Star

Star

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This article is about the astronomical object. For other uses, see Star (disambiguation).

The Pleiades, an open cluster of stars in the constellation of Taurus. NASA photoLook up Star in

Wiktionary, the free dictionary.A star is a massive, luminous ball of plasma. Stars group together to form galaxies, and they dominate the visible universe. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth, including daylight. Other stars are visible in the night sky, when they are not outshone by the Sun. A star shines because nuclear fusion in its core releases energy which traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created inside the cores of 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 that are determined by its evolutionary history include the diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H-R diagram), allows the current age and evolutionary state of a particular star to be determined.

A star begins as a collapsing cloud of material that is composed primarily of hydrogen along with some helium and heavier trace elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. These processes keep the star from collapsing upon itself and the energy generates a stellar wind at the surface and radiation into outer space.[1]

Once the hydrogen fuel at the core is exhausted, a star of at least 0.4 times the mass of the Sun[2] expands to become a red giant, fusing heavier elements at the core, or in shells around the core. It then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment where it will form a new generation of stars with a higher proportion of heavy elements.[3]

Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[4]

Contents [hide]

1 Etymology

2 Observation history

3 Star designations

4 Units of measurement

5 Formation and evolution

5.1 Protostar formation

5.2 Main sequence

5.3 Post-main sequence

5.3.1 Massive stars

5.3.2 Collapse

6 Distribution

7 Characteristics

7.1 Age

7.2 Chemical composition

7.3 Diameter

7.4 Kinematics

7.5 Magnetic field

7.6 Mass

7.7 Rotation

7.8 Temperature

8 Radiation

8.1 Luminosity

8.2 Magnitude

9 Classification

10 Variable stars

11 Structure

12 Nuclear fusion reaction pathways

13 See also

14 References

15 Further reading

16 External links

Etymology

The word star in English is derived from Greek aster, which comes from Hittite shittar, which is derived from Sanskrit sitara, "सितारा"[5].

Observation history

Stars have always been important to every culture. They have been used in religious practices and for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they were all but immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.[6] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.[7] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to the nearest star, the Sun.

In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear .[8] Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584 Giordano Bruno suggested that the stars were actually other suns, and may have other planets, possibly even Earth-like, in orbit around them,[9] an idea that had been suggested earlier by such ancient Greek philosophers as Democritus and Epicurus.[10] By the following century the idea of the stars as distant suns was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the solar system, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.[11]

The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley