StarsStars

What is a star? The objects that heat and light the planets in a system A star is a ball of plasma held together by its own gravity –Nuclear reactions occur in stars (H  He) –Energy from the nuclear reactions is released as electromagnetic radiation

Characteristics of Stars DISTANCE –Measured in light-years The distance which a ray of light would travel in one year About 6,000,000,000,000 (6 trillion) miles 186,000 miles per second

How Do we Measure Stars Distance? Parallax- the apparent change in position of an object when you look at it from different places

Parallax The closer the star is to Earth, the larger the shift will be. (No good past a 1000 Light Years.

Characteristics of Stars Magnitude (brightness) –A measure of brightness of celestial objects Smaller values represent brighter objects than larger values –Apparent magnitude How bright a star appears to be from Earth –Absolute magnitude (luminosity) How bright a star actually is

Characteristics of Stars Temperature & Color –The color of a star indicates the Temperature of the star –Stars are classified by Temperature Decreasing Temperature (bright to dim) O, B, A, F, G, K, M [Oh Be A Fine Girl, Kiss Me ]

Types of Stars Classification ClassTemperatureColor O20, ,000 KBlue B10,000 – 30,000 KBlue-white A7,500 – 10,000 KWhite F6,000 – 7,500 KYellow-white G5,000 – 6,000 KYellow K3,500 – 5,000 KOrange M2,000 – 3,500 KRed

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Main Sequence Stars A major grouping of stars that forms a narrow band from the upper left to the lower right when plotted according to luminosity and surface temperature on the Hertzsprung-Russell diagram

Hertzsprung-Russell Diagram

Life Cycle of Stars

Life Cycle Stars begin their lives as clouds of dust and gas called Nebulae (Greek for cloud) Gravity may cause the nebula to begin to contract

Life Cycle Matter in the gas cloud will begin to condense into a dense region called a Protostar The Protostar continues to condense, it heats up. Eventually, it reaches a critical mass and nuclear fusion begins.

Star Nurseries

Life Cycle After nuclear fusion begins, stars begin the main sequence phase. Most of its life is in this phase

The Sun: the nearest Main Sequence star to the Earth. This is an X-ray image from the Yohkoh satelite.

Logo for the Subaru car company. Pleiades or the 7 Sisters

Life Cycle Life span of a star depends on its size. –Very large, massive stars burn their fuel much faster than smaller stars – Their main sequence may last only a few hundred thousand years –Smaller stars will live on for billions of years because they burn their fuel much more slowly Eventually, the star's fuel will begin to run out.

Life Cycle It will expand into what is known as a red giant Massive stars will become red supergiants This phase will last until the star exhausts its remaining fuel At this point the star will collapse

BETELGEUSE –RED GIANT Only a few million years old, Betelgeuse is already dying. Astronomers predict that it's doomed to explode as a soon, within 1,000 years or so, an event that will be spectacular for Earth's future inhabitants. (Conceivably, it's already happened as Betelgeuse is 640 light- years away!)

Hubble image of Betelgeuse a Red Supergiant

Life Cycle Most average stars will blow away their outer atmospheres to form a planetary nebula Their cores will remain behind and burn as a white dwarf until they cool down What will be left is a dark ball of matter known as a black dwarf

Image of Sirius A and Sirius B taken by the Hubble Space Telescope Sirius A and Sirius B Hubble Space TelescopeSirius A and Sirius B Hubble Space Telescope

Life Cycle If the star is massive enough, the collapse will trigger a violent explosion known as a supernova If the remaining mass of the star is about 1.4 times that of our Sun, the core is unable to support itself and it will collapse further to become a neutron star

Crab Nebula

Life Cycle The matter inside the star will be compressed so tightly that its atoms are compacted into a dense shell of neutrons. If the remaining mass of the star is more than about three times that of the Sun, it will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole

Life Cycle of Stars