Stellar classification
From Memory-Zeta
There are billions of stars in the universe, most falling into one of the following categories.
Contents |
[edit] Main Sequence
The majority of stars in the universe fall into the Main Sequence of stars. This table goes from Red Dwarf stars (K-class) to Blue Giants (O-class).
Mass, Radius and Luminosity are expressed in multiples of Sol.
| Class | Temperature (K) | Stellar color | Apparent color | Mass (x Sol) | Radius (x Sol) | Luminosity (x Sol) |
|---|---|---|---|---|---|---|
| O | 30,000-60,000 | Blue | Bluish | 60 | 15 | 1,400,000 |
| B | 10,000–30,000 | Blue | Bluish | 18 | 7 | 20,000 |
| A | 7,500–10,000 | Blue-white | Bluish | 3.1 | 2.1 | 80 |
| F | 6,000–7,500 | White | Bluish | 1.7 | 1.3 | 6 |
| G | 5,000–6,000 | Yellow-white | White | 1.1 | 1.1 | 1.2 |
| K | 3,500–5,000 | Orange | Reddish | 0.8 | 0.9 | 0.4 |
| M | 2,000–3,500 | Red | Reddish | 0.3 | 0.4 | 0.04 |
[edit] Red Giants
Red Giants are usually main-sequence (spectral type O-K) stars that are nearing the end of their lives. Sol is expected to become a Red Giant in 4.5 billion years.
These stars are typically around 1 solar mass while expanding to 1000 times the volume. This causes the star’s photosphere to slowly blend into the corona – making it difficult to distinguish sharply-contrasting limbs as with a main-sequence star. Luminosity also increases 1000-10,000 fold.
[edit] Supergiants / Hypergiants
Supergiants can have masses from 10 to 70 solar masses and brightness from 30,000 up to hundreds of thousands times the solar luminosity. They vary greatly in radii, usually from 30 to 500, or even in excess of 1000 solar radii. The Stefan-Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants; thus, for a given luminosity red supergiants are larger than their blue counterparts. Hypergiants are at least as large as supergiants, having masses up to 100 times that of the Sun. Hypergiants are not necessarily larger than supergiants, but are usually more massive. This approaches the Eddington limit, a theoretical upper limit of stellar mass (about 120 solar masses) at which a star generates so much radiation that it throws off its outer layers. Some hypergiants appear to be more than 100 solar masses and may have initially been 200 to 250 solar masses, challenging current theories of star formation and evolution. Hypergiants are the most luminous stars, thousands to millions of solar luminosities; however, their temperatures vary widely between 3,500 K and 35,000 K. Almost all hypergiants exhibit variations in luminosity over time due to instabilities within their interiors at moderate temperatures and high pressures.
[edit] White Dwarfs
A white dwarf is the kind of star which a main-sequence star of low or medium mass will become in the last stage of its evolution. When such a star has become a red giant during its helium-burning phase, in which helium is fused to carbon and oxygen by the triple-alpha process, it generally has insufficient mass to generate the core temperatures required to fuse carbon. After shedding its outer layers to form a planetary nebula, it will leave behind an inert core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen, although some helium white dwarfs appear to have been formed by mass loss in binary systems. It is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen-neon-magnesium white dwarf may be formed. The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported against gravitational collapse by the heat generated by fusion. It is supported only by electron degeneracy pressure and is therefore extremely dense, with a typical mass on the order of the Sun's contained in a volume on the order of the Earth's. The physics of degeneracy yields a maximum mass for a white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by degeneracy pressure. A carbon-oxygen white dwarf which approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation. A white dwarf is very hot when it is formed, but since it has no source of energy, it will gradually radiate away its energy and cool down. This is the source of its faint luminosity, which may however have a high color temperature. Over a very long period of time, it will cool to temperatures at which it is no longer visible. However, since no white dwarf can be older than the age of the Universe (approximately 13.7 billion years) even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins. As a class, white dwarfs are fairly common; they comprise roughly 6% of all known stars in the solar neighborhood. White dwarfs are thought to be the final state of over 97% of all stars in our galaxy.
[edit] Neutron Stars
Neutron stars are one of many possible end states after the death of a star. These super-dense objects form from stars that are between 1.35 and 2.1 solar masses, and condense this matter into a sphere that is 10 to 20 km wide. Neutron stars also preserve the angular momentum of the parent star, and through the law of conservation of energy, the neutron star ends up with a rotational period between 1/700 sec to 30 sec.
The neutron star's compactness also gives it very high surface gravity, 2×10^11 to 3×10^12 times stronger than that of Earth. One measure of such immense gravity is the fact that neutron stars have an escape velocity of around 150,000 km/s, about 1/2 of the speed of light. Matter falling onto the surface of a neutron star would strike the star also at 150,000 km/s, and then be crushed under its own weight into a puddle less than an atom thick.
Magnetars
Magnetars are neutron stars with extremely powerful magnetic fields, on the order of more than 10^11 teslas (or 10^15 gauss).
