Page 320 - NS-2 Textbook

ASTRONOMY 315

itself five times is approximately 100) is the base of the LIFE CYCLE OF STARS
magnitude scale used in modern astronomy. Stars fainter
than the sixth magnitude can be seen from Earth only A star begins as a huge, cold, dark sphere of gas alld
through a telescope. dust. Precisely where or how this original star material
Our Sun has an absolute magnitude of +5. The giant was formed is not known. SOllle astrononlers believe it
stars are given an absolute magnitude of 0, thus making came from the ashes of stars long gone; others say it had
them 100 times brighter than the Still. There are even been present since the "beginning," at the Big Bang. No
brighter stars called supergiants, which are as much as 1 one knows exactly when the beginning was, of course j
million times as bright. Their absolute magnitudes alld there are those who think that the universe did not
would therefore be negative. have a beginning but has always existed. Cosmologists
are still trying to find the answers to these questions.
The most interesting class, howevel~ is the one called
white dwmfs. 11,ese stars are at least 100 times fainter than But we do know that stars are being born today out
the Sun but are much hotter. They have about the same of the gas and dust of the Milky Way, our galaxy of stars.
mass as the Sun, but they are smaller in size. The white The Hubble Space Telescope has provided many spectacu-
dwarfs have densities much greater than any substance lar photos of this process taking place. Because of ad-
on Earth. A cubic inch of material from one of these stars vances in astronomy alld nuclear physics, the major
could weigh as much as a ton. An ordinaty finger ring stages in the life of a star have been fairly well deter-
would weigh 75 pmillds on a white dwarf. Of the more mined.
familiar stars, Rigel, Polaris (the North Star), and Antares The main factor determining what kind of star will be
are supergiants; Arcturus and Capella are giants. Vega, born is how much gas alld cosmic dust become locked to-
Altair, and the Sun are medium-sized stars in the group gether by gravity in that particular area. If there is a lot of
called the main sequence on the spectrum-llll11inosity material available there, the star will probably end up as
scale. The main sequence comprises 98 percent of all a brilliant blue giant. If it is like most stars, it will become
stars presently observed by astronomers. a yellow star like our Sun, with a much longer "life" thall
a blue giant. With even less dust alld gas, it will become
all orange dwarf that will live on for billions of years.
CEPHEID STARS
lt perhaps seems somewhat odd that the brighter
One V\ray astronomers can determine a star's hrighhless stars have a shorter life. This is rather easily explained.
is by using nearby cepheid stars as a reference. 11,ese The more fuel there is to burn, the greater the heat alld
bright stars expand and contract with a definite rhythm, the consumption rate; so, comparatively speaking, the
and thus they are called cepheid variables. They are some- brighter star is burning itself out faster. The rate of fuel
times referred to as the "lighthouses of space." The constilllption is set at the beginning alld does not vary.
cepheid variables are hottest when they are at maximum Once the hydrogen-to-helitilll fusion cycle begins, it will
brightness and coolest when they are at mininltlll continue until the hydrogen is exhausted.
brightness. Their variations in brightness are usually In the introduction to this unit, we described the
about one magnitude. There also are changes in the spec- probable birth of the Sun. Thus we need not explain the
trum radiations as the star's brighhl€SS varies. entire cycle of the birth of a star again. By way of review,
When the star contracts, its internal pressure and though, we know that the radiation pressure eventually
temperature increase. The star must expand in a sort of builds up toward the center of the gas-dust ball. As the
explosion. Once the star has expanded, the balance is temperature atld pressure increase, the ball begins to
again upset-so the stat' contracts again, tmder its gravi- glow. When the nuclear furnace starts working full time,
tational attraction. Another name for the cepheid vari- the whole swirling gas ball glows, sending its energy out
able, because of this alternating phenomenon, is pulsating into space in the form of visible light and other electro-
star. magnetic radiation.
The discovery that there is a constant relationship Most astrononlers believe there is a °normal" evolu-
between the period of variation and the brightness of tion of stars. As the helium content builds up in the cen-
cepheid stars has enabled ash'onomers to use them not ter, leftover hydrogen accumulates, upsetting the inter-
only as brightness references but also as a means of mea- nal balance of the star. To compensate, the star increases
suring distances in space. By noting how many days it in size alld luminosity, until about one-eighth of the orig-
takes the star to grow bright alld dim, they can deduce inal hydrogen has been transformed to helitilll. The star
the absolute magnitude of the star, and consequently continues to increase in size, finally becoming a red
hm·v far mvay it is. TI1US, when other methods of deter- giallt. As a giatlt, the star consumes fuel at a tremendous
mining distances are not practical because of extreme rate, tmtil its hydrogen is used up.
distallce, cepheid variables provide a valuable method The helium produced in stellar fusion actually repre-
for calculating star distances. sents the star's "ashes." But the heliull1 ashes also fuse as