Changes

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>

<math>\,\!M = V + 5 \times \log \frac{s_0}{s}</math>

<math>\,\!M = V + 5 \times \log \frac{s_0}{s}</math>

where s₀ is the standard distance. This distance is ten parsecs, or about 2,062,650 AU.

where s₀ is the standard distance. This distance is ten [[parsec]]s, or about 2,062,650 AU.

Brightness declines with the square of distance, and squares correspond to doubling of logarithms. One must then multiply that result by 2.5 to stay within the magnitude scale.

Brightness declines with the square of distance, and squares correspond to doubling of logarithms. One must then multiply that result by 2.5 to stay within the magnitude scale.

==Variable stars==

==Variable stars==

Some stars vary in brightness and are known as variable stars. The star [[Algol]] in the  constellation of Perseus can drop from its normal magnitude of 2.3 to magnitude 3.5. This is now known to be caused by a dim companion star orbiting Algol, which occasionally passes between Algol and the Earth, blocking some of the light. Other variable stars vary in brightness due to actual variations in the luminosity of the star itself. The time taken from one maximum brightness to the next one is called the '''period'''. The most famous of the variable stars is delta Cepheus, the first-found member of the [[Cepheid]] group of variable stars. In 1908 [[Henrietta Swan Leavitt]] noticed that the variable stars in the [[Magellenic Clouds]] (two nearby galaxies in the [[Local Group]]) had a relationship between their period and their apparent brightness. At that time galaxies outside our own (the [[Milky Way]]) had been discovered, but it was not possible to measure the distances to them. It was soon realized that the variable stars in the Magellenic Cloud were of the Cepheid type. Since Cepheid variables also occur in our [[galaxy]] it was possible measure their distances and thus convert (using the inverse square law) Leavitt's relationship between apparent brightness and period to one of actual brightness and period. Once this formula was discovered, it became possible to apply to Cepheids of unknown distance. By observing their periods, their actual brightness can be calculated and, by the inverse square law, their distance. Through observations of Cepheids in [[globular cluster]]s (compact bunches of stars) in our galaxy it was shown that our galaxy is about 300,000 light-years in diameter.

Some stars vary in brightness and are known as [[variable star]]s. The star [[Algol]] in the  constellation of Perseus can drop from its normal magnitude of 2.3 to magnitude 3.5. This is now known to be caused by a dim companion star orbiting Algol, which occasionally passes between Algol and the Earth, blocking some of the light. Other variable stars vary in brightness due to actual variations in the luminosity of the star itself. The time taken from one maximum brightness to the next one is called the '''period'''. The most famous of the variable stars is delta Cepheus, the first-found member of the [[Cepheid]] group of variable stars. In 1908 [[Henrietta Swan Leavitt]] noticed that the variable stars in the [[Magellanic Clouds]] (two nearby galaxies in the [[Local Group]]) had a relationship between their period and their apparent brightness. At that time galaxies outside our own (the [[Milky Way]]) had been discovered, but it was not possible to measure the distances to them. It was soon realized that the variable stars in the Magellanic Cloud were of the Cepheid type. Since Cepheid variables also occur in our [[galaxy]] it was possible measure their distances and thus convert (using the inverse square law) Leavitt's relationship between apparent brightness and period to one of actual brightness and period. Once this formula was discovered, it became possible to apply to Cepheids of unknown distance. By observing their periods, their actual brightness can be calculated and, by the inverse square law, their distance. Through observations of Cepheids in [[globular cluster]]s (compact bunches of stars) in our galaxy it was shown that our galaxy is about 300,000 light-years in diameter.

== Energy production ==

== Energy production ==

[[Image:CNO_Cycle.png|300px|thumb|CNO cycle]]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 [[Sun#Energy production and transport|here]].

[[Image:CNO_Cycle.png|300px|thumb|CNO cycle]]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 [[Sun#Energy production and transport|here]].

In 1938 and 1989, two physicists, Carl F. von Weizsäcker<ref name=Weiz>Von Weizsäcker, Carl F. ''Physik. Zeitsch.'' 39:633, 1938.</ref> and Hans Bethe<ref name=Bethe>Bethe, Hans A. "[http://prola.aps.org/abstract/PR/v55/i5/p434_1 Energy Production in Stars]." ''Physics Review'' 55(5):434-456, 1939. {{doi|10.1103/PhysRev.55.434}} Accessed June 27, 2008.</ref> 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 [[positron]]s and two [[neutrino|electron neutrino]]s.<ref name=Krane>Krane, Kenneth S. ''Introductory Nuclear Physics''. New York: John Wiley and Sons, 1988, p. 537. ISBN 9780471805533</ref>

In 1938 and 1989, two physicists, Carl F. von Weizsäcker<ref name=Weiz>Von Weizsäcker, Carl F. ''Physik. Zeitsch.'' 39:633, 1938.</ref> and Hans Bethe<ref name=Bethe>Bethe, Hans A. "[http://prola.aps.org/abstract/PR/v55/i5/p434_1 Energy Production in Stars]." ''Physics Review'' 55(5):434-456, 1939. {{doi|10.1103/PhysRev.55.434}} Accessed June 27, 2008.</ref> independently proposed a [[nuclear fusion]] process, the '''[[Carbon-nitrogen-oxygen cycle|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 [[positron]]s and two [[neutrino|electron neutrino]]s.<ref name=Krane>Krane, Kenneth S. ''Introductory Nuclear Physics''. New York: John Wiley and Sons, 1988, p. 537. ISBN 9780471805533</ref>

The equations for the cycle are as follows:

The equations for the cycle are as follows:

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]]