Hertzsprung-Russell Diagram
Created by Captain Alistar McKeon on Sun Jul 24th, 2016 @ 10:15pm
The Hertzsprung-Russell Diagram (HRD) is a plot of a star's brightness and temperature, and can be used to determine its spectral classification.
• ABSOLUTE MAGNITUDE. A very distant, very bright star might appear dimmer than a nearby star that is actually dim because some of the brighter star's light is diffused by interstellar dust and gas. Absolute magnitude is the measure of how bright a star truly is, when observed from a standard distance of exactly 10 parsecs (32.6 light years). Each increment of 5 corresponds to a factor of 100 in brightness, so a star with an absolute magnitude of -10 is 100 times brighter than a star with an absolute magnitude of -5. A star with an absolute magnitude of -5 is 100 million times brighter than a star with an absolute magnitude of +15.
• LUMOSITY. The total amount of light energy emitted by a celestial object when compared to the sun. The sun emits 384,600,000,000,000,000,000,000,000 watts (an ordinary light bulb might emit 100 watts). Thus, an object with a lumosity of 1 emits the same amount of wattage as the sun. An object with a lumosity of 10,000 is 10,000 times brighter than the sun, and an object with a lumosity of 0.0001 emits just a tiny fraction of the light (so it's really dim).
• SURFACE TEMPERATURE. The temperature of a star's surface determines its color. Very hot stars are blue and purple, while cooler stars are orange and red. Though it's not pictured on the above image because it would be too confusing, every spectral class is further divided into one of ten subclasses based solely upon temperature, with 0 being the hottest and 9 being the coolest. So a star classed as type G1 would be a smokin' hot G-type star, while a G9 would at the cooler end of the G-type spectrum.
• SPECTRAL CLASS. A letter assigned to a star to denote its type. There are nine class types (O, B, A, F, G, K, M, L, and T), each of which is associated with a different color; for example, any red star is considered Class M, and any yellow star (like the sun) is Class G. However, since there can be vast differences in size and brightness within a class, stars are further divided into eight more specific groups based upon lumosity.
• Ia: HYPER GIANTS
• Ib: SUPER GIANTS
• II: BRIGHT GIANTS
• III: GIANTS
• IV: SUB-GIANTS
• V: MAIN SEQUENCE
• VI: SUB-DWARFS
• VII: WHITE DWARFS
The first five groups are fairly self-explanitory, with each type representing different varieties of very large stars. These stars are generally quite rare, and most stars fall into the sixth category, the main sequence.
A star in the main sequence is generally a stable, middle-aged star whose dominant energy production process is the fusion of hydrogen in its core. Over the course of its life, a star will spend most of its time within the main sequence, remaining near its initial position on sequence until it has expended a significant amount of hydrogen in its core. At that point, the star will begin to evolve out of the main sequence--typically moving up and to the right on the HR diagram (a class G star like the sun would evolve into a sub-giant or a bona fide giant, while a class O star would transition into a hyper giant).
Class O and B stars that evolve out of the main sequence will eventually die violent deaths in the form of a supernova and become either a black hole or a neutron star (and completely removing it from the HR diagram in the process). Smaller stars like the sun have far less violence in their futures; after a period of vast expansion, the star will just fizzle away, becoming a white dwarf and eventually a lifeless brown dwarf.
So, using the HR diagram, the sun would be classified as type G2V... where G indicates a yellow star, the 2 indicates that it's a pretty hot when compared to other yellow stars, and the V places it on the main sequence.
• ABSOLUTE MAGNITUDE. A very distant, very bright star might appear dimmer than a nearby star that is actually dim because some of the brighter star's light is diffused by interstellar dust and gas. Absolute magnitude is the measure of how bright a star truly is, when observed from a standard distance of exactly 10 parsecs (32.6 light years). Each increment of 5 corresponds to a factor of 100 in brightness, so a star with an absolute magnitude of -10 is 100 times brighter than a star with an absolute magnitude of -5. A star with an absolute magnitude of -5 is 100 million times brighter than a star with an absolute magnitude of +15.
• LUMOSITY. The total amount of light energy emitted by a celestial object when compared to the sun. The sun emits 384,600,000,000,000,000,000,000,000 watts (an ordinary light bulb might emit 100 watts). Thus, an object with a lumosity of 1 emits the same amount of wattage as the sun. An object with a lumosity of 10,000 is 10,000 times brighter than the sun, and an object with a lumosity of 0.0001 emits just a tiny fraction of the light (so it's really dim).
• SURFACE TEMPERATURE. The temperature of a star's surface determines its color. Very hot stars are blue and purple, while cooler stars are orange and red. Though it's not pictured on the above image because it would be too confusing, every spectral class is further divided into one of ten subclasses based solely upon temperature, with 0 being the hottest and 9 being the coolest. So a star classed as type G1 would be a smokin' hot G-type star, while a G9 would at the cooler end of the G-type spectrum.
• SPECTRAL CLASS. A letter assigned to a star to denote its type. There are nine class types (O, B, A, F, G, K, M, L, and T), each of which is associated with a different color; for example, any red star is considered Class M, and any yellow star (like the sun) is Class G. However, since there can be vast differences in size and brightness within a class, stars are further divided into eight more specific groups based upon lumosity.
• Ia: HYPER GIANTS
• Ib: SUPER GIANTS
• II: BRIGHT GIANTS
• III: GIANTS
• IV: SUB-GIANTS
• V: MAIN SEQUENCE
• VI: SUB-DWARFS
• VII: WHITE DWARFS
The first five groups are fairly self-explanitory, with each type representing different varieties of very large stars. These stars are generally quite rare, and most stars fall into the sixth category, the main sequence.
A star in the main sequence is generally a stable, middle-aged star whose dominant energy production process is the fusion of hydrogen in its core. Over the course of its life, a star will spend most of its time within the main sequence, remaining near its initial position on sequence until it has expended a significant amount of hydrogen in its core. At that point, the star will begin to evolve out of the main sequence--typically moving up and to the right on the HR diagram (a class G star like the sun would evolve into a sub-giant or a bona fide giant, while a class O star would transition into a hyper giant).
Class O and B stars that evolve out of the main sequence will eventually die violent deaths in the form of a supernova and become either a black hole or a neutron star (and completely removing it from the HR diagram in the process). Smaller stars like the sun have far less violence in their futures; after a period of vast expansion, the star will just fizzle away, becoming a white dwarf and eventually a lifeless brown dwarf.
So, using the HR diagram, the sun would be classified as type G2V... where G indicates a yellow star, the 2 indicates that it's a pretty hot when compared to other yellow stars, and the V places it on the main sequence.
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