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A star is usually made of gas and a substance known as plasma. The Sun (Sol) is a star. A portion of the stars, however, those called white dwarfs and neutron stars, are composed of  tightly packed atoms or subatomic particles and are much more dense than anything on Earth.
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A '''star''' is usually made of gas and a substance known as plasma. The [[Sun]] (Sol) is a star. A portion of the stars, however, those called white dwarfs and neutron stars, are composed of  tightly packed atoms or subatomic particles and are much more dense than anything on Earth.


Stars range widely in size. Our Sun’s radius is about 432,000 miles (695,500 kilometres). Because there are other stars that are much larger, our sun is classified as a dwarf star. Stars classified as supergiants have a radius about 1,000 times that of the sun. Neutron stars are the smallest stars and have a radius of only about 6 miles (10 kilometres).
Stars range widely in size. The Sun’s radius is about 432,000 miles (695,500 kilometres). Because there are other stars that are much larger, the Sun is classified as a dwarf star. Stars classified as supergiants have a radius about 1,000 times that of the Sun. Neutron stars are the smallest stars and have a radius of only about 6 miles (10 kilometres).


Our sun is a single star. However, about 75 % of all stars are binary stars, two stars that orbit each other. The star nearest our sun, Proxima Centauri, is part of a multiple-star system which includes '''Alpha Centauri A''' and '''Alpha Centauri B'''.<ref>The distance to Proxima Centauri is over 25 trillion miles (40 trillion kilometres) and light from '''Proxima Centauri''' must travel 4.2 years to travel to us before we can see it. One light-year, the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46 trillion kilometres).</ref>
The Sun is a single star. However, about 75 % of all stars are binary stars, two stars that orbit each other. The star nearest the Sun, [[Proxima Centauri]], is part of a multiple-star system which includes [[Alpha Centauri A]] and [[Alpha Centauri B]].<ref>The distance to Proxima Centauri is over 25 trillion miles (40 trillion kilometres) and light from Proxima Centauri must travel 4.2 years to travel to us before we can see it. One light-year, the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46 trillion kilometres).</ref>
 
A star is so massive that its gravity pulls it in on itself and would cause it to collapse if there was nothing to oppose this gravity. The gravity is opposed by [[hydrostatic support]] created by the pressure of hot gas and radiation in the star's interior. The reason for the heat and radiation is [[nuclear reaction]] near the centre of the star. While these two forces oppose each other, the star is in the [[main sequence]] phase of its evolution. Stars contract or collapse in on themselves before the main sequence phase because they are not hot or dense enough for nuclear reactions to begin. Heat generated during contraction provides hydrostatic support and counteracts the pull of gravity. After the main sequence, most of the fuel is used up and the lack of sufficient hydrostatic support results in the star collapsing under its own gravity.<ref>[http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html What is a star?] (2004) Imagine the Universe, Goddard Space Flight Center, NASA</ref>
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==Categories==
==Categories==
Characteristics of stars
==Characteristics of stars==


Stars are categorised by their five main attributes;
Stars are categorised by their five main attributes;
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:(5) mass (amount of matter).  
:(5) mass (amount of matter).  


Colour is dependent of surface temperature, brightness depends on surface temperature and the size. Mass affects how fast a stars produces energy and effects surface temperature.
Colour is dependent of surface temperature, brightness depends on surface temperature and the size. Mass affects how fast a star produces energy and effects surface temperature.


Astronomers use a diagramme to show how these are all related, the '''Hertzsprung-Russell''' (H-R) diagramme
Astronomers use a diagram to show how these are all related, the [[Hertzsprung-Russell]] (H-R) diagram.


===Magnitude===
===Luminosity===
Based on a numbering system used by '''Hipparchus''' (125 B.C. Greece) stars are numbered according to their brightness as they are seen from Earth. The brightest is of the first magnitude, the next brightest of the second magnitude and so on to the sixth magnitude, which are very faint stars. Today this is called '''apparent magnitude'''. There is now a measure of '''absolute magnitude''', what the apparent magnitude would be if the star was 32.6 light-years from Earth
A star's luminosity is the measure of all energy it radiates each second (just as power is measured), usually understood to be at all wavelengths ([[bolometric luminosity]] or L<sub>bol</sub>) and is measured as ergs per second. Star luminosity is commonly compared to the Sun whose luminosity is indicated as ''L<sub>sun</sub> = 4 x 10<sup>33</sup> ergs/s''. <ref>[http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryL.html] NOrthern Arizona University</ref>


