1ISP 205 - Astronomy Gary D. Westfall Lecture 1

The Universe on a Large ScaleThe 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

2ISP 205 - Astronomy Gary D. Westfall Lecture 1

Farther AwayFarther Away• 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

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• Farther away are quasars Energetic centers of galaxies

Black holes?

• Beyond the quasars is the cosmic microwave radiation left over from the Big Bang

3ISP 205 - Astronomy Gary D. Westfall Lecture 1

The Universe of the Very SmallThe 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

4ISP 205 - Astronomy Gary D. Westfall Lecture 1

Composition of the UniverseComposition 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

5ISP 205 - Astronomy Gary D. Westfall Lecture 1

The Celestial SphereThe 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

6ISP 205 - Astronomy Gary D. Westfall Lecture 1

Celestial PolesCelestial 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

7ISP 205 - Astronomy Gary D. Westfall Lecture 1

Rising and Setting of the SunRising 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

8ISP 205 - Astronomy Gary D. Westfall Lecture 1

Tilt of Earth’s AxisTilt of Earth’s Axis

• The Earth rotates on an axis tilted 23 degrees from the vertical

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9ISP 205 - Astronomy Gary D. Westfall Lecture 1

Planets and StarsPlanets 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

10ISP 205 - Astronomy Gary D. Westfall Lecture 1

ConstellationsConstellations• 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

11ISP 205 - Astronomy Gary D. Westfall Lecture 1

Constellations in the SkyConstellations in the Sky

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12ISP 205 - Astronomy Gary D. Westfall Lecture 1

Galactic EquatorGalactic 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

13ISP 205 - Astronomy Gary D. Westfall Lecture 1

Locating Objects on a SkymapLocating Objects on a Skymap

Galactic Equator

Ecliptic

14ISP 205 - Astronomy Gary D. Westfall Lecture 1

Locating the Celestial North PoleLocating 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

15ISP 205 - Astronomy Gary D. Westfall Lecture 1

Early Greek and Roman CosmologyEarly 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

16ISP 205 - Astronomy Gary D. Westfall Lecture 1

Measurements of the EarthMeasurements 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

To zenithat Alexandria

To Sunat noonJune22

To Sunand zenithat Syene

SyeneAlexandria7°7°• 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

17ISP 205 - Astronomy Gary D. Westfall Lecture 1

HipparchusHipparchus• 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

18ISP 205 - Astronomy Gary D. Westfall Lecture 1

PtolemyPtolemy• 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

19ISP 205 - Astronomy Gary D. Westfall Lecture 1

EpicyclesEpicycles

EarthCenter of eccentricEquant pointDeferent

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