HZ Astronomy 101 Midterm #2

If the wavelength of a wave decreases, does the velocity of the wave increase or decrease?

Decrease; v \= 𝜆 × 𝑓

What are the two light equations?

(1) c \= 𝜆 × 𝑓 (2) E \= h × 𝑓 (energy of photon \= Planck's constant * frequency

List the spectrum of light from shortest wavelength to longest.

Gamma rays, x-rays, ultraviolet, visible light, infrared, microwaves, radio waves.

Do x-rays have longer or shorter wavelengths than infrared?

Shorter

What was Galileo able to discover in 1609 with his telescope?

(1) Jupiter has moons. (2) Venus has phases and changes in size. (3) The Moon has mountains.

What increases a telescope light gathering power?

Increasing the diameter of the lens thus increasing the area of the lens.

Jeopardy round: Light at different wavelengths focus differently.

What is chromatic aberration?

How to avoid chromatic aberration?

Use mirrors instead of lenses to direct all the light to a focal point.

Who discovered infrared light in 1800?

William Hersehl.

What is bad seeing?

When Earth's atmosphere interferes with seeing.

How did astronomers avoid bad seeing?

They put the Hubble space telescope in space to bypass Earth's atmosphere.

What advantage do infrared telescopes have over visible light telescopes?

Infrared telescopes can bypass dust/gas clouds and see stars within dust clouds.

Which telescope uses infrared light?

James Webb Space Telescope

What are the 3 types of spectra?

(1) Continuous spectra. (2) Emission line spectra. (3) Absorption spectra.

T/F: All colors are emitted with the same wavelength.

False

Why can't we see the light emitted off of ourselves?

We emit infrared.

Jeopardy round: It is each element's specific "code".

What is emission line spectra?

Who determined the composition of the sun in 1859?

Joseph von Fraunhofer.

What did Joseph von Fraunhofer use to determine the composition of the sun?

Absorption spectra.

What are Kirchoff's Laws?

(1) Hot opaque objects (solid/dense gasses) emit a continuous spectra. (2) Hot thin gasses emit an emission line spectra. (3) Cool thin gasses between hot opaque objects and the observer emit an absorption spectra.

What can spectral lines tell us?

Chemical composition, temperature, density, magnetic fields, motion towards/away from the observer.

T/F: All waves exhibit the doppler effect.

True

Explain why the doppler effect happens.

An object that is moving is producing waves of equal wavelength but at different positions. Thus, because of the relative motion to the observer, it seems like the wavelengths are changing.

T/F: If the sound producing object is moving away, the wavelengths of the sound waves will seem shorter.

False; they'll seem longer when the object is moving away and shorter when the objecting is moving towards you.

T/F: You will notice the doppler effect if you're moving with the source or moving perpendicular to the source.

False

What is red shift?

The shift of the absorption spectra of a celestial object towards longer wavelengths when it is moving away from the observer.

What is the cause of red and blue shift?

The doppler effect.

What is the equation for the doppler effect for light?

z \= (received - emitted )/ emitted \= v/c

A star's spectral line normally at 𝜆 \= 600 nm is observed to be 𝜆 \= 700 nm. That means that the star is \_____.

Moving away from the observer.

A star's spectral line normally at 𝜆 \= 600 nm is observed at 𝜆 \= 605 nm. How fast is the star moving?

1/120 of the speed of light away from us.

What is the definition of temperature?

The average speed of random interval motion of the molecules in a substance.

What happens when the temperature of a substance increases?

(1) The amplitude/intensity of every wavelength increases. The radiation of the entire spectrum increases. (2) The wavelength of the peak intensity decreases (higher frequency).

Jeopardy point: The point at which there is no random interval motion of molecules in a substance.

Absolute zero on the Kelvin scale.

How did Max Planck solve the blackbody problem?

He came up with the quantum hypothesis which is the idea that energy can only be exchanged in discrete amounts called quanta, where you can only have whole numbers of energy.

