Beyond Our Solar System – The Universe in a Nutshell! Chapter 25

Star Temperature Color is a clue to a star’s temperature Very hot (30,000 K) stars emit their light in the blue spectrum, red stars are much cooler, stars with temperatures between 5000 and 6000 K appear yellow

Binary Stars Binary Stars – pairs of stars, pulled together by gravity, that orbit each other Binary stars are used to determine the star property most difficult to calculate – its mass

Parallax The nearest stars have large parallax angles, while those of distant stars are too small to calculate Light-Year – unit used to express stellar distance, the distance light travels in one year (~9.5 trillion kilometers) Our closest star (besides the sun), Proxima Centauri, is about 4.5 light-years away from the sun

Stellar Brightness Apparent Magnitude – a star’s brightness as it appears to Earth Absolute Magnitude – how bright a star actually is

A Hertzsprung-Russell diagram shows the relationship between the absolute magnitude and temperature of stars

Nebulae – clouds of dust and gases in space Reflection Nebula in Orion Dark Nebula – Horsehead Nebula Emission Nebula – N11

The Birth of a Star The birthplaces are dark, cool interstellar clouds (nebulae) The initial contraction of the nebula can be triggered by the shock wave from an explosion of a nearby star The Orion Nebula in normal color and infrared

Protostars in the Horsehead Nebula are circled Protostar – a developing star not yet hot enough to engage in nuclear fusion When the core of a protostar has reached about 10 million K, pressure within is so great that nuclear fusion of hydrogen begins, and a star is born Protostars in the Horsehead Nebula are circled

Main-Sequence Stage – From the moment of birth until the star’s death The more massive a main-sequence star, the shorter its life span A yellow star, like our sun, can remain in the main-sequence for approximately 10 billion years

The Sun will spend less than 1 billion years as a Red-Giant Red-Giant Stage The Sun will spend less than 1 billion years as a Red-Giant Globular Star Cluster, some of the oldest stars in the universe

White Dwarf – remains of low and medium mass stars, extremely small stars with densities greater than anything on Earth

Life Cycle of a Sun-like Star

Neutron Stars – remnants of supernova events, stars that are smaller and more massive than white dwarfs

Black Hole – A massive star that has collapsed to such a small volume that its gravity prevents the escape of everything, including light

Stellar Evolution

The Milky Way Galaxy Galaxies – groups of stars, dust, and gases held together by gravity The Milky Way is a large spiral galaxy whose disk is about 100,000 light-years wide and about 10,000 light-years thick at the nucleus It has at least three distinct spiral arms, the sun lies about 2/3 of the way from the center on one of these arms and orbits the nucleus about every 200 million years There may be more than 100 billion stars in our galaxy alone

Structure of the Milky Way

Spiral Galaxies – like our galaxy, these galaxies have multiple arms that sweep out from a central nucleus

Elliptical Galaxies – do not have spiral arms, makes up ~60% of known galaxies, can range from round to oval

Irregular Galaxies – consist mostly of younger stars, appear as clouds of stars

Galaxy Cluster – a system of galaxies containing from several to thousands of member galaxies

The Expanding Universe Hubble’s Law – galaxies are retracting from us at a speed that is proportional to their distance The red shifts from distant galaxies indicate that the universe is expanding

Big Bang Theory – The universe began as a violent explosion from which the universe continues to expand, evolve, and cool The big bang theory states that at one time, the entire universe was confined to a dense, hot, supermassive ball. Then, about 13.7 billion years ago, a violent explosion occurred, hurling this material in all directions