There are also stars that are brighter than the first magnitude and they are classed by numbers less than 1. Rigel’s apparent magnitude is 0.12. Extremely bright stars have numbers that are less than zero or negative numbers. The brightest stars in the night sky is Sirius with a apparent magnitude of -1.46 and the absolute magnitude of Rigel is -8.1. At the present time there are no stars classified with a absolute magnitude brighter than -8.0 Some very dim stars have an apparent magnitude of 28, but absolute magnitude can be no fainter than about 16.<ref name=PGreenSmithsonian>Paul J. Green,  (2005) "Star." World Book Online Reference Center.. World Book, Inc. [http://www.worldbookonline.com/wb/Article?id=ar529540] Reprinted by NASA at [http://www.nasa.gov/worldbook/star_worldbook.html] Paul J. Green, PhD. is an Astrophysicist with Smithsonian Astrophysical Observatory.</ref>
The luminosity of a star is proportional to its temperature and its surface area. This means that a hotter star is more luminous than a cooler star if they have the same radius and a bigger star more luminous than a smaller star if they have the same temperature. This, in turn, means that a cool (red) giant star is more luminous than the Sun since it is much larger even though it is much cooler.<ref>[http://www.astro.cornell.edu/academics/courses/astro201/lumin_star.htm Luminosity of a star] Cornell University Astronomy</ref>


===Luminosity===
Stars radiate visible light but they also radiate energy the human eye can not see. The term for all radiation from a star is bolometric magnitude or bolometric flux which means the total energy out put over the entire electromagnetic spectrum, not just visible light.
How bright a star looks is controlled by its actual brilliance and its distance from Earth. A star’s brilliance is the amount of visible light it produces. So a nearby star can be quite dim if it is not generating a great deal of light while a star much further away can be brighter because it actually is much brighter. The star '''Alpha Centauri''' appears brighter than another star called '''Rigel'''. It would seem that Alpha Centauri produces more energy. However, Rigel is actually much brighter but it is much further away fro Earth. Alpha Centauri is about 4.4 light years away while Rigel is 1,400 light years from Earth.
 
The Sun has a luminosity of 400 trillion trillion watts (10<sup>24</sup>). Commonly in astronomy however, the luminosity is measured another way, by comparing it with the Sun. In this way Alpha Centauri is about 1.3 times as luminous as the Sun and Rigel is 150,000 times as luminous.<ref>[http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryL.html] Northern Arizona University,  College of Engineering and Natural Sciences.</ref>
 
How bright a star looks is controlled by its actual brilliance and its distance from Earth. A star’s brilliance is the amount of visible light it produces. So a nearby star can be quite dim if it is not generating a great deal of light while a star much further away can be brighter because it actually is much brighter. The star [[Alpha Centauri]] appears brighter than another star called [[Rigel]]. It would seem that Alpha Centauri produces more energy. However, Rigel is actually much brighter but it is much further away from Earth. Alpha Centauri is about 4.4 light years away while Rigel is 1,400 light years from Earth.
 
There is also a ''luminosity''-''absolute magnitude'' comparison: 5 on the absolute magnitude scale equals 100 on the luminosity scale. So, if a star has an absolute magnitude of 2, it is 100 times as luminous as a star with an absolute magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a star with an absolute magnitude is 2 and 10,000 times more luminous than a star with an absolute magnitude of 7.


Stars radiate visible light but they also radiate energy the human eye can not see. A stars luminosity is the measure of energy it radiates<ref>[http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryL.html] Northern Arizona UniversityCollege of Engineering and Natural Sciences. Luminosity is defined as the total energy radiated by a star each second, at all wavelengths (measured as ergs per second). Star luminosity is commonly compared to our Sun (L<sub>sun</sub> = 4 x 10<sup>33</sup> ergs/s). Luminosity is measured in the same way as power which is energy per second.</ref>
Luminosity is variable. Small changes may be responsible for the [[Little Ice Age]] of the 15th through 18th centuries when there may have been a decrease in the Sun's luminosity. One possible explanation of these changes is that they are triggered by modifications of the magnetic field produced in a star's interior. The mechanism of these modifications are yet to be revealed.<ref>[http://www.ast.obs-mip.fr/users/donati/press/v374peg_more_eng.html The simple magnetic field of an ultra-cool star]DonatiJ.F. & Cameron, A.C. (2006)</ref>
Our sun has a luminosity of 400 trillion trillion watts (10<sup>24</sup>). Commonly in astronomy however, the luminosity is measured another way, by comparing it with our Sun. In this way Alpha Centauri is about 1.3 times as luminous as our Sun and Rigel is 150,000 times as luminous.


There is a luminosity absolute magnitude comparison: 5 on the absolute magnitude scale equals 100 on the luminosity scale. So, if a star has absolute magnitude of 2, it is 100 times as luminous as a star with an absolute magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a star with an absolute magnitude is 2 and 10,000 times more luminous than a star with an absolute magnitude of 7.
The Yerkes classification is a schema for [[Stellar classification (astrophysics)|classifying stars by their luminosity]],<ref>Yerkes Observatory</ref> also known as M-K established in 1943.<ref>[http://www.journals.uchicago.edu/AJ/journal/issues/v121n4/200462/200462.html The physical basis of luminosity classification in the late A-, F-, and early G-type Stars. I. Precise spectral types for 372 stars] Gray, R.O., Napier, M.G., Winkler, L.I. (2001) The Astronomical Journal April. 121:2148-2158. The American Astronomical Society; MKK are the initials of the authors Morgan, William W., Keenan, Philip C. and Kellman, Edith </ref> This classification scheme measures surface gravities of stars by measuring the shape and nature of certain spectral lines.
<ref name=PGreenSmithsonian/>


===Colour===
===Colour===
With or without a telescope or binoculars, stars display a range of colours--reddish, yellowish and bluish--not very strong colour, but the variations are visible. Our sun is yellow, as is Pollux, Betelgeuse looks reddish and Rigel looks bluish. Star colour is a function of its surface temperature.<ref> Star temperatures are given in units called kelvin. One kelvin (1 K) equals exactly 1 degree Celsius (or 1.8 Fahrenheit). Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 C: 0 K equals -273.15 degrees C, or -459.67 degrees F. A  temperature of 0 degrees C (32 degrees F) equals 273.15 K.</ref> Dark red stars have surface temperatures of about 2500 K and bright red stars reach about 3500 K. For yellow stars, e.g. the sun, surface temperature is approximately 5500 K. Blue stars are in the range of 10,000 to 50,000 K in surface temperature.
With or without a telescope or binoculars, stars display a range of colours--reddish, yellowish and bluish--not very strong colour, but the variations are visible. The Sun is yellow, as is Pollux, Betelgeuse looks reddish and Rigel looks bluish. Star colour is a function of its surface temperature.<ref> Star temperatures are given in units called kelvin. One kelvin (1 K) equals exactly 1 degree Celsius (or 1.8 Fahrenheit). Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 C: 0 K equals -273.15 degrees C, or -459.67 degrees F. A  temperature of 0 degrees C (32 degrees F) equals 273.15 K.</ref> Dark red stars have surface temperatures of about 2500 K and bright red stars reach about 3500 K. For yellow stars, e.g. the sun, surface temperature is approximately 5500 K. Blue stars are in the range of 10,000 to 50,000 K in surface temperature.


