Transcript Chapter 22 - Stars part one

Chapter 22
Stars
Distance to the Stars
The closest star to Earth is, of course, the sun. the average distance between
Earth and the sun is about 150 million kilometers. This distance defines on
astronomical unit, or one AU.
How far away is the nearest star? Imagine that Earth is a dot 1 centimeter form
the sun. the next nearest star, Alpha Centauri is about 40 trillion kilometers
away, nearly 300,000 times as far from Earth as is the sun.
Kilometers are not very satisfactory units for expressing the great distances in
space. Neither are astronomical units. Instead, astronomers use a unit called a
light year (LY). Despite the name, the distance that a ray of light travels in on
year. The speed of light is about 300,000 kilometers per second. At this rate light
can travel about 9.5 trillion kilometers in 1 year. Alpha Centauri is about 4.3
light-years from Earth while Betelgeuse, the red super giant in Orion, is nearly
490 light-years away.
Physical Properties of the Sun
The sun is an average start in may ways. Its diameter is about 1,
380,000 kilometers. Its average dsenisty is about 1.4 times that of
water. Its mass is about 300,000 times that of Earth. How do the other
starts in the universe compare with the sun?
Star sizes vary over a great range. The smallest stars may be smaller
than Earth. The largest star known is more than 2,000 times the
diameter of the sun.
Stars differ even more in density. Betelgeuse is one-ten millionth as
dense as the sun. Sirius has a neighbor so dense that one tea spoonful
of it would eight more than a ton on Earth.
Stars differ in mass. Masses larger than 50 times that of the sun are probably very rare. The smallest is
about one hundredth the mass of the sun. most stars are fairly close to the sun in mass.
The color of the star depends on its surface temperature. Betelgeuse is red, the sun is yellow, and Sirius is
blue. Stars radiate all colors, but hotter stars emit more blue and less red. The same color changes can be
seen when an iron bar is heated. As it gets hotter, its color changes from red to orange to yellow to white
to blue-white. In stars, red-hot may mean a temperature of only 3,000 degrees at the surface. Stars that
are blue-hot may be over 30,000 degrees. The sun has a surface temperature of about 5500 degrees.
(all temperatures are in Celsius)
Elements in Stars
Spectrum analysis helps astronomers
determine the composition of stars. Stars
are mainly hydrogen and helium. One or
two percent of a star’s mass may be heavier
elements such as iron, titanium, calcium,
sodium, and others. The sun appears to be
around 70 percent hydrogen and 28 percent
helium. The remaining two percent is
heavier elements.
The spectrum radiated by a star depends on
both its composition and its temperature.
No two stars have exactly the same
composition and temperature. Each star has
its own individual spectrum.
Star Brightness
Astronomers have several ways of talking about eh brightness of a star. One way is the star’s apparent
magnitude. The apparent magnitude is how bright the star appears to an observer on Earth. The brighter
stars are first-magnitude stars. The faintest stars that can be seen with the unaided eye are sixth
magnitude.
In the star-magnitude system, each magnitude differs from the next by a factor of approximately 2.5. this
means that a first-magnitude star is 2.5 times brighter than a second-magnitude star. A second-magnitude
star is 2.5 times brighter than a third magnitude star, and so on. A first-magnitude star is 100 times
brighter than a sixth-magnitude star
(2.5 X2.5X2.5X2.5X2.5). The apparent magnitudes of stars brighter than first magnitude are expressed as
values less than 1.0 such as 0 magnitude. Some stars are even brighter than 0 magnitude and have
negative values. For example, Sirius, the brightest star in our sky, has an apparent magnitude of -1.43
Apparent magnitudes are also used to express the apparent brightness
of the planets. At their brightest, Venus, Mars and Jupiter are brighter
than any star. Their brightness apparent magnitudes are -4.4, -2.5 and 2.8 respectively.
Apparent magnitude indicates how bright the star appears to us. It
does not tell how bright the star actually is. For example, the apparent
magnitude of Sirius is about 10 times brighter than that of Antares. Yet
Antares is actually about 250 times brighter than Sirius. Antares is
much farther away from Earth than Sirius. The actual or true
brightness of a star is its luminosity.
Luminosity of a star depends only upon its size and temperature.
Apparent magnitude also depends upon a star’s distance from Earth.
Consider this example. Viewed from the same distance away a 100 watt
bulb is much brighter than a flashlight bulb. The 100 watt bulb has
greater luminosity. However, the flashlight bulb up close would look
brighter than the 100 watt bulb a kilometer away. Under those
conditions the apparent magnitude of the flashlight is greater than that
of the 100-watt bulb.
How do astronomers express the true brightness or luminosity of a
star? If all stars could be placed at the same distance from Earth, their
true brightness could be compared. Astronomers use the term absolute
magnitude to express the luminosity of stars as if they were seen from
the same distance. Absolute magnitude is the apparent magnitude a
start would have if placed at a distance of 32.6 light-years from the sun.
the sun is an average star. Its absolute magnitude is 4.8. by contrast, a
very bright star such as Rigel in Orion has an absolute magnitude of
-6.4