UW Madison Astronomy 103 Exam 1

What is astronomy?

the scientific study of celestial objects, space, and the Universe as a whole.

What is astrology?

the study of the movement and position of celestial objects and their supposed influence on the lives of humans

What is the speed of light?

3.00 x 10^8 m/s

How many stars are in the Milky Way?

100 billion

What is the distance from the earth to the sun?

8 light minutes

What is the diameter of the Milky Way?

100,000 light years

What is a constellation?

is an imaginary image formed by a pattern of stars in the sky

All the stars that we can see with the naked eye belong to?

The Milky Way

What is the celestial sphere?

The celestial sphere is a representation of how the entire sky looks as seen from Earth.

What is the zenith?

the point in the sky or celestial sphere directly above an observer

What is Meridian?

Longitude. An arc drawn between the North and South poles.

The motions we observe from Earth depend on our

latitude (position North-South)

What would we see standing on the North Pole?

the North Celestial Pole (and the North Star) would appear directly overhead- stars near the Celestial Equator would be on our horizon- all the stars would appear to move counter clockwise about the pole and would take 24 hours to complete a full rotation- no star rises or sets, but only half of the celestial sphere is visible

What would we see standing on the Equator?

stars near the North and South Celestial Poles would appear on the horizon- stars near the Celestial Equator would be straight overhead- all stars would rise and set - all stars rotate into view at some point (though we can't see them during daylight)

What would we see at a point between the Equator and the Pole?

one pole would above the horizon: the angle between the pole and the horizon would be equal to the observer's latitude- some stars would compete full rotations about the pole, never rising or setting - we call these "circumpolar" stars- some stars would rise and set- some stars near the opposite pole would never be visible to us

What did we use for centuries to observe time?

the sky

What are the three timescales in the sky?

Daily, Yearly, and Monthly

What is the reason for seasons?

The tilt of the Earth's axis

What is Earth's tilt?

23.5 degrees

What is the reason for hotter seasons?

The other reason is that the sun is higher in the sky in summer. The Sun's rays strike the ground closer to perpendicular, so there is more heating per unit area

The fastest spacecraft we have ever launched travels 20,000 times slower than the speed of light. How long will the spacecraft take to reach a star 4 light years away?

80,000 years

Beijing and Philadelphia have roughly the same latitude but very different longitudes. Assuming it's clear, tonight's night sky in these two places will

look about the same

Over the course of the year, as the Earth orbits the sun, we see ...

different constellations overhead at midnight

A planet very similar to Earth is discovered around a distant star similar to the Sun. The planet has a perfectly circular orbit and an axial tilt of 45 degrees. Which of the following is the best inference you could make about its climate?

the planet has more extreme seasonal variations (summer-winter differences) than Earth

A first quarter moon will

Rise at noon and set at midnight

What is light?

electromagnetic radiation

Light often behaves like a

a wave

Light can be described as

wave and particle

What is the wavelength of light?

Wavelength is the distance between successive crests (or troughs) in an electromagnetic wave.

What is the frequency of light?

number of wave crests that pass a given point in one second

How does light relate to color?

wavelength of light is related to the color that we perceive with our eyes- long wavelength (low frequency) light is red- short wavelength (high frequency) light is violet

X-rays photons penetrate our skin because they are more energetic than visible light photons. Compared to visible light, the wavelength of X-rays is

Shorter

The nucleus of an atom is made

of protons

Why are gamma rays generally harmful to life forms but radio waves generally harmless?

Gamma rays carry a lot more energy per photon than radio waves.

If a photon of exactly the right energy (corresponding to the energy difference between orbitals) strikes an electron

that electron will absorb the photon and move into the higher orbital

If an electron moves from a higher to a lower orbital

it emits a photon with an energy corresponding to the energy difference between orbitals.

What is a spectrum?

a band of colors, as seen in a rainbow, produced by separation of the components of light by their different degrees of refraction according to wavelength.

What three temperature scales are commonly used?

Fahrenheit, Celsius, & Kelvin

What is a diffuse gas?

the individual atoms are so spread out that that they don't influence one another appreciably

What is a dense opaque gas?

In a dense object like a light bulb filament or the surface of a star, the individual atoms are packed so closely that their energy levels get perturbed Photons tend bounce around inside a dense object, being absorbed and re-emitted many times before finally escaping

What are the laws of thermal radiation?

Wien's Law: • Hotter bodies emit more strongly at shorter wavelengths• The peak wavelength (λmax) is inversely proportional to the temperature.

