Star (astronomy)

I

Introduction

Star (astronomy), massive shining sphere of hot gas. Of all the stars in the universe, our Sun is the nearest to Earth and the most extensively studied. The stars visible to the naked eye all belong to the Milky Way Galaxy, the massive ensemble of stars that contains our solar system (the Sun and its nine planets).

About 5,000 stars can be seen with the naked eye, although not all of these stars are visible at any given time or from any given place. With a small telescope, hundreds of thousands of stars can be seen. The largest telescopes disclose millions of galaxies, which may each contain over 200 billion stars. Modern astronomers believe there are more than 1 x 10²² stars in the universe (this number is very large, a 1 followed by 22 zeros). The largest stars, if placed at the Sun's position, would easily engulf Earth, Mars, Jupiter, and Saturn. The smallest white dwarf stars are about the size of Earth, and neutron stars are less than about 20 km (about 10 mi) in diameter.

All stars are composed of hot glowing gas. The outer layers of some stars are so empty that they can be described as red-hot vacuums. Other stars are so dense that a teaspoonful of the material composing the outer layers would weigh several tons. Stars are made chiefly of hydrogen and a smaller amount of helium. Even the most abundant of the other elements present in stars—oxygen, carbon, neon, and nitrogen—are generally present in very small quantities.

The Sun, our nearest star, is about 150 million km (about 93 million mi) from Earth. It appears different from the stars visible in the night sky because it is about 250,000 times closer to Earth than the next closest star. The next nearest star is Proxima Centauri, which is more than 30 trillion km (20 trillion mi) from Earth. While light from the Sun takes only about eight minutes to reach Earth, the farthest stars are so distant that their light takes billions of years to reach Earth.

---

---

The color of stars—ranging from the deepest red through all intermediate shades of orange and yellow to an intense white-blue—depends directly on their temperature. The coolest stars are red and the hottest stars are blue. Most stars make light by several different kinds of thermonuclear fusion, a process in which the nuclei of atoms combine to form a heavier element and release energy (see Nuclear Energy). One of the most common thermonuclear fusion processes occurs in stars when four hydrogen atoms combine into a helium atom, releasing energy that is transformed into light and heat.

In the 1990s astronomers discovered planets orbiting stars outside our solar system. Planets outside our solar system are difficult to detect, because they are much fainter than stars are. However, astronomers located these planets by measuring the wobble of a star’s motion created by the slight gravitational pull that is exerted on the star by a planet. Although scientists can only speculate how many Earthlike planets with continents and oceans exist in the universe, they believe that many stars have planetary systems (See also Gravity).

II

Characteristics of Stars

Astronomers learn about the physical and chemical properties of a star by studying the energy it emits. They can directly observe the atmosphere, or outermost gaseous layer, of the star. Astronomers can infer many of the properties of the star’s interior by studying the star’s atmosphere, its size, and the energy the star releases. The properties more closely related to the interior structure of the star are its mass and chemical composition. Astronomers also observe the motions of stars to learn more about star and galaxy formation.

A

Stellar Atmosphere

The only visible part of a star is its gaseous outer region, or atmosphere. The atmosphere of the Sun is about 320 km (about 200 mi) thick, while the Sun’s diameter is 1,392,000 km (865,000 mi). Even though the atmosphere is relatively small compared to the size of the entire star, astronomers can learn a great deal about a star by studying its atmosphere.

Light emitted by a star has several properties of interest to astronomers. Magnitude is how astronomers measure a star’s brightness. Luminosity is the total amount of light a star emits. Astronomers analyze a star’s light to classify the star’s spectral type, which provides clues to the temperature and chemical composition of the star.

A

1

Magnitude

Astronomers first ranked stars in the night sky by their apparent magnitude, or relative brightness. They grouped stars visible to the naked eye into six classes, or magnitudes, that each correspond to a factor of about 2.5 in brightness. The brightest stars are classified as 1st magnitude, 2nd-magnitude stars are about 2.5 times less bright, while stars just visible to the naked eye on a clear night are 6th magnitude. The faintest stars observable with the Hubble Space Telescope are fainter than the 28th magnitude.

Astronomers distinguish between apparent magnitude and absolute, or intrinsic, magnitude. Apparent magnitude is the brightness of a star as viewed from Earth, and the absolute magnitude of a star is its actual brightness as viewed from a set distance away from the star. The difference between these two types of magnitude helps astronomers distinguish between stars that appear bright only because they are relatively close to Earth and stars that appear bright because they are intrinsically bright or highly luminous. For example, Sirius, which is only 8.6 light-years distant, has the greatest apparent magnitude of any nighttime star. (A light year is the distance light travels in a year—9.5 trillion km, or 5.9 trillion mi.) Other stars may appear fainter because they are farther away, even if they actually shine much more brightly than Sirius does. Such stars have a greater absolute magnitude. The absolute magnitude is closely related to the physical conditions of the star.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain