A star is a huge, shining ball that produces a large amount of energy in form of light and other forms. Stars are very far from us, thats why they look like twinkling points of light. Our sun is also a star. A lot of stars are like our sun. Some differ in mass, size, brightness and temperature.
Stars come in many sizes. Some of the stars have a radius of about 1,000 times that of the sun. The smallest stars are the neutron stars, some of which have a radius of only about 6 miles (10 kilometers). About 75 percent of all stars are members of a binary system, a pair of closely spaced stars that orbit each other. The sun is not a member of a binary system. However, its nearest known stellar neighbor, Proxima Centauri, is part of a multiple-star system that also includes Alpha Centauri A and Alpha Centauri B. Proxima Centauri is 4.2 light years far form sun.
Stars are grouped in huge structures called galaxies. Telescopes have revealed galaxies throughout the universe at distances of 12 billion to 16 billion light-years. Our sun is in a galaxy called the Milky Way that contains more than 100 billion stars. There are more than 100 billion galaxies in the universe, and the average number of stars per galaxy may be 100 billion.
Stars have life cycles. They born, pass through several phases, and finally die. The sun was born about 4.6 billion years ago and will remain much as it is for another 5 billion years. Then it will grow to become a red giant. Late in the sun's lifetime, it will cast off its outer layers. The remaining core, called a white dwarf, will slowly fade to become a black dwarf.
Other stars will end their lives in different ways. Some will not go through a red giant stage. Instead, they will merely cool to become white dwarfs, then black dwarfs. A small percentage of stars will die in spectacular explosions called supernovae.
Brightness of star seen from Earth depends on two factors:
A nearby star that is actually dim can appear brighter than a distant star that is really extremely brilliant. For example, Alpha Centauri A seems to be slightly brighter than a star known as Rigel. But Alpha Centauri A emits only 1/100,000 as much light energy as Rigel. Alpha Centauri A seems brighter because it is only 1/325 as far from Earth as Rigel is -- 4.4 light-years for Alpha Centauri A, 1,400 light-years for Rigel.
These characteristics are related to one another in a complex way. Color depends on surface temperature, and brightness depends on surface temperature and size. Mass affects the rate at which a star of a given size produces energy and so affects surface temperature. To make these relationships easier to understand, astronomers developed a graph called the Hertzsprung-Russell (H-R) diagram.
The surface temperature of a 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 color 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 temperature in the core region of a star is several million kelvins. The stellar temperature determines the rate of energization or ionization of different elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star.
Star temperatures are measured in a metric unit known as kelvin. One kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), but the Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 degrees C. Therefore, a temperature of 0 K equals -273.15 degrees C, or -459.67 degrees F. A temperature of 0 degrees C (32 degrees F) equals 273.15 K.
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.
Color of star is the color of light the star is emitting. A star can appear red, if it emits light more towards red part of visible band of electromagnetic spectrum, and can appear blue if it emits light more towards blue part of visible spectrum. Our sun emits light equally among all wavelengths of visible spectrum. That is why our sun appears white to us.
Color of the star depends on its surface temperature.
To understand how the color of a star depends on its temperature, we have to understand the concept of blackbody. A blackbody is a theoretical object that absorbs all radiation fall upon it. As a result it gets heated by absorbing all radiation, and starts emitting electromagnetic radiation at every wavelength. A plot of intensity versus wavelength of electromagnetic radiation emitted by blackbody is called blackbody spectrum. Figure 1 shows an example of blackbody spectrum.
Figure 1 can be understood using Wien's law, which states the following relationship:
|λ =||2.9 x 10-6|
λ is wavelength of maximum blackbody emission and
T is the temperature of blackbody (in Kelvin)
As we can see that the relationship between wavelength and temperature is inverse. This means that as temperature will increase, wavelength will decrease or vice versa. Now if we take a look at figure 1, we will get the same relationship. There are 2 blackbody plots in figure 1, A and B. Body A is at a temperature of 20,000 K and body B is at a temperature of 5,000 K. Curve for Blackbody A peaks at a shorter wavelength than curve for blackbody B (Wavelength is increasing from left to right in figure 1), and blackbody A is at higher temperature than blackbody B.
Until this point we have seen that temperature is inversely proportional to the wavelength at which maximum emission from blackbody occurs. But this is not the end of story. Consider the following equation:
L = 4πR2σT4
L is the luminosity in J/s,
4πR2 is the surface area of the blackbody in m2,
σ is the Stefan-Boltzmann constant and
T is the temperature of the blackbody in Kelvin
Above equation is called Stefan-Boltzmann law and it describes relationship between luminosity, temperature and size of blackbody. In figure 2, there are 3 blackbodies. Body A and C are at same temperature. We can see that intensity peaks for body A and C are same (at same height along Y-axis), but these 2 blackbodies are at different temperatures. This doesn't seem possible at first, but Stefan-Boltzmann law explains this. According to Stefan-Boltzmann law, intensity of emitted radiation not only depends on temperature, but on size of blackbody too.
Now consider blackbodies B and C in figure. Both are at same temperature, but intensity or brightness is different. According to Stefan-Boltzmann law, we can conclude that blackbody B is smaller than C.
Another thing that should be noted from Stefan-Boltzmann law is that effect of change in temperature is more on the intensity of emitted radiation than change in size.
