Encyclopedia
A
star is a massive, compact body of
plasma in
outer space that is held together by its own
gravity and, unlike a
planet, is sufficiently massive to sustain
nuclear fusion in a very dense, hot core region. This fusion of
atomic nuclei generates the
energy that is continuously radiated from the outer layers of the star during much of its life span.
Individual stars differ in their total mass, composition, and age. The total mass of a star is the principal determinant in its evolution and eventual fate. A
Hertzsprung-Russell diagram shows the pattern of the temperature of stars against their absolute magnitude, and can be used to determine the age of a star and the stage in its evolution. Initially, stars are composed primarily of
hydrogen, with some
helium and heavier trace elements that determine their metallicity. Over the course of a star's evolution, a portion of the hydrogen is converted into helium and smaller quantities of heavier elements through the process of nuclear fusion. Part of the matter is then recycled into the interstellar environment and used to form a new generation of more metal-rich stars.
Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. For example, a
nova occurs when a white dwarf accretes matter from a companion star.
Observation history
Stars have been important to every culture. They have been used in religious practices and for
celestial navigation and orientation. The
Gregorian calendar, used nearly everywhere in the world, is a solar calendar based on the position of the
Earth relative to the nearest star, the
Sun.
Early astronomers such as
Tycho Brahe identified new stars in the heavens, suggesting that the heavens were not immutable. In 1584
Giordano Bruno suggested that the stars were actually other suns, and may have Earth-like planets in orbit around them. By the following century the idea of the stars as distant suns was reaching a consensus among astronomers, and it would be the theologian
Richard Bentley who would prompt
Isaac Newton to suggest that the stars were equally distributed in every direction, resulting in no net gravitational pull.
The Italian astronomer
Geminiano Montanari recorded seeing variability in the star
Algol 1667.
Edmond Halley would then publish the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed position from the time of the ancient Greek astronomers
Ptolemy and Hipparchus. But it would not be until 1838 that the first direct measurement of the distance to the star 61 Cygni was made by
Friedrich Bessel using the
parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens. These pre-main sequence stars are often surrounded by a
protoplanetary disk. The protostar then follows a
Hayashi track on the
Hertzsprung-Russell diagram. The contraction will proceed until the Hayashi boundary is reached, and thereafter contraction will continue on a Kelvin-Helmholtz timescale with the temperature remaining stable. Stars with less than 0.5 solar masses thereafter join the main sequence. For more massive protostars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the Henyey track. The period of gravitational contraction lasts for about 10-15 million years.
Early stars of less than 2 solar masses are called
T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as
Herbig-Haro objects.
Main sequence
Stars spend about 90% of their lifetime
fusing hydrogen to produce
helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the
main sequence. Starting at zero age main sequence, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity. The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.
Every star generates a
stellar wind of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible. The sun loses 10
-14 solar masses every year, or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10
-7 to 10
-5 solar masses each year, significantly affecting their evolution. Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.
The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to burn and the rate at which it burns that fuel. In other words, its initial mass and its luminosity. For the Sun, this is estimated to be about 10
10 years. However, the luminosity of a star is also determined by its mass. Consequently the total main sequence lifetime of a star can be estimated from its mass relative to the Sun's as follows:
where is the mass of the star and is the star's estimated main sequence lifetime in years.
Large stars burn their fuel very rapidly and are short-lived. Small stars burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs. However, since the lifespan of such stars is greater than the current age of the universe , no black dwarfs exist yet.
Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. This metallicity can influence the duration that a star will burn its fuel; control the formation of magnetic fields, and modifies the strength of the stellar wind. Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed.
Post-main sequence
As most stars exhaust their supply of hydrogen at their core, their outer layers expand and cool to form a
red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume Mercury and
Venus. Models predict that the Sun will expand out to about 99% of the distance to the Earth's present orbit . By that time, however, the orbit of the Earth will expand to about 1.7 AUs due to mass loss by the Sun and thus the Earth will escape envelopment.
In a red giant, hydrogen fusion proceeds in a shell-layer surrounding the core. Eventually the core is compressed enough to start
helium fusion, and the star heats up and contracts. In low mass stars the helium fusion process begins with an explosive burst of energy generation known as a helium flash. The energy resulting from this event is equivalent to the luminosity of 10
8 Suns, but it lasts only a few minutes. However, this energy goes into the elimination of the electron degeneracy at the core, and is not visible from the exterior.
