Light bulbs are designed with color temperature in mind. This is likely a familiar concept to photographers when selecting back lighting for a shoot. For a “candlelight” type background, a bulb of around 2000 K will be best, while a “natural daylight” background is best achieved with a bulb at 4500 K. Most fluorescent light bulbs in classrooms are around 5000 K while those in a library will be closer to 3000 K.

C0lor Temperature

Putting this value into the flux equation, we find that we receive 1360 W/m2 of sunlight on the Earth.

25

You observe two stars, named Sol-2 and Sol-3, that have the exact same spectral type and luminosity class, G2V, as the Sun (this means they have the same temperature, radius, and luminosity as the Sun). You measure the brightness of both stars with the same instrument and find that the light from Sol-2 is twenty-five times brighter than the light from Sol-3. Which star is closer to the Earth and by how much? Explain your reasoning.

This idea—that the apparent brightness of a source (how bright it looks to us) gets weaker with distance in the way we have described—is shown in Figure 10 below. At point 1, the light is concentrated into one box. By the time the light reaches point 2, which is twice as far as point 1, it is now spread out into four squares.

flux

In the same way that a 100 W bulb will always put out 100 Watts

Pure purple stars are also never seen for similar reasons — the blue and violet mix into a more deep blue color.

This makes sense when you consider that the Sun is also emitting red light and blue light; these all mix together as a nearly white color.

Note that the temperatures we associate with different colors in science are not the same as the ones artists use. In art, red is often called a “hot” color and blue a “cool” color.

a rough

a rough sort

d is:

, it turns out,

since humans act like blackbodies, does that mean that our bodies emit dangerous X rays and gamma rays?

solid objects

Earth is not a perfect blackbody, since clouds do reflect some sunlight.

Note that any objects that reflect light, such as a book with a red cover, are not blackbodies.

are believed to

This is probably most apparent when you are outside on a sunny day and forgot to bring some sunblock — your skin will absorb some of the UV (specifically, UVA) light.

extraterrestrial life

visible

dust

IR telescopes are primarily in space.

heat

night

white light was sent through a prism and he found that the invisible region beyond the red edge of the rainbow was hotter than the temperature of the rainbow!

heat

Microwaves are also low energy radiation and have sizes that range from about 1 mm up to 300 mm.

The most abundant element in the cosmos — hydrogen — also naturally emits radio waves at a very specific wavelength of 21 cm (which corresponds to a frequency of 1420 MHz).

461,538,461,538,461

These properties of light are summarized by the beautifully simple equation that relates the energy of a photon to its frequency (or wavelength), where h is Planck’s constant: This relationship shows the wave-particle duality of light, as the energy of photon (a particle of light) is directly related to its frequency (a wave property). Since h has a constant value, you can immediately get the energy of a particular color of light simply by knowing its frequency (or wavelength, which can be expressed as ).

Test

Among the colors of visible light, violet-light photons have the highest energy and red-light photons have the lowest.

as a wave

when you think about it as a wave.

The frequency of a wave is a measure of how many waves pass by in one second. Let’s imagine that the amount of time that elapsed for each of the waves in Figure 3 is 1 second. Looking at red light, two full waves can pass by in this 1 second period, so we say the frequency is 2 cycles per second, or 2 Hz. (Actually, a bit more than two red light wave cycles can pass by, about 2.5, but we will say 2 full waves to keep this example simple.)

Today, we know that there is no aether and that EM waves have no trouble at all moving through empty space (as all the starlight visible on a clear night must surely be doing).

one for which there was not a single shred of evidence

Albert Einstein proposed in 1905 that light can indeed behave both ways, thus solidifying the concept of wave-particle duality, one of the tenets of quantum mechanics

radiation

In general, the amount of the energy in the universe is always the same and it is continuously being changed from one form into another. This is the essence of the law of conservation of energy. Other quantities in nature, such as mass and momentum, are also conserved.

