Useful Notes: Stars
Stars are large balls of mostly hydrogen. They are so massive that their insides grow really hot, and fusion occurs. For specific examples, see the Useful Notes pages on:
Magnitude- Apparent and AbsoluteMagnitude is the brightness of a star in the form of a number. Created by Hipparchus or Ptolemy, a star of magnitude 1 is about 2.512 times (the 5th root of 100) brighter than one of magnitude 2. It used to be that 1 was the highest, and 6 was the lowest, but they found that this didn't quite work and had to go into negative numbers, hence the Crab Supernova having -6. Most famous stars are between 0 and 2. The human eye can see to about 6, with 7.72 being the current world record. Vega is approximately magnitude 0. Sirius, the brightest star in the night sky is at -1.46. -3.9 is the faintest you can see in daylight, Venus ranges from -3.8 to -4.9 (which means it's always brighter than any star apart from the Sun), the Moon is -12.6 when it's full and the Sun is -26.73. This is the apparent magnitude of course. The farther away the same kind of star is, the dimmer it's going to appear. If a star that's 10 light-years away has an apparent magnitude of 1.5, that same star would have an apparent magnitude of 3.0 if you moved it 20 light-years away. To judge the true, intrinsic brightness of stars, the magnitude that they would be at if 10 parsecs (32.6 light years) away is determined and called absolute magnitude. For example, Alpha Centauri has an absolute magnitude of 4.38, but an apparent one of -0.27, as it is very close to Sol, relatively speaking (4.3 light years). Far-off (700-900 light years) Rigel has an apparent magnitude of 0.18, but at the 10 parsec distance it shines at absolute magnitude -6.7. (There is also "planetary absolute magnitude", which is the brightness a planet would show at full illumination from 1 astronomical unit - that is, if the planet was moved to the Earth's orbit and viewed from the center of the Sun.)
Distance determinationNearby stars have it determined by something called trigonometric parallax. It's basically trigonometry that uses the relative movements of objects against a "fixed" background of far-off stars and galaxies- the closer the near star is, the more it will seem to move (you can see this on a train or in a car). Because of the distances involved, the change in angle is very small, less than a second of arc (or 1/3600 of a degree). This is the source of the unit of distance known as a "parsec" it's the distance [3.26 light years] at which an object viewed from two points one AU apart would show a parallax of one second of arc. Most stellar distances are calculated by taking two observations six months apart (with a baseline of 2 AU), but plans exist to send out probes to provide baselines of tens or even hundreds of AU. But after a certain point current instruments simply can't measure the infinitesimal parallax of truly far away bodies. That's where we turn to an indirect means called the standard candle — we look for an object whose intrinsic brightness (i.e. absolute magnitude) is known, then use how dim it appears (i.e. its apparent magnitude) to calculate how far away it must be. Not many objects have predictable brightnesses, but one of the few that do is a type of giant star known as a Cepheid variable. These stars pulse regularly at a rate proportional to their intrinsic luminosity, and some of them are also close enough to have measurable parallax. Therefore, far-off Cepheids (and objects such as the nebulae or galaxies they reside near or within) can have their distance judged simply by noting their pulse rate. Some stars that aren't Cepheid variables can at least give us a ballpark hint as to their intrinsic brightness by certain clues in their spectra. When we look at the spectrum of a red star, for example, we can tell whether it's a nearby red dwarf or a distant red giant. While we can't calculate its intrinsic brightness precisely, we can figure it within a couple of orders of magnitude. This trick goes by the rather confusing name of "spectroscopic parallax", even though no actual parallax is involved.
