Giant Stars Giants are brighter and larger than

Giant Stars • Giants are brighter and larger than a Main Sequence star of the same temperature. • The distinguishing characteristic of giants is that they are relatively cool, but bright

Supergiant Stars • Supergiants are even brighter than giants. • Supergiants have a wide range of temperatures. • Supergiants distinguishing characteristic is that they are very bright, but relatively cool

White Dwarf Stars • White dwarfs are less bright and smaller than a Main Sequence star of the same temperature. • The distinguishing characteristic of white dwarfs is that they are very hot, but dim

Out of all the stars in the universe, how many fall into each category of the H-R Diagram? • Take a random sample of 1, 000 stars from our galaxy. On average, this sample would consist of: • 900, 000 Main Sequence Stars • 95, 000 White Dwarfs • 4000 Giants • 1 Supergiant • The numbers don’t add up to exactly 1, 000 because they have been rounded.

• Although supergiants are extremely rare, we know of their existence because they are extremely bright and can be seen for great distances.

• Has every star in the universe been there since the Big Bang and will every star always be there? • A: No, stars are born, they “live” a “life, ” and they die. They go through life stages called a lifecycle.

The Lifecycle of a Star • The lifecycle of a star depends on the star’s size. • Lifecycles for average stars, massive stars, and very massive stars are different

Lifecycle of an Average Star (star similar in mass to our sun): • Nebula Protostar Main Sequence Star Red Giant White Dwarf Black Dwarf

Stage 1: Nebula • This phase describes a place where a star could form (a cloud of gas and dust created from a supernova).

Stage 2: Protostar • This phase is a star being formed when a nebula starts to spin and gravitational contraction starts squeezing atoms together until they start to undergo fusion.

Stage 3: Main Sequence Star • This phase starts the moment that fusion begins. • Fusion refers to the condensation of 4 hydrogen nuclei to form 1 helium in the core of the star. • Fusion is the process of combining two heavier elements to make one lighter element and the extra mass gets turned into energy (E=mc 2). This releases heat and light energy and is why stars shine. • Stars in this stage usually stay about the same size because the outward force from fusion balances the inward pull of gravity. This is called equilibrium. • Nearly 90% of all stars, including our sun, are in this “midlife” phase, fusing hydrogen into helium. • This stage ends when all hydrogen has been converted to helium.

Stage 4: Red Giant • This stage begins when there is only helium left in the star’s core. • The helium can undergo fusion into carbon and then into oxygen. This also releases heat and light energy. • The star gets very big because when the hydrogen ran out, the temperature in the star’s core increased causing its outer layers to expand. However, the giant is now cooler than it was a Main Sequence star. • The only reason it is brighter is because it has more surface area. • When only carbon and oxygen make up the core, the temperature of the star is not high enough to start the fusion of carbon and oxygen into heavier elements. • This stage ends when all of the helium has been converted to carbon and oxygen.

Stage 5: White Dwarf • This stage begins when all helium is converted into carbon and oxygen. • The outer layers of the star continue to expand but the remaining core becomes a white dwarf. • The squeezing of these carbon and oxygen atoms being pulled together in the core causes pressure and temperature to increase. Eventually, the white dwarf cannot squeeze together any more. • This stage is very hot since heat is being radiated, but very dim because fusion has stopped and the core remaining is much smaller than the original star.

Sirius B

Stage 6: Black Dwarf • By this stage, the star has lost all of its heat into space and is now cold, dark, and considered to be officially dead.

Lifecycle of a Massive Star • (stars with mass greater than 1. 5 times, but less than 3 times that of our sun): • Nebula Protostar Main Sequence Star Red Supergiant Supernova Neutron Star • *Stages 1 -3 are the same as those for an average star

Stage 4: Red Supergiant • Similar to a giant but more massive and does not stop with carbon fusion. • These stars produce heavier elements until their core becomes iron

Stage 5: Supernova • Iron cannot be fused together to make heavier elements because that requires the input of energy instead of the release of energy. • Fusion stops because there is no more fuel and there is no longer any outward pressure to balance the inward gravitational force. The star can no longer stay at equilibrium. • The core collapses, which blows the outer layers away from the core. This causes a very violent explosion to occur known as a supernova. • 90% of a supernova (all the outer layers of the star) is released to create new stars. 10% (the core) remains as a neutron star.

