Earth Science Today
Russ Colson
Minnesota State University Moorhead

Stars and Planets:

Understanding the heavens is a puzzle in logic and exploration that has taught us humans to both wonder and to learn.  In this course, we will only dip our toe into the deep waters of how we have figured out the heavens.

Let's begin.  Here is a puzzle for you to do with someone else.  Come up with a list of objects in the sky.  How many can you think of?

Your list probably includes at least many of the following:  Sun, clouds, moon, planets, galaxies, stars, airplanes.  Now, list these in order of increasing distance from the surface of the Earth.  When you are done, you can continue to read.

How did you know which things were closer and which were farther away?  For example, how do you know that the clouds are closer than the Moon, or the Sun?
 

• How can we tell how far away things in the sky are?:
Superposition:  We can tell that one thing is closer than another because it passes in front of another.  For example, clouds pass in front of the Moon and Sun, the Moon passes in front of the Sun during an eclipse, the Moon passes in front of stars, and airplanes may pass either in front of or behind clouds.

Relative motion:  In the same way that objects close to a speeding car appear to move more quickly, we might expect closer objects in space to move more quickly.  The Moon and Sun appear to move very quickly.  The planets move  quickly relative to the stars (The world planet even means "wanderer", reflecting their character that allowed them to be distinguished from the stars).

Triangulation:  Triangles and logic revealed to the Greeks a great deal about our universe.  With shadows, angles, and a keen mind, Aristarchus discerned the great size and distance of the Sun 2300 years ago.  Around a hundred years later, Erotosthenes used shadows and angles to measure the size of the Earth.  Triangles are the foundation of our modern understanding of the size of space.  The idea of triangulation is simple:  if you can measure one side of a right triangle, and one angle of the right triangle (other than the 90 degree angle), all other dimensions and angles of the triangle can be calculated.   The lab activity for this topic will deal extensively with the idea of triangulation.  But here is a simple puzzle illustrating a case where a distance can be measured using either trigonometric relationships or similar triangles.  (how high is the tree, est3a1.html).

Standard candles:  If you had 2 lightbulbs of the same wattage and one was 10 feet away and one was 100 feet away, you could figure out which one was closer based on how bright each appeared.   If you knew the wattage of the lightbulbs, you could determine quantitatively how far away each was based on its apparent brightness.  The wattage is a measure of how bright it really is, and we can compare this to how bright it appears, which tells us how far away it is (the lightbulbs appearing progressively dimmer as it gets farther away).   We know how bright certain galaxies, supernovae, and stars really are, and we can then tell how far away they are by looking at how bright they appear.  This is called the standard candle method.

Cepheid variables (Sef ee id):  One type of standard candle is the Cepheid variable star.  Some stars in the sky change their brightness, getting brighter and dimmer in a regular and periodic way.  How fast or slow a star changes its brightness is related to how bright the star actually is.  The brighter the star, the longer it takes it to cycle from bright to dim and back again.  Generally, Cepheid variables cycle from bright to dim back to bright in anywhere from a day to over 2 months.  Because we know how bright the star really is, we can determine its distance from us by considering how bright it appears.

Red Shift:  We have observed that the more distant an object in space is from us (once we get away from the neighbors in our immediate corner of the universe), the more rapidly it is moving away from us.  This produces a shift in the wavelength of light reaching us from those objects (much like the frequency of sound changes as a car approaches, then passes us).  The wavelengths of light are shifted toward redder (or longer) wavelengths.  Once this relationship is established based on other evidence, we can use it to determine the distance an object is from us based on its redshift.

Some objects (distant galaxies) are so far away that it takes millions or billions of years for light to travel from them to us.  Thus, looking at light from these objects is like looking into the ancient past.  What we see happening in those galaxies actual happened millions or billions of years ago when the light left there.
 
 

•  How do we know what things in the sky are made of?:
Light:  Light gives us a unique fingerprint of the composition of stars and other objects in space.  Each element has its own unique fingerprint which can be determined in a laboratory.  We can then examine the spectrum from each star, match it up with the spectra of all the elements, and figure out what a star is made of.  (puzzle:  What are two elements that the star, Sirius, is made of?, est3a2.html)
     Light also tells us about the temperature of stars.  Everything is glowing, as we learned in our first topic.  This glow is called black-body radiation.  Hotter objects glow mainly in shorter wavelengths.  Colder objects glow at longer wavelengths.

Density:  By observing the size of objects in space, and measuring their mass (by how other bodies or spacecraft are attracted to them by gravity, for example)  a body's density can be determined (density = mass/volume).  A rocky body will have a density somewhere around 3 grams per cubic centimeter.  An icy body will have a density around that of ice (near 1 g/cc).  A body with an iron core, like Earth, will have a higher density, perhaps near 5g/cc.  More detailed understanding of composition can be determined from consideration of density as well.
 