Since the human eye can only discern colours of the visible spectrum we do not see other wavelengths of the spectrum and stars emit a broad spectrum (bands) of colours. The colours in the visible spectrum, all the colours of the rainbow,  can be viewed with a prism which separates the colour bands. Red is produced by particles of light (photons) with the least energy, all the way to violet, the photons with the most energy.
Since the human eye can only discern colours of the visible spectrum we do not see other wavelengths of the spectrum and stars emit a broad spectrum (bands) of colours. The colours in the visible spectrum, all the colours of the rainbow,  can be viewed with a prism which separates the colour bands. Red is produced by particles of light (photons) with the least energy, all the way to violet, the photons with the most energy.
[[Image:3 spectra types.gif|right|thumb|350px|{{#ifexist:Template:3 spectra types.gif/credit|{{3 spectra types.gif/credit}}<br/>|}}Continuous, absorption and emission spectra.]]
{{Image|3 spectra types.gif|right|350px|Continuous, absorption and emission spectra.}}
Including the electromagnetic band of seven visible colours,<ref> red, orange, yellow,  green, blue, indigo and violet</ref> there are six bands of electromagnetic radiation in the electromagnetic spectrum, in range from the least energetic to the most energetic:<ref> least energetic also corresponds with the lowest frequency and the longest wave length and highest energy corresponds with the highest frequency and shortest wavelength </ref>
Including the electromagnetic band of seven visible colours,<ref> red, orange, yellow,  green, blue, indigo and violet</ref> there are six bands of electromagnetic radiation in the electromagnetic spectrum, in range from the least energetic to the most energetic:<ref> least energetic also corresponds with the lowest frequency and the longest wave length and highest energy corresponds with the highest frequency and shortest wavelength </ref>
*radio waves
*radio waves
Line 47: Line 53:
*X rays  
*X rays  
*gamma rays
*gamma rays
[[Image:Fraunhofer lines-1.gif|right|thumb|350px|{{#ifexist:Template:Fraunhofer lines-1.gif/credit|{{Fraunhofer lines-1.gif/credit}}<br/>|}}Fraunhofer Lines of common elements.]]
{{Image|Fraunhofer lines-1.gif|right|350px|Fraunhofer Lines of common elements.}}
Studying the electromagnetic spectrum of stars is important function of astronomy.<ref> a common instrument in this sort of analysis is an ''interferometer''</ref> Displaying the spectrum of a star shows dark bands called absorption lines, or Fraunhofer lines, where the radiation energy is weaker, caused by a cooler gas between the observer and the and the hotter emitting gas.<ref>[http://science.msfc.nasa.gov/ssl/pad/solar/suntime/slshow8.stm The Sun in Time] Solar Physics,  Marshal Space Flight Center, NASA</ref>  
Studying the electromagnetic spectrum of stars is an important function of astronomy.<ref> a common instrument in this sort of analysis is an ''interferometer''</ref> Displaying the spectrum of a star shows dark bands called absorption lines, or Fraunhofer lines, where the radiation energy is weaker, caused by a cooler gas between the observer and the hotter emitting gas.<ref>[http://science.msfc.nasa.gov/ssl/pad/solar/suntime/slshow8.stm The Sun in Time] Solar Physics,  Marshal Space Flight Center, NASA</ref>  


Occasionally there are also bright emission lines were radiation energy is especially strong.
Occasionally there are also bright emission lines where radiation energy is especially strong.
[[Image:PIA04940 modest.jpg|right|thumb|350px|{{#ifexist:Template:PIA04940 modest.jpg/credit|{{PIA04940 modest.jpg/credit}}<br/>|}}Spectrum for PIA04940: Embedded Star in Herbig-Haro 46/47.]]
{{Image|PIA04940 modest.jpg|right|350px|Spectrum for PIA04940: Embedded Star in Herbig-Haro 46/47.}}
Absorption lines exist because a chemical element or compound absorbs radiation with the same energy at that particular line on the spectrum. Our sun, for example, has absorption bands in the green range of the spectrum because there is calcium in the outer layer of the sun absorbing radiation at the same energy. While most stars have absorption lines in the visible spectrum, emission lines are more common in other bands of the spectrum. Nitrogen in a sun’s atmosphere, for example, will result in emission lines in the ultraviolet band.
Absorption lines exist because a chemical element or compound absorbs radiation with the same energy at that particular line on the spectrum. The Sun, for example, has absorption bands in the green range of the spectrum because there is calcium in the outer layer of the sun absorbing radiation at the same energy. While most stars have absorption lines in the visible spectrum, emission lines are more common in other bands of the spectrum. Nitrogen in a sun’s atmosphere, for example, will result in emission lines in the ultraviolet band.
<ref name=PGreenSmithsonian/><ref>[http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html Electromagnetic spectrum] (2006). ''Imagine the Universe.'' High Energy Astrophysics Science Archive Research Center, Goddard Space Flight Center, NASA</ref><ref>[http://ls7pm3.gsfc.nasa.gov/whatsRM/em.html What is the electromagnetic spectrum?] Landsat 7, Goddard Space Flight Center, NASA</ref>
<ref>[http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html Electromagnetic spectrum] (2006). ''Imagine the Universe.'' High Energy Astrophysics Science Archive Research Center, Goddard Space Flight Center, NASA</ref><ref>[http://ls7pm3.gsfc.nasa.gov/whatsRM/em.html What is the electromagnetic spectrum?] Landsat 7, Goddard Space Flight Center, NASA</ref>