Energy

As a final note, the idea of parallel universes or the multiple universes (multiverses) turned from science fiction to a possible scientific reality via the “many worlds interpretation” to emerge from quantum mechanics. The physicist Hugh Everett III showed mathematically in 1957 that multiverses could exist. Such ideas, although inherently unprovable, perhaps seemed more feasible in light of the observation made by Edwin Hubble in the 1920s that the universe contains other galaxies and that the fabric of the universe itself is expanding with time, and that there is a limit to the “observable” universe.

colonized

turned out to be

rightful

Copernican

Bruno was tried and accused of heresy and accordingly burned at the stake in the year 1600, a decade before the telescope would show that the cosmos was unimaginably vast.

thus

The observation of Venus’ phases was actually direct proof that the geocentric model of the world was wrong.

He had improved the optics of his telescope and the magnification was now 20×.

The British physicist Robert Hooke coined the term “cells” after observing chambers inside of a piece of cork (cork is made of dead plant tissue) in 1665. By the late seventeenth century, the Dutch biologist Antonie van Leeuwenhoek had made pioneering strides in microbiology, particularly in the magnification of lenses, and discovered single-celled organisms (“animalcules”).

The exact origin of the microscope is unknown but it likely emerged from the same setting in Denmark that saw the telescope emerge.

nearer

does not actually turn backwards

In reality, though, the slower car is not moving backwards.

like a faster race car on the inside track. From the perspective of the driver of the faster car, a slower car on the outside track will appear to move backwards when it is passed.

The fact that closer planets orbit faster is a consequence of Kepler’s Third Law of Motion.

of Mars on the sky

Kepler’s Three Laws of Planetary Motion are discussed in detail in the Laws of Motion and Gravity chapter.)

contemporary of Galileo

is a

One of the most important events of the Renaissance was the displacement of Earth from the center of the universe, an intellectual revolution initiated by

The Middle Ages

Ancient Greek Cosmologies

()

So

contains

,

light

In general,

To explore the chemistry and possible biology of other worlds, ideally we would send humans there to run experiments. Humans have been to the Moon to collect rocks, and there are currently several rovers that are controlled from Earth analyzing rocks and soil on the surface of Mars. There are plans to send humans to Mars in the next decade or two. Why not send humans sooner? There turn out to be many challenges to physically traveling beyond the Earth, but the distances involved are foremost.

In general,

A tennis ball and a billiard ball have roughly the same volume, but the tennis ball is filled with air and is much less dense than a billiard ball of the same size.

For example, two planets could have the exact same volume and have the same size and shape in a picture, but their densities depend on what kind of matter is inside.

hus, we might say that the order of magnitude scale of a grapefruit is about 101 inches or 10-4 miles.

gassy surface

Regarding distances, the question of how long it takes to get someplace is a critical measure in our ability to explore other worlds in the cosmos.

Lengths of objects and distances to them are important measures in astrobiology.

There is also a prefix for 1021, called zetta and abbreviated by a Z. The Milky Way’s size can be expressed as 1021 m, 1 Zm or 1 sextillion m. This link to the National Institute of Standards and Technology lists the common prefixes that we use. In this table, we see that a yoctosecond is a very small measure of time – it is a mere septillionth of a second or 10-24 s. We will make regular use of powers of ten notation in this course, as we will be dealing with immensely large and small numbers.

M and G are sometimes used, where M stands for a million and G for a billion

The number is more manageable when expressed as one billion trillion

Clearly, the grapefruit is larger than the bacterium

All that matters

one-dimensional

One more very important point to note before comparing the sizes of any objects are units.

Of course not all objects are symmetric so you may need to be more specific about how you are defining length, depending on what is being compared. For example, bacteria come in different shapes, such as rods, spheres, and spirals. In specifying a bacterium’s size, you would need to be clear about the start and end points of the measurement. In astronomy, galaxies also come in different shapes – ellipsoids, spirals, and irregular shapes – so it is important to know which dimension (say left to right or top to bottom) the length is referring to.

was also create

as well

just

!