The star Proxima Centauri is about 100 million times farther away from Earth than the moon, yet it is the closest star to Earth other than the sun Constellation  word used to designate an area of the sky that contains a specific pattern of stars Properties of Stars

Parallax The slight shifting in the apparent position of a nearby star due to the orbital motion of Earth The nearest stars have the largest parallax angles, while those of distant stars are too small to measure Light-Year The distance that light travels in one year Measuring Distance to Stars

The measure of a star’s brightness is its magnitude Apparent Magnitude  a star’s brightness as it appears from Earth Absolute Magnitude  how bright a star actually is Stellar Brightness

A Hertzsprung-Russell diagram shows the relationship between the absolute magnitude and temperature of stars Red Giants  a group of very bright stars Supergiants  extremely large stars Betelgeuse is a bright red supergiant in the Orion constellation that has a radius 800 times that of our sun Hertzsprung-Russell Diagram

Star Birth The birthplaces of stars are dark, cool interstellar clouds made up of dust and gases For some reason not yet fully understood, some nebulae become dense enough to begin to contract Once the process begins, gravity squeezes particles in the nebula, pulling every particle towards the center Stellar Evolution

Protostar Stage The initial contraction stage spans a million years or so. The temp increases until it is hot enough to radiate energy from its surface in the form of long- wavelength red light 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 Stellar Evolution

Main Sequence Stage An average star spends 90% of its life as a hydrogen burning, main sequence star Once the hydrogen fuel in the star’s core is depleted, it evolves rapidly and dies However, with the exception of the least-massive red stars, a star can delay its death by fusing heavier elements and becoming a giant Stellar Evolution

Red Giant Stage This stage occurs because the zone of hydrogen fusion continually moves outward, leaving behind a helium core As the energy runs out, the core no longer has enough pressure to support itself against the inward force of gravity and the core begins to contract The contraction force the core to heat up, which in turn heats the outer layer and causes it to expand hundreds to thousands of times its main sequence size Stellar Evolution

All stars, regardless of their size, eventually run out of fuel and collapse due to gravity Low Mass Stars Become white or black dwarf stars Medium Mass Stars Become red giants or planetary nebulae Massive Stars Become supernova  the brilliant explosion of a dying star Burnout and Death

White Dwarfs The remains of low-mass and medium-mass stars Extremely small stars with densities greater than any known material on Earth Although they can be no larger than Earth, their masses can equal 1.4 times that of the sun The sun began as a nebula, will spend much of its life as a main sequence star, and then will become a red giant, planetary nebula, white dwarf, and finally, a black dwarf Stellar Remnants

Neutron Stars Stars smaller and more massive than white dwarfs that are thought to be the remnants of supernova events The star is so massive that the electrons are forced to combine with protons and produce neutrons If Earth were to collapse to the density of a neutron star, it would have a diameter equal to the length of a football field A pea size sample would weigh 100 million tons Stellar Remnants

Supernovae During a supernova, the outer layer of the star is ejected, while the core collapses into an extremely hot neutron star As the star collapses, it will rotate faster and faster Radio wave energy are concentrated at two poles, meaning that the star seems to pulse as it rotates round and round Pulsar  a spinning neutron star that appears to give off pulses of radio waves Stellar Remnants

Black Holes Dense objects with gravity so strong that not even light can escape their surface Many scientists think there are supermassive black holes in the center of most galaxies We are thought to have one at the center of the Milky Way with a mass of 1 to 2 billion suns Stellar Remnants

Galaxies Large groups of stars, dust, and gases held together by gravity May be 400 billion stars in the Milky Way galaxy alone Our galaxy looks milky because the solar system is located within a flat disc – the galactic disk. We view it from inside and see stars in every direction The Universe

Size The Milky Way is a large spiral galaxy whose disk is about 100,000 light-years wide and about 10,000 light years thick Structure Has at least three distinct spiral arms with some signs of splintering Our solar system is about half way in one of these arms All the arms rotate around the center, which is most likely occupied by a supermassive black hole The Milky Way Galaxy

Spiral Galaxies Usually disk shaped with a somewhat greater concentration of stars near their centers Many different variations Generally quite large About 30% of all galaxies are though to be some variation of spiral galaxies Types of Galaxies

Elliptical Galaxies Range in shape from round to oval without the presence of spiral arms Most are very small, however, the largest known galaxies are elliptical as well About 60% of all galaxies are classified as elliptical Types of Galaxies

Irregular Galaxies Composed mostly of young stars Only 10% of known galaxies are classified as irregular galaxies In addition to shape and size, one of the major differences among different types of galaxies is the age of their stars Types of Galaxies

Galaxy Clusters Many different galaxies grouped relatively close together in space Some may contain thousands of galaxies Our own cluster (Local Group) contains at least 28 galaxies Make up huge groups called superclusters, which make up vast thread-like structures called filaments (largest known structures in the universe) Types of Galaxies

Quasars Objects that are very bright and very far away Discovered in the 1960’s, they were called quasi-stellar objects because they looked like stars Since it takes their light billions of years to reach Earth, they must have existed when the universe was very young Types of Galaxies

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 All matter and space were created at that instant The Big Bang

Supporting Evidence Galaxies and other structures in space are moving which provides evidence that the universe is still expanding Cosmic microwave background radiation has been discovered coming from every direction in space This is thought to have been originally produced during the big bang The Big Bang Theory