What is a star? (Not in Notes Page)

1) A star is a hot ball of mostly hydrogen gas; the Sun is an example of a typical, ordinary star.

2) Gravity keeps the gas from evaporating into space, and pressure due to the star's high temperature and density keeps the ball from shrinking. 3) In the core of the star, the temperature and densities are high enough to sustain nuclear fusion reactions, and the energy produced by these reactions works its way to the surface and radiates into space as heat and light.

•When the fuel for the fusion reactions is depleted, the structure of the star changes. The process of building up heavier elements from lighter ones by nuclear reactions, and adjusting the internal structure to balance gravity and pressure, is called stellar evolution.

Why Stars Evolve (Start of Notes)

•The stars' fuel for energy generation is the stuff they are made of -- hydrogen, helium, carbon, etc. -- which they burn by converting these elements into heavier elements.

•"Burning" in this context does not refer to the kind of burning we are familiar with, such as the burning of wood or coal, which is chemical burning.

• It refers to nuclear burning, in which the nuclei of atoms fuse into nuclei of heavier atoms.

How Stars Evolve

•When stars start their lives, they consist mostly of hydrogen, some helium, and small amounts of heavier elements, such as carbon, nitrogen, and oxygen.

•They generate energy by converting nuclei of hydrogen into nuclei of helium in their hot, central cores (Fusion).

•The energy produced by nuclear burning heats its interior to many millions and, in some instances, hundreds of millions to billions of degrees Fahrenheit.

•Emission nebulae are clouds of high temperature gas and dust.

•Emission nebulae are usually red, because hydrogen, the most common gas in the universe, most commonly emits red light.

It all starts with a nebula (interstellar clouds of dust and gas).•Some nebulas are several light years in diameter and could be 100 times bigger than our own solar system.

•Stars begin their lives by burning hydrogen in their cores. The product of this burning is helium. A star that burns hydrogen in its core is called a main sequence star. The Sun is an example of a main sequence star.

                                                                                                                 

The Sun in white light. Most of the radiation from the Sun is white light -- namely, light that is a combination of all of the rainbow colors. The conspicuous dark splotches are sunspots.

The Sun's brightness is equivalent to four trillion trillion 100-watt light bulbs! (A trillion trillion is a one followed by 24 zeros.) All of this energy is generated in the Sun's core by hydrogen burning.

Image Credit: National Solar Observatory/Sacramento Peak, Sunspot, New Mexico.

Life Cycle of a Star

I. Main Sequence Stars (Young Stars)

• Main Sequence Stars are the central band of stars on the Hertzsprung-Russell Diagram.

• 90% of all stars are Main Sequence stars.

• For these stars, the hotter they are, the brighter.

• The sun is a typical Main Sequence Star.

Life cycle of a star

II. Giants and Supergiant Stars (Old, Large Stars)

Depending of the size of the nebula, as a star approaches “old” age, it can become a:

1) Red Giant

a) 100 times bigger than it was originally.

b) Is cooler than it was originally.

c) Frequently orange in color.

d) 20 times as massive as the sun and 14,000 brighter.

e) When they die, they become dwarfs.

Example: Betelgeuse

Betelgeuse

Life cycle of a star

2) Supergiants

a) Largest known type of star. Some are almost as large as our entire solar system.

b) When they die, they explode as a supernova and become:

a) black hole

b) neutron star (pulsar)

Example: Rigel

The pulsating blue supergiant Rigel has a diameter of about 100 million kilometres, some seventy times that of the Sun. In the far distance a double blue star is visible - Rigel's much less luminous companions.

Life Cycle of a star

III. Faint, Virtually Dead Stars

1) Dwarf stars (from RED GIANTS)

a) White Dwarf

- occurs after a RED GIANT loses its outer layers (nuclear cores are depleted)

- small, very dense, made of carbon.

- about the size of Earth, but much heavier.

- They will eventually turn into a cold and dark…

b) Black Dwarf

Example: Sirius

Constellation

Sirius are Canicula - which is Latin and means "The Little Dog"

Sirius A is hotter, bluer, and younger

than our Sun, Sol. This Hubble image

also shows white dwarf companion Sirius B, at lower left.

Life Cycle of a Star

2) Neutron Star

a) Very small, super dense, tightly packed neutrons.

b) Has a thin atmosphere of hydrogen.

c) Has a diameter of 5 – 10 miles.

Pulsar – a rapidly spinning neutron star that emits energy as pulses.

Black Hole Swallowing Neutron Star

Life cycle of a star

3) Black Holes

• Occurs after a supernova of a Supergiant.

• A region of space containing a huge amount of mass compacted into an extremely small volume.

• A black hole's gravitational influence is so strong that nothing, not even light, can escape its grasp.

September 5, 2001 (CNN) -- Direct observations are offering the first authoritative evidence that a black hole resides in the heart of our galaxy. Black Hole Spotted

Burning of Elements Heavier than Helium1) Stars that start their lives with masses less than about eight solar masses stop their nuclear burning history with core helium burning. (Red Giants)2) Stars that start their lives with masses greater than about eight solar masses continue their nuclear burning history beyond that of core helium burning. (Super Giants) a) Eventually, silicon and sulfur ignite in the star's core to form iron, nickel, and other elements of similar atomic weight. b) The star's structure now resembles an onion: The central core of the onion consists of iron. Surrounding it is a shell in which silicon and sulfur burn, adding more iron to the iron core. In additional shells further out, lighter elements burn -- oxygen, carbon, helium, and hydrogen.

                                   

• The structure of a highly evolved star of 20 solar masses.

• The letters H, He, C, O, Ne, Mg, Si, S, and Fe are the chemical symbols for:

•Hydrogen, Helium, Carbon, Oxygen, Neon, Magnesium, Silicon, Sulfur, and Iron (Fe from Ferrum – Latin term for Iron).

•The evolution from main sequence to red giant occurs at different times for different stars.

•Stars that are much heavier and hotter, like O-stars, become red giants in only 10 million years.

•Cooler, lighter stars like our sun take 10 billion years to become red giants.