# Stars

animated sun from Welcome On The Verge!

Stars are huge balls of gas that radiate energy produced by nuclear-fusion reactions. They range in mass from about 0.06 to 100 solar masses, one solar mass being equivalent to the mass of the Sun. The properties of a star and the manner in which it evolves depend principally on its mass.  Our sun is a fairly typical star.

### Magnitude

Magnitude is a measure of the star's brightness.  Magnitude is measured on a logarithmic scale, so that a difference of 5 in magnitude is equivalent to a factor of 100 in brightness.  The lower the magnitude the brighter the star.   So a star of magnitude +1 is 2.512 times brighter than a star of magnitude +2, 2.5122 squared (= 6.310) times brighter than a star of magnitude +3, and 2.512 to the fifth power (= 100) times brighter than a star of + 6.  Very bright objects can have negative magnitudes.  .  There are two forms, Apparent magnitude and Absolute magnitude

Apparent magnitude is an artificial measurement that indicates the brightness of a star as it appears to us here on Earth.  So a dim close star will have a higher apparent magnitude them a bright but distant star.  The  faintest stars that can be seen with the naked-eye on a very clear night are about apparent magnitude + 6. The world's largest telescopes can detect objects as faint as magnitude + 27.  On the other hand the planet Venus has an apparent magnitude of upto - 4.4, the full Moon - 12.0 and the Sun as seen from Earth -26.8.

Absolute magnitude measures the true brightness (luminosity) of a star.  It gives the value of a star as it would appear at a distance of 10 parsecs.   One parsec is the distance at which appear to change position of opne second of arc (1/3,600 of a degree) against it's background (parallax) as viewed from the Earth over an interval of 6 months (the diameter of the Earth's orbit).  As you can imagine, a  parsec is a pretty big distance.  It works out at 3.2 light years.  10 parsecs equals 32 light years.  So the Sun which has an apparent magnitude of -26.8 (very high because it is so close) would have an absolute magnitude of only +4.8.  The bright nearby star Sirius has an apparent magnitude of -1.5, but an absolute magnitude is +1.3.

### Stellar Types

Stars are classified according to surface temperature and luminosity.  The temperature of a star can be deduced by it's colour.  Hot stars are blue or white, while cool stars are red.  The colours also refer to specific spectral types.  Each colour has a letter as shorthand.  The letters are (from hot to cool) O, B, A, F, G, K, M.  The mnemonic Oh Be A Fine Girl Kiss Me is sometimes used in older astronomy books.

 Type Colour approximate Surface Temperature, (Kelvin) O Blue 25,000 - 40,000 B Blue 11,000 - 25,000 A Blue-White 7,500 - 11,000 F White 6,000 - 7,500 G Yellow 5,000 - 6,000 K Orange 3,500 - 5,000 M Red 3,000 - 3,500

These spectral types are further divided divided from 0 (hotter) to 9 (cooler).  Our Sun is rated a G2 on this scale.

### The Hertzsprung-Russell diagram

Hertzsprung-Russell diagram from Astronomy - the Cosmic Journey

The Hertzsprung-Russell diagram - after it's creators the Danish astronomer Ejnar Hertzsprung (1873-1967) and the American Henry Norris Russell (1877--1957) - is used for classifying stars..  It arranges stars according to luminosity (absolute magnitude) and temperature (spectral type).   It turns out that most stars fit into a diagonal band which has been called the main sequence

The Hertzsprung-Russell Diagram - has some nice graphics here

### Binary, multiple and variable stars

Over 75% of stars are members of binary or multiple star systems.  Binary stars consist of two stars each orbiting around a common centre of gravity.

An eclipsing binary can occur where one component of the system periodically obscures, and is obscured by, the other (as seen from Earth). This leads to a reduction in the light intensity seen from Earth.  This is how binary stars were first discovered. Some stars are actually complex multiple stars.  For example, the 'star' Castor in the constellation of Gemini has six individual components.

### Variable Stars

Variable stars are stars that vary in brightness, alternately brightening and fading. The variability can be caused by a line-of-sight effect, as in eclipsing binaries, or in changes in the energy output. of star itself.  Variable stars can have periods ranging from a few hours to several years.

### The Birth of Stars

Stars are formed within nebulae gas-clouds. Patches of gas and dust inside a nebula collapse under gravity forming dark regions called protostars. As they continue to collapse, the protostars become denser and hotter.  Finally they may become hot enough for nuclear-fusion reactions to ignite.  They then become a star.  Smaller aggregations of particles become planets, moons, and asteroids.

### Stellar evolution

Stellar evolution depends pretty much upon a protostar's mass.

Protostars with mass less then 0.06 of the never become hot enough for nuclear ignition.  They become so-called "brown dwarfs.  It is believed that brown dwarfs mnmay account for at least some of the so-called "missing mass" in the universe.

Protostars with masses of between 0.06 and 1.4 times the sun move on to the main sequence and may remain there for some 10 000 million years or more.  When all the available hydrogen is used, the core contracts, which increases the temperature to 100 million degrees kelvin (celsius).  This allows the helium to ignite and the star expands about 100 times in diameter to become a red giant. Finally, the outer layers of the star are ejected, forming a planetary nebula. The remaining core shrinks to become a small white dwarf, only about one hundredth the diameter of the Sun, but very dense, which will continue to burn for some hundreds of millions of years.

Stars of between 1.4 and 4.2 solar masses have much briefer life spans.  They burn much hotter and faster, may remain on the main sequence for only about a million years before entering the red giant phase.  The temperature increases even further, to around 700 million degrees kelvin, allowing synthesis of heavy elements.  These are finally ejected as the star goes nova or supernova.  The ejected gas cloud becomes the basis of further nebular, solar systems, planets, and even life.  All that remains of the original star is an incredibly dense, rapidly rotating neutron star, which may have a diameter of only 10-20 km and wigh 250 million tonnes per cubic centimeter.

Stars of greater than four solar masses may end their lives by producing a black hole - an object so dense that not even light can escape The only means of detecting a black hole is by observing its gravitational effects on surrounding objects.  It is believed that the X--ray source Cygnus X-1 may consist of gas from a red giant being sucked into a black hole.

### The Nearest Stars

The Nearest 27 Stellar Systems (Stars Within 4 Parsecs)
good table - gives info on all the nearest stars

a map of the nearest stars.- only lists the most important stars

ChView - 3-D Star Maps * Interstellar Navigation * Data on Stars & Extrasolar Planets This site is really cool!  Not only does it include star maps of the ists all the nearest stars, and a complete list of possible planets, it also includes a program for displaying stars, which you can edit for a customisable universe.  An invaluable resource for any sci fi writer writing a story about interstellar space.  Although this proggy is based on the writings of the Hugo-winning author, C.J. Cherryh, it is customisable so you don't have to be limited to one particular future universe.

SolStation.com - based on the above - interactive map of the nearest satrs, with info on each. Excellent!

Barnard's Star