Stars

Stars are celestial bodies of hot gases that radiate energy derived from thermonuclear reactions in the interior. Stars are composed of Plasma, a substance similar to gas in which a certain portion of the particles are ionized. Stars are Luminous and are held together by gravity. A Star appears Luminous and shining because of the thermonuclear fusion in its core which releases energy that traverses the Star's interior and then radiates into outer space. There are billions and trillions of Stars in the Milky Way Galaxy but only a very small fraction are visible to the naked eye. Some Stars are visible at night as the Sun's light doesn't outshine them at that time. Sun is the closest Star to Earth. Stars are not spread uniformly across the universe, but are normally grouped into Galaxies along with Interstellar Gas and dust. Stars vary greatly in brightness (magnitude), colour, temperature, mass, size, chemical composition, and age. A typical Galaxy contains hundreds of billions of Stars, and there are more than 100 billion (1011) Galaxies in the observable Universe.

Life Cycle of a Star

Every Star has certain span of life time which varies from Star to Star. The lifespan of Stars vary from thousands of years for massive Stars to billions for smaller Stars. Larger Stars have more fuel, but they have to burn (fuse) it faster in order to maintain Equilibrium. Because Thermonuclear Fusion occurs at a faster rate in massive Stars, Large Stars use all of their fuel in a shorter length of time. Therefore the bigger the Star the shorter is it's life. In contrast a Smaller Star has less fuel, but its rate of fusion is not as fast. Therefore, Smaller Stars live longer than Larger Stars because their rate of fuel consumption is not as rapid.  The Sun which has an average mass, is predicted to be about 10 billion years old.

Main Sequence - Stars spend tens of millions of years forming before joining the Main Sequence. Main Sequence is a phase in which Stars live out the majority of their lives. During this phase, Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Stars in this phase are known as Dwarf Stars. The duration of a Star's life in the Main Sequence depends primarily on the amount of fuel it has to fuse and the rate at which it fuses that fuel, i.e. its initial mass and its luminosity. Once achieving nuclear fusion, Stars radiate (shine) energy into space. The Star slowly contracts over billions of years to compensate for the heat and light energy lost. As this slow contraction continues, the Star’s temperature, density, and pressure at the core continue to increase. The temperature at the centre of the Star slowly rises over time because the Star radiates away energy, but it is also slowly contracting. This battle between gravity pulling in and gas pressure pushing out will go on over the entire life span of the Star.

After or Post-Main Sequence - After or Post-Main Sequence is the end stage of a Star. A Star will eventually use up most of it's hydrogen and be left with helium. At this time there is not enough pressure crushing down on the Star to create a nuclear reaction with helium. Nuclear reactions cease inside the Star, and because there is no longer any outward push from fusion, the Star begins to collapse upon its self. This is the phase when the Star leaves the Main Sequence and enters After or Post-Main Sequence. This collapse begins to create more and more pressure inside the Star until it is sufficient to have the fusing process of helium begin in the core, while some of the remaining hydrogen burns just outside of it. The products of this helium burning is carbon and oxygen. The Star swells, and depending on its size, either becomes a Red Giant or a Red Super Giant. It will then eventually collapse and explode. This explosion is also known as Supernova explosion. Depending upon its original mass, the Star will become either a Black Dwarf, Neutron Star, or Black Hole. The blown-off outer layers of dying Stars include heavy elements which may be recycled during new Star formation. These heavy elements also allow the formation of Rocky Planets.

Spectral Classification

Stars are classified by their Spectra i.e. the elements that they absorb.

This Classification can be simplified to learn, through this phrase 'Oh Be A Fine Girl And Kiss Me'.

Types of Stars

T Tauri Star - It is a type of Star when the Star is just evolving. Actually it is a stage in a Star's formation, right before it becomes a Main Sequence Star. This type of Star occurs at the end of the Protostar Phase, when the gravitational pressure holding the Star together is the source of all its energy. T Tauri Stars don't have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble Main Sequence Stars; they're about the same temperature but brighter because they're a larger. T Tauri Stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years.

Main Sequence Star - The Main Sequence Star is a Star which occurs in the Main Sequence Star Phase. These are also known as Young Stars because of their recent origin. Main Star vary in size, mass and brightness. These Stars convert hydrogen into helium in their cores, releasing a tremendous amount of energy.  A Star in the main sequence is in a state of Hydrostatic Equilibrium. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward. Majority of all Stars in our galaxy, and even the Universe, are Main Sequence Stars. The Sun is a Main Sequence Star, and so are our nearest neighbours, Sirius and Alpha Centauri A.