Massive stars
Very high mass stars with more than nine solar masses can continue to fuse elements heavier than helium. The core contracts until the temperature and pressure are sufficient to fuse
carbon. This process continues, with the successive stages being fueled by
oxygen,
neon,
silicon, and
sulfur. Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell burns a different element, with the outermost shell burning hydrogen; the next shell burning helium, and so forth.
The final stage is reached when the star begins producing
iron. Since iron nuclei are more
tightly bound than any heavier nuclei, if they are fused they do not release energy — the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by
fission. These too will fade into brown, and then black dwarfs over a very long stretch of time. Electron degenerate matter is not plasma, even though stars are generally referred to as being spheres of plasma.
In larger stars, defined as having more than 1.4 solar masses after explosion, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse
beta decay, or electron capture. The
shockwave formed by this sudden collapse causes the rest of the star to explode in a
supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the
Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.
Eventually, most of the matter in a star is blown away by the supernovae explosion and what remains will be a
neutron star or, in the case of the largest stars , a
black hole. In neutron stars and black holes, the star is not in a plasma state of matter, but either neutron degenerate matter or a state of matter not currently understood within the black hole.
The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky
planets. The outflow from supernovae and the
stellar wind of large stars play an important part in shaping the interstellar medium.
It has been a long-held assumption that the majority of stars occur in gravitationally-bound, multiple-star systems, forming
binary stars. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the portion of single star systems increases for smaller stars, so that only 25% of
red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.
Larger groups called
star clusters also exist. Stars are not spread uniformly across the
universe, but are normally grouped into
galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion galaxies in the observable universe.
Astronomers estimate that there are at least 70 sextillion stars in the known universe. That is 230 billion times as many as the 300 billion in our own
Milky Way.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light-years away. Light from Proxima Centauri takes 4.2 years to reach Earth. Travelling at the orbital speed of the
Space Shuttle , it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the solar system is located. Stars can be much closer to each other in the centres of galaxies and in
globular clusters, or much farther apart in galactic halos.
Because of their low density, collisions of stars in the galaxy are thought to be rare. However in dense regions such as the core of stellar clusters or the galactic center, collisions can be more common. Such collisions can produce what are known as
blue stragglers. These abnormal stars appear on a different part of the evolutionary track of the HR-diagram, effectively forming a merged star that has a higher surface temperature than the other main sequence stars in the cluster with the same luminosity.
Small,
dwarf stars such as the
Sun generally have essentially featureless disks with only small
starspots. Larger,
giant stars have much bigger, much more obvious starspots, and also exhibit strong stellar
limb darkening. That is, the brightness decreases towards the edge of the stellar disk. Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.
Age and size
Almost everything about a star is determined by its initial mass, including its destiny and fate, as well as its essential characteristics, such as lifespan, luminosity, and size. Stars range in size from
neutron stars no bigger than a city to supergiants like
Betelgeuse in the
Orion constellation, which has a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometres. However, Betelgeuse has a much lower density than the
Sun.
Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old — the observed age of the universe. The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass burn their fuel very slowly and last tens to hundreds of billions of years.
Most of our understanding of stars comes from theoretical models and simulations based on spectral observations and measurements of the diameters of stars. The first measurement of the diameter of a star other than the Sun was made in 1921 by
Albert Abraham Michelson on the
Hooker telescope.
One of the most massive stars known is
Eta Carinae, with 100–150 times as much mass as the Sun; its lifespan is very short — only several million years at most. A recent study of the
Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe. The reason for this limit is not precisely known, but is partially due to Eddington luminosity.
The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than
lithium in their composition. This generation of supermassive,
population III stars is long extinct, however, and currently only theoretical.
With a mass only 93 times that of
Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of
Jupiter. When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter. and as a steady stream of
neutrinos emanating from the star’s core.
The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an
atomic nucleus of a new heavier element deep inside the core of a star,
photons of electromagnetic energy are released from the nuclear fusion reaction, which are then converted to
visible light in the star’s outer layers.
The peak
frequency and
color of the visible light depends on the temperature of the star’s outer layers, including its
photosphere. Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans across the entire electromagnetic spectrum, from the longest
wavelengths of
radio waves and
infrared to the shortest wavelengths of
ultraviolet,
X-rays, and
gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.
Using the
stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and
rotation velocity of a star. If the distance of the star is known, such as by measuring the
parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. With these parameters, astronomers can also estimate the age of the star.
