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[[Image:Moooghj.jpg|300px|right]]'''Stars''' are extremely large, luminous bodies of gas. They are the most obvious features found in the [[universe]].  They are principally composed of [[hydrogen]] that is undergoing nuclear [[fusion]] to become [[helium]]. Our sun, ([[Sol]]), is the nearest star to Earth, at a distance averaging 93 million miles. The Earth orbits the sun in a period of approximately 365.25 days, and this defines the [[year]]. The diameter of the sun, which is a typical star, is about 870,000 miles and its power output is about 10²⁶ watts. The temperature inside the sun is estimated to be in excess of ten million degrees, and this is hot enough for [[nuclear reactions]] to occur.

[[Image:Moooghj.jpg|300px|right]]'''Stars''' are extremely large, luminous bodies of gas. They are the most obvious features found in the [[universe]].  They are principally composed of [[hydrogen]] that is undergoing nuclear [[fusion]] to become [[helium]]. Our sun, ([[Sol]]), is the nearest star to Earth, at a distance averaging 93 million miles. The Earth orbits the sun in a period of approximately 365.25 days, and this defines the [[year]]. The diameter of the sun, which is a typical star, is about 870,000 miles and its power output is about 10²⁶ watts. The temperature inside the sun is estimated to be in excess of ten million degrees, and this is hot enough for [[nuclear reactions]] to occur.

In Genesis, the stars were made in the fourth day<ref>[Genesis 1-8 (Translated)|Gen 1:14]</ref>, and their number is compared to the number of descendants of Abraham<ref>[Genesis 9-16 (Translated)|Gen 15:5]; an earlier count of the number of descendants of Abraham was the number of grains of dust of the Earth (Gen 13:16)</ref>.

In Genesis, the stars were made in the fourth day,<ref>[Genesis 1-8 (Translated)|Gen 1:14]</ref> and their number is compared to the number of descendants of Abraham.<ref>[Genesis 9-16 (Translated)|Gen 15:5]; an earlier count of the number of descendants of Abraham was the number of grains of dust of the Earth (Gen 13:16)</ref>

The [[Bible]] implies that the number of stars is virtually countless<ref>[Jeremiah 27-34 (Translated)|Jeremiah 33:22]; similarly to Genesis, the number of descendants of David is compared to the number of stars and the number of grains of sand</ref>, but for many years this was not accepted.  [[Hipparchus]] in 128 B.C. stated there were 1,026 stars in the sky.  [[Kepler]] in 1600 A.D. did his own count and found the number to be 1,005.  Today, thanks to telescopes (especially the [[Hubble Telescope]]) showing many stars previously too dim to be seen, we are now aware of some 70,000,000,000,000,000,000,000,000 (7*10²⁵) stars.<ref>[http://www.cnn.com/2003/TECH/space/07/22/stars.survey Star survey reaches 70 sextillion</ref>

The [[Bible]] implies that the number of stars is virtually countless,<ref>[Jeremiah 27-34 (Translated)|Jeremiah 33:22]; similarly to Genesis, the number of descendants of David is compared to the number of stars and the number of grains of sand</ref> but for many years this was not accepted.  [[Hipparchus]] in 128 B.C. stated there were 1,026 stars in the sky.  [[Kepler]] in 1600 A.D. did his own count and found the number to be 1,005.  Today, thanks to telescopes (especially the [[Hubble Telescope]]) showing many stars previously too dim to be seen, we are now aware of some 70,000,000,000,000,000,000,000,000 (7*10²⁵) stars.<ref>[http://www.cnn.com/2003/TECH/space/07/22/stars.survey Star survey reaches 70 sextillion]</ref>

==Measuring stellar positions==

==Measuring stellar positions==

where p is measured in seconds of arc.

where p is measured in seconds of arc.