Heavy element abundanceAbout 1% of the mass of the sun consists of elements heavier than hydrogen and helium: lithium, beryllium, carbon, nitrogen, oxygen, iron, et cetera. Collectively, astronomers refer to all elements heavier than helium as "metals", even those that aren't metals in the chemical sense. And as it turns out, the abundance of these heavy elements varies from star to star, depending on the chemical composition of the primordial gas-and-dust cloud from which the star originally condensed. We can tell how much of an element is present in the photosphere of a star by looking at its spectrum. Every element has a characteristic signature, a fingerprint if you will, of absorption/emission lines (sometimes called Fraunhofer lines after their discoverer) at very specific frequencies. Sodium, for example, has two very strong lines right next to each other at 5895.92 and 5889.95 Εngstroms. By looking at how prominent these lines are in the spectrum of a star, relative to their prominence in the spectrum of the sun, we get a pretty good measure of the relative abundance of that element. The overall picture of every heavy element's relative abundance is known as the "metallicity" of the star. Most of the stars that lie along the galactic plane are rich with heavy elements, like the sun is. These stars are now known as the galactic plane population; in earlier times, they were referred to as "Population I". These stars are second- or third-generation — that is, the gas-and-dust cloud out of which they formed was spewed out by the death throes of an earlier star, in which heavy elements had been synthesized. First generation stars, such as those that orbit the galaxy 'way out in the galactic halo, are much poorer in heavy elements, and used to be referred to as "Population II". The heavy element abundance of a star is important for two reasons:
- More massive stars can take advantage of carbon in their cores, and use it as a nuclear fusion catalyst to vastly speed up the rate at which they burn hydrogen into helium. This nuclear catalyst process is known as the CNO cycle. This makes them much brighter, but also severely shortens their main-sequence lifetimes. A star like the sun (too small for the CNO cycle) will burn for 10 billion years before it evolves off the main sequence, but a big bright star like Sirius won't last longer than about 300 million years total. A Sirius-mass star that lacked heavy elements, though, might last for a few billion years.
- It's not just the star that formed out of that original gas-and-dust cloud. Any planets that form around that star will have also formed out of the same cloud. Thus, if the star is poor in heavy elements, the planets orbiting it will also be poor in heavy elements. There might not be enough carbon, oxygen, silicon, etc. to form rocky planets at all; you might just be left with a few hydrogen-helium gas balls orbiting it.
NamesStars have a variety of naming methods:
- Traditional or "proper" names: Most ancient cultures had names for the brighter stars, but generally the ones assigned by the ancient Greeks (Arcturus, Sirius, Procyon, etc.) and medieval Arabs (Betelgeuse, Altair, Deneb, Rigel, etc.) are the ones you'll find on star charts today. Many of these star names start with "Al", because "al" is the Arabic word for "the." Others have received proper names more recently - Alpha Pavonis was named "Peacock" by the Royal Air Force because it didn't have a well known one yet, and it was needed on maps as a navigation star for pilots operating in the Southern Hemisphere.
- The Bayer designation: A Greek letter followed by the genitive case of the Latin constellation name, such as "Alpha Centauri" or "Epsilon Eridani". The brightest star in the constellation is named Alpha, the second-brightest Beta, the third-brightest Gamma, et cetera. (There are exceptions, though Beta Orionis (Rigel) is usually brighter than Alpha Orionis (Betelgeuse), for example, and the stars in the Big Dipper are given Greek letters according to their position in the dipper rather than their relative brightness.) Since this naming scheme pre-dates the astronomical use of the telescope, they are only used for stars visible with the naked eye. Some of these objects turned out to be 2 different stars that lay along the same line of sight; their modern names carry an additional superscript to tell the two objects apart, e.g. Zeta1 Reticuli and Zeta2 Reticuli.
- The Flamsteed designation: A number followed by the genitive constellation name, such as "51 Pegasi" or "40 Eridani." As with the Bayer designations, this naming scheme pre-dates the use of telescopes in astronomy, and thus only applies to stars visible with the naked eye. Flamsteed designations
seem to be arbitrarywere originally supposed to have the number increase from west to east, but precession and proper motion have changed the position of some stars relative to celestial longitude since the time the designations were made. Some of them weren't stars at all; for example, what Flamsteed included in chart as "34 Tauri" turned out to be the planet Uranus - as a sidenote, this error left that designation available for the star at the heart of a certain 'Verse...
- Variable stars that don't already have Bayer or Flamsteed designations have their own wonky naming scheme. They're named in order of discovery, starting with "R" — R Ceti was the first variable star discovered in Cetus, S Ceti was the second variable star discovered in Cetus, etc.. When they get to Z, they go back and use RR through RZ, then SS through SZ, then TT through TZ, etc.. When they get to ZZ, they go back and use AA through AZ, then BB through BZ, etc., all the waynote to QQ through QZ. Finally, if still more variable stars are discovered in the constellation, they give up and name the next ones V335, V336, V337, etc..
- Catalog numbers - usually a prefix followed by an ID number or coordinate reference. Some catalogs include the Henry Draper (HD), Smithsonian Astrophysical Observatory (SAO) and "Bonner Durchmusterung" (BD). Early catalogs contained fewer stars, and are only referred to if the star in question first appeared there, e.g. "Wolf 359."