An example of a spectrum for a particular star is an embedded star in Herbig-Haro 46/47 as determined by the Spitzer Space Telescope (SST). This is a low mass protostar ejecting a jet of supersonic gas and creating a bipolar, or two-sided, outflow which interacts with the surrounding interstellar medium--bright, nebulous regions of gas and dust that are buried within a dark dust cloud. Elements and compunds present in the protostar as determined from the EM spectrum include water ice, methyl alcohol, silicates and carbon dioxide ice.<ref>[http://photojournal.jpl.nasa.gov/catalog/PIA04940 PIA04940: Spectrum from Embedded Star in Herbig-Haro 46/47] Photo Journal, Jet Propulsion Lab, California University of Technology, NASA</ref>
An example of a spectrum for a particular star is an embedded star in Herbig-Haro 46/47 as determined by the Spitzer Space Telescope (SST). This is a low mass protostar ejecting a jet of supersonic gas and creating a bipolar, or two-sided, outflow which interacts with the surrounding interstellar medium--bright, nebulous regions of gas and dust that are buried within a dark dust cloud. Elements and compounds present in the protostar as determined from the EM spectrum include water ice, methyl alcohol, silicates and carbon dioxide ice.<ref>[http://photojournal.jpl.nasa.gov/catalog/PIA04940 PIA04940: Spectrum from Embedded Star in Herbig-Haro 46/47] Photo Journal, Jet Propulsion Lab, California University of Technology, NASA</ref>


===Surface temperature===
===Surface temperature===
===Size===
===Size===
===Mass===
===Mass===
A star's mass is its most fundamental property. Mass governs the evolution of a star, what fuels it will burn, how long it will live, its colour. Determining a star's mass is an essential part of understanding a specific star. Knowing the mass of main sequence stars determines the answers to a number of problems, such as the mass content of a galaxy and the manner in which it evolves. The mass-luminosity relation (MLR) is a fundamental relationship in numerous areas of astronomy.<ref>[http://planetquest.jpl.nasa.gov/SIM/science_henry.pdf A MASSIF Effort To Determine The Mass-Luminosity Relation For Stars of Various Ages, Metallicities, and Evolution States]Todd J. Henry (Principle investigator, Georgia State Univ.). Masses and Stellar Systems with Interferometry Team</ref>


==Fusion==
==Fusion==
Stars have limited amounts of fuel. As each type of their fuel is consumed, the star must burn successively different types of fuel. In the beginning a star burns [[hydrogen]] in a process that synthesises [[helium]]. When the hydrogen is consumed, the helium is then burned and that in turn synthesises carbon. To continue to burn fuel produced at each stage, the star must be hot enough to convert progressively heavier elements. Smaller stars cannot generate sufficient heat to do so and eventually lose energy.


==Life Cycle==
Eventually, as the fuel is consumed, stars less than 5 solar masses swell into red giants which eventually eject their outer layers forming planetary nebulae, leaving only the inner core, a white dwarf.<ref name=StarsCornell>[http://curious.astro.cornell.edu/stars.php Stars] (2003) Curious about Astronomy? Cornell University</ref>
Stars have a beginning and an end. Our sun became a star about 4.6 billion years ago and may last another 5 billion years. After that it will become a '''red giant''', later it will lose its outer layers and the remaining core of the star will become a '''white dwarf''', eventually fading to a '''black dwarf''', dense and without light.
 
Not all stars end this way. Some will cool off and without expanding to a red giant stage will become white dwarfs, then black dwarfs..
 
Another possibility for a small percentage of stars is a spectacular explosion called a '''supernovae'''. The star explodes and loses most of its material. An even smaller percentage, undergo a very rare occurrence, they completely explode and nothing is left.


===Intermediate-mass stars===
==Life cycle==
Stars have a beginning and an end. The Sun formed about 4.6 billion years ago and may last another 5 billion years. After that it will become a [[red giant]]. Later it will lose its outer layers and the remaining core of the star will become a [[white dwarf]], eventually fading to a [[black dwarf]], dense and without light.


===T-Tauri phase===
Not all stars end this way. Some will cool off and without expanding to a red giant stage will become white dwarfs, then black dwarfs.