Red Giant Star - It is a type of Star which is formed during the later stages of the evolution of a Star. It is also known as Ageing Star as it has reached old age.  When a Star has used its stock of hydrogen in its core, fusion stops and the Star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the Star, but causes it to increase in size dramatically. A Red Giant Star can be 100 times larger than it was in its Main Sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. The Red Giant phase of a Star's life will only last a few hundred million years before it runs out of fuel completely and becomes a White Dwarf.

White Dwarf Star - It is a type of Star which has reached the last stage in it's life cycle. In this last stage a Star has completely exhausted hydrogen fuel in its core and lacks the mass to force higher elements into fusion reaction. The outward light pressure from the fusion reaction stops and the Star collapses inward under its own gravity. A White Dwarf shines because it was a hot Star once, but there's no fusion reactions happening any more. A White Dwarf will just cool down until it because the background temperature of the Universe. This process will take hundreds of billions of years, so no White Dwarfs have actually cooled down that far yet.

Red Dwarf Star - It is type of a Star which are Main Sequence Stars but with a low mass. Red Star are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other Stars. Therefore these are the most common kind of Stars in the Universe. According to Astronomers some Red Dwarf Stars will burn for up to 10 trillion years. The smallest Red Dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to half of the Sun.

Neutron Star - It is a type of Star whose core is entirely composed of neutrons. When Star has a mass size between 1.35 and 2.1 times to that of the Sun, it doesn't form a White Dwarf when it dies. Instead, the Star dies in a catastrophic Supernova explosion, and the remaining core becomes a Neutron Star. A Neutron Star is composed of Neutrons because the intense gravity of the Neutron Star crushes protons and electrons together to form Neutrons. If Stars are even more massive, they will become Black Holes instead of Neutron Stars after the Supernova goes off.

Super Giant Star - It is a type of Star which have a massive mass. Their size is dozens of times as compared to the size of the Sun. They are the largest Stars in the Universe. Super Giants consume hydrogen fuel at an enormous rate and consume all the fuel in their cores within just a few million years. Super Giant Stars have a short life span and die young, detonating as Supernovae; completely disintegrating themselves in the process.

Binary Stars - They are pairs of Stars moving in orbit around their common centre of mass. The brighter Star is called the primary and the other is its Companion Star, Comes, or Secondary Star. They are also known as Double Stars. An optical pair appears to be double because 2 Stars lie in the viewer's line of vision. Examples of Double Stars are Phakt in Columba and Arcus in Crux.

Binary Stars are classified into 4 Types:

1. Visual Binary Star - A Visual Binary Star is a Binary Star for which the angular separation between the two components is great enough to permit them to be observed as a Star in a telescope, or even high-powered binoculars.
2. Spectroscopic Binary Star - This type consists of a pair of Stars where the spectral lines  in the light emitted from each Star shifts first toward the blue, then toward the red, as each moves first toward us, and then away from us, during its motion about their common centre of mass, with the period of their common orbit.
3. Eclipsing Binary Star - An Eclipsing Binary consists of 2 close Stars moving in an orbit so placed in space in relation to Earth that the light of one can at times be hidden behind the other. On of the example of Eclipsing Binary Star is Algol (Beta Persei).
4. Astrometric Binary Star - Astrometric Binary Star is relatively nearby Stars which can be seen to wobble around a point in space, with no visible companion. Sometimes a Binary System is too far away, or the Stars are too close, or one Star is so much brighter than the other that we cannot distinguish the two Stars visually. To detect the presence of an unseen companion, the gravitational influence on the Primary Star is studied. A Binary discovered in this way is termed an Astrometric Binary Star.

Variable Star - A Star whose brightness as seen from Earth changes over time. These changes are due to variations in the Star's actual luminosity, or to variations in the amount of the Star's light that is blocked from reaching Earth.

Variable Stars are of 2 types:

1.  Intrinsic Variables - Stars whose variability is caused by changes in the physical properties of the Stars themselves. These changes can be periodic swelling and shrinking of the Star.

Intrinsic Variables can be divided into iii subgroups:

i)  Pulsating Variables - Pulsating Variable Stars are those Stars whose radius alternately expands and contracts as part of their natural evolutionary ageing process.

