Outline - Jan. 19, 2010Units of measurement for the course (pg. 6)

Our place in the universe (pgs. 2-5)

The universe as a time machine (pgs. 7 and 8)

Motion in the universe (pgs. 15-20)

What can we learn by looking at the sky without a telescope? (Ch. 2)

To get the notes, go tohttp://firedrake.bu.edu/AS102/AS102.htmland click on Class Notes

Units of MeasurementTime: seconds, minutes, years, millions of years (106 years = 1 Myr), billions of years (109 years = 1 Gyr)

SI (kilograms, kilometers) + some astronomers favorite units

Solar masses (Msun) = the mass of an object measured in units of the suns mass

Solar luminosities (Lsun) = the intrinsic brightness of an object measured in units of the suns intrinsic brightness

Astronomical Unit (AU) = distance between the earth and the sun = 1.5x108 km

Light year (ly) = the distance light can travel in 1 year = 9.46x1012 km

Parsec (pc) = 3.26 light years

Examples: Solar Masses and Soar LuminositiesMass of the sun = 1.99x1030 kg = MsunLuminosity (intrinsic brightness) of the sun = 3.84x1026 W = Lsun

Mass of Sirius = 4.02x1030 kg = MsiriusLuminosity of Sirius = 9.75x1027 W = Lsirius

In units of the suns mass, the mass of Sirius is4.02x1030 / 1.99x1030 = 2.02Msirus = 2.02 Msun

In units of the suns luminosity, the luminosity of Sirius is9.75x1027 / 3.84x1026 = 25.4Lsirius = 25.4 LsunSite: Alborz Moutains (Iran), morning of Jan. 25, 2007

Our place in the universeEarth is a small, rocky planet orbiting a rather ordinary star (the Sun) at a distance of 1.5x108 km = 1 AU = 1.58x10-5 ly = 8.3 light minutes

Artists conception of the Milky Way galaxy

Sun (and all the planets) orbit the center of the Milky Way at a distance of about 28,000 ly (about halfway across the disk of the galaxy)

One orbit of the sun around the galaxy takes about 230 million years to go once around the galaxy

Sun has made about 20 trips around the galaxy since it was born

The Local Group of GalaxiesLarge Magellanic Cloud (LMC)Small Magellanic Cloud (LMC)Andromeda / M31About 30 galaxies in total, dominated by two large spiral galaxies (Milky Way and M31); remaining galaxies are dwarf galaxies. Local Group is about 2.5x106 ly = 2.5 Mly in diameter. Milky Way and M31 are approaching each other at a relative speed of about 300 km/s.

Galaxies are Gregarious(prefer to live in small groups or big clusters)Small portion of the Virgo Cluster, the closest large galaxy cluster (contains about 1,500 galaxies, distance about 60 Mly)

The Local Network of Superclusters and VoidsGalaxies cluster together to form groups or clusters of galaxies

Clusters of galaxies cluster together to form superclusters of galaxies

We belong to the Virgo Supercluster

Voids are regions of space where, for some reason, galaxies had a hard time growingEach white dot in the picture represents a galaxy.

Pause for reflectionWe keep quoting distances in units of light years (and even millions of light years), what does this really mean when we look through the telescope at these objects?Spiral galaxy M87; distance = 12 Mly = 12,000,000 ly

The Universe as a Time MachineWhen you look up at the sky tonight, you will see light from the stars that is arriving at Earth tonight. The farther an object is from Earth, the longer it has taken the light to arrive at your eyes (and the farther back in time you are seeing when you look at the more distant object).The Big Dipper is visible in the Northern Sky every night, all night long

DubheMerakPhecdaMegrezAliothMizarAlkaidThe light that you see tonight from Dubhe left the star before your Grandmother was born! It also left Dubhe 45 years before the light that you see tonight from Merak left Merak.Note: the star that is the farthest away is also seen to be the brightest in the sky. What does that tell you?

How can we study the evolution of the universe?(the universe is REALLY old - 13.8 billion years)Problem: Humans live for a short time (100 years) compared to the age of the universe

We dont have the luxury of watching the universe undergo changes over our lifetimes (it actually changes very little on time scales less than a few million years)

Tactic: Study vast collections of objects in the universe (i.e., galaxies) that are located at different distances (= different look back times) and compare them to each other

Are the galaxies that are located 5 to 10 billion light years from us significantly different from the galaxies that are only a few million light years away?

The universe is full of galaxies!There are over 2000 galaxies in the image, and in the observable universe there are at least 100 billion galaxies.

The look back times to the galaxies in this image range from 0.5 billion to 9 billion years.

In astronomy, we cannot separate distance from time. This means the observable universe corresponds to how far back in time we can see.

Motion in the UniverseIs it ever possible to be truly still on Earth?

Observed MotionsRotation of the Earth on its axis

Revolution of the Earth about the Sun

Revolution of the Sun about the center of the Milky Way

Motions of galaxies in the Local Group

Recessional velocities of galaxies outside the local group due to expansion of the universeThe first 4 motions above are related to conservation laws in physics (angular momentum) and law of gravity. The last is related to the birth of the universe in the Big Bang.

How to Get a Law Named After You(discover something really fundamental)Edwin HubbleHubbles Law: the farther a galaxy is from us, the faster it is receding from usHubbles Law is proof that the universe as a whole is expanding (getting larger)!

