People look at the sky for many reasons. For the Astronomer it’s a journey filled with endless possibilities. For the Cosmologist it’s about creation and what lies beyond. There’s mythology linked with the constellations, folklore, history, and archaeoastronomy that link humanity to the heavens, as if it is all connected by a sacred geometric stream of consciousness. Some love to relax and stargaze, finding it a healing tool. Pseudoscience takes us to Astrology, Ancient Alien Theory, or perhaps an inner knowing that our souls traveled here from a distant star to which we will one day return. Whatever the reason, since the beginning of time, stars have always set our galactic course, and now play a role in its final outcome.
About Stars
A star is a massive, luminous ball of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun. which is the source of most of the energy on Earth. Other stars are visible from Earth during the night when they are not outshone by the Sun or blocked by atmospheric phenomena.
Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.
For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen in its core releasing energy that traverses the star’s interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium were created by stars, either via stellar nucleosynthesis during their lifetimes or by supernova nucleosynthesis when stars explode. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space.
The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H-R diagram), allows the age and evolutionary state of a star to be determined.
A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. The remainder of the star’s interior carries energy away from the core through a combination of radiative and convective processes.
The star’s internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun expand to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.
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. Stars can form part of a much larger gravitationally bound structure, such as a cluster or a galaxy.
Observation History
Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they were immutable.
By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun. The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth’s rotational axis relative to its local star, the Sun.
The oldest accurately dated star chart appeared in ancient Egyptian astronomy in 1534 BC.
The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (ca. 1531-1155 BC).
The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis.
The star catalog of Hipparchus (2nd century BC) included 1020 stars and was used to assemble Ptolemy’s star catalogue. Hipparchus is known for the discovery of the first recorded nova (new star). Many of the constellations and star names in use today derive from Greek astronomy.
In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.
In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185.
The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers.
The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers.
Medieval Islamic astronomers gave Arabic names to many stars that are still used today, and they invented numerous astronomical instruments that could compute the positions of the stars. They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues.
Among these, the Book of Fixed Stars (964) was written by the Persian astronomer Abd al-Rahman al-Sufi, who discovered a number of stars, star clusters (including the Omicron Velorum and Brocchi’s Clusters) and galaxies (including the Andromeda Galaxy).
In the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.
The Andalusian astronomer Ibn Bajjah proposed that the Milky Way was made up of many stars which almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.
Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable.
In 1584 Giordano Bruno suggested that the stars were like the Sun, and may have other planets, possibly even Earth-like, in orbit around them, an idea that had been suggested earlier by the ancient Greek philosophers, Democritus and Epicurus, and by medieval Islamic cosmologists such as Fakhr al-Din al-Razi.
By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.
The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667.
Edmond Halley published the first measurements of the proper motion of a pair of nearby “fixed” stars, demonstrating that they had changed positions from the time of the ancient Greek astronomers Ptolemy and Hipparchus. The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.
William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he performed a series of gauges in 600 directions, and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction. In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines – the dark lines in a stellar spectra due to the absorption of specific frequencies by the atmosphere.
In 1865 Secchi began classifying stars into spectral types. However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius, and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104 day period. Detailed observations of many binary star systems were collected by astronomers such as William Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of the orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.
The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star, and hence its temperature, could be determined by comparing the visual magnitude against the photographic magnitude.
The development of the photoelectric photometer allowed very precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope.
Important conceptual work on the physical basis of stars occurred during the first decades of the twentieth century.
In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. The spectra of stars were also successfully explained through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.
With the exception of supernovae, individual stars have primarily been observed in our Local Group of galaxies, and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy).
But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth. In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Cluster – the most distant stars resolved have up to hundred million light years away.
However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located one billion light years away – ten times the distance of the most distant star cluster previously observed.
Constellations – Designations
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. Historically, the term was also used to refer to a perceived pattern formed by prominent stars within apparent proximity to one another, and this practice is still common today.
The concept of the constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology. Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
As well as certain constellations and the Sun itself, stars as a whole have their own myths. To the Ancient Greeks, some “stars”, known 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 Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)
Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star’s right ascension was invented and added to John Flamsteed’s star catalogue in his book “Historia coelestis Britannica” (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.
Under space law, the only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU). A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise. However, the IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU nor used by them. One such star naming company is the International Star Registry, which, during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud, and the New York City Department of Consumer Affairs issued a violation against ISR for engaging in a deceptive trade practice.
Astrology Crystalinks
Star Formation and Evolution
As learned by star formation astronomers, stars are born in molecular clouds, large regions of slightly higher density of matter (though still less dense than the inside of an Earthly vacuum chamber), and form by gravitational instability inside those clouds triggered by shockwaves from supernovae.
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-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, far longer than the time elapsed in the universe so far.
Dwarf Stars Red Dwarfs
Red dwarfs are small stars that never really managed to get fired up. With masses of about 40% that of our sun, they are relatively cool, with a surface temperature of less than 3,200C, so they have a dimmer, reddish appearance. Red dwarfs collectively make up the vast majority of all stars in the universe. Stars that are smaller and dimmer still are known as brown dwarfs. These generally have a mass of less than 7% of our sun, making them too small to sustain hydrogen-burning fusion reactions at their core.
