Stellar Evolution: Aim: The Life Cycle of a Star Notepack 23 Stellar Evolution: Aim: The Life Cycle of a Star Do Now: Where do stars come from? What are they made of?

What is a Star?

What is a Star? A star is a massive ball of gas that has ignited due to tremendous heat and pressure. Stars “burn” as the results of 2 Hydrogen atoms fusing together to create a Helium atom. A stars fuel is hydrogen; its waste product is helium, heat and light. The Sun is a star.

How do stars put hydrogen atoms together? To put hydrogens atoms together, the Star must perform a nuclear reaction. There are 2 types of nuclear reactions: Fission reactions are nuclear reactions that splits atoms apart. This creates a lot of energy Makes harmful waste produces (radioactive) Used by human to power nuclear power plants and nuclear bombs.

How do stars put hydrogen atoms together? Fusion reactions combines two or more atoms to make a new one. Fusion reactions creates enormous energy They need a lot of energy to start it. (usually a nuclear fission explosion is needed to start it) This type of reaction produces waste products that are “clean” Fission reactions are used by stars.

Age of a Star Scientists can predict the age of a star based on the ratio of Hydrogen and Helium it has. Stars that have a lot of hydrogen (majority) is said to be a young star. Stars that have a lot of helium is said to be an old star. Scientists use a spectrometer to measure color emitted from the star to determine how much hydrogen it has.

Types of Stars Due to the different ages of different stars and the amount of hydrogen atoms the star started out with, we get many different types of stars

Types of Stars There are many different types of stars Hot Stars Blue Cold Stars Red Large Stars Supergiants Giants These stars have a high luminosity – they are very bright Small Stars Dwarfs These stars have a low luminosity – they are very dim

Luminosity The word, Luminosity, is used to describe the brightness of a star. The reason why some stars shine brighter than others is because these stars are creating enormous energy. The unit of measurement used to describe luminosity is 1 Sol or the brightness of our Sun. A star with a luminosity of 1000 means the star is 1000 times brighter than our Sun. The color of the star tells you it’s temperature.

Color The color of the star tells you the star’s temperature. Stars that shine blue are very hot stars with temperatures ranging up to 30,000 K. Stars that shine red are cool stars with temperatures ranging down to 3,000 K. Brown stars are very cold. Their surface is estimated as 78 degrees F. You could walk on a brown star.

Our Sun Description of our Sun Our sun is an average star Our Sun is known as a Main Sequence star It is Yellow And has a temperature of about 6,000oK Our sun is an average star Not too big / Not too small Not too hot / Not too cold Not too bright / Not too dim

Medium Size Stars Take about 1 million years to form Spend about 10 billion years on the main-sequence Similar to our sun Diameter: 1,391,400 Mass: 1.99 x 1030 kg (1 solar mass)

Supergiant Largest and Hottest stars Form quickly (100,000 years) Die quickly (average 100 million years) Contain about 2100 times (or more) the mass of the Sun When they die, they explode This explosion is called a supernova

Brown Dwarfs Brown dwarfs are the smallest and coldest stars Take a long time to form (millions of years) Live for a very long time (trillions of years?)

How Stars Form Stars begin their lives as Nebula A Nebula is a giant cloud of gas and dust in space. These clouds are very big. They range from 93 million miles long to over 1000 million miles long. They are very cold a the typical temperature of a nebula is 100 K or (-279°C)

How Stars Form (gravity) Every atom has gravity. Gravity is an attraction between atoms. As atoms get closer to each other, the attraction between them increase. Gravity increases as the atoms combine. The more atoms accumulate, the stronger gravitational pull becomes. As the gravitational pull increase– the more atoms are attracted.

How Stars Form Every piece of dust in the nebula has gravity. As the dust move closer to each other, their attraction to each other increases. This attraction causes them to combine, thereby increasing the gravity pull. Has the gravity pull increases, so is the amount of atoms being pulled to the center. This action causes the nebula to be less like a gas and more like a solid.

1st Stage of Life - The Nebula Ball of gas and dust are pulled together by gravity The shrinking gas cloud starts to heat up an glow due to the energy released by the atoms colliding into each other from the gravitational pull. When the clouds shrink to almost a solid ball it ignites and nuclear fusion starts.

Depending on how much gas was in the nebula and how large in size the nebula was will depend on how long the star will stay in each stage of its life. Stage 2 - Main Sequence Stage 3 - Giants Stage 4- Super Giants Stage 5 - White Dwarfs or Nova

2nd stage - Main Sequence (the Sun) Stage 2 is the Longest stage (usually) of a star’s life. The star has a lot of hydrogen to change into helium and it creates enormous amounts of energy The size of the star does not change much during this stage The star is somewhat stable

3rd and 4th stages - Giants and Supergiants As the star ages, the Main Sequence star will eventually become a red giant During the Red giant stage (stage 3) the star begins to expands and cools as it starts to use up all of its hydrogen. The star is mostly made up of helium. Center of the star shrinks while the atmosphere grows larger and cools In the future, our Sun will become a Red Giant. It will grow so large that its size will overlap the Earth’s orbit, consuming the Earth.

A Possible 5th stage - first possibility The Red giant may begin to shrink forming a White Dwarf . White Dwarfs a small but very hot star that is the leftover center (core) of an older star. White Dwarfs can shine for billions of years before they extinguish, leaving only a helium core.

Possible 5th stage - second possibility The Red Giant uses all of the hydrogen and begins to collapse on itself. The collapsing star then explodes (a nova) The remnants of the star’s core will form either: Black hole A neutron star

A Supernova is a gigantic explosion in which a massive star collapses Occurs after a massive star uses up its fuel source A Neutron Star is a star that has collapsed to a point at which all particles are neutrons A neutron star that spins and sends out beams of radiation is called a pulsar. A Black Hole is a star that has shrunk to the size of a golf ball but still contains all its mass. Because black holes are so dense, they have tremendous gravity. The gravity is so strong that light cannot escape.

Hertzprung-Russell Diagram Shows the relationship between a star’s surface temperature and absolute magnitude Used to study the lives of stars Most stars lie along the main sequence portion of the diagram