Stars and Constellations

For other uses, see Star (disambiguation).

The Pleiades star cluster

A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Stellar astronomy is the study of stars.

Star formation and evolution

Main article: stellar evolution

Star formation occurs in molecular clouds, large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). Star formation begins with gravitational instability inside those clouds, often triggered by shockwaves from supernovae or collision of two galaxies (as in a starburst galaxy). High mass stars powerfully illuminate the clouds from which they formed. One example of such a nebula is the Orion Nebula.

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.

Small stars (called red dwarfs) 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 (13.6 billion years), no black dwarfs exist yet.

As most stars exhaust their supply of hydrogen, 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 both Mercury and Venus. Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. Larger stars will also fuse heavier elements, all the way to iron, which is the end point of the process. Since iron nuclei are more tightly bound than any heavier nuclei, they cannot be fused to release energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission. In old, very massive stars, a large core of inert iron will accumulate in the center of the star.

An average-size star will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. These too will fade into black dwarfs over very long stretches of time.

The Crab Nebula, remnants of a supernova which occurred around 1100 AD.

In larger stars, 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. 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 explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars, a 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.

Appearance and distribution of stars

All stars except the Sun appear to the human eye as shining points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere. Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight.

Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars. The majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist.

Astronomers estimate that there are at least 70 sextillion (7×10²²) stars in the known universe [1]. That is 70 000 000 000 000 000 000 000, or 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 kilometers, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos.

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 (the brightness decreases towards the edge of the stellar disk).

Age and size of stars

The Sun is the nearest star to Earth.

Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the observed age of the universe. (See Big Bang theory and stellar evolution.) They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometers. However, these have a much lower density than the Sun.

One of the most massive stars known is η Carinae, with 100–150 times as much mass as the Sun. Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The reason for this limit is not precisely known, but the Eddington limit is part of the answer. The very 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 star 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. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.

Star 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 can be easily remembered using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Jump Cannon. There are many other mnemonics for star classification. 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 (hottest to coldest) 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 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). 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.

Our Sun is a G2V (yellow dwarf), being of intermediate temperature and ordinary size. The Sun is taken as the prototypical star (not because it is special in any way, but because it is the closest and most studied star), and most characteristics of other stars are usually given in solar units.

solar mass: M_Sun = 1.9891×10³⁰ kg

solar luminosity: L_Sun = 3.827×10²⁶ W.

Naming of stars

Most stars are identified only by catalogue numbers; only a few have names as such. The names are either traditional names (mostly from Arabic), Flamsteed designations, or Bayer designations. The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union (IAU). A number of private companies (e.g. the "International Star Registry") purport to sell names to stars; however, these names are not recognized by the scientific community, nor used by them, and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.

See star designations for more information on how stars are named. For a list of traditional names, see the list of stars by constellation.

Energy production

The energy produced by stars radiates into space as electromagnetic radiation, as a stream of neutrinos from the star's core, and as a stream of particles from the star's outer layers (its stellar wind). The peak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically significant. The apparent brightness of a star is measured by its apparent magnitude.

Nuclear fusion reaction pathways

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition (see Stellar nucleosynthesis).

Stars begin as a cloud of mostly hydrogen with about 25% helium and heavier elements in smaller quantities. In the Sun, with a 10⁷ K core, hydrogen fuses to form helium in the proton-proton chain:

4¹H → 2²H + 2e⁺ + 2ν_e (4.0 MeV + 1.0 MeV)

2¹H + 2²H → 2³He + 2γ (5.5 MeV)

2³He → ⁴He + 2¹H (12.9 MeV)

These reactions result in the overall reaction:

4¹H → ⁴He + 2e⁺ + 2γ + 2ν_e (26.7 MeV)

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle.

In stars with cores at 10⁸ K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process:

⁴He + ⁴He + 92 keV → ^8*Be

⁴He + ^8*Be + 67 keV → ^12*C

^12*C → ¹²C + γ + 7.4 MeV

For an overall reaction of:

3⁴He → ¹²C + γ + 7.2 MeV

Star mythology

As well as certain constellations and the Sun itself, stars as a whole have their own mythology. They were thought to be the souls of the dead or gods and goddesses. In Greco-Roman pantheism, some "stars", later identified as planets, represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken. (Uranus and Neptune were also Roman gods, but neither planet was known to the Romans as they are not visible with the naked eye from Earth. Their names were assigned by later astronomers.)

References

• Cliff Pickover (2001) "The Stars of Heaven", Oxford University Press
• John Gribbin, Mary Gribbin (2001) "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press.