• What causes the phases of the Moon?
Get a ball on a stick and a single light source to use as a thought-aid for the following thought puzzles (illustration).  These puzzles are intended to be solved in discussion with your group (a prize to the winning group, thus the answers aren't given here, but you CAN figure them out!).

What percent of the Moon's surface is lit by the Sun?
Why are there phases of the Moon?
How fast does the Moon orbit the Earth?
How fast does the Moon rotate?
How long is one period of daylight on the Moon?
What causes New Moon?
How is an eclipse different from New Moon?
What does "the dark side of the Moon" mean?

There are all kinds of misconceptions about phases of the Moon and eclipses.  One of my elementary teachers got confused about the "man in the moon", telling us that it was caused by the shadows of Earth's mountains....I spent years trying to make sense of that before I finally realized it didn't make sense!

Here a little activity to practice understanding how the phases of the Moon work.  Have one person be the Earth, one the Moon, one the Sun and, if you have a fourth person in your group, that person can help orchestrate it all.  Try to move relative to each other as the Sun, Moon, and Earth move, to generate days and phases of the Moon.
 

• What causes seasons on Earth?
Seasons are caused by changes in the amount of solar energy that is reaching the Earth's surface.   We might think that this is a function of how far away we are from the Sun.  But, as it turns out, this is only a minor factor (the Earth is actually closest to the Sun in January).  The main factors that change with seasons are 1)  how direct the sunlight is, and 2) the length of daylight.

1)  Directness of sunlight:  The average energy that strikes the Earth's surface is about 350 watts per square meter (think of nearly 6 60 watt light bulbs for each square meter or land).  However, this varies greatly with location (quick!  which do you think gets more, the equator or the north pole!)  Where the sun is directly overhead (near the equator), the energy is about 1390 watts/square meter.  However, where the sun is not directly overhead, this energy gets spread out over a bigger area, causing the energy per square meter to decrease (illustration).  Where the sunlight is tangent to the Earth, the energy falls to 0 watts/square meter.  Because of the Earth's tilt, the angle at which sunlight strikes the Earth at a particular spot changes with the season (think of how the sun is lower toward the horizon at noon in the winter than in the summer).
2)  length of daylight:  The average length of time in which the sun is hitting a particular place on Earth also changes with the season.  Daylight is longer in summer than in winter (except at the equator where daylight is always 12 hours long).  This is because the Earth is tilted, and as the Earth orbits around the Sun, the part of the Earth that is tilted toward from the Sun changes.  This causes more energy to strike the surface in a 24 hour period in summer than in winter.  (illustration)
 

• The Lumpy Universe and the life cycle of stars
Matter and energy in the universe are not evenly distributed (it's lumpy).  This is true on almost any size-scale one thinks of, from clusters of galaxies, to galaxies, to star clusters, to solar systems, to planets, to material on planets, to molecules, to atoms.
    In the late 1800's, when people had newly discovered spectroscopy and photography and for the first time were able to measure the composition of stars (how composition is measured is discussed above), it was not simply the composition that people found exciting, but rather what that composition revealed about how stars change with time.  In the early part of that century, Hutton and Lyell had revolutionized our understanding of the Earth with their realization that the Earth changes through time.  People wanted to find out the life cycle of stars and galaxies as well, how they change with time.  The distribution of matter in stars and galaxies, and the types of stars gave clues to that.
    We don't have time in this class to delve into the story of stars in detail, but here is a simple puzzle for you that illustrates how we can figure out things about the lifetime of stars.   Think of stars as like people, with a childhood, a middle age, and an old age.  Suppose that their characteristics, such as color, size, composition, and brightness, change with age so that we can tell their age (just like people change with age and we can tell old from the young).  With people, there are always new ones being born and old ones dying so that we stay roughly in balance.  Suppose stars are this way too.  Now consider people:  if each year one new person is born and one old person dies (no one dies until they are old in this thought puzzle), and you have 18 children, 42 middle aged folks, and 15 oldsters, how long does it take for a child to grow up (answer)?.  This puzzle is analogous to how we can figure out how long it takes for stars to go through different stages of their life cycle.  We can look at how many stars are in different stages of their life cycle today and how often stars die (for example, how frequently supernovae occur), and figure out how long they spend in each stage of their life cycle.  For stars, the part of their life cycle that is longest, where most stars are therefore found, is called the Main Sequence.  Our Sun is a Main Sequence star.

Lab:  Moon phase journal (in MSword)
 Lab Activity:  Distance to Stars, est3a4.html
Parallax class activity (in MSword)
(An alternative lab will be a visit to the MSUM planetarium.  However, you should still look over this lab as an exercise in measuring the distance to stars).
 

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