The cotangent of one second (1/3600 of a degree) of arc is approximately 206,264.81. No parallax angle for any star will be larger than one second. Therefore astronomers initially defined a unit of stellar distance, the ''parsec'' (symbol pc), from this relationship. One parsec is the distance corresponding to a parallax angle of one second of arc. Hence:

The cotangent of one second (1/3600 of a degree) of arc is approximately 206,264.81. No parallax angle for any star will be larger than one second. Therefore, astronomers initially defined a unit of stellar distance, the ''parsec'' (symbol pc), from this relationship. One parsec is the distance corresponding to a parallax angle of one second of arc. Hence:

<math>1 pc \approx 206,264.81 AU</math>

<math>1 pc \approx 206,264.81 AU</math>

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<ref name=Cannon>"[http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/LC_main_p8.html Life Cycles of Stars]." ''Goddard Space Flight Center'', November 21, 2002. Accessed April 22, 2008.</ref> 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.<ref name=Swinburne>"[http://astronomy.swin.edu.au/cosmos/H/Harvard+Spectral+Classification Harvard Spectral Classification]." ''Study Astronomy Online at Swinburne University''. Accessed April 22, 2008.</ref><ref name=Seattle>Irizarry, David. "[http://www.seattleastro.org/webfoot/feb00/pg2.htm The Secrets of the Harvard Classification Revealed]." ''The Webfooted Astronomer'', Seattle Astronomical Society, February 2000. Accessed April 22, 2008.</ref>

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<ref name=Cannon>"[http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/LC_main_p8.html Life Cycles of Stars]." ''Goddard Space Flight Center'', November 21, 2002. Accessed April 22, 2008.</ref> 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.<ref name=Swinburne>"[http://astronomy.swin.edu.au/cosmos/H/Harvard+Spectral+Classification Harvard Spectral Classification]." ''Study Astronomy Online at Swinburne University''. Accessed April 22, 2008.</ref><ref name=Seattle>Irizarry, David. "[http://www.seattleastro.org/webfoot/feb00/pg2.htm The Secrets of the Harvard Classification Revealed]." ''The Webfooted Astronomer'', Seattle Astronomical Society, February 2000. Accessed April 22, 2008.</ref>

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).  There is an additional spectral class for the smallest and dimmest stars (Class L), that still fuse hydrogen, although warmer [[brown dwarf|brown dwarfs]] also fall into this class (but referred to as L dwarfs instead of L stars). Cooler still methane dwarfs are classified as [[Brown_dwarf#Spectral_class_T|T dwarfs]].<ref>http://adsabs.harvard.edu/abs/2007arXiv0704.1522K</ref>  A proposed spectral class Y has been suggested for the coolest brown dwarfs, which also have a different spectra from T class dwarfs.<ref>http://xxx.lanl.gov/abs/astro-ph/0607305</ref>

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).  There is an additional spectral class for the smallest and dimmest stars (Class L), that still fuse hydrogen, although warmer [[brown dwarf]]s also fall into this class (but referred to as L dwarfs instead of L stars). Cooler still methane dwarfs are classified as [[Brown dwarf#Spectral class T|T dwarfs]].<ref>http://adsabs.harvard.edu/abs/2007arXiv0704.1522K</ref>  A proposed spectral class Y has been suggested for the coolest brown dwarfs, which also have a different spectra from T class dwarfs.<ref>http://xxx.lanl.gov/abs/astro-ph/0607305</ref>

{| class="wikitable"

{| class="wikitable"

| Yellow

| Yellow

| [[Calcium]], [[hydrogen]], other [[metal]]s

| [[Calcium]], [[hydrogen]], other [[metal]]s

| Balmer lines weaker still. K lines dominant. Metals now appearing. Contains the sun. <ref>www.astrometry.org/starclassification.php  

| Balmer lines weaker still. K lines dominant. Metals now appearing. Contains the sun.<ref>http://www.astrometry.org/starclassification.php  

