astronomers recognized one class of stars hotter than the O stars (the very hot Wolf-Rayet stars) and three classes of stars (the N, R, and S stars) cooler than the M stars. (Some astronomers include the N and R stars in one class, the C stars, for the carbon compounds that their spectra exhibit.)
In addition to the spectral type, astronomers today add a luminosity class, which varies from I to VI in order of decreasing brightness. The sun's spectral type is G2 and its luminosity class is V (five).
|W||106,000 K||Violet||Ionized helium, carbon, oxygen, nitrogen||Wolf-Rayet stars. Additional subclasses include WC (overabundant carbon and oxygen) and WN (overabundant nitrogen)|
|O||30,000 K||Blue||Ionized Helium, nitrogen, oxygen||Weak Balmer lines (hydrogen) at higher subclasses.|
|B||13,000 K to 20,000 K||Blue||Neutral helium; ionized silicon, oxygen and magnesium.||Hydrogen (Balmer lines) appear in strength|
|A||75,00 to 10,000 K||Blue-white||Hydrogen, calcium, helium||Balmer lines dominant. K lines (calcium) now appearing.|
|F||7,000K to 9,000K||White-yellow||Hydrogen, calcium, iron, manganese, sodium||Balmer lines weakening. K lines stronger.|
|G||5,200 to 6,000K||Yellow||Calcium, hydrogen, other metals||Balmer lines weaker still. K lines dominant. Metals now appearing.|
|K||4000K to 5100K||Orange||Calcium, neutral metals, titanium oxide|
|M||3000K||Red||Titanium oxide, iron iodide||Strong molecular bands|
|N,R||2300K to 2600K||Red||Carbon compounds|
|S||2300K to 2600K||Red||Hydrogen, zirconium oxide|
In the early twentieth century, astronomers Ejnar Hertzsprung and Henry Norris Russell prepared the first plot of stellar temperature as a function of luminosity, or brightness. Other astronomers have since prepared versions of the diagram showing absolute magnitude as a function of color. This diagram shows a "main sequence" of stars for which brightness declines as temperature increases, but also shows a "white dwarf" population of very hot but dim stars, and the population of giants and supergiants that are far brighter than their temperatures would indicate.
Along with studies of luminosity, studies of the motion of stars (star kinematics) have been very important for deducing the structure of our Milky Way Galaxy. In 1944, astronomers noted that speed naturally divides stars into groups of low velocity stars (called population I) and high velocity stars (called population II). Since these speeds are relative to the sun, population I stars orbit in the disk of the galaxy as the sun does, whereas population II stars are more associated with the halo of stars above and below the flatter disk of the galaxy. Interestingly, population I stars tend to have a higher metallicity than population II stars, and the bright stars are generally all population I stars.
Faulkner notes that this does fit very naturally in to the evolutionary theory of stellar formation, and needs to be answered by creationists. The reasonableness of the theory in explaining the two populations of stars needs to be investigated by creationists, though creationist concepts such as the white hole cosmology fit in with this kind of evidence.
Recently these assumptions of stellar populations recognized as challenges to creationists by Faulkner has been challenged with recent observations gleaned by Giampaolo Piotto representing the University of Padua in Italy. Piotto led a group of scientists that observed and analysed a globular cluster dubbed NGC 2808. Piotto told the New Scientist that, "NGC 2808 was just considered a normal globular cluster and no one was expecting this ability to see three distinct stellar populations..." adding that, "This result says globular cluster stars are not as simple as we are teaching to our students." The stars that are problematic are found to be Helium rich and not population III stars.
Faulkner also notes that the Big Bang cosmology presumes that there should have been a group of "population III" stars that have no metals. Searches have found some remnants of such populations but present another difficulty in that it was found in a globular cluster with the two other populations. This represents massive complexity that the Big Bang has trouble explaining. The theoretical difficulty of how these first stars could have formed is also one which the Big Bang has trouble.
