The Birth and Death of Stars

1. The Birth and Death of Stars

2. A small to medium mass star forms a planetary nebula leaving a core white dwarf in the center Nebula where new stars are born A new star is born and grows and lives as a main sequence star Massive stars explode into supernovas and compress into a neutron star and or black hole As star dies it loses its energy and grows to a red giant

3. What are Stars? • Stars are large balls of hot gas. • They look small because they are a long way away, but in fact many are bigger and brighter than the Sun. • The heat of the star is made in the center by nuclear fusion reactions that create heavier and heavier elements. • There are lots of different colours and sizes of stars.

4. How are stars made? • Stars are made (or “born”) in large, cold clouds of dust and hydrogen gas called nebulas. These nebulas are found in dark spaces between stars. They do not give off their own visible light, but can be seen with infrared wavelengths. Some nebulas are the remains of a supernova (Ex. Crab nebula). Some are star nurseries. During the star formation process, nebulas glow brightly.

5. Protostars • part of a nebula cloud shrinks because of cold and gravity. • As it shrinks it becomes hotter and begins spinning, forming a dense sphere. • The more matter it pulls into itself, the larger the resulting star will be. • As pressure increases, heat increases • Extreme heat and pressure can cause the hydrogen atoms to fuse together (nuclear reaction) and produce helium and energy (heat and light). • The mass of the material must be at least 80x the size of Jupiter to create enough pressure for fusion to occur. • Once fusion begins, the radiation burns off surrounding gas and... A Star is Born! This new little star is known as a “protostar”. • This stage lasts for about 100,000 years or so depending on the size of the star.

7. Main sequence • About 80% of all stars are in this stage • When the gas pressure pushing outward from inside a star equals the force of gravity pulling atoms inward, the star becomes stable and enters its main cycle of life burning hydrogen gas to create energy to keep it stable. This process is called “fusion”. It is when the hydrogen atoms are fused together to form helium. During its “life” a star will not change much. Different stars are different colors, sizes, and brightnesses! • Lower mass stars (1/10 to about 10x our sun’s diameter—most are medium sized) can remain in this stage for billions (even trillions!) of years. Smaller stars are less bright and usually cooler. Medium stars are yellow or orange and small, cool stars are red to brown. Larger mass stars ( about 10-30x the size of our sun/ supermassive stars are more than 30x the size of our sun) These large stars can run of fuel in just a few million years. • Because they are so hot, the bigger stars actually have shorter lives than the small, cool ones. • The bigger a star, the hotter and brighter it is. Hot stars are Blue or white.

9. Stars have different colors—Blue hottest, green warm, red cool

10. RED GIANTS (and supergiants) • Hydrogen continues to burn in the star’s outer shell, but the core is mostly helium now • Core continues to get hotter and hotter until He begins to fuse into carbon (about 200,000,000 degrees Centigrade) • Outer shell expands rapidly as it runs out of H • As it expands, it cools and reddens

12. PLANETARY NEBULA • In a small to medium mass star the last of the hydrogen gas surrounding the red giant begins to drift away forming a ring around the center core—this ring is called a planetary nebula (it has nothing to do with planets) • WHITE DWARF • The small core remaining is called a white dwarf • The last of the star’s matter begins to collapse inward and is squeezed very tightly • 1 teaspoon may have a mass of one ton or more • No fusion is occurring at this point, but a white dwarf still burns very hot for billions of years • It takes about 10 billion years for a medium sized star such as our sun to die ( a smaller star could take 100 billion years or so) • BLACK DWARF • White dwarfs gradually use all the energy they have left (they can not contract any further so they can not create any more heat) and finally end their lives as cold, dark bodies called black dwarfs. These are small and do not give off any light. The universe is not really old enough for any of these to have formed yet and they will be very hard to find.

15. RED SUPERGIANT • Can expand up to 1000x its original size • In a massive star gravity is much stronger and will continue to pull carbon atoms together—under the extreme pressure and heat (about 1,000,000,000 degrees C), carbon atoms will continue to fuse together and form heavier atoms such as oxygen and nitrogen—intense heat continues fusion process until iron forms in the core—these atoms are too dense to fuse—energy is lost and star expands to red supergiant

16. Supernova • Larger stars take only a few million to a few billion years to die • Fe atoms begin to absorb energy and then gravity causes the core to collapse inward • Part of the core bounces back outward and creates a tremendous explosion • Heat can reach up to 1 billion degrees and Fe is then fused to form the heavier elements as they are exploded outward • This explosion will create more energy in a few days than our sun will create in 100 billion years • The light from the supernova can be brighter than the original star and can light up the sky for weeks (as bright as a million stars at once) • The remaining gas and dust will form a new nebula

17. Neutron star • After a large mass star (blue giant) explodes, a small, dense core is left • Less than 16 km in diameter, but 1 tsp of this material will have the mass of 100 billion tons or more • This is called a neutron star—it emits very little visible light but strong x-ray wavelengths • Neutron stars do not contract any furtherso they just continue to burn out the remaining energy and finally end as cold, dark bodies PULSAR-- • If the explosion causes this core to spin, it will give off energy in the form of radio waves and pulse light toward Earth as it spins—this is called a “pulsar” • It spins very rapidly, much like a blender

18. Black holes • Stars considered blue supergiants leave a core that has and enormous mass that cannot be supported • Without the energy being created from the fusion process, the core is actually swallowed by its own gravity • The gravitational pull is so great that even light particles are sucked in • Scientists believe a black hole is funnel shaped • The process heats gases to where x-ray wavelengths are released before being lost • Scientists have been able to find black holes by the swirling x-ray waves around the “event horizon” • massive stars that had companion stars sometimes pull the gases off them as they die, also letting scientists know where they are in space • Black holes maintain the same gravitational pull as the original star had before it collapsed