See also

• Black hole
• Blue straggler
• Overview of star constellations
• Nursery rhyme Twinkle twinkle little star
• sidereal clock
• Star count
• Star clocks
• Stars with articles in Wikipedia
• Stellar navigation
• Stellar evolution
• Timeline of stellar astronomy
• Variable star

Related lists

• List of brightest stars (apparent & absolute magnitude)
• List of heaviest stars (by solar mass)
• List of largest stars (by diameter)
• List of mnemonics for star classification
• List of nearest bright stars
• List of nearest stars
• List of the most important stars
• List of stars by constellation
• List of stars with confirmed extrasolar planets

External links

• Images of starspots on the surface of Betelgeuse
• Find out what is known about any given star by entering its name or position
• Star Classification

For other uses, see Constellation (disambiguation).

Orion is a remarkable constellation, visible from most places on the globe (but not always the whole year long).

A constellation is a group of stars visibly related to each other in a particular configuration.

Explanation

In three-dimensional space, most of the stars we see have little relation to one another, but can appear to be grouped on the celestial sphere of the night sky. Humans excel at finding patterns and throughout history have grouped stars that appear close to one another into constellations.

An "unofficial" constellation, that is, one that may be widely known but is not recognized by the International Astronomical Union, is called an asterism. An example is the grouping called the Big Dipper (US) or the Plough (UK).

The stars in a constellation or asterism rarely have any astrophysical relationship to each other; they just happen to appear close together in the sky as viewed from Earth and typically lie many light years apart in space. However, one exception to this is the Ursa Major moving group.

The grouping of stars into constellations is essentially arbitrary, and different cultures have had different constellations, although a few of the more obvious ones tend to recur frequently, e.g., Orion and Scorpius.

The International Astronomical Union (IAU) divides the sky into 88 official constellations with precise boundaries, so that every direction belongs to exactly one constellation. In the northern celestial hemisphere, these are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages, and contains the signs of the zodiac.

The constellation boundaries were drawn up by Eugène Delporte in 1930, and he drew them along vertical and horizontal lines of right ascension and declination. However, he did so for the epoch B1875.0, which means that due to precession of the equinoxes, the borders on a modern star map (eg, for epoch J2000) are already somewhat skewed and no longer perfectly vertical or horizontal. This skew will increase over the years and centuries to come.

History of the Constellations

Main article: List of Constellations

Our current list is based on those listed by the Roman astronomer, Claudius Ptolemy, who lived in Alexandria, Egypt. (Claudius Ptolemy, the astronomer, was not related to the Greek kings of Egypt named Ptolemy.)

In more recent times this list has been added to, to fill gaps between Ptolemy's patterns. The Greeks considered the sky as including both constellations and dim spaces between. But Renaisance star catalogs by Johann Bayer and John Flamsteed required every star to be in a constellation, and the number of visible stars in a constellation to be manageably small.

Twelve of the constellations in the southern celestial hemisphere were not observable by the Greeks, and were created by Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman in the sixteenth century and first cataloged by Johann Bayer.

Other proposed constellations didn't make the cut, most notably Quadrans Muralis (now part of Boötes) for which the Quadrantid meteors are named. Also the ancient constellation Argo Navis was so big that it was broken up into several different constellations, for the convenience of stellar cartographers.

Constellations in variant cultures

Main article: Chinese constellation

Chinese constellations are different from the western constellations, due to the independent development of ancient Chinese astronomy. Ancient Chinese skywatchers divided their night sky in a different way, but there are also similarities. The Chinese counterpart of the 12 western zodiac constellations are the 28 "Xiu" (宿) or "mansions" (a literal translation).

Star names

All modern constellation names are Latin proper names or words, and some stars are named using the genitive of the constellation in which they are found. The genitive is formed using the usual rules of Latin grammar, and for those unfamiliar with that language the form of the genitive is unpredictable and must be memorized. Some examples include: Aries → Arietis; Taurus → Tauri; Gemini → Geminorum; Virgo → Virginis; Libra → Librae; Pisces → Piscium; Lepus → Leporis.

These names include Bayer designations such as Alpha Centauri, Flamsteed designations such as 61 Cygni, and variable star designations such as RR Lyrae. However, many fainter stars will just be given a catalog number designation (in each of various star catalogs) that does not incorporate the constellation name.

For more information about star names, see Star designations and the list of stars by constellation.

See also

• List of constellations
• List of constellations by area
• Chinese constellation
• lunar mansion

External links

Wikimedia Commons has media related to:

Constellations

• The Constellations
• Photographic Atlas of the Constellations
• Celestia free 3D realtime space-simulation (OpenGL)
• Stellarium realtime sky rendering program (OpenGL)
• Strasbourg Astronomical Data Center Files on official IAU constellation boundaries (the older NASA ADC service does not function anymore)
• Interactive Sky Charts (Allows navigation through the entire sky with variable star detail, optional constellation lines)
• http://www.astronomical.org/constellations/obs.html
• http://www.seds.org/Maps/Stars_en/Fig/const.html
• Constellations Articles
• Full constellation diagrams resembling their names
• Images of constellations