===Main-sequence stars===
Another possibility for a small percentage of stars is a spectacular explosion called a [[supernova]]. The star explodes and loses most of its material. In an even smaller number of cases, the star explodes completely, leaving nothing - but this is a very rare occurrence.<ref>[http://chandra.harvard.edu/resources/illustrations/supernova.html Supernovas and remnants] Chandra X-ray Observatory</ref>
 
===Red giant phase===
 
===Horizontal branch phase===
 
===Asymptotic giant phase===
 
===White dwarf phase===
 
===Black dwarf phase===
 
===Supernovae===
 
===Neutron stars===
 
===Black holes===


==Galaxies==
==Galaxies==
Stars are grouped in galaxies with rare exceptions.<ref>[http://www.nasa.gov/topics/universe/features/intergalatic_shot.html 'Shot in the Dark' Star Explosion Stuns Astronomers] NASA</ref> Astronomical observations to date have recorded galaxies at distances of 12 to 16 billion light years from our sun. Our sun is in the galaxy called the '''Milky Way''' which contains an estimated 100 billion stars. It is estimated that there are more than 100 billion galaxies in the universe and average about  100 billion stars per galaxy. This means that there is an estimated 10 billion, trillion stars in the universe (10<sup>21</sup>). From Earth however, without using binoculars or a telescope, it is only possible to see about 3,000 of them.
Stars are grouped in galaxies with rare exceptions.<ref>[http://www.nasa.gov/topics/universe/features/intergalatic_shot.html 'Shot in the Dark' Star Explosion Stuns Astronomers] NASA</ref> Astronomical observations to date have recorded galaxies at distances of 12 to 16 billion light years from the Sun. The Sun is in the galaxy called the [[Milky Way]] (because of its appearance as a faint band of light encircling the night sky) which contains an estimated 100 billion stars. It is estimated that there are more than 100 billion galaxies in the universe and average about  100 billion stars per galaxy. This means that there is an estimated 10 billion, trillion stars in the universe (10<sup>21</sup>). From Earth however, without using binoculars or a telescope, it is only possible to see about 3,000 of them.


==Notes==
==Notes==
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</div>[[Category:Suggestion Bot Tag]]
 
[[Category: CZ Live|CZ Live]]
[[Category: Astronomy Workgroup|Astronomy Workgroup]]
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A star is usually made of gas and a substance known as plasma. The Sun (Sol) is a star. A portion of the stars, however, those called white dwarfs and neutron stars, are composed of tightly packed atoms or subatomic particles and are much more dense than anything on Earth.

Stars range widely in size. The Sun’s radius is about 432,000 miles (695,500 kilometres). Because there are other stars that are much larger, the Sun is classified as a dwarf star. Stars classified as supergiants have a radius about 1,000 times that of the Sun. Neutron stars are the smallest stars and have a radius of only about 6 miles (10 kilometres).

The Sun is a single star. However, about 75 % of all stars are binary stars, two stars that orbit each other. The star nearest the Sun, Proxima Centauri, is part of a multiple-star system which includes Alpha Centauri A and Alpha Centauri B.[1]

A star is so massive that its gravity pulls it in on itself and would cause it to collapse if there was nothing to oppose this gravity. The gravity is opposed by hydrostatic support created by the pressure of hot gas and radiation in the star's interior. The reason for the heat and radiation is nuclear reaction near the centre of the star. While these two forces oppose each other, the star is in the main sequence phase of its evolution. Stars contract or collapse in on themselves before the main sequence phase because they are not hot or dense enough for nuclear reactions to begin. Heat generated during contraction provides hydrostatic support and counteracts the pull of gravity. After the main sequence, most of the fuel is used up and the lack of sufficient hydrostatic support results in the star collapsing under its own gravity.[2]

[edit intro]

Categories

Characteristics of stars

Stars are categorised by their five main attributes;

(1) brightness, (magnitude or luminosity);
(2) colour;
(3) surface temperature;
(4) size (diameter);
(5) mass (amount of matter).

Colour is dependent of surface temperature, brightness depends on surface temperature and the size. Mass affects how fast a star produces energy and effects surface temperature.

Astronomers use a diagram to show how these are all related, the Hertzsprung-Russell (H-R) diagram.

Luminosity

A star's luminosity is the measure of all energy it radiates each second (just as power is measured), usually understood to be at all wavelengths (bolometric luminosity or Lbol) and is measured as ergs per second. Star luminosity is commonly compared to the Sun whose luminosity is indicated as Lsun = 4 x 1033 ergs/s. [3]

The luminosity of a star is proportional to its temperature and its surface area. This means that a hotter star is more luminous than a cooler star if they have the same radius and a bigger star more luminous than a smaller star if they have the same temperature. This, in turn, means that a cool (red) giant star is more luminous than the Sun since it is much larger even though it is much cooler.[4]

Stars radiate visible light but they also radiate energy the human eye can not see. The term for all radiation from a star is bolometric magnitude or bolometric flux which means the total energy out put over the entire electromagnetic spectrum, not just visible light.

