Stars


Magnitudes

The magnitude scale was invented by an ancient Greek astronomer named Hipparchus
in about 150 BC He ranked the stars he could see in terms of their brightness,
with 1 representing the brightest down to 6 representing the faintest. Modern
astronomy has extended this system to stars brighter than Hipparchus\' 1st
magnitude stars and ones much, much fainter than 6.

As it turns out, the eye senses brightness logarithmically, so each increase in
5 magnitudes corresponds to a decrease in brightness by a factor 100. The
absolute magnitude is the magnitude the stars would have if viewed from a
distance of 10 parsecs or some 32.6 light years. Obviously, Deneb is
intrinsically very bright to make this list from its greater distance. Rigel, of
nearly the same absolute magnitude, but closer, stands even higher in the list.
Note that most of these distances are really nearby, on a cosmic scale, and that
they are generally uncertain by at least 20%. All stars are variable to some
extent; those which are visibly variable are marked with a "v".

What are apparent and absolute magnitudes? Apparent is how bright the appear to
us in the sky. The scale is somewhat arbitrary, as explained above, but a
magnitude difference of 5 has been set to exactly a factor of 100 in intensity.
Absolute magnitudes are how bright a star would appear from some standard
distance, arbitrarily set as 10 parsecs or about 32.6 light years. Stars can be
as bright as absolute magnitude -8 and as faint as absolute magnitude +16 or
fainter. There are thus (a very few) stars more than 100 times brighter than
Sirius, while hardly any are known fainter than Wolf 356.

Star, large celestial body composed of gravitationally contained hot gases
emitting electromagnetic radiation, especially light, as a result of nuclear
reactions inside the star. The sun is a star. With the sole exception of the sun,
the stars appear to be fixed, maintaining the same pattern in the skies year
after year. In fact the stars are in rapid motion, but their distances are so
great that their relative changes in position become apparent only over the
centuries. The number of stars visible to the naked eye from earth has been
estimated to total 8000, of which 4000 are visible from the northern hemisphere
and 4000 from the southern hemisphere. At any one time in either hemisphere,
only about 2000 stars are visible. The other 2000 are located in the daytime sky
and are obscured by the much brighter light of the sun. Astronomers have
calculated that the stars in the Milky Way, the galaxy to which the sun belongs,
number in the hundreds of billions. The Milky Way, in turn, is only one of
several hundred million such galaxies within the viewing range of the larger
modern telescopes. The individual stars visible in the sky are simply those that
lie closest to the solar system in the Milky Way. The star nearest to our solar
system is the triple star Proxima Centauri, which is about 40 trillion km (about
25 trillion mi) from earth. In terms of the speed of light, the common standard
used by astronomers for expressing distance, this triple-star system is about
4.29 light-years distant; light traveling at about 300,000 km per sec (about
186,000 mi per sec) takes more than four years and three months to travel from
this star to earth (see LIGHT-YEAR). Physical Description The sun is a typical
star, with a visible surface called a photosphere, an overlying atmosphere of
hot gases, and above them a more diffuse corona and an outflowing stream of
particles called the solar (stellar) wind. Cooler areas of the photosphere, such
as the sunspots (see SUN) on the sun, are likely present on other typical stars;
their existence on some large nearby stars has been inferred by a technique
called speckle interferometry. The internal structure of the sun and other stars
cannot be directly observed, but studies indicate convection currents and layers
of increasing density and temperature until the core is reached where
thermonuclear reactions take place. Stars consist mainly of hydrogen and helium,
with varying amounts of heavier elements.

The largest stars known are supergiants with diameters that are more than 400
times that of the sun, whereas the small stars known as white dwarfs have
diameters that may be only 0.01 times that of the sun. Giant stars are usually
diffuse, however, and may be only 40 times more massive than the sun, whereas
white dwarfs are extremely dense and may have masses about 0.1 times that of the
sun despite their small size.