The Characteristics of Stars

Classifying Stars Stars are classified by their size, temperature and brightness. The sun is neither the largest nor the brightest star but it is the closest to us.

3 STAR BRIGHTNESS

LUMINOSITY - the total amount of energy produced by a star per second

Brightness of stars The brightness of a star depends upon its size and temperature. Betelgeuse is fairly cool but it is very large so it gives off a lot of light. Rigel is much smaller but it is very hot so it appears to shine as brightly as Betelgeuse.

Apparent Magnitude A star’s apparent magnitude is its brightness as seen from Earth. How does it appear. The sun appears to be the brightest

Absolute Magnitude A star’s absolute magnitude is the brightness the star would have if it were at a standard distance from Earth.

The Hertzsprung-Russel Diagram The H-R diagram is a graph of stars temperature and brightness. Most stars fall within a diagonal band called the main sequence. In the main sequence surface temperature increases as brightness increases.

The Life Cycle of Stars

12 Nebulas and Protostars A star is made up of a large amount of gas in a relatively small volume. All stars begin as nebulas which are large amounts of gas and dust spread out over an immense (huge) volume. Gravity can pull some of the gas and dust in a nebula together This contracting cloud is called a protostar which is the earliest stage in the life of a star

Eagle Nebula /2009/01/hubble-eagle-nebula-wide-field-04086y.jpg Protostars images/ JPG

14 Birth of a Star A star is “born” when the contracting gas and dust become so hot that nuclear fusion begins to occur. How long a star lives depends on how much mass it has. Stars with less mass burn their fuel more slowly and last longer than stars with more mass

Lifetimes of Stars Stars with little mass can live as long as 200 billion years. Stars that are 15 times bigger than the sun might last only 10 million years. The sun is medium-sized and should live for about 10 billion years, since the sun is about 4.6 billion years old it is almost halfway through its lifetime.

16 Deaths of Stars When a star begins to run out of fuel, the center of the star shrinks and the outer part expands. The star becomes a red giant or supergiant All main sequence stars eventually become red giants or supergiants, what happens next depends on the mass of the star. When a star runs out of fuel it becomes a white dwarf, a neutron star, or a black hole.

Life Cycle of a Star 1.Nebula/Protostar Small or Medium Star Giant or Supergiant Star Red Giant Supernova Black Dwarf Black HoleNeutron Star

18 White Dwarfs Small- and Medium-sized stars become red giants, then the outer layers drift off into space. The blue-white hot core is left behind and is a white dwarf.

and Black Dwarfs A White dwarf has the mass of the sun but is the size of Earth, it is one million times as dense as the sun. When a white dwarf runs out of fuel and energy it becomes a black dwarf. A black dwarf has stopped glowing because fusion has stopped. It is a “dead” star, not the Death Star that is Darth Vader’s spacecraft.

Neutron Stars Dying giant or supergiant stars can explode, this is called a supernova. Left behind is an incredibly dense star called a neutron star, only about 20 kilometers across. A spoonful of matter would have as much mass as a large truck

Black Holes Only the most massive stars with 40 times the mass of the sun become black holes. After a supernova more than 5 times the mass of the sun can be left. The gravity is so strong that gas is pulled inward, eventually all of this mass is contained with in a sphere only 30 kilometers in diameter.

22 Black Holes The gravity becomes so strong that nothing can escape, not even light. This is a black hole. No light, radio waves, or any form of radiation can get out of a black hole. Astronomers can not see black holes directly. Astronomers can detect black holes indirectly –Gas pulled in rotates so fast that it heats up and gives off X-rays –Scientists can calculate the mass of a black hole by it effect on nearby stars

Quasars In the 1960’s astronomers discovered very bright objects that are very far away, about 12 billion light-years away Astronomers concluded that quasars are distant galaxies, each with a black hole at its center

Medium-sized Star (The Sun)

Red Giants

Giants and Supergiants Red giant Supergiants

27 Supernova

28 White Dwarf and Black Dwarf NYC063368//blackdwarf.gif /images/screenshots/various/extrasolar_ stars_White_Dwarf_Sirius_B_1__ Frank_Gregorio.jpg

Neutron Star

Black Hole

Quasar DeepSpace/Quasars/Quasar3C273Hubble.jpg