How did Planck's quantum theory differ from classical physics?

In classical physics, you could have fractional energy. In quantum theory, you can only have whole numbers of energy.

What is Wien's Law?

max λ (in nm) \= 3x10^6 (in nm * K)/ T (in K)

What is Stefan-Boltzmann Law?

E is proportional to T^4

What causes blackbody/continuous/thermal spectra?

Electrons vibrating in dense objects.

T/F: All blackbodies at the same temperature emit the same spectra.

True; composition doesn't matter.

T/F: Hotter blackbodies emit more of their light in higher wavelengths.

False; in lower wavelengths/higher frequencies/higher energy.

T/F: Hotter blackbodies emit less total energy.

False; use common sense. If you don't have that like me, use Stefan-Botlzmann Law.

Who discovered the mathematical pattern to explain the emission spectrum of hydrogen in 1885?

J. J. Balmer

What gives the illusion of solidity?

Electrical repulsion between electrons.

T/F: Atoms are mostly empty space.

True

T/F: According to Bohr's hydrogen model, electrons can be at different energy levels at the same time.

False

What happens when hydrogen's electrons jump from a high to low energy level?

The process will release a photon at a certain wavelength.

How does Bohr's model explain the emission line spectrum?

When an electron jumps from a higher energy level to a lower energy level, it'll release light that corresponds to the emission line spectrum of hydrogen. The energy of each single line equals the difference between energy levels.

What is the law of conservation of energy?

Energy cannot be created or destroyed.

How does Bohr's model explain the absorption spectrum?

When

What must be true about the photon's energy when it hits the electron to be absorbed?

The energy must match exactly with the energy needed to make the jump (the energy difference between the different energy levels). E3 - E2 \= hf \= (hc)/λ

What is the inverse square law of light?

b \= L / (4 pi d^2) where b is observed brightness, L is the luminous intensity, and d is the distance from the light source.

What is the photosphere?

The "surface" of the sun. Where light we see emerges from the sun.

T/F: There is a solid surface of the sun.

False; it's dense gas.

What is hydrostatic equilibrium?

When equilibrium is reached as a result of the weight of the top most layers crushing the layers beneath. This increases the gas pressure of the lower layers to the point where the lower layers can support the higher layers.

What is the process that fuels the sun?

Nuclear fusion reactions.

What is the nuclear fusion process of a 1 solar mass star?

Proton-proton process. At the core (and only in the core), H fuses with He. Photos slowly work their way outward from the core.

Explain how gamma rays produced by nuclear fusion eventually turn into photons.

When the gamma rays are produced, they bounce around and hit molecules as they make their way to the surface causing it to lose energy to become visible light.

What is the other nuclear reaction that powers stars?

The CNO cycle. It also turns hydrogen into helium. It's theorized to be more prominent in larger stars.

T/F: Our sun can produce elements beyond He.

False; it's not hot enough to create anything else.

How does a star become a red giant?

As He builds up from the nuclear fusion, H "burning"/conversion moves to the shell outside of the dead core. Eventually, He ignites in the core and makes carbon. As H continues to "burn" in the shell outside the He shell the core begins to shrink. This increases the temperature which leads to He + He reaction. Because the gas has spread out so much, the gamma rays created from the nuclear fusion have lost so much energy that the wavelength increases and it looks red.

How do planetary nebulae form?

After a star becomes a red giant, it'll eventually disperse and become a planetary nebula.

What is a white dwarf?

Remnants of stars formerly like our sun.

T/F: White dwarfs are supported by nuclear fission.

False; Electron degeneracy pressure.

Explain why the more massive a white dwarf is, the smaller it is.

As the mass of the white dwarf increases, the gravitational force squishes it even more.

What is Chandrasekhar's limit?

The upper limit for the mass of a white dwarf, which is equal to 1.4 solar masses.

What happens when the mass of a white dwarf exceeds 1.4 solar masses.

It collapses.

Describe the structure of a 1 solar mass red giant.