</ref>

</ref>

|-

|-

| Ia - Iab - Ib

| Ia - Iab - Ib

| [[Supergiant|supergiants]]

| [[supergiant]]s

|-

|-

| IIa - IIab - IIb

| IIa - IIab - IIb

|-

|-

| VII

| VII

| [[white dwarf|white dwarfs]]

| [[white dwarf]]s

|}

|}

==Origins==

==Origins==

Christian scientists assert that [[materialism|materialistic]] explanations of the origin of stars are errant and contra-evidence and reports of stars forming are invalid. <ref>http://www.icr.org/article/403/</ref><ref>http://www.answersingenesis.org/creation/v18/i2/stars.asp</ref><ref>http://www.creationscience.com/onlinebook/AstroPhysicalSciences21.html</ref><ref>http://www.answersingenesis.org/Docs/399.asp#55</ref><ref>http://www.answersingenesis.org/creation/v19/i1/feedback.asp</ref>  In addition, creationists cite the secular scientific literature in order to make the case that materialist explanations of star formation are inadequate:

Christian scientists assert that [[materialism|materialistic]] explanations of the origin of stars are errant and contra-evidence and reports of stars forming are invalid.<ref>http://www.icr.org/article/403/</ref><ref>http://www.answersingenesis.org/creation/v18/i2/stars.asp</ref><ref>http://www.creationscience.com/onlinebook/AstroPhysicalSciences21.html</ref><ref>http://www.answersingenesis.org/Docs/399.asp#55</ref><ref>http://www.answersingenesis.org/creation/v19/i1/feedback.asp</ref>  In addition, creationists cite the secular scientific literature in order to make the case that materialist explanations of star formation are inadequate:

“We don’t understand how a single star forms, yet we want to understand how 10 billion stars form.” Carlos Frenk, as quoted by Robert Irion, “Surveys Scour the Cosmic Deep,” Science, Vol. 303, 19 March 2004, p. 1750. <ref>http://www.sciencemag.org/cgi/content/summary/303/5665/1750</ref>

“We don’t understand how a single star forms, yet we want to understand how 10 billion stars form.” Carlos Frenk, as quoted by Robert Irion, “Surveys Scour the Cosmic Deep,” Science, Vol. 303, 19 March 2004, p.&nbsp;1750.<ref>http://www.sciencemag.org/cgi/content/summary/303/5665/1750</ref>

“Nobody really understands how star formation proceeds. It’s really remarkable.” Rogier A. Windhorst, as quoted by Corey S. Powell, “A Matter of Timing,” Scientific American, Vol. 267, October 1992, p. 30. <ref>http://adsabs.harvard.edu/abs/1992SciAm.267Q..26P</ref>

“Nobody really understands how star formation proceeds. It’s really remarkable.” Rogier A. Windhorst, as quoted by Corey S. Powell, “A Matter of Timing,” Scientific American, Vol. 267, October 1992, p.&nbsp;30.<ref>http://adsabs.harvard.edu/abs/1992SciAm.267Q..26P</ref>

==Habitable zone==

==Habitable zone==

A star's habitable zone is the region in which a [[terrestrial planet]] of the right size could have a surface temperature that might allow for liquid water and potentially life. <ref>Bennet, Jeffrey, et al. "Life Around Stars." <u>The Essential Cosmic Perspective</u>. 4th ed. San Francisco: Pearson Education, Inc., 2008. 508-13.</ref>

A star's habitable zone is the region in which a [[terrestrial planet]] of the right size could have a surface temperature that might allow for liquid water and potentially life.<ref>Bennet, Jeffrey, et al. "Life Around Stars." <u>The Essential Cosmic Perspective</u>. 4th ed. San Francisco: Pearson Education, Inc., 2008. 508-13.</ref>

For example, if a star much similar our [[Sun]] has a lifetime of one million years and temperature of 6094K. Its habitable zone lies within 1.02AU and 1.49AU. <ref>"Exploring the Habitable Zone and Central Star", CADRE design Pty. Ltd.</ref>

For example, if a star much similar our [[Sun]] has a lifetime of one million years and temperature of 6094K. Its habitable zone lies within 1.02AU and 1.49AU.<ref>"Exploring the Habitable Zone and Central Star", CADRE design Pty. Ltd.</ref>

==See also==

==See also==

The ''Cosmological Distance Ladder'', by Michael Rowan-Robinson. Published by Freeman. 1985.

The ''Cosmological Distance Ladder'', by Michael Rowan-Robinson. Published by Freeman. 1985.

[[category:astronomy]]

[[Category:Astronomy]]