- Single stars can also be named after the individual who discovered them - usually they have some notable property. Examples include Barnard's Star (which has the highest proper motion of any star) and Herschel's Garnet Star (also known as Mu Cephei, it possesses a vivid red hue).
Types of starsMost stars are found along something called the Main Sequence, characterized by their balance between inward gravity and outward pressure generated by hydrogen fusion. Other stars exist that are off of it and fusing other elements, or else are dead or dying. These types have varying letters (spectral classifications) applied to them, with numerical sub-groups and a corresponding informal color (you can see the color in a good telescope). The higher up the scale, the bigger (and brighter) the star, but the faster the rate at which the hydrogen in them is used up and so the shorter their lifespan. For a number of reasons, very large stars - called giants, supergiants and hypergiants - like to live at the extreme ends of the spectral scales. Giant stars really do not like to be classes F or G, seeming to stay there for a very short time while heating up or cooling down to either extreme. There are a few known, but are decidedly in the minority. In general, most of the visible giants are either class-M or rather cool class-K (Betelgeuse, Arcturus) red giants, or class-B (Eta Carinae, Rigel, Deneb) blue giants.
- Main Sequence Classifications, in order from hottest to coolest:
- O Blue-violet stars. The hottest and most massive main sequence stars, with most of their energy output in the ultraviolet regions of the spectrum. Pretty rare, but also conspicuous. Delta Orionis and Zeta Puppis (Naos) are examples.
- B Blue-white stars, e.g. Rigel, or all the bright stars in the Pleiades.
- A White stars. Sirius A and Vega are examples.
- F Yellow-white stars. Upsilon Andromedae and Procyon are of this type. Canopus is a rare class F giant.
- G Yellow stars. The most famous is a G2V type known in Latin as Sol, and in English as The Sun. Alpha Centauri A, Tau Ceti and Zeta Reticuli are this type as well.
- K Orange stars. Alpha Centauri B, Epsilon Eridani.
- M Red dwarf stars. (No, not that one.) Have very long life-spans i.e. a trillion years. Proxima Centauri and Barnard's Star are of this type. Luminosity class is always V or VI, as more massive types are actually red giants, an entirely different kind of creature.
- Non-Main Sequence Classifications:
- W Wolf-Rayet stars, former O-types which have long expended the hydrogen in their core and are using more exotic heavy element fusion, shedding mass rapidly and likely to go supernova sooner or later. They undergo Type-Ib/c core-collapse supernovae, generating spectacular gamma ray bursts. They have an onion like structure with helium on the outside and progressively heavier elements all the way to their core, which ultimately becomes iron just before exploding.
- K or M - Red giant stars - classified as having a similar spectrum to M-type or cooler K-type red dwarfs but with luminosity classes I through IV. These are actually former B- through K-types that have expended their hydrogen and are fusing helium now. Their surfaces are cooler than they were in their hydrogen-fusion days, causing them to shift down the color spectrum to look like red dwarfs even though they may have originally been larger and hotter. They are quite diverse: types of red giant include ordinary, still hydrogen-fusing red giant, red clump giant that starts to fuse helium, assymptotic red giant - a bright giant that already has a carbon core, and red supergiant, which is many-layered and fuses various elements up to silicon. They may either shed their outer layers and become white dwarfs upon expending their helium, or else undergo a Type-II core-collapse supernova. The main difference with Wolf-Rayets, apart from being less massive, is that they still have hydrogen in their outer layers when they explode, and their explosions are less energetic. For eg., Arcturus, Antares, Betelgeuse. Betelgeuse, in particular, is presently on course for a supernova in the near future (read: sometime between tomorrow and a million years) that from Earth would appear brighter than the full Moon.
- D White dwarfs. (No, not that one.) Small, dead stars that aren't undergoing fusion at all, but are held up against gravity by electron degeneracy pressure, created by quantum effects regarding electrons. Their average densities are on the order of a ton to the cubic centimeter, and the surface gravity is usually around 100,000 G's. They are generally made of a dense core of carbon and oxygen and lack the gravity to fuse these elements further. Sirius B is one of these, there's another orbiting 40 Eridani. This is the final fate of most small to medium stars, including the Sun. Binary white dwarfs can do a bit more though - they may siphon off hydrogen from their companion stars, heating it on their surfaces until it undergoes flash fusion in an event called a nova. If the white dwarf accretes enough material to exceed 1.44 solar masses, electron degeneracy will no longer be able to support the star's weight, and the whole star will collapse in a spectacular explosion called a Type-Ia supernova.