The two most important types Pulsating Variables are:

a)  Cepheids and Cepheid-like Stars - They have short periods (days to months) and their luminosity cycle is very regular.

b)  Long Period Variables - Their period is longer, on the order of a year, and much less regular.

ii)  Eruptive Variables - Eruptive Variable Stars are those Stars which are characterised by eruptions on their surfaces like flares or mass ejections.

iii)  Cataclysmic or Explosive Variables - Cataclysmic Stars are those Stars which undergo a cataclysmic change in their properties like Novae and Supernovae.

2.  Extrinsic Variables - Stars whose variability in brightness is caused due to changes in the amount of their light that reaches Earth. Example, a Star has an orbiting companion that sometimes eclipses it.

There are ii main subgroups of Extrinsic Variables

i)  Eclipsing Binaries - Eclipsing Binary Stars are those Stars which occasionally eclipse one another as they orbit. These are also known as Double Stars.

ii)  Rotating Variables - Rotating Variable Stars are those Stars whose variability is caused by phenomena related to their rotation. For examples some Stars have 'Sunspots' which affect the apparent brightness or Stars that have fast rotation speeds causing them to become Ellipsoidal in shape.

Characteristics and Physical Features of Stars

• Luminosity - Luminosity is the amount of light that a Star radiates. The size of the Star and its surface temperature determine its luminosity. The apparent brightness of a Star is measured by its apparent magnitude, which is the brightness of a Star with respect to the Star’s luminosity, distance from Earth, and the altering of the Star’s light as it passes through Earth’s Atmosphere. Intrinsic or absolute magnitude is directly related to a Star’s luminosity and is what the apparent magnitude a Star would be if the distance between the Earth and the Star were 10 parsecs (32.6 light-years). Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times (the 5th root  of 100 or approximately 2.512). This means that a first magnitude (+1.00) Star is about 2.5 times brighter than a second magnitude (+2.00) Star, and approximately 100 times brighter than a sixth magnitude (+6.00) Star. The faintest Stars visible to the naked eye under good seeing conditions are about magnitude +6. On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the Star; the larger the magnitude number, the fainter. The brightest Stars, on either scale, have negative magnitude numbers.

• Colour - A Star's colour depends on its surface temperature. Cooler Stars tend to be redder in colour, while hotter Stars have a bluer appearance. Stars in the mid ranges are white or yellow, such as our sun. Stars can also blend colors, such as red-orange Stars or blue-white Stars. For example, Betelgeuse looks reddish, Pollux is yellowish and Rigel is bluish. A Star's colour depends on its surface temperature. The temperature of a Star is measure in a metric unit known as the Kelvin. Dark Red Stars have surface temperatures of about 2500 K. The surface temperature of a bright Red Star is approximately 3500 K; that of the Sun and other Yellow Stars, roughly 5500 K. Blue Stars range from about 10,000 to 50,000 K in surface temperature.

• Temperature - The surface temperature of a main Sequence Star is determined by the rate of energy production at the core and the radius of the Star and is often estimated from the Star's colour index. It is normally given as the effective temperature, which is the temperature of an idealized black body  that radiates its energy at the same luminosity per surface area as the Star. The effective temperature is only a representative value, however, as Stars actually have a temperature gradient that decreases with increasing distance from the core. The temperature in the core region of a Star is several million kelvins. The stellar temperature will determine the rate of energization or ionization of different elements, resulting in characteristic absorption lines in the spectrum. Massive Main Sequence Stars can have surface temperatures of 50,000 K. Smaller Stars such as the Sun have surface temperatures of a few thousand K. Red Giants have relatively low surface temperatures of about 3,600 K, but they also have a high luminosity due to their large exterior surface area. The coolest, Reddest Stars are approximately 2,500 K, while the Hottest Stars can reach 50,000 K. The Sun's temperature is about 5,500 K.

• Size - Stars range in size from Neutron Stars, which vary anywhere from 20 to 40 km in diameter, to Super Giants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun—about 0.9 billion kilometres. The size of Stars is measured in terms of the Sun's radius. Alpha Centauri A, with a radius of 1.05 solar radii (the plural of radius), is almost exactly the same size as the Sun. Rigel is much larger at 78 solar radii, and Antares has a huge size of 776 solar radii.