Luminosity
In astronomy, luminosity is the amount of
light, and other forms of radiant energy, a star radiates per unit of
time. The luminosity of a star can be approximated by treating the emitted energy as a
black body radiation. So:
-
where
L is the luminosity,
s is the Stefan-Boltzmann constant,
R is the stellar radius and
T is the effective temperature. This same formula can be used to compute the approximate radius of a main sequence star relative to the sun:
-
Magnitude
The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star’s luminosity, distance from Earth, and the altering of the star’s light as it passes through Earth’s atmosphere.
Number of stars brighter than magnitudeApparent
magnitude | Number
of Stars |
| 0 | 4 |
| 1 | 15 |
| 2 | 48 |
| 3 | 171 |
| 4 | 513 |
| 5 | 1,602 |
| 6 | 4,800 |
| 7 | 14,000 |
Intrinsic or absolute magnitude is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs , and it is directly related to a star’s luminosity, measured from the standard distance of 10 parsecs.
Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times
Classification
There are different classifications of stars according to their
spectra ranging from type
O, which are very hot, to
M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are
O, B, A, F, G, K, and
M.
A variety of rare spectral types have special classifications. The most common of these are types
L and
T, which classify the coldest low-mass stars and
brown dwarfs. Each letter has 10 subclassifications numbered from
0 to
9. This system matches closely with temperature, but breaks down at the extreme hottest end; class
O0 and
O1 stars may not exist.
In addition, stars may be classified by their "luminosity effects", which correspond to their spatial size. These range from
0 through
III to
V and
VII . Most stars fall into the
main sequence which consists of ordinary hydrogen-burning stars. These fall along a narrow band when graphed according to their absolute magnitude and
spectral type.tional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an "
e" can indicate the presence of emission lines; "
m" represents unusually strong levels of metals, and "
var" can mean variations in the spectral type.
Variable stars
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
Pulsating variables are stars that vary in radius over time, expanding and contracting as a result of the stellar aging process. This category includes
Cepheid and cepheid-like stars, and long-period variables such as Mira.
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.clysmic or explosive variables undergo a dramatic change in their properties. This group includes
novae and
supernovae. A binary star system that includes a nearby white dwarf
can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova. Some novae are also recurrent, having periodic outbursts of moderate amplitude.s can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.
In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving a shell within the star will exactly match the incoming flux.
The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very
high energy fluxes occur, as near the core, or in areas with high opacity, as in the outer envelope.occurrence of convection in the outer envelope of a main sequence star depends on the spectral type. Massive stars several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers. The convective zones will also vary over time as the star ages and the constitution of the interior is modified. The existence of a corona appears to be dependent on a convective zone in the outer layers of the star. the corona, a
stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium.
Nuclear fusion reaction pathways
A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of
stellar nucleosynthesis. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is converted into energy, according to the mass-energy relationship
E=mc².he Sun, with a 10
7 °K core, hydrogen fuses to form helium in the
proton-proton chain reaction:
- 41H ? 22H + 2e+ + 2?e
- 21H + 22H ? 23He + 2?
- 23He ? 4He + 21H
These reactions result in the overall reaction:
- 41H ? 4He + 2e+ + 2? + 2?e
In more massive stars, helium is produced in a cycle of reactions
catalyzed by
carbon, the
carbon-nitrogen-oxygen cycle.tars with cores at 10
8 K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the
triple-alpha process:p>4He +
4He + 92 keV ?
8*Be
- 4He + 8*Be + 67 keV ? 12*C
- 12*C ? 12C + ? + 7.4 MeV
For an overall reaction of:
- 34He ? 12C + ? + 7.2 MeV
In massive stars, heavier elements can also be burned in a contracting core through the Neon burning process and Oxygen burning process. The final stage in the stellar nucleosynthesis process is the
Silicon burning process that results in the production of the stable isotope
iron-56. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.
Fuel
material | Temperature
| Density
| Burn duration
τ |
|---|
| H | 37 | 0.0045 | 8.1 million years |
| He | 188 | 0.97 | 1.2 million years |
| C | 870 | 170 | 976 years |
| Ne | 1,570 | 3,100 | 0.6 years |
| O | 1,980 | 5,550 | 1.25 years |
| S/Si | 3,340 | 33,400 | 11.5 days |
References
See also
General topics:Unusual stars:Time and navigation:Other:External links