Studies of star formation found that a cloud of dust and gas will not normally condense into a star because any slight heating causes the dust and gas to disperse faster than gravity can draw it in. Thus, stars are thought to begin when the pressure wave of a supernova passes through a cloud of gas that is ready to condense. This means it takes stars (or at least something like a supernova) to make stars. This reminds some creationists of trees in a forest. Trees can reproduce themselves if you have trees to start with. If stars need stars to form, how did the first stars form?
The Sun, and stars as massive as the Sun or less massive, commonly use a nuclear fusion process called the proton-proton chain reaction to produce energy. A full description of that process appears here.
In 1938 and 1989, two physicists, Carl F. von Weizsäcker and Hans Bethe independently proposed a nuclear fusion process, the Carbon-Nitrogen-Oxygen cycle, by which stars more massive than the sun produce energy. In this process, stars convert hydrogen to helium using carbon, nitrogen, and oxygen as catalysts. The reaction also produces two positrons and two electron neutrinos.
The equations for the cycle are as follows:
The last reaction reproduces the nucleus that the first reaction consumes. The end result of this process is:
Rarely, this cycle branches into a somewhat different cycle involving fluorine, and that second cycle is thought to branch again in some of the most massive stars.
- Star cluster's triple baby boom puzzles astronomers "Astronomers are puzzling over a strange, ancient star cluster that hosts three generations of stars instead of the normal one."
- V838 Mon: Mystery Star "Observations indicate that the erupting star transformed itself over a period of months from a small under-luminous star a little hotter than the Sun, to a highly-luminous, cool supergiant star undergoing rapid and complex brightness changes. The transformation defies the conventional understanding of stellar life cycles."
- World Book at NASA: Star by NASA (United States).
- Speedy star changes baffle long-agers Creation 19(4):7–9. September 1997
- "Star: Determining stellar distances." Encyclopedia Britannica. 2008. Encyclopedia Britannica Online. Accessed 21 Apr. 2008
- Weisstein, Eric W. "Right Ascension." Eric Weisstein's World of Astronomy, 2007. Accessed April 21, 2008.
- Weisstein, Eric W. "Declination." Eric Weisstein's World of Astronomy, 2007. Accessed April 21, 2008.
- Haworth, David. "Star Magnitudes." Observational Astronomy, 2003. Accessed April 21, 2008.
- Some cosmological models call for an expansion of space itself, not merely the matter in it. According to these models, a redshifted star is in a part of space that was still expanding as the incident light was generated.
- "Life Cycles of Stars." Goddard Space Flight Center, November 21, 2002. Accessed April 22, 2008.
- "Harvard Spectral Classification." Study Astronomy Online at Swinburne University. Accessed April 22, 2008.
- Irizarry, David. "The Secrets of the Harvard Classification Revealed." The Webfooted Astronomer, Seattle Astronomical Society, February 2000. Accessed April 22, 2008.
- "Hertzsprung-Russell Diagram." Study Astronomy Online at Swinburne University. Accessed April 22, 2008.
- The Role of Stellar Population Types in the Discussion of Stellar Evolution by Danny Faulkner. CRSQ 30(1):8-11, June, 1993.
- Star cluster's triple baby boom puzzles astronomers by David Shiga. NewScientist news service. 02 May 2007
- Rogue hot stars discovered in 'boring' clusters New Scientist magazine. Sep. 2006. issue 2567 p.15
- CNO cycle by Wikipedia
- Von Weizsäcker, Carl F. Physik. Zeitsch. 39:633, 1938.
- Bethe, Hans A. "Energy Production in Stars." Physics Review 55(5):434-456, 1939. doi:10.1103/PhysRev.55.434 Accessed June 27, 2008.
- Krane, Kenneth S. Introductory Nuclear Physics. New York: John Wiley and Sons, 1988, p. 537. ISBN 9780471805533
- Evolution of Stars Cartage
- The First Stars, by Volker Bromm and Richard Larson. Ann Rev Astron Astrophysics 2004: 42