Stars are also known to astronomers as blackbodies. So blackbody characteristics apply on stars as well. As we know that color of the blackbody is the color of the light it emits or light emitted in a particular part of visible spectrum (if light emitted is in red part of visible spectrum, then color will be red; if light emitted is in blue part of visible spectrum, then color will be blue), and since color of light is due to its wavelength, we can say that color of star depends on its temperature as wavelength is related to temperature (Wein's law).
Luminosity is the brightness of star - i.e. total light emitted by star. The scientific term for rate of energy emission is power, and power is measured in watts. But in case of stars, luminosity is not measured in watts. Luminosity of a star is measured in terms of the luminosity of the sun, which is called Solar Luminosity. Luminosity of sun is 3.82 * 1026 watts. For example, luminosity of Alpha Centauri A is about 1.3 times solar luminosity.
Magnitude is basically a measure of star's brightness. In ancient times, when there were no modern astronomical instruments, astronomers classify stars by their brightness. This system is now called stellar magnitude scale. Ancient astronomers developed this system by watching sky after sun set. The first stars that appeared were classified as first magnitude stars, second stars that appeared are classified as second magnitude stars and so on, upto six magnitude stars. Stars with magnitude six were the faintest stars.
Modern astronomers further classified this magnitude scale into apparent magnitude and absolute magnitude. Apparent magnitude of a star is related to its brightness and absolute magnitude is related to its luminosity. Modern astronomers also extended magnitude scale to include negative and decimal numbers and faintest stars upto 26th magnitude.
Apparent magnitude is a measure of star's brightness, or how bright a star appears to human eye. But in modern days, astronomers do not depend on their eyes. They use modern instruments like telescopes, photometers etc. So we can say that apparent magnitude is a measure that how bright a star appears to viewer or observer.
Brightness of a star depends on the distance of star, size of star and energy emitted by star. If 2 stars have same size, distance and emitting same amount of energy, then their brightness or apparent magnitude will be same. If 2 stars have different brightnesses and their size and distance are equal, then they will be emitting different amount of energy. If we know star's apparent magnitude and two out of three qualities(size, distance and amount of energy emitted), we can get idea about the remaining third quality.
Unlike apparent magnitude, absolute magnitude does not depend on star's size or distance. Absolute magnitude is the measure of how bright a star actually is, or how much energy a star is emitting. Absolute magnitude does not provide any idea about star's radius or its distance. Absolute magnitude is also called luminosity.
Modern magnitude scale starts with the brightest object and goes to the faintest object. Apparent and absolute magnitudes of sun are -26.8 and 4.8 respectively.
Luminosity is related to absolute magnitude in a simple way. A difference of 5 on the absolute magnitude scale corresponds to a factor of 100 on the luminosity scale. Thus, a star with an absolute magnitude of 2 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 whose absolute magnitude is 2 and 10,000 times as luminous as a star that has an absolute magnitude of 7.
Magnitude is based on a numbering system invented by the Greek astronomer Hipparchus in about 125 B.C. Hipparchus numbered groups of stars according to their brightness as viewed from Earth. He called the brightest stars first magnitude stars, the next brightest second magnitude stars, and so on to sixth magnitude stars, the faintest visible stars.
Modern astronomers refer to a star's brightness as viewed from Earth as its apparent magnitude. But they have extended Hipparchus's system to describe the actual brightness of stars, for which they use the term absolute magnitude. For technical reasons, they define a star's absolute magnitude as what its apparent magnitude would be if it were 32.6 light-years from Earth.
Astronomers have also extended the system of magnitude numbers to include stars brighter than first magnitude and dimmer than sixth magnitude. A star that is brighter than first magnitude has a magnitude less than 1. For example, the apparent magnitude of Rigel is 0.12. Extremely bright stars have magnitudes less than zero -- that is, their designations are negative numbers. The brightest star in the night sky is Sirius, with an apparent magnitude of -1.46. Rigel has an absolute magnitude of -8.1. According to astronomers' present understanding of stars, no star can have an absolute magnitude much brighter than -8. At the other end of the scale, the dimmest stars detected with telescopes have apparent magnitudes up to 28. In theory, no star could have an absolute magnitude much fainter than 16.
Astronomers measure the size of stars 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.
A star's size and surface temperature determine its luminosity. Suppose two stars had the same temperature, but the first star had twice the radius of the second star. In this case, the first star would be four times as bright as the second star. Scientists say that luminosity is proportional to radius squared -- that is, multiplied by itself. Imagine that you wanted to compare the luminosities of two stars that had the same temperature but different radii. First, you would divide the radius of the larger star by the radius of the smaller star. Then, you would square your answer.
Now, suppose two stars had the same radius but the first star's surface temperature -- measured in kelvins -- was twice that of the second star. In this example, the luminosity of the first star would be 16 times that of the second star. Luminosity is proportional to temperature to the fourth power. Imagine that you wanted to compare the luminosities of stars that had the same radius but different temperatures. First, you would divide the temperature of the warmer star by the temperature of the cooler star. Next, you would square the result. Then, you would square your answer again.
Mass of a star is measured in terms of the solar mass, which is the mass of the sun, which is 1.98 * 1030 kilograms. For example, we can say that mass of Alpha Centauri A is 1.08 solar masses, and mass of Rigel is 3.50 solar masses.
Stars that have similar masses can be different in size i.e; they can have different densities. Density is the amount of mass per unit of volume. For instance, the average density of the sun is 1,400 kilograms per cubic meter. Sirius B has almost exactly the same mass as the sun, but it is 90,000 times denser than sun. As a result, its radius is only about 1/50 of a solar radius.