StarsA. The Brightness of Stars

-Star: A hot glowing sphere of gas that produces energy by fusion.

-Fusion: The joining of separate nuclei. Common in nature, but not on Earth.

Actual vs. Apparent Brightness

• Variables which affect a star’s brightness:

1. Star size

2. Distance from Earth

3. Star temperature

• Apparent Brightness: The amount of light received on Earth from a star.

• Actual Brightness: How large and hot a star is in relation to other stars.

Star Brightness• Example: (Fig.

20.1) Sirius has a greater apparent brightness then Rigel, even though Rigel is a much hotter and brighter star.

• Why?

B. The Birth of a Star

• Stars form in dark, cool clouds called nebula.

• Something happens to make these clouds contract and spin.

• Orion Nebula

More Nebulas

• Gravity pulls all of the pieces close to the center and squeezes them creating immense pressure.

• This contraction could last over a million years.

• Another view of Orion

Protostar Stage

• The temperature of this gaseous body continues to rise until it starts emitting red light, like the burner of a stove.

• Stars that are red have the lowest surface temperature and the longest wavelength light.

• The inside of the protostar continues to increase in temperature. When it reaches about 20,000,000 degrees Fahrenheit, nuclear fusion of hydrogen begins and a star is born.

Main-Sequence Stage• Stars have an

enormous amount of pressure inside of them that pushes them out.

• Gravity holds this pressure back and keeps the star together.

• It is this balance between pressure and gravity that forms a main sequence star.

Main Sequence Cont.• Different stars age at

different rates. • Large hot stars produce a

large amount of short wavelength, high energy blue light. These stars burn up their fuel very quickly and burn out within a few million years.

• A yellow star like our Sun, burns its’ fuel much slower and lasts about 10 billion years.

• Small red stars that burn slowly and produce long wavelength, low energy red light can last for hundreds of billions of years.

Red Giant Stage• As a star gets older, the

hydrogen fusion slows in the core and moves out toward the outer part of the star. When this happens, the Helium core contracts, becomes very hot and increases the hydrogen fusion on the outer part of the star.

• This makes the star grow 100’s or 1,000’s of times its normal size.

• Eventually the giant uses all of its’ Hydrogen and Helium fuel and shrinks under the force of gravity.

Burnout and Death

• There are many ways that a star can die. The way in which this happens depends on the stars’ size.

A) Low Mass Stars

• A low mass star never burns the helium in its’ core, so it never becomes a red giant.

• Low mass stars collapse under the force of gravity and form white dwarfs.

B) Medium Mass Stars

• Planetary Nebula• Medium mass stars

become red giants and fuse helium and hydrogen at a fast rate.

• Medium mass stars shrink to white dwarfs too, but before they do, they eject their outer layer of gas. This is called a planetary nebula

C) Massive Stars

• Massive stars become red supergiants. They are very large when they finish consuming their hydrogen and helium. They collapse with such force that they explode, producing a supernova.

• The intensity of a massive star collapsing produces a super-dense star called a neutron star. Imagine a star 1,000,000 times bigger than our sun becoming a star that is 20 km across. This would be like taking the Earth (8,000 miles across) and squeezing it down to 100 yards across. One pea sized sample of a neutron star weighs over 100 million tons.

• In some rare cases, the red supergiant is so massive that when it collapses it produces a black hole.

• Black Hole • Neutron Star

Black Holes• Black holes should be

very hot. Usually hot things glow brightly, like a star. However, the gravity pulling the surface of a black hole toward the center is so strong that not even light can escape from its’ surface, so they completely disappear from sight.

• Anything that gets too near to a black hole is swept in by its’ gravity and lost forever.

Study of Stars

• Scientist study stars by using the Hertsprung Russell Diagram.

• The HR Diagram compares a stars Surface Temperature with its’ Absolute Magnitude.

Hertsprung-Russell Diagram

Determining a Star’s Temperature

• A star’s temperature can be determined by its color.

• All objects will glow a different color when heated differently

• Colors hottest to coolest: Blue/white yellow orange red.

Determining a Star’s Composition

• Starlight is separated into a spectrum with a spectrometer

• A star’s light has dark bands along the spectrum, these bands are caused by the absorption of certain wavelengths of light by specific gases in the star.

• Different bands show what elements are in the star’s atmosphere.

A star’s spectrum

Light-Years

• Light-year: Distance light travels in one year. (Equal to about 9.5 trillion kilometers)

• Approximate distances:

-Sun to edge of solar system = 5.5 light hours

-Nearest star (Alpha Centauri) = 4.3 light years

-Center to edge of Milky Way = 50,000 light years

The Sun and You

• Our sun is a main sequence star according to the H-R Diagram.

• The actual brightness is average for a star of its average size.

Layers of the Sun

• Dense inner core which is the site of hydrogen fusion.

• Radiation zone: Energy bounces back and forth before escaping.

• Convections zone: Cooler layer of gas that is constantly rising and sinking.

Anatomy of Sun

• Photosphere: Bright source of much of the light we see.

• Chromosphere: Active layer which is home to many significant displays.

Anatomy of Sun

• Corona: Outer layer which is a gradual boundary between sun and space.

Sunspots

• Sunspots: Cool dark areas on the sun’s surface.

-First discovered by Galileo

-Not permanent features—Will appear and disappear

Cycle of Solar Activity

• Cycle of Solar Activity: 11 year cycle which see number of sunspots change.

• Sunspot Maximum: Time of many large sunspots.

• Sunspot Minimum: Time of few sunspots.

Prominences and Flares

• Prominence: A huge arching column of gas.

Prominences and Flares

• Solar Flares: Violent eruptions near a sunspot which suddenly brighten and shoot outward at high speed.

Prominences and Flares

• The interaction of solar flares with Earth’s magnetic field causes the aurora borealis/ aurora australis (Northern/Southern Lights)