What is a Star

Tuesday 19 September 2017 - 23:53:03 Posted by  Bobby

What Is A Star?

  A star is a rotating sphere of plasma - hot ionized gas. After a newborn star's core is heated by gravitational contraction to 10 million°K, nuclear fusion of hydrogen and helium increases to a significant level. The thermal energy produced eventually builds up enough pressure to counteract the crushing force of gravity, & a normal star, like the Sun, settles into the longest stable period of its existence. Late in a star's life, after the hydrogen fuel in its core is depleted, and if the star has sufficient mass, the fusion of heavier elements occurs.

Molecular Clouds

  Stars are formed inside giant molecular clouds, which are huge regions of gas and dust, scattered throughout the galaxy. Our own Sun was born in one.. Pockets of denser gas form, and then begin to collapse in on themselves. Eventually the pressures become so great that nuclear fusion reactions commence, & a star is born. The clouds themselves are the byproduct of earlier generations of stars which, at the end of their lives, exploded and seeded space with their gas and heavy elements.

Stellar Nurseries

  The Great Nebula in Orion is perhaps the best known example of a nearby star forming region, being only 1,500 light years from Earth. Just visible to the unaided eye from a dark site as a tiny smudge of light, a telescope reveals it to be a stunningly beautiful cloud of gas and dust. Deep inside, in regions where the density has increased, new star systems are being born. Astronomers have caught glimpses of this process, in the form of "proplyds" - protoplanetary discs. Studying the Orion Nebula is giving science tremendous insights into the evolution of stars.

Star Clusters

  There are two types of star clusters - Open Clusters and Globular Clusters. All of the stars in a particular cluster were formed from the same collapsing molecular cloud. While the most massive stars form faster than low-mass stars, all of the stars were formed at approximately the same time - within a few million years - and have the same chemical composition. These two facts make star clusters valuable astrophysical laboratories.

Open Clusters

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   Open clusters are inhabitants of the Milky Way's disc. They contain anywhere from tens, up to hundreds of stars, and, very rarely, thousands. The unusually rich open clusters, Messier 37 in Auriga, and NGC 2477 in Puppis, each have almost 2,000 stars. Many new open clusters are still being formed in the Milky Way today.
 

Globular Clusters

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  Globular clusters are great spheres of stars, with a typical membership of hundreds of thousands, all packed into a diameter of about 60 to 150 light-years. Science fiction authors delight in writing about the sky's appearance from a planet located in the core of one of the great globular clusters. The sky would feature thousands of stars rivaling Sirius, and many that are comparable to the brighter planets!

Double Stars

  Just over half the stars within our galaxy are in a binary or multiple star system. This means that about a quarter of the stars in the sky are actually two or more stars orbiting each other. It is only the nearest and most separated pairs that can be seen as distinct stars in a telescope - the visual binaries. The more distant, and/or more closely bound pairs are detected indirectly either by their spectrum - spectroscopic binary, or by the light changes as one star passes in front of the other - eclipsing binary.

Visual Binaries

  Since 1752, repeated measurements of the angular separation in the direction of the line joining these two stars have shown that the fainter star goes around the brighter every 80 years on a path tilted only 11 degrees from edge-on. Both stars are orbiting about their common center of mass. By mapping out the orbits, and knowing the distance to the binary, it is possible to calculate all the properties of the orbits - size, shape, orientation - and then most importantly, the masses of the individual stars. In the case of Alpha Centauri, both stars are similar to the Sun; only slightly more massive and thus hotter and brighter, the other slightly less massive and so fainter and cooler.

Spectroscopic Binaries

  Most binaries are detected not using an image, but using a spectrograph to measure the spectrum. Each star of the binary produces its own spectrum. As they orbit, the spectral lines shift back and forth in frequency, and so in wavelength, because of the Doppler Effect. As one star approaches us, its spectral lines are shifted to higher frequencies - toward the blue end of the spectrum. The other star in the system would then be moving away from us and so its spectral lines are moved to lower frequencies - toward the red end of the spectrum. By measuring the time it takes for the lines to shift back and forth and back again, we know the orbital period. The size of the shift tells us about the orbital velocities, and hence, the total mass of the stars. The Doppler shift gives us no information about the tilt of the orbit. When the orbits are face-on, we would detect no shifts. When the orbits are edge-on, we would measure the actual orbital velocities. Generally we can only put a limit of the total mass.

