Astronomy Test 2

What kind of galaxy is the Milky Way?

a barred spiral galaxy, so it’s elongated

Where is our solar system in the Milky Way galaxy?

the Orion Arm of the galaxy 

What is special about the orientation of our galaxy?

angled 60° to the horizontal

How long does 1 orbit around of our sun around the center of the Milky Way take?

 225 million years

What is the order of the planets from the Sun?

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

Terrestrial Planets

made of solids like rock and metal. Have solid surfaces, so a spaceship could land on it.

Are relatively small, closer to the sun (so relatively warmer), do not have rings

What are the terrestrial planets?

mercury, venus, earth, mars

Gas giant (jovian) planets

are made primarily of gas, mostly hydrogen. Do not have solid surfaces, so a spaceship couldn’t land on it

Are relatively large, further from the sun (so relatively cooler), and have rings

What are the Jovian planets?

jupiter, saturn, uranus, neptune

Asteroid Belt

a belt of meteoroids and asteroids (metals, ices, rock) between mars and Jupiter, possibly debris from the early solar system

What are the theories of the asteroid belt?

Jupiter’s gravitational pull is the reason the objects couldn’t coalesce into a planet, broke things apart

 something crashed into a forming planet

The Sun

• Contains 99.8% of the mass of the solar system

• Converts roughly 600 million tons of hydrogen (H) into 596 million tons of helium (He) in its core each second

• Solar wind of charged particles (protons and electrons) streams away from its surface

How far is Mercury from the Sun?

Closest planet to the sun, .4 AU

What are land features of Mercury?

• Barely larger than Earth’s moon

• Made of rock with a large iron core, is heavily cratered

• Long, tall cliffs (up to 5 km high)

What are characteristics of Mercury?

• Has the most extreme temperature range in the solar system

• Thin atmosphere 

• 1 day on mercury = 176 earth days (around 6 months)

How far is Venus from the Sun?

.78 AU from the sun

Why is Venus’ atmosphere unique?

• Has clouds of water droplets in a thick carbon dioxide atmosphere

• Has sulfuric acid rain

• Venus’ thick atmosphere traps heat which is why it’s so ho

What are characteristics of Venus?

• 85% Earth’s size

• Pressure at surface is comparable to .5 miles deep in Earth’s oceans

• Surface temperature is 800 degrees F

• Venus is the only planet that rotates opposite than the others (like it got knocked upside down)

Earth and Moon

• Only world in our solar system with abundant liquid water on its surface

• Only rocky planet with a large moon

How far is Mars from the sun?

1.5 AU

What are land features of Mars?

• Covered in red dust

• has 2 tiny moons, Phobos and Diemos

• About half the diameter of Earth and only 1/10 of Earth’s mass

• Ice caps of frozen water and CO2 which migrate from pole to pole

Why does Mars no longer have water?

Solar wind stripped away Mar’s atmosphere, leading to the disappearance of its surface water

Chaotic Obliquity

Mars; Axial tilt changes about every 1 million years

The Martian Dichotomy

half of Mars is a smooth deep basin, half is heavily cratered and higher elevation

How far is the asteroid belt from the Sun?

2.2-3.3 AU

What makes up the asteroid belt?

Some are small piles of rubble but others, such as Ceres and Vesta are much larger

Ceres

mostly a rocky dwarf planet like Pluto and other moon-sized icy worlds of outer solar system

shows evidence of past ice/mud volcanoes

How far is Jupiter from the sun?

5 AU

How much mass does Jupiter have?

Contains 50% of the mass of the solar system that is not the sun

Why are Jupiter’s moons unique?

Its four largest moons, Io, Europa, Calisto, and Ganymede rival the size of the planet Mercury

What are the characteristics of Jupiter?

• Almost entirely gas and liquid hydrogen with a tiny solid core 

• Violent storms and clouds made of various chemical ices give Jupiter its colors

• Red spot that’s larger than Earth 

Metallic Hydrogen

molecular hydrogen compressed under extreme pressure until its electrons move freely, making it act like a shiny, electrically conductive metal 

Found in the centers of planets

How far is Saturn from the Sun?

10 AU

What are characteristics of Saturn?

• Like jupiter but smaller and less dense

• Main rings consist of water ice particles ranging from a few mm to a few meters in diameter

• Dusty rings consist of particles the size of smoke particles which glow brightly when lit from behind

How far is Uranus from the Sun?

19.8 AU

What is unique about Uranus’ orbit?

• Axial tilt is almost 90°

• Daylight falls on one hemisphere for half of its 84-year orbit and darkness for the other half.

• 84 year orbit spin axis tilt forces seasons

What are characteristics of Uranus?