The Stefan-Boltzmann Law: • The total light output of a hot body increases with temperature.• A hotter object emits more radiation at anywavelength.

Suppose we have an object that emits thermal radiation that peaks at visible wavelengths. If that object is heated it becomes

brighter and bluer

The hot end of these iron rods are glowing orange. Why is the cooler end black?

The cool end of the rod is glowing in the infrared but we can't see it

What are the three basic types of spectra?

Continuous spectrum, emission line spectrum, absorption line spectrum

Each chemical element produces a unique pattern of emission or absorption lines because

each element has a unique configuration of electron orbitals

The star Rigel has a surface temperature that is two times higher than the Sun's. How do their energy fluxes compare?

Rigel's energy flux is 16 times larger than the Sun's

A star has thermal spectrum that peaks at 500 nm. What is the star's approximate surface temperature?

5800 K

Two photons are detected -- let's call them photon A and B. The wavelength photon A is 3 times larger than the wavelength of photon B. How do the energies of the photons compare?

The energy of photon A is 3 times lower than the energy of photon B

What is an emission spectrum?

Light emitted by an element that is passed through a prism. It is an energetic fingerprint, and the bright lines have specific wavelengths and energies.

What is absorption spectra?

a spectrum of electromagnetic radiation transmitted through a substance, showing dark lines or bands due to absorption of specific wavelengths.

What is the Doppler effect?

an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other.

What is refraction?

the bending of waves due to a change in speed

What are reflector telescopes?

They use a curved mirror to focus light• Mirrors can be supported from behind so it's easier to make really big ones• Polished glass is coated with silver or aluminum.

What is the biggest telescope in the world?

The FAST Radio Telescope

D = 500 m = 5.5 football fields across!

What is a CCD (charge-coupled device)?

array of small light-sensitive elements that can be used in place of photographic film to obtain and store a picture

If light from a star passes through a cool diffuse interstellar cloud, the observed spectrum would show

dark absorption lines on a bright background

You observe a star with a certain absorption line of hydrogen at 489.1 nm. You know that in your laboratory on Earth this line has a wavelength of 486.1 nm. You infer that the star is

moving away from Earth at 1851 km/s

Which kinds of light can penetrate Earth's atmosphere?

visible, some infrared, and radio

The average distance from Earth to Mars is 225 million km. How far is this distance in light years?

2.4*10-5 light years

A friend is studying abroad in New Zealand. Will she see all the same stars and moon phase as you?

Same moon phase, different stars.

An evil graduate student has changed the Earth's axial tilt from 23.5 degrees to 45 degrees for their thesis. What will happen to the seasons?

The seasons will become more intense.

The star Arcturus is about 35 light years away. If a spacecraft orbiting Arcturus pointed a large radio antenna toward Earth, they would receive:

Our radio broadcasts from 1987.

How does the light gathering ability of a telescope with a mirror diameter of 30 meters compare to a telescope with a mirror of diameter 3 meters?

100 times more

At what time does the third quarter moon rise?

Midnight (12AM)

What does the double-slit experiment prove?

light is simultaneously a particle and a wave

A purple light (λ = 400 nm) is moving such that it appears orange (λ = 600 nm). How fast is it going?

Away from us at 1/2 the speed of light

Two stars are separated by an angle of 0.05 arcseconds. What is the smallest diameter telescope that could observe these two stars as separate objects when viewing them at a wavelength of 500 nm?

2.5 meters

The Sun has an angular diameter of about 1 10-2radians. Its radius is approximately 7 108meters. About how far away is it?

1.4 * 1011 meters

A star has the same luminosity as the Sun, but its radius is 4 times larger than the Sun. How does the star's temperature compare to the Sun's?

The Sun is 2 times hotter than the star

The star Rigel has a measured peak wavelength at 145 nm. The star Betelgeuse has a measured peak wavelength of 855 nm. Which star emits more red light?

Rigel

A star emits light via thermal radiation. As the light travels through the galaxy, it encounters a cloud of hydrogen gas. What happens to the stellar photons with energies that are different than any of the energy differences between hydrogen's electron orbits?

Nothing happens; the photons travel through the gas as if it wasn't there.

We observe an element radiating with two emission lines: in ultraviolet at 350 nm and red at 700 nm. How does the energy of the electronic transition that produced the ultraviolet line compare to the energy of the electronic transition that produced the red line?

2 times greater