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SPECTRAL CLASS: A star's spectral class depends on the temperature, and thus basically upon its mass. More massive stars are hotter. But more massive stars are also less common. Spectral classes are subdivided into numeric distinctions from 0 to 9, where a star with 0 is hotter than one with 9. O: These very massive blue stars are also very rare. One of the closest to Earth is Mintaka in the belt of Orion, almost 1000 LY away. B: Blue-white massive stars. B-stars are also uncommon. One of the closest to Earth is Alpha Gruis, slightly more than 100 LY away. A: White stars. Sirius, Vega and Altair are of this type. F: Yellow-white stars slightly larger than the Sun. Usually considered the most massive stars capable of harboring Earth-like life. G: Yellow stars. Our sun is class G2, while Alpha Centauri is G0. K: Orange stars. They are less massive and cooler than the Sun. Epsilon Eridani is a typical example. M: Small red stars often called red dwarves. They are very common and faint. Proxima Centauri and Barnard's Star are typical. Some very cool red stars are "L"-class, and these could well be brown dwarves or their close relatives. SPECIAL STELLAR TYPES White Dwarves: These are very dense, hot and small (in size, not mass) stars which are formed from old stars. White dwarves are common - about 10% of all stars. A young white dwarf is very hot but it gradually cools off, so the surface temperature indicates age of the dwarf, not primarily mass. Interestingly enough, the more massive a white dwarf is the smaller it also is. Very young white dwarves are among the hottest stars known, but that phase is fairly short. White dwarves don't have habitable planets, but it is possible that distant cold worlds could survive the stellar evolution. The closest white dwarf to the sun is Sirius B. Brown Dwarves: "Stars" which are too small to ignite stellar fusion in earnest. They range in size from 0.013 to 0.08 solar masses. They have a brief period of deuterium burning, but after that generate energy by gravitational contraction. Brown dwarves are cooler than real stars and they, like white dwarves, cool off with time. Young brown dwarves are substantially brighter and hotter than the older ones found. Unlike planets, brown dwarves can be formed separately, like stars. (Sometimes "brown dwarf" is used for all objects larger than 1.5 Jupiter masses and below 80 Jupiter masses, but here the term is limited to the bigger objects that can form independently). Brown dwarves radiate infrared heat much more than visible light. In radius, brown dwarves are probably smaller than Jupiter, or about as large - despite being more than ten times more massive. This may seem strange, but the gravitation of a brown dwarf is enough to compress it to much greater densities. (And small red stars are much denser than the Sun - or the Earth for that matter). Few brown dwarves have been found, but they are believed to be common though hard to detect. Wolf-Rayet Stars: These are O-class stars which seem to have a gas envelope. Such envelopes are also sometimes found around B-stars and is probably ejected from the star. Coal Stars: Spectral classes R, N and S are uncommon very cool stars. C-stars (carbon stars): Of spectral types R & N, these stars seem to be rich in carbon and carbon compounds. Many are found in the Magellanic clouds, where they are more common than in our galaxy. C-stars roughly conform to G4-M9 spectral classes in terms of size. S-stars: These are usually very cool red giant stars with an abundance of zirconium oxide and lantanum oxide. Peculiar A-stars: These A-class stars have very strong absorbtion lines of metals. Flare Stars: These are M-class stars (ranges M3 to M9) which periodically increase in luminosity by 1d10 x 50% for a short time. About half (1-5 on 1d10) of all dim red stars may be flare stars. The increase is due to large solar flares being considerably hotter than the star, and thus richer in visible, UV and X-ray radiation. Flare stars may provide problems for life on close planets to cope with the increased radiation. Flare stars near us include Proxima Centauri, UV Ceti B, Wolf 359 and Ross 154, all closer than 10 LY. Protostars: These are stars in the process of initial contraction towards the main sequence. Protostars are brighter but also cooler than the star they eventually will become as they generate heat by gravitational contraction and not by nuclear fusion, and they have not formed any real planetary systems. Contraction goes much faster for a massive star than for a red dwarf, which will take hundreds of millions of years to contract. A young (<Gy) red dwarf is thus very similar to a brown dwarf. Strong Magnetic Fields: About 1 in 10 A & B-class stars have very strong and variable magnetic fields. Hot stars often rotate very rapidly (up to 100 times faster than the sun) and may be noticeably flattened. Novae: A nova is a white dwarf which has a companion which loses mass to it ... usually a giant star. When a white dwarf receives this matter it eventually sets off hydrogen burning and blows off gas. The longer time between these flashes the stronger it tend to be. A nova can reach up to 100,000L, but generally this is less. If the white dwarf receives so much matter it passes the 1.4 solar mass limit it becomes a