Star

A star is usually made of gas and a substance known as plasma. The Sun (Sol) is a star. A portion of the stars, however, those called white dwarfs and neutron stars, are composed of tightly packed atoms or subatomic particles and are much more dense than anything on Earth.

Stars range widely in size. The Sun’s radius is about 432,000 miles (695,500 kilometres). Because there are other stars that are much larger, the Sun is classified as a dwarf star. Stars classified as supergiants have a radius about 1,000 times that of the Sun. Neutron stars are the smallest stars and have a radius of only about 6 miles (10 kilometres).

The Sun is a single star. However, about 75 % of all stars are binary stars, two stars that orbit each other. The star nearest the Sun, Proxima Centauri, is part of a multiple-star system which includes Alpha Centauri A and Alpha Centauri B.

A star is so massive that its gravity pulls it in on itself and would cause it to collapse if there was nothing to oppose this gravity. The gravity is opposed by hydrostatic support created by the pressure of hot gas and radiation in the star's interior. The reason for the heat and radiation is nuclear reaction near the centre of the star. While these two forces oppose each other, the star is in the main sequence phase of its evolution. Stars contract or collapse in on themselves before the main sequence phase because they are not hot or dense enough for nuclear reactions to begin. Heat generated during contraction provides hydrostatic support and counteracts the pull of gravity. After the main sequence, most of the fuel is used up and the lack of sufficient hydrostatic support results in the star collapsing under its own gravity.

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Characteristics of stars

Stars are categorised by their five main attributes;

(1) brightness, (magnitude or luminosity);

(2) colour;

(3) surface temperature;

(4) size (diameter);

(5) mass (amount of matter).

Colour is dependent of surface temperature, brightness depends on surface temperature and the size. Mass affects how fast a stars produces energy and effects surface temperature.

Astronomers use a diagram to show how these are all related, the Hertzsprung-Russell (H-R) diagram.

Magnitude

Based on a numbering system used by Hipparchus (sometime around 120-125 B.C. in Greece) stars are numbered according to their brightness as they are seen from Earth. The brightest is of the first magnitude, the next brightest of the second magnitude and so on to the sixth magnitude, which are very faint stars. Today this is called apparent magnitude. Without a telescope, Hipparchus created a catalogue of 1080 stars that could be seen in each constellation. He noted their positions, and rated their brightness but could only see stars as dim as 6th magnitude.

Hipparchus' catalogue was later edited and increased by Ptolemy which he published in the Almagest (possibly between 127 and 150 A.D.), one of the most prominent works in the history of astronomy. Today, with ground based telescopes, we can see to about 22^nd magnitude.

In 1856, Norman Pogson replaced the system developed by Hipparchus and Ptolemy, with one based on mathematics but matching, as much as he could, the old system. He used the formula:

$m=-2.5*log{\frac {F}{F_{stand}}}\,$

• m - the magnitude
• F - is the flux from our star
• F_stand - is the flux from a standard star

Flux denotes the amount energy emitted from a star that reaches Earth

• log - is a function denoting powers of ten in a number.

When the flux of the star is the same as the standard, then we have:

m = -2.5 log(1)

m = -2.5 * 0

m = 0

So if a star has the same flux as the standard star, its magnitude is zero. A negative magnitude indicates a star is brighter than the standard, usually the result when absolute magnitude is calculated. Absolute magnitude calculates apparent magnitude taking into consideration the distance of a star. By calculating the magnitude of all the stars as if they were a uniform distance away, the magnitude is measured in a uniform way. That distance is ten parsecs, about 32.6 light years distant. Apparent magnitudes are written in lower case m and absolute magnitudes are written with a upper case M.

Today, the standard star is Vega, with a defined absolute magnitude of M=0.0. Stars that have a brighter magnitude have a negative number and those that are dimmer have positive numbers.

Later investigations showed that the difference between each magnitude as denoted by Hipparchus was about 2.5 times brighter than the next greatest magnitude. In other words, a difference in 5 units of magnitude, e.g. from magnitude 1 to magnitude 6, corresponds to a change in brightness of 100 times. As technology developed to allow more accurate measurments, astronomers started assigning partial values rather than whole numbers, 2.45 or 2.75 for example, rather than rounding off to magnitude 2 or 3.

Stars that are brighter than the first magnitude are classed by numbers less than 1. The apparent magnitude of Rigel is m=0.12. Extremely bright stars have numbers that are less than zero or negative numbers. The brightest stars in the night sky is Sirius with an apparent magnitude of m=-1.46 and the absolute magnitude of Rigel is M=-8.1. The Sun has an apparent magnitude of m=-26.7, since it is close, but an absolute magnitude of M=+4.8, the magnitude it would have if it were 10 parsecs from Earth.

At the present time there are no stars classified with an absolute magnitude brighter than -8.0 Some very dim stars have an apparent magnitude of 28, but absolute magnitude can be no fainter than about 16.

Luminosity

A star's luminosity is the measure of all energy it radiates each second (just as power is measured), usually understood to be at all wavelengths (bolometric luminosity or L_bol) and is measured as ergs per second. Star luminosity is commonly compared to the Sun whose luminosity is indicated as L_sun = 4 x 10³³ ergs/s.

The luminosity of a star is proportional to its temperature and its surface area. This means that a hotter star is more luminous than a cooler star if they have the same radius and a bigger star more luminous than a smaller star if they have the same temperature. This, in turn, means that a cool (red) giant star is more luminous than the Sun since it is much larger even though it is much cooler.

Stars radiate visible light but they also radiate energy the human eye can not see. The term for all radiation from a star is bolometric magnitude or bolometric flux which means the total energy out put over the entire electromagnetic spectrum, not just visible light.