- L or T Brown dwarfs. (No, not Gary... wait, never mind.) Collections of gas that never got big enough to start proper fusion reactions. They usually have some sub-par fusion like deuterium-deuterium and leftover heat of formation to give off a dull magenta glow, but cannot fuse ordinary hydrogen (protium, i.e, raw protons). Sort of intermediate between huge gas giant planets and true stars. The exact amount of mass needed to sustain fusion and not just peter out like this is unclear. A very few actual hydrogen-fusing red dwarfs may be cool enough to fall under the early L-class, as well.
- Neutron stars. The dead remains of heavier stars, known for being extraordinarily dense (a billion times denser than a white dwarf) and occasionally spinning at extraordinary speeds while blasting out radio noise and generating intense magnetic fields (a.k.a., pulsars and magnetars). They are formed from core-collapse supernovae and are composed of pure neutronium, as their name suggests, with whatever charged particles that may be left strewn over their surfaces. Like white dwarfs, they are stabilized by quantum effects, except with neutrons instead of electrons. Also like white dwarfs, neutron stars in binary systems can accrete material from their companion, and eventually ignite it — except that while a white drawrf that does so can erupt as a nova every few years, a neutron star that does so will erupt as an X-ray burster every few minutes. A neutron star will be the final fate of most of the larger red/blue giant stars, like Spica, Betelgeuse or Rigel.
- The Magnetar is a common neutron star variation - a star with a magnetic field thousands of times stronger than a vanilla neutron star. However, their magnetic fields decay extremely rapidly in stellar terms, and after about 10,000 years their activity ceases, which leads to estimates of up to 30 million inactive magnetars in the galaxy. Their magnetic field is strong enough to instantaneously rip all of the iron from a person's blood cells from a thousand kilometers away.
- Pulsars pulse electromagnetc radiation at extremely precise intervals along a thin beam as the star rotates, effectively turn it into an enormous stellar lighthouse. Their period of rotation is precise enough to rival atomic clocks, and they make for useful interstellar navigation points - the Voyager Golden Record◊ and the Pioneer plaque note have a pulsar map of relative locations and frequencies which can be used to locate the Sun, if intelligent aliens were to discover it.
- Black Holes. The ultimate product of core-collapse supernovae, these were once stars whose core gravity was so strong that no force in the known universe could oppose it, so they contracted all the way down into gravitational singularities. This is the final fate of extremely large stars, like Eta Carinae or probably Deneb.
- B VI - Blue subdwarfs. They're spectral class B, and are hence blue-white, but are much smaller and less luminous than the main sequence B-types. They form either when two white dwarfs merge and core gravity needed to restart reactions, or when a companion star strips a relatively small red giant of its outer layers. As a result, they have no hydrogen, and are fusing helium, or carbon/oxygen. They are longer-lived than the usual kind of blue stars, but still no more than some millions of years. More commonly found in globular clusters than in the main galaxy, since star densities there are higher and make the interactions needed to create blue subdwarfs more common.
- Black dwarfsnote . White dwarfs that have finally radiated away all their residual heat. As best we can tell, there aren't any of these yet, since best estimates indicate it would take at least a quadrillion years for one to come about.
- Blue dwarfs. There aren't any of these yet, either. A blue dwarf is what you get when a small red dwarf is out of fuel. It turns hotter for a brief time, becoming a blue dwarf, then shrinks and becomes inert, turning into a small white dwarf. Such stars never had the mass to even enter the red giant phase, and so instead have this as their intermediate stage before death. Such stars however are also extremely long-lived, and none of them have died yet.
- 0. Hypergiants like Eta Carinae or the Pistol Star.
- I. Supergiants like Betelgeuse or Rigel.
- II. Bright giants like Dabih (which is a rare class G giant).
- III. Giants like Arcturus.
- IV. Subgiants like Procyon. These tend to be main sequence stars that have run out of hydrogen to fuse and are expanding toward their red giant stage.
- V. Main sequence stars like the Sun or Sirius. These are sometimes called "dwarf stars", though in fact, those of spectral class G or hotter outshine 80-90% of all stars in the galaxy. (Because most of them are dim red dwarfs).
- VI. Subdwarf stars. There aren't very many of these known - this class is mostly used to designate odd, metal-poor stars. Their low metallicity lets radiation escape easier; consequentially there's less support for the star's outer layers and they tend to shrink and become hotter. This results in a net 1-2 magnitude drop from the brightness a "normal" dwarf star would demonstrate; hence the term "subdwarf".