• Mass - Astronomers measure the mass of a Star in terms of the solar mass, the mass of the sun. For example, they give the mass of Alpha Centauri A as 1.08 solar masses; that of Rigel, as 3.50 solar masses. The mass of the sun is 2 Ž 1030 kilograms, which would be written out as 2 followed by 30 zeros. Stars that have similar masses may not be similar in size i.e. they may have different densities. Density is the amount of mass per unit of volume. For instance, the average density of the sun is 88 pounds per cubic foot (1,400 kilograms per cubic meter), about 140 percent that of water. Sirius B has almost exactly the same mass as the sun, but it is 90,000 times as dense. As a result, its radius is only about 1/50 of a solar radius. When the metallicity is very low, however, a recent study of the faintest Stars found that the minimum Star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter. Smaller bodies are called Brown Dwarfs, which occupy a poorly defined grey area between Stars and Star. The combination of the radius and the mass of a Star determines the surface gravity. Giant Stars have a much lower surface gravity than Star, while the opposite is the case for degenerate, compact Stars such as Star. The surface gravity can influence the appearance of a Star's spectrum, with higher gravity causing a broadening of the absorption lines.

• Longevity of Age - Most Stars are very old, as old as 1 billion-10 billion years old. Some Stars are considered to be even as old as the Universe itself. The age of Stars i dependant on it's mass. The bigger the Stare, the shorter its lifespan. This is because massive Stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive Stars last an average of about one million years, while Stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.

• Abundance of Hydrogen -  In nearly all Stars, hydrogen is the most abundant element. It has been found that most Stars are composed of around 70% hydrogen and 28% helium by mass. Other materials by which Stars may be composed of include Silicon, Magnesium, Neon, Iron, Sulphur etc. All Stars have a steady state period during their life cycle, in which phase they transform hydrogen to helium. When the hydrogen gets exhausted the Star reaches the end of its life, then the formed helium is transformed into larger elements like carbon, oxygen or neon.

• Occurence in Pairs - Most Stars occur in pairs and multiple systems. Most of the Stars are not solitary like the Sun, but occur in pairs (binaries) or multiple systems. According to some studies as many as 85% of Stars are not just single points of light, but instead, belong to double or multiple Star systems. In some cases, a Dwarf Star is revolving around a Giant sun. In others, two Stars of equal size orbit a common point in between the two, while in other cases still, several Stars orbit one another. In some cases, as many as six or seven Stars belong to a single system.

• Spherical Shape - All the Planets in our Solar System are spherical in shape. The gravity compresses the Star into a shape that will most evenly distributes the gravitational force among the Star’s mass. Any object in weightless space larger than a couple of hundred miles in diameter has enough mass for its gravity to overcome large-scale irregularities and force it into a spherical shape. Gravity is pulling the Star inward, and the light pressure from all the fusion reactions in the Star are pushing outward. The inward and outward forces balance one another out, and the Star maintains a spherical shape.

Some of the Major Stars

Sirius.
Canopus.
Arcturus.
Alpha Centauri.
Vega.
Capella.
Rigel.
Procyon.
Achernar.
Betelgeuse.
Beta Centauri.
Altair.
Alpha Crucis.
Aldebaran.
Spica.

Origin and Evolution of Stars

Nebulae are the birthplaces of Stars. A Nebula is a cloud of gas (hydrogen) and dust in space. Star Formation takes place when these dense Clouds of hydrogen and dust grains collapse under their own gravity. This collapse maybe caused by the shock waves from Supernovae (massive stellar explosions) or the collision of two Galaxies (as in a star burst galaxy). As the Cloud collapses, individual conglomerations of dense dust and gas form. These are are known as 'Bok Globules'. As a Globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. The internal temperature increase until they are hot enough to trigger nuclear fusion in its core. When the Protostellar Cloud (a fragment of a bigger cloud) has approximately reached the stable condition of Hydrostatic Equilibrium (balancing of pressure - outward and gravity - inward, a Protostar forms at the core. These pre-main sequence Stars are often surrounded by a Protoplanetary Disk. The period of gravitational contraction lasts for about 10–15 million years. Early Stars of less than 2 solar masses are called T Tauri Stars, while those with greater mass are Herbig Ae/Be Stars. These newly born Stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing Star and result in small patches of nebulosity known as Herbig-Haro objects. These jets, in combination with radiation from nearby massive Stars, may help to drive away the surrounding cloud in which the Star was formed.