A magnetic field above 10 gigateslas is strong enough to wipe a credit card from half the distance of the Moon from the Earth. A small neodymium based rare earth magnet has a field of about 1 tesla, Earth has a geomagnetic field of 30-60 microteslas, and most media used for data storage can be erased with a millitesla field at very short range.
The magnetic field of a magnetar would be lethal at a distance of up to 1000 km, tearing tissues due to the diamagnetism of water. Tidal forces of a 1.4 solar mass magnetar would also be lethal at such a distance, pulling an average-sized human apart with a force of over 20 kilonewtons (over 4500 pounds-force). As described in a 2003 Scientific American cover story, remarkable things happen within a magnetic field of magnetar strength:
X-ray photons readily split in two or merge together. Also, when entering the magnetic field, photons of polarized light change speed (and therefore wavelength). Since the field can prevent electrons from vibrating as they normally do in response to light, light waves can slip past electrons without losing energy. Empty space itself experiences vacuum birefringence, gaining the ability to split light into different polarizations (like an immaterial version of a calcite crystal).
The field also stretches atoms into long cylinders. In a field of about 105 teslas atomic orbitals deform into cigar shapes. At 10^10 teslas, a hydrogen atom becomes a spindle 200 times narrower than its normal diameter. The active life of a magnetar is short. Their strong magnetic fields decay after about 10,000 years, after which point activity and strong X-ray emission cease. The Galaxy is thus probably full of dead magnetars.
Pulsars
Pulsars are rotating neutron stars which emit detectable electromagnetic radiation in the form of radio waves. They may rotate at up to hundreds of times a second. The radiation intensity varies with a regular period, believed to correspond to the rotation period of the star. Pulsars also exhibit a so-called lighthouse effect, which occurs when the light and other radiation from a pulsar are only seen at specific intervals and not all of the time.
[edit] Quark Star
These theoretical objects are formed from the collapse of stars of densities between 2 and 5 solar masses. These stars would be completely comprised of Quarks and as such would have unusual characteristics.
[edit] Protostars
A Protostar is an object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a solar-mass star it lasts about 100,000 years. It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star, which then develops into a main-sequence.
[edit] Black Hole (Singularity)
Black holes are formed by the collapse of stars in excess of 5 solar masses. Black holes have extremely large mass and are extremely dense. Gravity is so strong with these objects that their escape velocity is in excess of the speed of light. Therefore, nothing can escape.
Singularity at a single point
According to general relativity, a black hole's mass is entirely compressed into a region with zero volume, which means its density and gravitational pull are infinite, and so is the curvature of space-time which it causes. These infinite values cause most physical equations, including those of general relativity, to stop working at the center of a black hole. So physicists call the zero-volume, infinitely dense region at the center of a black hole a "singularity".
The singularity in a non-rotating, uncharged black hole is a point, in other words it has zero length, width and height.
But there is an important uncertainty about this description: quantum mechanics is as well-supported by mathematics and experimental evidence as general relativity, and does not allow objects to have zero size - so quantum mechanics says the center of a black hole is not a singularity but just a very large mass compressed into the smallest possible volume. At present we have no well-established theory which combines quantum mechanics and general relativity; and the most promising candidate, string theory, also does not allow objects to have zero size.
The rest of this article will follow the predictions of general relativity, because quantum mechanics deals with very small-scale (sub-atomic) phenomena and general relativity is the best theory we have at present for explaining large-scale phenomena such as the behavior of masses similar to or larger than stars.
Photon sphere
A non-rotating black hole's photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times larger than the radius of the event horizon. No photon is likely to stay in this orbit for long, for two reasons. First, it is likely to interact with any infalling matter in the vicinity (being absorbed or scattered). Second, the orbit is dynamically unstable; small deviations from a perfectly circular path will grow into larger deviations very quickly, causing the photon to either escape or fall into the hole.
Other extremely compact objects such as neutron stars can also have photon spheres. This follows from the fact that light "captured" by a photon sphere does not pass within the radius that would form the event horizon if the object were a black hole of the same mass, and therefore its behavior does not depend on the presence of an event horizon.
Accretion disk
Space is not a pure vacuum - even interstellar space contains a few atoms of hydrogen per cubic centimeter. The powerful gravity field of a black hole pulls this towards and then into the black hole. The gas nearest the event horizon forms a disk and, at this short range, the black hole's gravity is strong enough to compress the gas to a relatively high density. The pressure, friction and other mechanisms within the disk generate enormous energy - in fact they convert matter to energy more efficiently than the nuclear fusion processes that power stars. As a result, the disk glows very brightly, although disks around black holes radiate mainly X-rays rather than visible light.
Accretion disks are not proof of the presence of black holes, because other massive, ultra-dense objects such as neutron stars and white dwarfs cause accretion disks to form and to behave in the same ways as those around black holes.
[edit] Multiple star systems
Star systems composed of two or more stars have their own interesting characteristics. A large percentage of stars are part of a multiple star system. Nearly half of these are triple-star systems, or trinaries. The highest number of stars known in one system is six, the system known as Castor, 50 light-years from Sol. Castor has three pairs of stars orbiting a barycenter.
Multiple star systems can be comprised of any combination of stellar objects, including main sequence stars, neutron stars, black holes, and others.
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