Raisin Bread Analogy to the Expanding Universehttp://spiff.rit.edu/classes/phys301/lectures/lambda/RaisinBread.swfThe program plots the distance of each of the raisins, as viewed from one particular raisin, and the rate at which the distances to the raisins is increasing (i.e., the velocity)

Getting to know the skyWhat things (not related to weather) can you see in the sky without the aid of a telescope?

A few things you can see in the sky without a telescope

With the exception of stars that are near the north and south poles, all astronomical objects rise & set

Constellations - 88 in totalSome constellations are easy to recognize from the patterns of stars. Very few are easy to imagine as their corresponding image (mythological character or animal). Orion (the hunter) is one of the exceptions!

Cassiopeia - The Queen (mother to Andromeda); see Clash of the Titans (theyre all in the sky)Constellations have well-defined borders on the sky (set by the International Astronomical Union in 1928)Cassiopeia is one of the northern circumpolar constellations, which means she never rises or sets

Constellations of the Zodiac (the sun sign constellations of astrology)Called the sun signs because they are all located along the ecliptic, the path of the sun through the sky (= the earths orbit around the sun).

Over the course of a year, the sun (as viewed from the earth), travels through each of these constellations.

Zodiacal Constellations and the SunWhich constellations could you SEE (i.e., on a given night)?

What determines which constellations you can see on a given night?Time of year (summer vs. winter constellations)

Latitude (cant see below your own horizon; e.g., if you live in Lima, Peru you will never see Cassiopeia)Since the constellations you see repeat over and over with the same seasons, its easy to notice a 1-year pattern to the seasons.

Cause of the SeasonsOver the course of the year in Boston, what will you notice about:

The height of the sun at noonThe rising and setting points of the sun (i.e., is it always due east and due north, respectively?)Day after day, the sun appears to take a slightly different track in the sky, and the track correlates with the seasons.

Path of the Sun at Different Times of Year From March 21-22 (spring equinox) to September 22-23 (autumnal equinox), the sun rises and sets north of due east/due west, so the days are long.

From September 22-23 to March 21-22, the sun rises and sets south of due east/due west, so the days are short.

Equniox = equal nightThe height of the sun at noon and its rising/setting point depend upon your latitude on earth. The poles are truly extreme (constant day/night for months).

Midnight SunNear the earths poles, the sun never sets during mid-summer. You pay for all this summer sun by never seeing the sun in mid-winter!

Tilt of the Earth Relative to the Suns RaysThe earths equator is inclined at 23.5o to the plane of its orbit around the sun. This is the true cause of the seasons.

What season is it in South America?

What season is it in North America?

Solar Irradianceequatormid-latitude (similar to Boston)polar climatesThe greater is the angle between the surface of the earth and the suns rays, the more power per unit area the surface of the earth receives.

Seasons and the Earths OrbitYou experience winter when your hemisphere is pointed away from the direction of the sun, and summer when your hemisphere is pointed toward the direction of the sun.

Precession of the EquinoxesLike a spinning top, the earths rotation axis precesses, constantly changing the direction of the North pole with respect to the sky.

This minor motion is very slow (takes 26,000 years to complete), but is important to navigation by the stars!

Right now, the North Star is Polaris (the tail star of the Little Dipper). Five thousand years ago the North Star was Thuban, and in 14,000 it will be Vega.

Phases of the MoonNames of phases, left to right:Waning Crescent, Third Quarter, Waning Gibbous, Full, Waxing Gibbous, First Quarter, Waxing CrescentThe amount of the moons face that is lit up increases daily from New Moon to Waxing Crescent to Full, then decreases daily from Full Moon to Waning Crescent to New Moon.

Shadow of the Earth does not cause the phases of the moonEarth and moon, both showing the First Quarter phase, seen from space.

Cause of Phases of the Moon:Changing angle between the earth and sunAt new moon, you look in the same direction to see both the sun and moon. At new moon the far side of the moon is in total daylight!

At full moon, you look on opposite sides of the sky to see the sun and the moon. At full moon, the far side of the moon is in total darkness!

At the quarter moons, the earth, moon, and sun make a right-angle triangle.

When does the moon rise and set?(make it simple - assume 12 hours from rise to set)Sunrise = 6am, Sunset = 6pmThe moon rises about 50 minutes later from one day to the next.New moon must rise and set with the sun (6am and 6pm), respectively.Full moon must rise when the sun is setting (6pm), and must set at sunrise the following day (6am).First quarter is mid-way between new and full, so it must rise at noon (i.e. 6 hours later than the new moon rises) and set at midnight (i.e., 12 hours after it rose).Third quarter is mid-way between full and new, so it must rise at midnight (i.e., 3 hours later than the full moon rises), and set at noon (i.e., 12 hours after it rose).

On the diagram, you can see anything that is 180o from you (the extent of your horizon)Choose a spot on the earth to stand, then ride along with the earth as it rotates counter-clockwise

Solar and Lunar EclipsesTotal solar eclipse time sequenceTotal lunar eclipse time sequence

Why dont we get a solar eclipse with every new moon and a lunar eclipse with every full moon?Moons orbit is inclined at 5o to Earths orbit. It is only when the moon is precisely in the same plane as the earths orbit that eclipses can occur.

Minute PaperA few sentences on one of the following:

* something you found particularly interesting today

* something you found particularly confusing today

* questions on things from today that you would like to know more about

Be sure to PRINT your name legibly