• 4 Earths across, but 14 times as massive 

• High pressure and temperature ocean of water, methane, and ammonia lurks below a thick later

What are characteristics of Neptune?

• Sun looks like a bright star, less than 0.1% of its brightness at Earth

• Similar to Uranus in both size and composition with similar rings and moons.

• Triton orbits Neptune in the opposite direction of its other moons and is most likely a captured dwarf planet like Pluto.

• Fastest winds in the solar system at 1,300 mph

How far is the Kuiper Belt from the Sun?

30 – 55 AU

What are characteristics of the Kuiper belt?

• many dwarf planets have been discovered

• Comets come from edge of solar system where it is cold, they’re like dirty snowballs. They have highly elliptical orbits

What are characteristics of Pluto?

• Pluto has a tenuous atmosphere, glaciers, mountains, and possibly even ice volcanoes.

• Pluto’s heart-shaped smooth region, called Sputnik Planitia, may even have a liquid water ocean deep below the cold, icy surface.

• Pluto’s orbit “intersects” Neptune’s, so it doesn’t“clear” its own orbit. Another reason it is not a planet

What are the four key patterns we need to explain to figure out how our solar system formed?

1. Patterns of motion of large bodies: Orbit in same plane and direction

2. Existence of 2 types of planet by size and composition (Jovian vs terrestrial)

3. Existence of swarms of smaller bodies (asteroids, comets, dwarf planets)

4. Exceptions: Venus rotates upside down, Uranus its tilted on its side, Earth’s large moon

Nebular theory

our solar system formed from the gravitational collapse of a giant interstellar gas cloud—the solar nebula

Immanuel Kant and Pierre-Simon Laplace proposed the nebular hypothesis 200+ years ago 

Close Encounter Hypothesis

states that another star passed within the Roche limit of the Sun. The star’s gravity would pull a filament of the material from the sun as it passed. This unstable filament would break into globs which would end up forming the planets

What is the downfall of the close encounter hypothesis?

it couldn’t explain observed motion

Roche Limit

distance at which tidal forces will rip a body apart

Stellar Recycling

elements that formed planets were made in previous generations of stars and then recycled throughout interstellar space

What causes orderly motion?

Rotation and disk flattening

How does rotation and disk flattening create the orderly motion we see in our universe?

• Primordial nebula (gas cloud) had turbulent motion and collapses 

• Rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted (conservation of angular momentum)

• Collisions between particles on inclined/eccentric orbits caused it to flatten into a disk with the gas and dust moving in nearly circular orbits

Protoplanetary Disk

observations of disks around other stars support the nebular hypothesis

Why are there two major types of planets?

Product of temperature conditions, which creates building blocks of the planets

What happens inside the frost line?

only rocks and metals solidify, so there’s not much material to build with. Small rocky cores can’t grab surrounding gases

What happens outside the frost line?

rocks and ices solidify, which leads to more building blocks. you get bigger chunks of ice which have larger gravity and start pulling other things in like gas

What happened when the rocky planets formed?

gas went outwards in the galaxy towards Jupiter and Saturn

Why is ice important in planet formation?

It adds mass fast. Without it (inside frost line), planets stay small and rocky. With ice, you can build giant cores that pull in the gas

How did terrestrial planets form?

• Small particles of rock and metal were present inside the frost line. They built up as rock/metal particles collided

• Gravity and collisions eventually assembled these into terrestrial planets (process called accretion)

Why are terrestrial planets different from gas giants?

• Too small: Rocky planets don’t have enough solid material inside the frost line to build up a massive core.

• Too hot: Close to the Sun, hydrogen and helium move too fast to be held by small planets’ gravity.

• Too late: By the time rocky cores grow a little, the solar wind has already blown away most of the leftover gas.

How do Jovian planets form?

• Ice could also form small particles outside the frost line. Larger bodies were able to form

• Larger gravity of these larger planets was able to draw in surrounding hydrogen and helium gases from solar nebula (Earth is too light)

• Extra rocks and ice can form moons of jovian planets (smaller-scale Nebular theory)

Why don’t gas giants just keep growing?

A combination of photon radiation pressure and solar wind blew away leftover gases, so Jovian planets had no more gas to accrete

Why are there smaller bodies in our universe?

Leftover accretion

• Rocky asteroids inside the frost line

• Icy comets outside the frost line

• Some mixing of rocky and icy objects thanks to Jupiter’s gravitational tugs

Giant Impact Hypothesis

• Evidence suggests a Mars-sized planet, Theia, destroyed early Earth in an off-axis impact

• Ejected material fell back to Earth and some accreted into our moon 

• This hypothesis explains the Moon’s similar chemical makeup to Earth, its lack of an iron core, and the thicker crust on its far side

What are other theories of the moon’s formation?

capture or co-formation

Why is the moon’s surface special?

