The Universe on a Large Scale The nearest galaxy to the Milky Way is 75,000 light years away (controversial) Toward the constellation Sagittarius Magellanic Clouds Two small galaxies that appear to be clouds to the naked eye Nearest large galaxy is Andromeda Galaxy, M31 A spiral galaxy similar to the Milky Way 2 million light years away Part of a cluster of 40 galaxies called the Local Group Lecture 1

Farther Away Farther away are quasars At a distance of 15 million light years we find other small galaxy groups At 50 million light years there is the Virgo Cluster Many thousands of galaxies Our Local Group and the Virgo Cluster are part of supercluster that has a diameter of 60 million light years Farther away are quasars Energetic centers of galaxies Black holes? Beyond the quasars is the cosmic microwave radiation left over from the Big Bang Lecture 1

The Universe of the Very Small Most of the universe is empty 1 atom/cm3 in our galaxy 1 atom/m3 between galaxies Air has about 1019 atoms/cm3 Even atoms are mostly empty Most of the mass of an atom is in the atomic nucleus The electrons around the nucleus are at a distance 10,000 times the size of the nucleus There are 92 naturally occurring elements (atoms) Evidence has been found for man-made elements up to 118 Lecture 1

Composition of the Universe The universe is roughly 75% hydrogen and 25% helium (by weight) The remaining 90 elements are relatively rare Carbon, oxygen, calcium, etc. Hydrogen and helium come from the Big Bang The remaining elements were “cooked” in the centers of stars These elements were ejected when stars used up their hydrogen fuel and exploded Supernova Lecture 1

The Celestial Sphere When we look up it seems that the Earth is the center of the universe Geocentric The point above your head is called the zenith Where the sky-dome intersects the ground is the horizon The sky seems to rotate around us Celestial sphere We know that the Earth’s rotation on its axis every 24 hours causes this illusion but the concept of the celestial sphere is still useful Lecture 1

Celestial Poles The points where the axis of the Earth intersects the celestial sphere are called the north and south celestial poles The celestial equator lies halfway between the two poles The circumpolar zone is always above the horizon, day and night Lecture 1

Rising and Setting of the Sun The sun does more than just rise and set It changes position with respect to the background stars Approximately 1 degree per day to the east The sun moves through the celestial sphere along the ecliptic The ecliptic is not aligned with the equator of the Earth because the Earth’s axis is tilted 23 degrees with respect to the plane of its orbit around the sun Lecture 1

Tilt of Earth’s Axis The Earth rotates on an axis tilted 23 degrees from the vertical Lecture 1

Planets and Stars Not only does the Sun move but the stars, the moon, and the planets move as well The moon and the planets move with respect to the stars on the celestial sphere The moon and the planets move on paths close to the ecliptic as well as the sun The Sun, the moon, and the planets are all found in a 18 degree wide belt in the sky called the zodiac Lecture 1

Constellations The stars form a fixed background Distinctive patterns of stars have been recognized throughout the ages These patterns were usually embellished and given names and drawings These constellations are not necessarily nearby stars but merely appear to be related Today the term constellation refers to 88 sectors in the sky Lecture 1

Constellations in the Sky Lecture 1

Galactic Equator The galactic equator is defined by the Milky Way galaxy The ecliptic and zodiac are not aligned with the Galactic equator nor with the celestial equator In fact, the Galactic equator is very different from the ecliptic and the celestial equator Lecture 1

Locating Objects on a Skymap Ecliptic Locating Objects on a Skymap Galactic Equator Lecture 1

Locating the Celestial North Pole The Celestial North Pole is currently located near the star Polaris (aptly named!) Locate the constellation Ursa Major, the Big Dipper Locate the outer two stars of the dipper Those 2 stars point at Polaris The distance to Polaris is about 5 times the spacing between the two stars Lecture 1

Early Greek and Roman Cosmology Cosmology is the concept of the basic structure and origin of the cosmos The Greeks and Romans knew The Earth was round The Moon was a sphere The Sun was farther away than the Moon The Greeks and Romans thought that the Earth was the center of the universe and all heavenly objects circled the Earth Geocentric Lecture 1

Measurements of the Earth The Greeks measured the size of the Earth Eratosthenes noticed that on the first day of summer, sunlight struck the bottom of a vertical well in Syene at noon At the same time in Alexandria, the Sun was not directly overhead but made an angle of 7 degrees 360 degrees divided by 7 degrees is about 50 50 times the distance between Syene and Alexandria would be the circumference of the Earth 50 times 5000 stadia is 250,000 stadia Lecture 1

Hipparchus Roman astronomer, 150 BC Compiled detailed star catalog Invented the system of magnitudes to describe the brightness of stars 1 is the brightest, 5 is the dimmest, with each magnitude about a factor 2 dimmer In modern astronomy, each magnitude varies by a factor 2.512 By comparing with previous observations, Hipparchus showed that the Earth’s north pole changes Precession Lecture 1

Ptolemy Roman astronomer around 140 AD Most important contribution was a geometric model of the solar system capable of predicting the positions of planets This model lasted more than 1000 years One puzzling observation that he explained was retrograde motion of the planets Usually the planets move east light the moon and sun but occasionally stop and move westward for some time He invented a complicated mathematical model describing the motions of the planets with superposition of circles Epicycles Lecture 1

Epicycles Lecture 1