The Sun has a luminosity of 400 trillion trillion watts (1024). Commonly in astronomy however, the luminosity is measured another way, by comparing it with the Sun. In this way Alpha Centauri is about 1.3 times as luminous as the Sun and Rigel is 150,000 times as luminous.[5]

How bright a star looks is controlled by its actual brilliance and its distance from Earth. A star’s brilliance is the amount of visible light it produces. So a nearby star can be quite dim if it is not generating a great deal of light while a star much further away can be brighter because it actually is much brighter. The star Alpha Centauri appears brighter than another star called Rigel. It would seem that Alpha Centauri produces more energy. However, Rigel is actually much brighter but it is much further away from Earth. Alpha Centauri is about 4.4 light years away while Rigel is 1,400 light years from Earth.

There is also a luminosity-absolute magnitude comparison: 5 on the absolute magnitude scale equals 100 on the luminosity scale. So, if a star has an absolute magnitude of 2, it is 100 times as luminous as a star with an absolute magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a star with an absolute magnitude is 2 and 10,000 times more luminous than a star with an absolute magnitude of 7.

Luminosity is variable. Small changes may be responsible for the Little Ice Age of the 15th through 18th centuries when there may have been a decrease in the Sun's luminosity. One possible explanation of these changes is that they are triggered by modifications of the magnetic field produced in a star's interior. The mechanism of these modifications are yet to be revealed.[6]

The Yerkes classification is a schema for classifying stars by their luminosity,[7] also known as M-K established in 1943.[8] This classification scheme measures surface gravities of stars by measuring the shape and nature of certain spectral lines.

Colour

With or without a telescope or binoculars, stars display a range of colours--reddish, yellowish and bluish--not very strong colour, but the variations are visible. The Sun is yellow, as is Pollux, Betelgeuse looks reddish and Rigel looks bluish. Star colour is a function of its surface temperature.[9] Dark red stars have surface temperatures of about 2500 K and bright red stars reach about 3500 K. For yellow stars, e.g. the sun, surface temperature is approximately 5500 K. Blue stars are in the range of 10,000 to 50,000 K in surface temperature.

Since the human eye can only discern colours of the visible spectrum we do not see other wavelengths of the spectrum and stars emit a broad spectrum (bands) of colours. The colours in the visible spectrum, all the colours of the rainbow, can be viewed with a prism which separates the colour bands. Red is produced by particles of light (photons) with the least energy, all the way to violet, the photons with the most energy.

Continuous, absorption and emission spectra.

Including the electromagnetic band of seven visible colours,[10] there are six bands of electromagnetic radiation in the electromagnetic spectrum, in range from the least energetic to the most energetic:[11]

  • radio waves
  • infrared rays
  • visible light
  • ultraviolet rays
  • X rays
  • gamma rays
Fraunhofer Lines of common elements.

Studying the electromagnetic spectrum of stars is an important function of astronomy.[12] Displaying the spectrum of a star shows dark bands called absorption lines, or Fraunhofer lines, where the radiation energy is weaker, caused by a cooler gas between the observer and the hotter emitting gas.[13]

Occasionally there are also bright emission lines where radiation energy is especially strong.

Spectrum for PIA04940: Embedded Star in Herbig-Haro 46/47.

Absorption lines exist because a chemical element or compound absorbs radiation with the same energy at that particular line on the spectrum. The Sun, for example, has absorption bands in the green range of the spectrum because there is calcium in the outer layer of the sun absorbing radiation at the same energy. While most stars have absorption lines in the visible spectrum, emission lines are more common in other bands of the spectrum. Nitrogen in a sun’s atmosphere, for example, will result in emission lines in the ultraviolet band. [14][15]

An example of a spectrum for a particular star is an embedded star in Herbig-Haro 46/47 as determined by the Spitzer Space Telescope (SST). This is a low mass protostar ejecting a jet of supersonic gas and creating a bipolar, or two-sided, outflow which interacts with the surrounding interstellar medium--bright, nebulous regions of gas and dust that are buried within a dark dust cloud. Elements and compounds present in the protostar as determined from the EM spectrum include water ice, methyl alcohol, silicates and carbon dioxide ice.[16]

Surface temperature

Size

Mass

A star's mass is its most fundamental property. Mass governs the evolution of a star, what fuels it will burn, how long it will live, its colour. Determining a star's mass is an essential part of understanding a specific star. Knowing the mass of main sequence stars determines the answers to a number of problems, such as the mass content of a galaxy and the manner in which it evolves. The mass-luminosity relation (MLR) is a fundamental relationship in numerous areas of astronomy.[17]

Fusion

Stars have limited amounts of fuel. As each type of their fuel is consumed, the star must burn successively different types of fuel. In the beginning a star burns hydrogen in a process that synthesises helium. When the hydrogen is consumed, the helium is then burned and that in turn synthesises carbon. To continue to burn fuel produced at each stage, the star must be hot enough to convert progressively heavier elements. Smaller stars cannot generate sufficient heat to do so and eventually lose energy.

