Chapter 22 Stars

Distance to the StarsThe 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 BrightnessAstronomers 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