Stage 6: Neutron Star • The core of a supernova begins rotating very fast. • It is very small and has a strong magnetic field. • Pulsars are neutron stars that are spinning very rapidly.

Lifecycle of a Very Massive Star • (stars with mass over 3 times that of our sun) • Nebula Protostar Main Sequence Star Red Supergiant Supernova Black Hole • Stages 1 -5 are the same as those for a massive star above.

Stage 6: Black Hole • The core that remains after a supernova collapses into itself. • Gravity is so strong that nothing can escape, not even light. • All matter in the core collapse into an infinitely dense point called a singularity. • We can’t see black holes since no light comes out of them, but we believe they exist because they have a powerful gravitational pull and seem to influence other objects around them

What about our star? What is it made of and how does it work? • Our star, the Sun, works like any other star

Our Sun: Structure of the Sun • The sun has six layers. • • • core (inner most) radiation zone convection zone photosphere chromosphere corona

Inner Layers of the Sun: • 1. Core: The power plant • Where nuclear fusion takes place, reactions generate energy in the form of heat and light • Hydrogen is fused into helium

Radiation Zone • Photons are absorbed and readmitted

Convection Zone • Circulating gas transfers heat to the outer layers • It takes about 20 million years for energy produced in the core to surface and become sunshine

3 of the sun’s layers make up its atmosphere • • 1. Photosphere: Means “light ball” Inner layer of the atmosphere (closest to the core) Visible surface Hot, thin, opaque gas layer Temperature of about 5800 K or 10, 000 o F Where energy radiates into space 400 km thick

• • 2. Chromosphere Means “color ball” Middle layer of the atmosphere Thin and transparent Visible from Earth only in a total eclipse of Sun Average temperature about 15, 000 K About 2500 – 6000 km thick

• • 3. Corona Means “crown” Outermost layer of the atmosphere Largest layer of the Sun Several million km thick Solar wind leaves from the corona Temperatures of up to 2 million K Seen as a jagged white halo around the photosphere

Surface Features of the Sun Surface Features: Granulation Bright spots that look like rice grains Cells about 1000 km across (625 miles) Are the tops of hot gas currents from the convection zone • Last about 5 minutes each • •

Surface Features: Sunspots Dark cool spots on the photosphere They are magnetic storms on the Sun. Appearance is temporary Usually appear in groups of 2 (positive and negative pole like a magnet) • Each spot lasts a few hours to a few months • A typical spot is the size of the Earth • Look dark because the surrounding area is hotter • •

Surface Features: Prominences • Fiery arch • Prominences occur when charged particles flow between two sunspots. • May rise tens of thousands of km • Held above the surface by the magnetic field • Last about a month

Surface Features: Solar Flare • Sudden, explosive outburst of light • One great solar flare may release as much energy as the whole world uses in 100, 000 years • Occur when two sunspots come together and the charged particles flowing between them explode outward. • Energized by strong magnetic fields • Lasts a few minutes to a few hours

Sunspot Cycles • Cycles are approximately 11 years long between minimum and maximum. • Sunspot maximum – most active outbursts for about 4 years • Sunspot minimum – least active with minimal spots • There may be as many as 300 sunspots at one time, or none at all

Solar Wind • Stream of energetic, electrically charged particles • Flows from the sun at all times • Is faster, thinner, and hotter than any wind on Earth • Solar wind blasts occur during solar flares • Wind is strongest when many sunspots are visible and solar activity is great.

Rotation • Sun turns on its axis from west to east like the Earth • Rotation is fastest at the equator (about 25 days) • Rotation is slowest at the poles (about 35 days) • Uneven rotation may account for violent solar activity