Eventually, as the fuel is consumed, stars less than 5 solar masses swell into red giants which eventually eject their outer layers forming planetary nebulae, leaving only the inner core, a white dwarf.[18]

Life cycle

Stars have a beginning and an end. The Sun formed about 4.6 billion years ago and may last another 5 billion years. After that it will become a red giant. Later it will lose its outer layers and the remaining core of the star will become a white dwarf, eventually fading to a black dwarf, dense and without light.

Not all stars end this way. Some will cool off and without expanding to a red giant stage will become white dwarfs, then black dwarfs.

Another possibility for a small percentage of stars is a spectacular explosion called a supernova. The star explodes and loses most of its material. In an even smaller number of cases, the star explodes completely, leaving nothing - but this is a very rare occurrence.[19]

Galaxies

Stars are grouped in galaxies with rare exceptions.[20] Astronomical observations to date have recorded galaxies at distances of 12 to 16 billion light years from the Sun. The Sun is in the galaxy called the Milky Way (because of its appearance as a faint band of light encircling the night sky) which contains an estimated 100 billion stars. It is estimated that there are more than 100 billion galaxies in the universe and average about 100 billion stars per galaxy. This means that there is an estimated 10 billion, trillion stars in the universe (1021). From Earth however, without using binoculars or a telescope, it is only possible to see about 3,000 of them.

Notes

  1. The distance to Proxima Centauri is over 25 trillion miles (40 trillion kilometres) and light from Proxima Centauri must travel 4.2 years to travel to us before we can see it. One light-year, the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46 trillion kilometres).
  2. What is a star? (2004) Imagine the Universe, Goddard Space Flight Center, NASA
  3. [1] NOrthern Arizona University
  4. Luminosity of a star Cornell University Astronomy
  5. [2] Northern Arizona University, College of Engineering and Natural Sciences.
  6. The simple magnetic field of an ultra-cool starDonati, J.F. & Cameron, A.C. (2006)
  7. Yerkes Observatory
  8. The physical basis of luminosity classification in the late A-, F-, and early G-type Stars. I. Precise spectral types for 372 stars Gray, R.O., Napier, M.G., Winkler, L.I. (2001) The Astronomical Journal April. 121:2148-2158. The American Astronomical Society; MKK are the initials of the authors Morgan, William W., Keenan, Philip C. and Kellman, Edith
  9. Star temperatures are given in units called kelvin. One kelvin (1 K) equals exactly 1 degree Celsius (or 1.8 Fahrenheit). Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 C: 0 K equals -273.15 degrees C, or -459.67 degrees F. A temperature of 0 degrees C (32 degrees F) equals 273.15 K.
  10. red, orange, yellow, green, blue, indigo and violet
  11. least energetic also corresponds with the lowest frequency and the longest wave length and highest energy corresponds with the highest frequency and shortest wavelength
  12. a common instrument in this sort of analysis is an interferometer
  13. The Sun in Time Solar Physics, Marshal Space Flight Center, NASA
  14. Electromagnetic spectrum (2006). Imagine the Universe. High Energy Astrophysics Science Archive Research Center, Goddard Space Flight Center, NASA
  15. What is the electromagnetic spectrum? Landsat 7, Goddard Space Flight Center, NASA
  16. PIA04940: Spectrum from Embedded Star in Herbig-Haro 46/47 Photo Journal, Jet Propulsion Lab, California University of Technology, NASA
  17. A MASSIF Effort To Determine The Mass-Luminosity Relation For Stars of Various Ages, Metallicities, and Evolution StatesTodd J. Henry (Principle investigator, Georgia State Univ.). Masses and Stellar Systems with Interferometry Team
  18. Stars (2003) Curious about Astronomy? Cornell University
  19. Supernovas and remnants Chandra X-ray Observatory
  20. 'Shot in the Dark' Star Explosion Stuns Astronomers NASA