Stars
A star is a body of hot gas and dust that shines by its own light produced by nuclear fusion reactions. The nearest star to Earth is the Sun. When compared to other stars in the universe, the Sun is of average size, with a diameter of approximately 860,000 mi (1,380,000 km). Over a million Earth's could fit inside the volume of the Sun. The Sun is made up of gas (it is about 70% hydrogen and 28% helium), and is about 1.5 times as dense as water. The mass of the Sun is over 300,000 times that of Earth. Deep inside the Sun, nuclear reactions convert hydrogen to helium, changing matter to energy in a process called fusion. Fusion is responsible for converting over four million tons of the Sun's matter into energy every second.
The fusion reactions that take place within the Sun's, and any star's, core are the same as those that take place in a hydrogen bomb. These nuclear reactions require incredibly high temperatures. About 1% of a hydrogen atom's mass is converted into energy when two hydrogen atoms are converted into a single helium atom. This energy is used to maintain such high interior temperatures, sometimes up to millions of degrees Fahrenheit (hundreds of thousands of degrees Celsius). The surface temperatures are slightly lower and vary depending on the type of star. All stars are composed of at least 99% hydrogen and helium, fueling these nuclear reactions. The remaining mass of the star is composed of heavier elements such as nitrogen, carbon, or oxygen.
Stars can differ from each other in many ways. One difference can be in their densities. Some stars are thousands of times more dense than the sun, while others are thousandths of times less dense than air. Another notable difference between stars is size. Some stars, such as the red supergiants, are thousands of times larger than the Sun, while others, such as white dwarfs and neutron stars, are smaller than Earth.
The Sun is 93 million mi (150 million km) away from Earth. The next nearest star is 26 million million mi (42 million million km) away. It is difficult to comprehend such large numbers, so a scale other than miles or kilometers is used when talking about stars. Distances between stars are measured in terms of light years. One light year is the distance light travels in one year, approximately 6 million million mi (9,461 trillion km). One light year is the distance you would cover if you circled Earth 118 million times. Stars exist throughout the known universe, up to billions of light years away.
Stars appear brighter or dimmer not only because of their distance from Earth, but also because of a property called luminosity, or brightness. The star's apparent magnitude is how bright a star appears to be from Earth. The brightest stars are called first-magnitude stars, and the dimmest stars (to the naked eye) are sixth-magnitude stars. Each magnitude differs from the next by a factor of 2.5. A first-magnitude star is 2.5 times brighter than a second-magnitude star, which is 2.5 times brighter than a third magnitude star, and so on. Some stars are even brighter than first-magnitude stars. These stars have magnitudes less than one, such as zero-magnitude or even negative-magnitude stars. The brightest star seen from Earth is the Sun, with a magnitude of -27. The second brightest star is Sirius, with an apparent magnitude of -1.43.
The apparent magnitude does not tell how bright a star actually is; it only tells how bright it appears from Earth. The star's luminosity is its true brightness. Luminosity depends on the size and the temperature of the star. A star that has a higher temperature than a star of the same size will be more luminous. A star that is larger than another star at the same temperature will also be more luminous. A star's temperature not only determine its brightness, but also its color. Stars can be found in many different colors, ranging from red (the coolest stars) to blue-white (the hottest).
The term absolute magnitude is used to describe a star's true luminosity. This is the apparent magnitude a star would have if it were located 32.6 light years from the Sun. The most luminous stars are the red supergiants. These are the largest of all the stars. Red giant stars are less luminous than red supergiants, although their large size also makes them appear very bright. The dwarf stars are less luminous. Stars, then, can be classified according to their luminosity into one of several groups, listed in order of decreasing luminosity: the supergiants, giants, sub-giants, dwarfs, and white dwarfs.
Stars can also be classified according to their spectrum. There are seven spectral types of stars, in order of decreasing temperature: O, B, A, F, G, K, and M. A star's spectrum is related to its temperature because different elements will appear in a spectrum at different temperatures. Type O stars are the hottest stars, with temperatures over 55,000°F (30,000°C). These stars appear blue-white. Type B stars have temperatures around 36,000°F (20,000°C), and are also blue-white. Type A stars are those with temperatures around 20,000°F (11,000°C). These stars appear white. Type F stars are around 13,500°F (7,500°C), and are seen as yellowish white. Type G stars have temperatures around 11,000°F (6,000°C) and are seen as yellow stars, type K stars are around 7,500°F (4,100°C) and are orange stars, and type M stars are around 5,500°F (3,000°C) and are seen as red stars.
Other star types include variable stars and pulsars. A variable star, also known as a pulsing star, exhibits variable brightness. These stars appear brighter and dimmer as they expand and contract. This phenomenon usually occurs as a star is forming or dying. Pulsars are neutron stars that emit pulses of radio waves in a very periodic fashion. These stars produce powerful beams of radio waves due to their intense magnetic fields. As a pulsar rotates, the beam sweeps past Earth, which is measured as a pulse. Stars can be found in formations that, when viewed from Earth, appear to take on various shapes. These formations are called constellations. Stars can also be found in pairs, called binary or double stars, in clusters, and in groups of thousands of stars, called galaxies.
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