The Sun has a luminosity of 400 trillion trillion watts (10²⁴). Commonly in astronomy however, the luminosity is measured another way, by comparing it with the Sun. In this way Alpha Centauri is about 1.3 times as luminous as the Sun and Rigel is 150,000 times as luminous.

How bright a star looks is controlled by its actual brilliance and its distance from Earth. A star’s brilliance is the amount of visible light it produces. So a nearby star can be quite dim if it is not generating a great deal of light while a star much further away can be brighter because it actually is much brighter. The star Alpha Centauri appears brighter than another star called Rigel. It would seem that Alpha Centauri produces more energy. However, Rigel is actually much brighter but it is much further away from Earth. Alpha Centauri is about 4.4 light years away while Rigel is 1,400 light years from Earth.

There is also a luminosity-absolute magnitude comparison: 5 on the absolute magnitude scale equals 100 on the luminosity scale. So, if a star has an absolute magnitude of 2, it is 100 times as luminous as a star with an absolute magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a star with an absolute magnitude is 2 and 10,000 times more luminous than a star with an absolute magnitude of 7.

Luminosity is variable. Small changes may be responsible for the Little Ice Age of the 15th through 18th centuries when there may have been a decrease in the Sun's luminosity. One possible explanation of these changes is that they are triggered by modifications of the magnetic field produced in a star's interior. The mechanism of these modifications are yet to be revealed.

The Yerkes classification is a schema for classifying stars by their luminosity, also known as M-K established in 1943. This classification scheme measures surface gravities of stars by measuring the shape and nature of certain spectral lines.

Colour

With or without a telescope or binoculars, stars display a range of colours--reddish, yellowish and bluish--not very strong colour, but the variations are visible. The Sun is yellow, as is Pollux, Betelgeuse looks reddish and Rigel looks bluish. Star colour is a function of its surface temperature. Dark red stars have surface temperatures of about 2500 K and bright red stars reach about 3500 K. For yellow stars, e.g. the sun, surface temperature is approximately 5500 K. Blue stars are in the range of 10,000 to 50,000 K in surface temperature.

Since the human eye can only discern colours of the visible spectrum we do not see other wavelengths of the spectrum and stars emit a broad spectrum (bands) of colours. The colours in the visible spectrum, all the colours of the rainbow, can be viewed with a prism which separates the colour bands. Red is produced by particles of light (photons) with the least energy, all the way to violet, the photons with the most energy.

Continuous, absorption and emission spectra.

Including the electromagnetic band of seven visible colours, there are six bands of electromagnetic radiation in the electromagnetic spectrum, in range from the least energetic to the most energetic:

• radio waves
• infrared rays
• visible light
• ultraviolet rays
• X rays
• gamma rays

Fraunhofer Lines of common elements.

Studying the electromagnetic spectrum of stars is important function of astronomy. Displaying the spectrum of a star shows dark bands called absorption lines, or Fraunhofer lines, where the radiation energy is weaker, caused by a cooler gas between the observer and the hotter emitting gas.

Occasionally there are also bright emission lines where radiation energy is especially strong.

Spectrum for PIA04940: Embedded Star in Herbig-Haro 46/47.

Absorption lines exist because a chemical element or compound absorbs radiation with the same energy at that particular line on the spectrum. The Sun, for example, has absorption bands in the green range of the spectrum because there is calcium in the outer layer of the sun absorbing radiation at the same energy. While most stars have absorption lines in the visible spectrum, emission lines are more common in other bands of the spectrum. Nitrogen in a sun’s atmosphere, for example, will result in emission lines in the ultraviolet band.

An example of a spectrum for a particular star is an embedded star in Herbig-Haro 46/47 as determined by the Spitzer Space Telescope (SST). This is a low mass protostar ejecting a jet of supersonic gas and creating a bipolar, or two-sided, outflow which interacts with the surrounding interstellar medium--bright, nebulous regions of gas and dust that are buried within a dark dust cloud. Elements and compunds present in the protostar as determined from the EM spectrum include water ice, methyl alcohol, silicates and carbon dioxide ice.

Mass

A star's mass is its most fundamental property. Mass governs the evolution of a star, what fuels it will burn, how long it will live, its colour. Determining a star's mass is an essential part of understanding a specific star. Knowing the mass of main sequence stars determines the answers to a number of problems, such as the mass content of a galaxy and the manner in which it evolves. The mass-luminosity relation (MLR) is a fundamental relationship in numerous areas of astronomy.

Life cycle

Stars have a beginning and an end. The Sun formed about 4.6 billion years ago and may last another 5 billion years. After that it will become a red giant. Later it will lose its outer layers and the remaining core of the star will become a white dwarf, eventually fading to a black dwarf, dense and without light.

Not all stars end this way. Some will cool off and without expanding to a red giant stage will become white dwarfs, then black dwarfs.

Another possibility for a small percentage of stars is a spectacular explosion called a supernova. The star explodes and loses most of its material. In an even smaller number of cases, the star explodes completely, leaving nothing - but this is a very rare occurrence.

Galaxies

Stars are grouped in galaxies with rare exceptions. Astronomical observations to date have recorded galaxies at distances of 12 to 16 billion light years from the Sun. The Sun is in the galaxy called the Milky Way (because of its appearance as a faint band of light encircling the night sky) which contains an estimated 100 billion stars. It is estimated that there are more than 100 billion galaxies in the universe and average about 100 billion stars per galaxy. This means that there is an estimated 10 billion, trillion stars in the universe (10²¹). From Earth however, without using binoculars or a telescope, it is only possible to see about 3,000 of them.