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. They comprise the vast majority of stars and have a diameter and mass of less than one-third that of the Sun (down to 0.08 solar masses, which are brown dwarfs) and a surface temperature of less than 3,500 K. They emit little light, sometimes as little as 1/10,000th that of the sun. Due to the slow rate at which they burn hydrogen, red dwarfs have an enormous estimated lifespan; estimates range from tens of billions up to trillions of years.
Red dwarfs never initiate helium fusion and so cannot become red giants; the stars slowly contract and heat up until all the hydrogen is consumed. In any event, there has not been sufficient time since the Big Bang for red dwarfs to evolve off the main sequence.
The fact that red dwarfs remain on the main sequence while older stars have moved off the main sequence allows one to date star clusters by finding the mass at which the stars turn off the main sequence. In addition, the fact that no red dwarfs have evolved off the main sequence have been observed is evidence that the universe has a finite age.
One mystery which has not been solved as of 2004 is the lack of red dwarf stars with no metals (in astronomy a metal is any element other than hydrogen and helium). The Big Bang model predicts the first generation of stars should have only hydrogen, helium, and lithium. If such stars included red dwarfs, they should still be observable today, but are not. The conventional explanation is that without heavy elements, low mass stars cannot form and the first stars were extremely high mass population III stars which died quickly and produced the metals necessary for low mass stars to form later.
Red dwarf stars are believed to be the most common star type in the universe. Proxima Centauri, the nearest star to the Sun is a red dwarf, (Type M5, magnitude 11.0) as are twenty of the next thirty nearest. However, due to their low luminosity, few are known.
At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs – although none exist yet.
Black Dwarf
A Black Dwaef constitutes the remains of a Sun-sized star which has evolved to a Black Dwaef or stellar remnant, subsequently cooled down such that it only emits black body radiation. None are known to exist in our universe, as the time taken for a white dwarf to cool to such a degree is hypothesized to be longer than the lifespan of the universe to date.
Even at the epoch when black dwarfs exist they will be extremely difficult to detect, emitting thermal radiation at a temperature not much above that of the cosmic microwave background radiation. One of the only ways to detect them may be through their gravitational influence.
Black dwarfs should not be confused with brown dwarfs, which are formed when gas contracts to form a star, but does not possess enough mass to initiate and sustain hydrogen nuclear fusion. (NB: what we now refer to as brown dwarf were at times called black dwarfs in the 1960s.)
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 subsume 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.
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. It will fade into a black dwarf over very long stretches of time. In larger stars, fusion continues until collapse ends up causing the star to explode in a supernova.
This is the only cosmic process that happens on human timescales; historically, supernovae have been observed as “new stars” where none existed before. Most of the matter in a star is blown away in the explosion (forming nebulae such as the Crab Nebula) but what remains will collapse into a neutron star (a pulsar or X-ray burster) or, in the case of the largest stars, a black hole The blown-off outer layers include heavy elements, which are often converted into new stars and/or planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.
White Dwarfs
Contaminated white dwarfs: Scientists solve riddle of celestial archaeology Science Daily – March 26, 2014
A decades old space mystery has been solved by an international team of astronomers. The team put forward a new theory for how collapsed stars become polluted – that points to the ominous fate that awaits planet Earth. Scientists investigated hot, young, white dwarfs – the super-dense remains of Sun-like stars that ran out of fuel and collapsed to about the size of the Earth. It has been known that many hot white dwarfs’ atmospheres, essentially of pure hydrogen or pure helium, are contaminated by other elements — like carbon, silicon and iron. What was not known, however, was the origins of these elements, known in astronomical terms as metals.
A white dwarf, also called a degenerate dwarf, is a stellar remnant composed mostly
of electron-degenerate matter. They are very dense; a white dwarf’s mass is
comparable to that of the Sun, and its volume is comparable to that of the
Earth. Its faint luminosity comes from the emission of stored thermal energy.
The nearest known white dwarf is Sirius B, 8.6 light years away, the smaller
component of the Sirius binary star. There are currently thought to be eight
white dwarfs among the hundred star systems nearest the Sun. The unusual
faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell,
Edward Charles Pickering, and Williamina Fleming;, p. 1 the name white dwarf
was coined by Willem Luyten in 1922.
White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become a neutron star – over 97% of the stars in the Milky Way. After the hydrogenÐfusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen. If the mass of the progenitor is between 8 and 10.5 solar masses, the core temperature is sufficient to fuse carbon but not neon, in which case an oxygen-neonÐmagnesium white dwarf may be formed. Also, some helium white dwarfs appear to have been formed by mass loss in binary systems.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported by the heat generated by fusion against gravitational collapse. It is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the Chandrasekhar limit – approximately 1.4 solar masses – beyond which it cannot be supported by electron degeneracy pressure. A carbon-oxygen white dwarf that 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. (SN 1006 is thought to be a famous example.)
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. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit significant heat or light, and it will become a cold black dwarf. However, the length of time it takes for a white dwarf to reach this state is calculated to be longer than the current age of the Universe (approximately 13.8 billion years), and since no white dwarf can be older than the age of the Universe, it is thought that no black dwarfs yet exist. The oldest white dwarfs still radiate at temperatures of a few thousand kelvins.
Stellar Structure
The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.
As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant and energy production ceases at the core. Instead, for stars of more than 0.4 solar masses, fusion occurs in a slowly expanding shell around the degenerate helium core.
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 any layer within the star will exactly match the incoming flux from below.
Naming 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.
