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Astrophysics Tsokos Option E-1 Introduction to the universe

IB Assessment Statements Option E-1, Introduction to the Universe: E.1.1. Outline the general structure of the solar system. E.1.2. Distinguish between a stellar cluster and a constellation. E.1.3. Define the light year.

IB Assessment Statements Option E-1, Introduction to the Universe: E.1.4. Compare the relative distances between stars within a galaxy and between galaxies, in terms of order of magnitude. E.1.5. Describe the apparent motion of the stars/constellations over a period of a night and over a period of a year, and explain these observations in terms of the rotation and revolution of the Earth

Objectives Describe the main features of the solar system. Name the main objects making up the universe. Give the definition of a light year.

Objectives State the average distances between stars and between galaxies. Outline the main facts about the motion of stars as they appear to an observer on earth.

Order of Magnitude Scales of the Universe

The Solar System OBJECTIVE: Describe the main features of our solar system Called a solar system because it is a system of planets and moons that rotate around a star, specifically the sun

The Solar System The sun has a mass of 1.99x1030 kg and a radius of 6.96 x 108 m The Sun is a main sequence G2 star that contains 99.86% of the system's known mass. Its two largest orbiting bodies, Jupiter and Saturn, account for 91% of the remainder. Consists of 8 major planets in elliptical orbits around the sun Pluto now considered a “dwarf planet” The sun constitutes one of the two foci of the elliptical orbits The earth’s orbit is nearly circular The orbit of Mercury is the most elliptically shaped

The Solar System Consists of 8 major planets in elliptical orbits around the sun Pluto now considered a “dwarf planet”

Relative Orbit Radii This is the inner solar system The sun constitutes one of the two foci of the elliptical orbits The earth’s orbit is nearly circular The orbit of Mercury is the most elliptically shaped

Relative Orbit Radii This is the outer solar system

Relative Orbits The planes of the orbits of the planets differ only slightly from the plane of the earth’s orbit The exception to that is Mercury which has a 7-degree inclination All the planets and most of the comets orbit the earth in the same direction The most famous exception to this is Halley’s comet

Laws of Gravitation and Planetary Motion

Kepler’s Laws The motion of an object that is attracted and bound to a heavier mass is, according to the law of gravitation, an ellipse or circle. Planets sweep out equal areas in equal times. The period of a planet around the sun is proportional to the 3/2 power of the orbit radius. Motion of the planets and comets about the sun is dictated by Kepler’s Laws The moons that orbit the planets also follow Kepler’s laws

Moons All the planets except Mercury and Venus have moons that orbit around them The earth’s moon has a period of 27.3 days Jupiter has 16 moons, 4 of which are as big as our moon

Moons Saturn has 17 moons

Saturn Because of it’s size, Saturn attracts a large amount of objects (in the millions) that orbit the planet in a coplanar circular ring

Pluto Pluto was thought to have only one moon, Charon Since Charon is nearly as large as Pluto, the two may be thought of as a binary system where they rotate about their center of mass

Asteroid Belt In the region between the orbits of Mars and Jupiter is an asteroid belt Consists of thousands of small objects, really small planets, in orbit around the sun The largest of these is Ceres which has a diameter of 770 km

Characteristics of Planets Relative sizes of the planets Listed in order of their distance from the sun Mass is proportional to radius Period is proportional to radius IAW Kepler Note that Neptune has a period of just under 164 years

Characteristics of Planets List of the planets by size from smallest to largest

Characteristics of Planets Size and acceleration due to gravity relative to the earth In other words, if the earth’s radius is 1, Jupiter’s radius is 11.2 times the earth’s radius, it has 318 times the mass of the earth and the gravity on the surface is 2.6 times that of earth.

Objective Name the main objects making up the universe

Beyond the Solar System Here is a half of the list of terms which you will become acquainted with by the end of this unit The list is on page 490 of the book You should read through it now so that the terms are familiar when they come up

Beyond the Solar System Here is the other half of the list

Objective Give the definition of a light year

Light Year The distance light will travel in one year

Objective State the average distances between stars and between galaxies

Parsec (pc) Represents the average distance between stars in a galaxy Parsec is a useful number to have The average distance between stars is 1pc The distance to the nearest star, Proxima Centauri, is about 1.3pc from earth which is 4.3ly and 4x1016 m away The distance between galaxies in the same cluster is about 100pc, and a few Mpc between galaxies of different clusters

Objective Outline the main facts about the motion of stars as they appear to an observer on earth

Motion of the Stars Observing the movement of the stars is complicated by the fact that the earth is also moving You may notice that stars seem to move from east to west, but their relative position remains the same Early notion of a celestial sphere rotating about the earth Polaris, the north star, doesn’t appear to move because it is aligned with the North Pole of the earth’s axis of rotation

Motion of the Stars The earth’s axis precesses in a circle that traces out a cone The period of the precession is about 26,000 years In AD 14,000, the earth’s axis will point to the start Vega

Motion of the Stars Using a long exposure, this is the pattern stars will trace in a given night.

Motion of the Stars As the earth rotates about the sun in the course of the year, different portions of the night sky appear visible The position of the stars change by about 1-degree per day This is where the astrological charts come from The different months correspond to the different constellations that are visible

Objectives Describe the main features of the solar system. Name the main objects making up the universe. Give the definition of a light year.

Objectives State the average distances between stars and between galaxies. Outline the main facts about the motion of stars as they appear to an observer on earth.

IB Assessment Statements Option E-1, Introduction to the Universe: E.1.1. Outline the general structure of the solar system. E.1.2. Distinguish between a stellar cluster and a constellation. E.1.3. Define the light year.

IB Assessment Statements Option E-1, Introduction to the Universe: E.1.4. Compare the relative distances between stars within a galaxy and between galaxies, in terms of order of magnitude. E.1.5. Describe the apparent motion of the stars/constellations over a period of a night and over a period of a year, and explain these observations in terms of the rotation and revolution of the Earth

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Homework #1-17