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In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. The luminosity of a star is determined by the radius and the surface temperature. However, many stars do not radiate a uniform flux—the amount of energy radiated per unit area—across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux at its poles than along its equator.

Surface patches with a lower temperature and luminosity than average are known as star spots. Small, dwarf stars such as the Sun generally have essentially featureless disks with only small star spots. Larger, giant stars have much bigger, much more obvious star spots, and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk. Red dwarf flare stars such as UV Ceti may also possess prominent star spot features.

The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star’s outer layers) and as a steady stream of neutrinos emanating from the star’s core.

The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star’s outer layers, including its photosphere.- Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays,
and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.

Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is known, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be measured directly for stars in binary systems. The technique of gravitational micro lensing will also yield the mass of a star). With these parameters, astronomers can also estimate the age of the star.

A star has five main characteristics: (1) brightness, which astronomers describe in terms of magnitude or luminosity; (2) color; (3) surface temperature; (4) size; and (5) mass (amount of matter). 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. This graph, a version of which appears in this article, also helps astronomers understand and describe the life cycles of stars.

Why do stars shine?
The stars shine because they are hot. But where does a star get its energy to be hot? Until fairly recently, the answer to this question was unknown. It was Albert Einstein, developer of the theory of relativity at the beginning of this century who answered this question: a small quantity of matter can be transformed in to a large quantity of energy.

The stars are mostly made up of hydrogen. Hydrogen is the simplest natural element and the most common one in the universe. A hydrogen atom consists of a nucleus with a single proton and an electron. The matter in the center of the star is very compressed by the weight of the star itself. This causes the protons that form the nuclei of the hydrogen atoms to collide violently with each other. As a result of these collisions, four hydrogen nuclei can come together to form a Nucleus of helium. The helium nucleus is made up of two protons and two neutrons. During this process of fusion, two protons are transformed in to neutrons, emitting a positron (a light particle like an electron, but with a positive charge). The helium nucleus weights slightly less than the four hydrogen nuclei that formed it. This small quantity of matter that has disappeared releases a large quantity of energy.

The nucleus of most stars is a true nuclear reactor, where fusion reaction takes place. The hydrogen nuclei collide violently with each other. Four protons can merge and create a helium nucleus with two protons and two neutrons. Like a star a nuclear power station gets its energy from nuclear reactions. But instead of getting it from the fusion of light nuclei, the power station gets energy from fission-the splitting-of heavy nuclei.

Have you ever noticed the color of the stars?The light they put out varies some what in color from one star to another. In some cases you can see this difference easily. For example, in the constellation Orion, which is visible in the evening around December, the two brightest stars have distinct colors: Betelgeuse, in the north east corner, is red; while Rigel, in the south west corner, emits a blue-white light. The stars have a whole range of colors, from red to orange, yellow, white and finally blue-white.

These color differences tell us how hot the star is. Every warm body emits light and heat(these are two forms of electromagnetic radiation), whether it is a star or an incandescent light bulb. A light bulb filament, heated by electric current, emits radiation, a small part of which is visible light. If, for example, the temperature of the filament goes down, then the light it emits turns yellowish or even reddish. If, on the other hand, the temperature of the filament rises, the light is not only more intense but also whiter and more bluish. The color of the light emitted lets us know the temperature of the body that is emitting it.

In this way, we know that the material the stars are made of is so hot that it takes the form of a gas: stars are huge balls of gas, mostly of hydrogen. This gas is extremely hot, and because of its high temperature it emits light and heat. The color of the light tells us the temperature of the stars surface.

Astronomers measure star temperatures in a metric unit known as the 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.

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. Although a star appears to the unaided eye to have a single color, it actually emits a broad spectrum (band) of colors. You can see that starlight consists of many colors by using a prism to separate and spread the colors of the light of the sun, a yellow star. The visible spectrum includes all the colors of the rainbow. These colors range from red, produced by the photons (particles of light) with the least energy; to violet, produced by the most energetic photons.

A star is a massive, luminous ball of plasma that produces a tremendous amount of light and other forms of energy which is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun. The sun looks like a ball because it is much closer to Earth than any other star.

The sun and most other stars are made of gas and a hot, gas like substance known as plasma. But some stars, called white dwarfs and neutron stars, consist of tightly packed atoms or subatomic particles. These stars are therefore much denser than anything on Earth. For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.

Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertz sprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.

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. The distance from the sun to Proxima Centauri is more than 25 trillion miles (40 trillion kilometers). This distance is so great that light takes 4.2 years to travel between the two stars. Scientists say that Proxima Centauri is 4.2 light-years from the sun.

What is a light year? The distance that a ray of light travels in one year is known as a light year. Light is the fastest of all things known to man. A Danish astronomer, Olaus Roemer(1644-1710), first calculated the speed of light in 1676 as 1,86,000 miles(2,97,600kilo meters) per second. Later German born American physicist professor Albert Michelson (1852-1931) made a more accurate calculation of the speed of light as 1,86,284miles per second, for all practical purposes the speed of light is taken as 1,86,000miles per second. The distance covered by light at this speed in a year is about 6*10 ¹² miles or 9.6*10 ¹² (96,000,000,000,000) kilometers. This distance is one light year. The maximum width of our galaxy is 1,20,000 light years. Can you try to imagine such a huge formation?