(1) Carbon and oxygen filled core (2) He burning shell (3) H burning shell (4) H envelope.

T/F: More elements are formed in a one solar mass red giant than a supergiant star.

False; the temperature of the supergiant star is much higher so more elements are able to be formed.

What's the difference between fusión and fission?

Fission \= break bigger elements into smaller pieces (think nuclear bombs that break down uranium). Fusion \= combine smaller elements into bigger pieces (think combining hydrogen and hydrogen to form helium).

How do Type II supernovae happen?

When a massive star begins to form iron, the weight/gravitational force of the star upon itself becomes too heavy and it collapses. This leads to a supernova explosion, which creates elements. (We are made of supernova stardust :D)

What are the properties of supernovas?

(1) They can emit energy greater than ALL the stars in the galaxy combined. (2) They can be observed from extremely large distances. (3) Elements with high atomic numbers are created during the explosion. (4) Material ejected at v \= 10,000 km/sec.

What are the two types of supernovas?

(1) Type IA; occurs when a white dwarf "steals" matter from a companion star until the mass exceeds Chandrasekhar's limit. (2) Type II; occurs when a massive star (greater than 10 solar masses) begins to produce iron in its core.

Who organized stellar spectra by the strength of hydrogen lines?

Henry Draper; he ordered them from A → Q

Which astronomers improved Draper's system?

Wiliamina Fleming, Antonia Maury, Annie Jump Cannon (she got rid of some letters in the classification).

What is a mnemonic for Annie Jump Cannon's classification system?

OBAFGKM → Oh Be A Fine Girl Kiss Me

T/F: The temperature goes from low to high in OBAFGKM.

False; high to low.

What is the most important factor that determines a star's spectrum?

Temperature.

T/F: Stars are mostly made of hydrogen and helium.

True

What sequence of star is our sun?

G2

What relationship does the Hertzsprung-Russell diagram show?

Temperature (x-axis) vs. Luminosity (y-axis)

T/F: More than 90% of stars fall on the main sequence of the HR diagram.

True

What is the formula for luminosity?

L \= 4*pi*R^2*sigma*T^4

How is it possible for a celestial body to have the same temperature as other celestial bodies but not have similar luminosity?

They could have a much smaller or bigger radius. E.g. white dwarf or supergiant.

What is a standard candle and why are they useful?

Objects with a well-known luminosity. Astronomers can use them to measure the distance of objects (IDK if we talked about this).

What are 3 common standard candles?

(1) Supernova (Type IA in particular) (2) Any star whose spectrum can be measured. (3) Cepheid variable stars.

T/F: The greater the mass of a star, the shorter its lifespan it has.

True

What is the formula for luminosity?

L \= 4*pi*d^2*b (b \= apparent brightness)

Who was the first person to discover parallax in 1837?

Friedrich Struve

A parallax of 1 arcsec corresponds to a \_______ distance from Earth.

Parsec

What is the formula that relates distance to observed parallax and what are the units?

d \= 1/p (d in parsecs; p in arcsec)

T/F: There is a limit to how much parallax works/how far we can use parallax to measure celestial objects.

True

How does spectroscopic "parallax" work?

For any star whose spectrum can be obtained, you can compare their spectrum to known stars in the main sequence of the HR graph and find their luminosity. Knowing both the luminosity and apparent brightness of the star allows you to calculate the distance from Earth.

Who discovered the relationship between the luminosity of Cepheid Variable stars and their period/the period-luminosity relationship?

Henrietta Leavitt

How does the period-luminosity relationship measure the distance of celestial objects?

If you know the period of a star, you can know the luminosity of the star. Knowing both the luminosity and the apparent brightness, you can find out the distance.

How do Type IA supernovae help us measure extremely distant stars?

Type IA supernovae have a constant peak luminosity. When they occur near the distant star we are trying to measure, we can measure the apparent brightness and the luminosity of the nearby Type IA supernova to know the distance of the distant star.