• The far side of the moon shows few grey smooth regions, called mare, which are ancient lava floods compared to the near side 

• Impacts formed around the mare, but the moon’s crust must be thicker on the far side since impacts come from every direction

What could have caused details of individual planets to be different, even if formation of planets was inevitable?

different starting conditions

What can gravitational tugs cause in planets?

it can cause them to migrate in or out over many orbits

How can we discover planets?

• Try to image them

• Observe the motion of the star as it gets pulled by the planet (motion of star around center-of-mass)

• Look at stars and wait for planets to go in front of them (transit), making the stars temporarily dimmer by <1%

Why are exoplanets so hard to see?

star’s brightness makes the planets very hard to see, because of how bright they are

Why is it hard to direct image exoplanets?

• Very far away and small compared to stars (we can’t resolve most stars with telescopes except big and close ones like betelgeuse)

• Very close to extremely bright object (parent star)

How do scientists make it easier to direct image?

mask out the star with a coronograph

Coronograph

a telescope that has a dark spot in the middle to block the light

What are the easiest planets to image?

ones that are big and far away from their host star

How effective is direct imaging?

Only a few planets have been detected with this method (~20-30 planets)

Stellar Wobble/Doppler Technique

Stars and planets orbit around a center of mass. If there is a massive body/planet orbiting the star, then we can see the star is wobbling/doing a little orbit 

How do we observe stellar wobble?

• Measuring a star’s doppler shift (red/blue shift) can tell us its motion towards/away from us 

• Red/blue shift movements are periodic, which tells us it’s part of a system

• Only measures radial (towards/away from Earth speed)

What are the easiest planets to observe with stellar wobble technique?

• massive ones

• planets very close to their host star

• planets orbiting a low-mass star

Why are massive stars easier to detect with stellar wobble?

the massive planets would affect the host star’s gravity more

Why are planets close to their host star easier to detect with stellar wobble?

planets would have a shorter period, so they’re easier to see/detect

Why are planets orbiting a low-mass star easier to detect with stellar wobble?

There is more tugging of gravity between planets

What can you learn about a planet if you found it with wobble technique?

Orbital speed, minimum mass, orbital period/distance

51 Pegasi

• first extrasolar planet found

• Doppler shifts indirectly revealed a planet with a 4-day orbital period 

• Short period meant planet has small orbital distance

What are the limitations of stellar wobble?

• We don’t know the inclination of the planet’s orbit with this method 

• Only provides minimum mass of the planet if the orientation isn’t known

How effective is stellar wobble?

Hundreds of planets have been detected with this method 

Why would see more doppler shift from an edge-on planet?

because it would be moving away/towards you. If you’re looking at it face-on, it’s not moving towards/away from you

Planetary Transit

when a planet crosses in front of a star, resulting in a dip in brightness. An eclipse is also sometimes seen, when a planet passes behind the star

What is necessary to use planetary transits?

A special geometry (as seen from Earth) is needed for transits. You need a very specific range of inclination angles

Kepler Mission

discovered thousands of planets during its mission. Observed a patch of the sky and took data of many stars, looking for dips in brightness

Luminosity

the total energy output of the sun (or a star) over its whole surface each second (J/s = watts). Is an intrinsic property of a star

What is the lifetime of a star a product of?

how much fuel is available over the rate of fuel consumption

Why is the center of a star more dense?

As you get closer to the center of a star, there is more pressure from the layers above it which compresses the center

What does high pressure within a star cause?

high temperatures

Gravitational Equilibrium

energy supplied by fusion maintains the pressure that balances the inward crush of gravity

What causes a star to remain stable?

Gravitational equilibrium

Pressure and energy balance

the rate at which energy radiate from the surface of the sun must be the same as the rate at which it is released by fusion in the core

Cecilia Payne-Gaposchkin

Doctoral thesis laid the foundations for understanding the composition of the sun and other stars

Fission

big nucleus splits into smaller pieces (ex. Nuclear power plants)

Fusion

small nuclei stick together to make a bigger one (ex. stars)

Nuclear Fusion

the process by which stars produce their own energy

What does nuclear fusion require and why?

Requires very high temperatures, because then particles are moving fast enough to stick together (~108 K)

Proton-Proton Chain

how hydrogen fuses into helium in the Sun

What is the reaction of a proton-proton chain?

Take in 4 protons of hydrogen nuclei and produces a helium nucleus (2 p 2 n), 2 neutrinos, 6 gamma ray photons