Eclipsing Binaries

  If the orbit is edge-on - inclination 90 degrees - each star will alternately pass in front of the other, causing the total light from the system to dim. If the two stars are of the same brightness, then the total brightness drops by half during each possible eclipse. When one star is much fainter and bigger than the other, the effect can be quite dramatic, with a deep and long-lasting primary eclipse, and a shallower less obvious secondary eclipse. The bigger the cool star, the longer the eclipse lasts. There is a lot of information in the light curve - brightness changes with time - which, when combined with spectra taken throughout the orbits, makes it possible to measure the masses of both stars and their absolute sizes.

Accreting Binaries or Interacting Binaries

  If the stars are close enough, mass can be transferred from one star to the other. When mass is transferred between normal stars, the observable effects are slight - emission lines and some X-ray emission. The most spectacular interacting binaries consist of a normal, or giant, star and a compact object - a white dwarf, neutron star, a black hole. As the material leaves the normal star it does not fall directly onto the compact object, but spirals, forming a disc about the compact object. This is because that gas is deflected as the two objects orbit each other.
  As the gas falls it releases energy heating up the disk. For white dwarfs, the disk gets hot enough to shine in the IR, optical, and UV parts of the spectrum. For neutron stars and black holes, even more energy is released in the disk, which becomes much hotter, glowing mostly in X-rays. Changes in the amount of matter falling onto the compact object, or changes in the structure of the disk, cause all of these systems to be variable.

Variable Stars

  Variable stars are valuable as astrophysical laboratories. Depending on the process occurring, a variable star's light curve may reveal such properties as its mass, luminosity, distance, or even its diameter. While the data is normally interpreted by professional astronomers, amateurs play a major role in data collection. There is no other area of the science of astronomy in which the work of amateurs is so important.
  Changes in the brightness of stars is the most common kind of variability but other types also occur, particularly changes in the spectrum or light that comes from the star.

Eruptive Variables

  T Tauri variables have not yet reached the stable main sequence. These very young stars rotate rapidly, generating strong magnetic fields that cause intense bright flares and cool starspots. Their brightness berries are regularly by several magnitudes. Some magnitude drops may be due to their circumstellar discs partially a calling them.
  FU Orionis variables, also very young stars, are still located in their birthplace. They may have large amplitude magnitude changes on time scales of years.
  UV Ceti stars are the flare stars, red dwarfs of spectral Class M. While the mass and luminosity of these dwarf stars is far less than that of the Sun, they have much deeper convection zones. Those that also rotate rapidly twist their magnetic field lines far more than the Sun twists its own magnetic field, and thus they experience more powerful flares then our star does. With the luminosity of these stars being low, and the flares being very powerful, UV Ceti stars can have very brief spikes in brightness, sometimes of many magnitudes. Monitoring such unpredictable variables isn't an ideal task for amateur astronomers.
  R Coranae Borealis variables are cool red giants with significant carbon in their outer layers that has been brought up from the core. Carbon soot occasionally forms in their atmospheres or in their slow-moving stellar winds, and dims them by up to 9 magnitudes for months or years. Easily monitored with binoculars, or even the unaided eye, the prototype of the class, R Coronae Borealis, is a favorite of amateurs. Normally about magnitude 5.8, R CrB fades in a matter of weeks to anywhere between 9th and 15th magnitude at irregular intervals, typically every few years.

Cataclysmic Variables

  These are close binary stars in which there is an exchange of mass. They include novae and dwarf novae. Novae are binaries in which gas from the outer hydrogen-rich layers of a star, typically a red giant, is streaming into an excretion disk around a white dwarf, and ultimately onto the white dwarf's surface. As hydrogen builds up in a shell on the compact white dwarf, and is compressed by both the new material added on top, and the tremendous gravity of the white dwarf, the critical temperature of 10 million°K is eventually reached at the base of the hydrogen shell, and then the hydrogen ignites and runaway fusion occurs. Novae increase in brightness by 7 - 16 magnitudes in as little as a day or, rarely, as long as several hundred days. Although the main show is usually over in a few weeks, it takes years for the star to completely fade back to its pre-outbursts magnitude. Several novae have repeat performances, and it is thought that probably all novae are recurrent, although outbursts might be hundreds to thousands of years apart in some systems.
  U Geminiorum dwarf novae have sudden increases of 2 to 6 magnitudes, with tens of thousands of days spent sitting at their normal minimum magnitude between outbursts. The similar Ζ Camelopardalis stars exhibit occasional "standstills" - periods of constant brightness that are about 1/3 of the way from normal maximum to minimum.

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