Stars

Stars A star is a ball of different elements in the form of gases The elements and gases give off electromagnetic radiation (from nuclear fusion) in the form of light Scientists study the light coming from a star to determine: The composition of the star It’s temperature It’s lifespan

Composition of Stars Starlight is analyzed through spectrographs Spectrographs separate light (electromagnetic radiation) into different wavelengths or colors The display of colors of light coming from a star is called a spectrum

White Board What is electromagnetic radiation? Light What process produces electromagnetic radiation in the sun? Nuclear fusion

Composition of Stars Every element has its own pattern or spectra Scientists can determine the elements that make up a star by studying its spectrum So…how do we know what stars are made of? Check it out…

Composition of Stars The most common element in stars in hydrogen The second most abundant is helium Elements like carbon, oxygen and nitrogen make up the rest of the stars

Temperature of stars The temperature of a star is indicated by the star’s color Colors range from blue, which is the hottest, to red, which is the coolest

Stellar Motion Stars have an apparent motion you can see at night Apparent motion is caused by the movement of the Earth (rotation).

Stellar Motion Stars can have up to three types of actual motion Rotate on an axis Revolve around another star Move away or toward our solar system A stars spectrum tell astronomers how a star is moving

Stellar Motion First scientists determine what the star is made of and how hot it is Then they determine what color of light the star should be giving off Last they determine whether the star is moving towards us or further away

White Board How do astronomers determine the temperature of a star? Its color What kind/color of star is our sun? yellow

Doppler Effect If a star is moving closer to us, the light it gives off gets squeezed together, which makes it appear bluer than it actually is Blue shift The colors of the spectrum are shifted toward blue If the star is moving away from us, the light gets stretched out, which makes it appear more red than it really is Red shift The colors of the spectrum are shifted toward red This shift in light is known as the doppler effect

Distance to stars Scientists determine a star’s distance by measuring parallax The shift in a star’s position when viewed from different locations Distance = 1/p

White Board What happens to the light of a star as it moves towards us? The light gets compressed and appears blue

Stellar Brightness Apparent brightness of a star as seen from Earth depends on how much light it emits and how far away it is Absolute brightness is how bright they would be if all stars were equal distance from the Earth

White Board What is the difference between apparent motion and absolute brightness of a star? Apparent how much light it emits Absolute – if all stars were equal distance from Earth

Organizing the Stars: The HR Diagram Stars have different temperatures, brightnesses, sizes, chemical compositions, colors, etc. It’s a lot to try to put together, let alone compare star to star. Two astronomers, Hertzsprung and Russell (in 1908 and 1913 respectively), managed to find a way to put all the information about stars onto one graph. This graph is now know as the Hertzsprung-Russell (HR) Diagram. It is a graph of brightness vs. temperature. Brightness also takes into account size. Temperature takes into account color and chemical composition and thus spectral class.

Where different stars are located

There are 3 groups of stars: Giants: Very large, very bright, cool stars. Found in the top right. Dwarves: Very small, very dim, hot stars. Found in the bottom left. Main Sequence: wide-ranging stars. Found in the middle, running from top left to bottom right. Most stars can be found here on the diagram.

Star Formation A star begins in a nebula-an outside force compresses the cloud, particles condense from gravity The central concentration of matter is a protostar After millions of years electrons are stripped from parent atoms and gas turns into plasma The birth of a star occurs when the temperature reaches about 10,000,000°C and nuclear fusion occurs

Life cycle of a Star The main-sequence stage is the second and longest stage of a star’s life A star reaches equilibrium when the inward force of gravity is balanced by the outward pressure from fusion and radiation This is the stage that energy is produced by nuclear fusion When fusion stops a star leaves the main sequence

Life cycle of a Star A star enters the third stage when almost all of the hydrogen within the core fuses into helium As stars expand because of cooling gas, they grow massive Stars as big as our sun become giants Stars that are more massive than the sun will become super giants

Life cycle of a Star When fusion no longer produces energy a star enters its final stage Planetary nebulas form when the outer gases of dying sun-like stars drifts outwards

Life cycle of a Star Finally a dying star will collapse on itself and become a white dwarf Once a white dwarf no longer gives off light it becomes a black dwarf – in theory When a white dwarf is near a red giant the star can turn into a nova or a super nova Nova – large explosion due to the accumulation of gases

Life cycle of a Star Massive stars always become supernovas at the end of their life cycles Supernovas can emit energy equal to the output of 400 million suns After the supernova a very small but dense ball or neutron star may be formed

Life cycle of a Star Pulsars are neutron stars that emit beams of radio waves Some extremely massive stars will turn into black holes after the supernova explodes

Black Hole Massive stars produce ‘leftovers’ too massive to become neutron stars The star may contract under gravity crushing the core leaving a black hole Gravity is so great nothing escapes it, not even light Black Hole

Warm Up Take out a piece of paper Watch and listen Describe in in your own words what the Doppler effect is and give me an example of it.