supernova of the most violent kind, a Type Ia, which tears the white dwarf apart (along with the companion). The other kind of supernova exists when a massive star "dies" and forms a neutron star or black hole. Pulsar: A fast-rotating neutron star. Pulsars are young neutron stars which have not slowed down and a neutron star is an object so densely packed it is made up of neutrons ... far denser than even a white dwarf. When neutron stars undergo disasters ... perhaps collision or merger they produce large amounts of gamma radiation Magnetar: This is a kind of neutron star with an iron shell. It generates intense magnetic fields. Other Objects: Neutron stars and black holes are formed from very massive stars, and thus they are uncommon. The closest known neutron star (pulsar) is over 400 LY away, though one may find one or two within 100 LY. Protostars are stars in the process of contraction to stellar fusion - they are also rather uncommon as this stage is very short compared to a star's total lifetime. Black dwarves are white dwarves that have cooled off, but it is doubtful if any white dwarf has cooled off enough during the life span of our Galaxy. SIZE TYPE: Most stars are main sequence (size type V). The Sun, Sirius and Proxima are all main sequence. This is the stage where most stars spend the majority of their active "lives". When a star leaves the main sequence to become a red giant it becomes a subgiant (IV) and later a giant (III). Larger giants formed from massive stars can be bright giants or supergiants (II, Ib, Ia) but as we already have mentioned they are rare. Typical giant stars include Pollux and Arcturus (K-class III). Giants and subgiants are less predictable concerning size and luminosity - they vary considerably more within their parameters than the main sequence stars. Older classification used code VI for subdwarves (very old main sequence stars lacking in heavy elements) and VII for white dwarves. |
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HERTZSPRUNG-RUSSELL DIAGRAM: The Hertzsprung-Russell diagram is a way of showing the relation between spectral class (and thus temperature) and luminosity. |
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Brown dwarves would exist off the lower right corner of the H-R diagram. Sun-size stars spend most of their time on the main sequence, then move up and to the right on the diagram, becoming subgiants and red giants. More massive stars move more to the right than up. The actual development of stars in their final stages is complex, and involves several distinct processes which causes the star to vary in temperature, radius and luminosity. A decent astronomy textbook is recommended for further insight (see References at the end of the document for suggestions). Note how the subgiant branch stops at KO, as less massive stars not yet have had time to evolve to subgiants. SURFACE TEMPERATURE: Stars vary in temperature with spectral class. Subgiants and giants of a spectral class are cooler than main sequence stars of the same spectral class. The temperature of a star is primarily important to calculate its radius, as shown below. STELLAR RADIUS: Compared with the sun, use the following equation:
CONVERTING MAGNITUDE TO LUMINOSITY: If you try to detail a star system where you already know the
absolute magnitude of the star, it might be useful to know how to recalculate
that value into luminosity (compared to the Sun). As every step in magnitude
indicate a 2.512 increase in luminosity, this doesn't seem to be very hard.
However, for stars cooler or hotter than the sun much of their bolometric
luminosity is in the UV or IR part of the spectrum, and thus a small red star
would end up much less luminous than it really is for purposes of system
generation. To solve this a bolometric correction (BC) is added to the absolute
magnitude (a correction depending upon the temperature - i.e. spectral class -
of the star) and luminosity is calculated afterwards. The bolometric correction
has been fitted to: MASS-LUMINOSITY RELATIONSHIP: For main sequence (V) stars, there is a connection between mass and luminosity. If a star has a mass of 0.5 to 4 solar, it has an approximate luminosity of M4. For more or less massive stars, the connection is about M3.3. This can be used to calculate the mass of a main-sequence star from its luminosity. GENERATING STELLAR NEIGHBOURHOODS: Using our own stellar neighbourhoos as a template, and considering not all faint red dwarf stars nor nearly all brown dwarves are discovered, a 10LY cube (1000 cubic light years) would contain 1-5 star systems, including brown dwarves. About half of these systems would be binaries or multiple stars. For random generation, just make a cube, roll 1d5 (1d10/2) and place the systems by random generation of X, Y and Z-axis. INTERSTELLAR GAS & DUST: The galaxy contain not only stars but also gas, dust and molecular clouds. The solar neighbourhood (about a 300 LY radius) is sparse in interstellar gas, though there is a warm cloud of gas about 70 LY away, towards the center of the galaxy. About 20% of the galaxy is in such warm clouds, which may have an average radius of tens of LY. If gas is close to a hot star ir will be visible as a nebula, but most gas clouds are not visible as such. Interstellar dust is found everywhere, but the densest areas are dark nebulae. |