Earth Science Notes Stars and Galaxies
What is a Star? A star is a massive, luminous ball of plasma that is held together by gravity. Stars shines due to nuclear fusion in its core releasing energy that radiates into space. When stars form they are composed of about 70% hydrogen and 28% helium by mass There is a small fraction of heavier elements.
Classifying Stars Stars are classified by their luminosity (brightness) and the temperature. Hetzsprung and Russell developed a graph that shows the relationship between luminosity and temperature. Three main classification Main sequence Dwarfs Giants
The Hertzsprung-Russell Diagram Classifying Stars The Hertzsprung-Russell Diagram
Classifying Stars The Hertzsprung-Russell Diagram
Classifying Stars Main Sequence Stars Run diagonally across H-R diagram Brightness increases as temperature increases The sun is near the middle of the sequence Around 90% of all stars are main sequence
Classifying Stars Dwarf Stars White dwarfs – hot but small (bottom-left of H-R diagram) Red Dwarfs – cool but small (bottom-right of H-R diagram)
Classifying Stars Giant Stars Star with substantially larger radius and luminosity than a main sequence star of the same surface temperature Some Giants are large but not hot Super-Giants are even larger
Evolution of Stars All stars begin as begin as nebulas Nebulas – large clouds of gas and dust As particles in the nebula contract they increase in temperature until the reach 10 million K. This is when fusion begins. Energy given off from the fusion process powers the star
Evolution of Stars Two series for stars Nebula high mass star main sequence red supergiant supernova black hole or neutron star Nebula low mass star main sequence red giant white dwarf
Evolution of Stars
Evolution of Stars Main Sequence to Giant Stars When hydrogen in a star is depleted, the fusion process begins to stop. The core collapses and heats up. The outer layers of the star begin to cool and expand. This stage of the life cycle is called a giant.
Evolution of Stars White Dwarfs When a stars core uses up its fuel it contracts even more and its outer layers escape to space. This leave behind a hot, dense core At this stage the stars becomes a white dwarf White dwarfs are about the size of the Earth. They are dim and hot stars.
Evolution of Stars Supergiants In massive stars, the core heats up to extremely high temperatures and heavier elements form via fusion, the star expands into a supergiant.
Evolution of Stars Supernovas Once an iron core forms the star collapses and a shockwave travels through the star. The outer portion of the star explodes and creates a supernova.
Evolution of Stars Neutron Stars Sometimes when the core of a supernova collapses it will shrink from 3 times the size of the sun to about 20 km. Only neutrons can exist at this density. One teaspoon of a neutron star is about 600 million metric tons
Evolution of Stars Black Holes Sometimes a supernova’s core can collapse to a point. The gravity at this point is so great that is pulls everything in, even light.
Evolution of Stars Black Holes
Evolution of Stars
Constellations Constellations are patterns made of stars in the sky Stars appear close together in the sky, however, they are actually light years from each other
Constellations Some constellations are visible only in the northern or the southern hemisphere Circumpolar constellations appear to rotate around Polaris (north star) Some constellations are only visible during certain times of the year
Circumpolar Constellations
Constellations Some stars in constellations appear brighter than others. This can be due to two things: The stars actual luminosity (brightness) Actual magnitude – how much light is given off by star The stars distance Apparent magnitude – how much light is received on Earth Stars far away appear dimmer
Constellations
Constellations
Measurements in Space Parallax – the apparent change in position of object when viewed from different perspectives
Measurements in Space Light year – measurement of distance The distance light travels in 1 year 9.5 trillion km / year Astronomical Unit – distance from Earth to Sun = 1 AU Solar Mass – the mass of our sun = 1 solar mass
Measurements in Space Wavelengths of light can be measured to determine the composition of stars Spectra of elements are like fingerprints, they will tell you what elements are present
Galaxies and The Universe Galaxy – large group of stars, gas, and dust held together by gravity Galaxies are separated by great distances millions of light years
Galaxies and The Universe Our galaxy is the Milky Way We are located on one of the spiral arms There is a black hole located at the center
The Milky Way
Galaxies and The Universe Three types of Galaxies Spiral – have spiral arms that swing out from the center (Like Milky Way)
Galaxies and The Universe Elliptical – football shaped
Galaxies and The Universe Irregular – have many different shapes
Origin of the Universe Three Models the Universes Origin The Steady-State Theory Suggest universe is the same as it has always been The Oscillating Model Suggest that the universe expands and contracts over time The Big Bang Theory Suggest universe began as an explosion and has expanded ever since
Origin of the Universe The Big Bang Theory Evidence suggest that the universe is expanding. Scientist have theorized that it has expanded from one point According to this theory the universe began with an enormous explosion
Expansion of the Universe As the universe expands, some stars are moving toward us and some move away. This causes an apparent change in the light we see Red shift – light moving toward Earth appears more red (toward the red end of the spectrum) Blue shift - light moving away from Earth appears more blue (toward the blue end of the spectrum)
Expansion of the Universe Redshift and Blueshift
Summary Stars are large bodies of plasma held together by gravity The Hurtzsprung-Russel relates brightness to temperature of stars There are three main classifications of stars There are three models for the origin of the universe, the leading model is the big bang theory