Astronomy & Astrology - Star (astronomy)

Star (astronomy)

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 (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.

Spectral Class	Effective Tempterature - Star Color	Principal Characteristics
O	25,000 K - Blue star	This stage is characterized by lines of helium, oxygen, and nitrogen in the spectrum of the photosphere. O stars are extremely hot, very bright stars that emit large amounts of ultraviolet radiation.
B	11,000 K - 25,000 K - White-blue star	In this group the lines of helium in the spectrum reach a maximum intensity and then fade. The intensity of the hydrogen lines regularly increases in all the subdivisions of stage B. Type B stars are typically represented by the star Epsilon Orionis.
A	7500 K - 11,000 K - White star	This group comprises the so-called hydrogen stars. The spectra of these stars is dominated by absorption lines of hydrogen. Sirius, the Dog star, is a typical type A star.
F	6000 K - 7500 K - Yellow-white star	This group is composed of stars characterized by an elevated intensity of the H and K lines of calcium and of lines characteristic of hydrogen. A notable star in this category is Delta Aquilae.
G	5000 K - 6000 K - Yellow, solar star	This group is composed of stars with prominent H and K calcium lines and less prominent hydrogen lines. The spectra of numerous metals, in particular iron, are also present. The Sun belongs to this group, and therefore G stars are frequently called solar stars.
K	K 3500 K - 5000 K - Orange-yellow star	This group comprises stars having strong calcium lines and lines indicating the presence of other metals in their spectra.The violet light of class K stars is less intense than the stars' red light. This group is typically represented by Arcturus.
M	3500 K - Red star	This group is composed of stars whose spectra are dominated by bands resulting from the presence of metallic-oxide molecules, notably those of titanium oxide. The violet end of the spectra is less intense than that of K stars. The star Orionis is typical of this group.

Astronomers categorize stars according to the the characteristics of the light that the stars emit, which are related to the stars' temperatures. O stars are the hottest stars, and M stars are the coolest. Our Sun is a G star.

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.

Contributed By: Owen Gingerich, A.B., A.M., Ph.D. Professor of Astronomy and History of Science, Harvard-Smithsonian Center for Astrophysics. Author of The Great Copernicus Race and The Eye of Heaven. Asteroid #2658 named Gingerich.

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