AST101 Midterm

Planet

A moderately large object that orbits a star and shines primarily by reflecting light from its star

An object can be considered a planet only if it 1) orbits a star, 2) is large enough for its own gravity to make it round, and 3) has cleared most other objects from its orbital path; an object that meets the first two criteria but has not cleared it orbital path, like Pluto, is labelled a dwarf planet

Dwarf planet

An object that orbits the Sun and is massive enough for its gravity to have made it nearly round in shape, but that does not qualify as an official planet because it has not cleared it orbital neighbourhood

The dwarf planets of our solar system include the asteroid Ceres and the Kuiper belt objects Pluto, Eris, Haumea, and Makemake

Moon

An object that orbits a planet

Star

A large, glowing ball of gas that generates heat and light through nuclear fusion in its core; our Sun is a star; the term star is sometimes applied to objects that are in the process of becoming true stars (i.e. protostars) and to the remains of stars that have died (ie. neutron stars)

Solar system

A star (or sometimes more than one star) and all the objects that orbit it; the Sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies

Galaxy

A great island of stars in space, containing millions, billions or trillions of stars, all held together by gravity and orbiting a common center

Astronomical Units (AU)

The average distance of Earth from the Sun, which is around 150 million km (1 AU is the length of the semimajor axis of Earth's orbit)

Light Years (ly)

The distance that light can travel in 1 year, which is 9.46 trillion km or around 10 trillion km/s

Understand the relative size of the Earth to the rest of the Solar System

(From order of smallest to greatest) Earth, solar System, Milky Way Galaxy, Local Group, Local Supercluster

How many stars are in a galaxy and how many galaxies are in the observable universe?

There are more than 100 billion stars in the galaxy

The Milky Way is only one of more than 100 billion large galaxies (and many more small galaxies) in the observable universe

If we assume 100 billion galaxies and 100 billion stars per galaxy, the total number of stars in the observable universe is roughly 100 billion×100 billion, or 10,000,000,000,000,000,000,000 (1022)

State the approximate sizes of important astronomical structures, such as planets, stars, solar systems, and galaxies, in AU and ly

Earth is 1 AU from the Sun; table 7.1 illustrates the approximate sizes

Relationship between astronomical distance and lookback time

The farther away we look in distance, the further back we look in time

Correctly describe the shape and major motions (orbit, rotation) of objects in the solar system

The Sun, planets and large moons generally orbit and rotate in a very organized way; Planets all orbit the Sun in the same direction (counterclockwise); large bodies have orderly motions

Venus rotates clockwise, and Uranus rotates nearly on its side

Recognize the major patterns of motion in the solar system and, later, relate these to its formation mechanism

Comparative studies reveal similarities and differences that give clues to solar system formation and highlight underlying processes that give each planet its unique appearance

1. Patterns of motion among large bodies. The Sun, planets and large moons generally orbit and rotate in a very organized way; Planets all orbit the Sun in the same direction (counterclockwise); large bodies have orderly motions

2. Planets fall into two main categories: terrestrial (Mercury, Venus, Earth and Mars) and jovian (Jupiter, Saturn, Uranus and Neptune)

3. Vast numbers of asteroids (rocky) and comets (icy) orbiting the sun

4. A few notable "exceptions to the rules"

Distinguish between terrestrial planets and jovian planets

Terrestrial planets = the four planets of the inner solar system: Mercury, Venus, Earth and Mars

Small in mass and size

Close to the Sun

Made of metal and rock

Few moons and no rings

Jovian planets = the four large planets of the outer solar system: Jupiter, Saturn, Uranus, and Neptune

Large mass and size

Orbit far from the sun

Made mostly of H, He, and hydrogen compounds

Rings and many moons

Distinguish between asteroids, comets, and the Kuiper Belt.

Asteroids = rocky bodies that orbit the Sun much like planets, but are much smaller; most known asteroids are found within the asteroid belt between the orbits of Mars and Jupiter

Comets = small objects that orbit the Sun, but are made largely of ices (such as water ice, ammonia ice, and methane ice) mixed with rock

- The vast majority of comets never visit the inner solar system; instead they orbit the Sun in one of two distinct regions: the Kuiper Belt and the Oort Cloud

Kuiper Belt = the comet-rich region of our solar system that resides b/w around 30 and 100 AU from the Sun; Kuiper belt comets have orbits that lie fairly close to the plane of planetary orbits and travel around the Sun in the same direction as the planets

Oort Cloud = a huge, spherical region centered on the Sun, extending perhaps halfway to the nearest stars, where trillions of comets orbit the Sun with random inclinations, orbital directions and eccentricities; much farther from the sun

Constellation

A region of the sky w/ well-defined borders; 88 official constellations cover the celestial sphere

Celestial pole

North celestial pole = the point directly over Earth's North Pole

South celestial pole = the point directly over Earth's South Pole

Celestial sphere

the imaginary sphere on which objects in the sky appear to reside when observed from Earth

Celestial equator

a projection of Earth's equator into space, and makes a complete circle around the celestial sphere

Zodiac

the constellations on the celestial sphere through which the ecliptic passes

Ecliptic

the path the Sun follows as it appears to circle around the celestial sphere once a year; it crosses the celestial equator at a 23 ½ degree angle, because that is the tilt of Earth's axis

Describe the motion of the Earth, Sun, Moon, and stars relative to one another over time periods ranging from a day to a year

Except comets, most things have near circular orbits and are often rotating

Earth orbits the Sun, and Moon orbits the Earth

Earth takes 365 days to orbit the Sun, and 24 hours to rotate

The Moon has synchronous rotation (takes about the same amount of time to orbit and rotate)

Explain why we can see different fractions of the celestial sphere from different locations on Earth and how this affects the placement of research telescopes

If you go South of the equator you see a completely different set of constellations from that of the Northern hemisphere

Constellations do not just sit; they do not just stay still because Earth rotates

The SUn and stars follow different paths across the sky depending on the time of year and latitude

The constellations we see depend on latitude and time of year

The visible constellations vary with time of year because our night sky lies in different directions in space as we orbit the sun

The constellations vary with latitude because your latitude determines the orientation of your horizon relative to the celestial sphere

The sky does not vary with longitude (the angular east-west distance b/w the prime meridian and a location on Earth's surface)

Describe the cause of the seasons and predict what seasons would be like on a planet with a different axial tilt than Earth's

The tilt of Earth's axis causes the seasons; The axis points in the same direction throughout the year, so as Earth orbits the Sun, sunlight hits different parts of Earth more directly at different times of year

With a different axial tilt, we would have more or less extreme seasons

Earth's seasons are caused by the tilt of its rotation axis, which is why the seasons are opposite in the two hemispheres. The seasons do not depend on Earth's distance from the Sun, which varies only slightly throughout the year.

Lunar eclipse

A lunar eclipse occurs when Earth comes directly between the Sun and Moon, so that Earth's shadow falls on the Moon

an event that occurs when the Moon passes through Earth's shadow, which can happen only at full moon; a lunar eclipse may be total, partial or penumbral

Solar eclipse

A solar eclipse occurs when the Moon comes directly between the Sun and Earth, so that the Moon's shadow falls on Earth

an event that occurs when the Moon's shadow falls on Earth, which can happen only at new moon; a solar eclipse may be total, partial, or annular

what causes the phases of the Moon as seen from Earth

The Moon's phases are caused by the fact that we see different portions of its day and night sides at different times as it orbits around the Earth

half the Moon is always illuminated by the Sun, but the amount of this illuminated half that we see from Earth depends on the Moon's position in its orbit

Why is there not a solar eclipse at every New Moon and why are solar eclipses seen only from some parts of Earth?

Because Earth is much larger than the Moon, Earth's shadow can cover the entire Moon during a lunar eclipse; therefore, a lunar eclipse can be seen by anyone on the night side of Earth when it occurs

By contrast, the Moon's shadow can cover only a small portion of Earth at any one moment, so you must be located within the relatively narrow pathway through which the shadow moves to see a solar eclipse (This is why we tend to see lunar eclipses more often than solar eclipses, even though both types occur about equally often)

eclipses can only occur when:

1. The phase of the Moon is full (for a lunar eclipse) or new (for a solar eclipse) and

2. The new or full moon occurs when the Moon is very close to a node, which means it is during an eclipse season

The moon's orbit is tilted by about 5 degrees

Eccentricity

a measure of how much an ellipse deviates from a perfect circle; defined as the center-to-focus distance divided by the length of the semimajor axis

Orbital Period

the amount of time a given astronomical object takes to complete one orbit around another object

Orbital semimajor axis

half the distance across the long axis of an ellipse; usually referred to as the average distance of an orbiting object, abbreviated a in the formula for Kepler's third law

The semimajor axis is half of the distance across the ellipse in its longest direction (which means half of the major axis), which is also the planet's average distance from the Sun. Therefore, the ellipse that measures the longest across is the one with the largest semimajor axis.

State one major contribution to astronomy made by Copernicus, Kepler, and Galileo

Copernicus:

- Proposed that the Sun is the center of the Universe

- Planets orbit the sun in circles

- The moon orbits the Earth

Kepler:

- Kepler's key discovery was that planetary orbits are not circles but instead are a special type of oval called an ellipse (a type of oval that happens to be the shape of bound orbits)

Galileo:

Used a telescope to look at the sky and found that:

- Jupiter has moons (So not everything orbits the Earth)

- The Moon has craters (so not everything in our observable universe is perfect and without blemishes)

- observed the phases of Venus (Venus orbits the Sun and has phases consistent with the Heliocentric model and inconsistent with the Geocentric/Earth-centered model)

Thus, all three astronomers helped us move from Earth centered to Sun centered model

Kepler's first law

Kepler's 1st law: the orbit of each planet about the Sun is an ellipse with the Sun at one focus

- A planet's distance from the Sun varies during its orbit; its closes point is called perihellion (the point at which an object orbiting the Sun is closest to the Sun) and its farthest point is called aphelion (the point at which an object orbiting the Sun is farthest from the Sun)

- The average of a planet's perihelion and aphelion distances is the length of its semimajor axis (half the distance across the long axis of an ellipse; the average distance of an orbiting object); or more simply, the planet's average distance from the Sun

Kepler's second law

Kepler's second law: a planet moves faster in the part of its orbit nearer the Sun and slower when farther from the Sun, sweeping out equal areas in equal times

Kepler's third law

more distant planets orbit the Sun at slower average speeds, obeying the precise mathematical relationship, p2 = a3

This principle claims that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun (semimajor axis)

The letter p stands for the planet's orbital period in years and a for its average distance from the Sun in astronomical units

The square of each planet's orbital period (p2) is equal to the cube of its average distance from the Sun (a3)

Distinguish between mass and weight

Mass = amount of matter

Weight = force exerted by your mass

Mass does not change, weight changes depending on the force of gravity

State Newton's three laws of motion and apply them in the case of objects moving under gravity

Newton's first law = an object moves at constant velocity if there is no net force acting upon it (i.e. an airplane on a smooth flight)

Newton's second law = a net force affects an object's motion; specifically, force = rate of change in momentum, or force = mass x acceleration

- a net force will change an object's momentum, accelerating it in the direction of the force

Newton's third law = For any force, there is always an equal and opposite reaction force

Apply the concepts of conservation of energy and momentum to determine how the temperatures and rotation rates of celestial objects change when their sizes change

Conservation of energy = the principle that energy (including mass-energy) can be neither created nor destroyed, but can only change from one form to another

- Planetary interiors cool with time because they radiate energy into space, and the Sun became hot because of energy released by the gas that formed it

- a small amount of mass contains a huge amount of energy

Conservation of momentum = the principle that, in the absence of net force, the total momentum of a system remains constant

- Stars and galaxies are both born from clouds of gas that begin much larger in size; these clouds have small net rotation, and like a spinning skater as she pulls in her arms, they must spin faster as gravity makes them shrink in size

- conservation of momentum means that an object's momentum cannot change uness the object transfers momentum to or from other objects; when no force is present, no momentum can be transferred so an object must maintain its speed and direction

Describe how the force of gravity between two objects depends on their masses and the distance between them

There is a force between any two objects in the Universe

The force is proportional to the product of the masses of each object

The force is inversely proportional to the square of the distances

The force depends on M1 times M2

If you double the mass of either object, you double the force

If you half the mass of either object, you half the force

The force depends on the distance between the centers of the two objects

If you double the distance, you reduce the force by a factor of 2 x2 = 4

If you half the distance, you increase the force by a factor of 2x2 = 4

F = G M1M2 / d2

Explain the concept of escape speed and describe how the escape speed of a body depends on the properties of that body

Escape speed = the speed necessary for an object to completely escape the gravity of a large body such as a moon, planet or star

Escape velocity does not depend on the mass of the escaping object - any object must travel at a velocity of 11 km/s to escape from Earth, whether it is an individual atom or molecule, a spacecraft, or a rock

Escape velocity does depend on whether you start from the surface or from someplace high above the surface

- Because gravity weakens with distance, it takes less energy (and thus a lower velocity) to escape from a point high above Earth than from Earth's surface

List the four possible shapes for orbits and relate them to the escape speed and circular speed

Unbound hyperbolic orbit

Unbound parabolic orbit

Bound elliptical

Circle (A circle is an ellipse with zero eccentricity, and greater eccentricity means a more elongated ellipse)

Explain why there are two high tides and low tides within 24 hours at any given location on Earth

Earth's rotation carries us through the two bulges each day, giving us two daily high tides and two daily low tides

There are two tidal bulges

One is facing the moon, and one is on the opposite side

They are about the same size

Because of its orbital motion around Earth, the Moon reaches its highest point in the sky at any location about every 24 hours 50 minutes, rather than every 24 hours; as a result, the tidal cycle of two high tides and two low tides takes about 24 hours 50 minutes, so each high tide occurs about 12 hours 25 minutes after the previous one

Correctly describe how tides arise from differences in the force of gravity between two points

The Moon's gravity creates a tidal force that stretches Earth along the Earth-Moon line, causing Earth to bulge both toward and away from the Moon

Tides are created by the difference in the force of attraction between the Moon and different parts of the Earth. The two daily high tides occur as the location on Earth rotates through the two tidal bulges.

Explain why we can never see the far side of the Moon from Earth

Synchronous rotation - the rotation of an object that always shows the same face to an object that it is orbiting because its rotation period and orbital period are equal

Explain the long-term effects of tides on the motions (orbits and rotation) of both the Earth and the Moon

synchronous rotation is a natural consequence of tidal friction

Because Earth is more massive than the Moon, Earth's tidal force has a greater effect on the Moon than the Moon's tidal force has on Earth

This tidal force gives the Moon two tidal bulges along the Earth-Moon line, much like the two tidal bulges that the Moon creates on Earth

If the Moon rotated relative to its tidal bulges in the same way as Earth, the resulting tidal friction would cause the Moon's rotation to slow down

The Moon probably once rotated much faster than it does currently; as a result it did rotate relative to its tidal bulges, and its rotation gradually slowed

Once the Moon's rotation slowed to the point at which the Moon and its bulges rotated at the same rate (synchronously with the orbital period) there was no further source for tidal friction

Earth's rotation pulls its tidal bulges slightly ahead of the Earth-Moon line, leading to gravitational effects that gradually slow Earth's rotation and increase the Moon's orbital energy and distance.

The Moon's synchronous rotation was thus a natural outcome of Earth's tidal effects on the Moon

Name the major parts of the Sun, such as the core, photosphere, and corona

The Sun's interior:

- Convection zone (of a star) = a region where energy is transported outward by convection; energy generated in the solar core travels upward, transported by the rising of hot gas and falling of cool gas called convection

- Radiation zone = region of the interior where energy is transported primarily by radiative diffusion

- Solar core = source of the Sun's energy: nuclear fusion transforming hydrogen into helium

The Sun's atmosphere:

- Solar wind = a stream of charged particles ejected from the Sun; even at great distances from the Sun, you and your spacecraft can feel slight effects from solar wind

- Corona = the uppermost layer of the Sun's atmosphere; most of the Sun's x-rays are emitted from this region, where the temperature is about 1 million K

- Chromosphere = the layer of the Sun's atmosphere below the corona; most of the Sun's ultraviolet light is emitted from this region, where the temperature is about 10,000 K

- Photosphere = the visible surface of the Sun, where the temperature averages just under 6000 K

Apply the concept of hydrostatic equilibrium to the Sun

Gravitational equilibrium = a state of balance where the force of gravity pulling inward is precisely counteracted by pressure pushing outward; also called hydrostatic equilibrium

The Sun's internal pressure precisely balances gravity at every point within it, thus keeping the Sun stable in size

Explain how the Sun produces energy from matter

Einstein's equation = E = m x c squared

Energy equals mass time speed of light squared

When a nuclear reaction produces energy, that energy comes from converting mass into energy

Fusion of hydrogen into helium gneerates energy because a helium nucleus has a mass slightly less than the combined mass of four hydrogen nuclei (when four hydrogen nuclei fuse into a helium nucleus, a bit of mass disappears); the disappearing mass becomes energy in accord with Einsten's formula E = mc2

Distinguish between nuclear and chemical reactions

Chemical reaction:

Electrons in atoms or molecules change over to new partners (nuclei just observers)

Rearrangement often releases energy

Ex. photosynthesis, fire, battery power

Nuclear reaction:

Nuclei combine or split apart

Now electrons are just observers

Ex. hydrogen bombs, radioactivity

Distinguish between fission and fusion

Fission = the process where a larger nucleus splits into two (or more) smaller particles

Fusion = the process when two (or more) smaller nuclei slam together and make one larger nucleus

Describe the main nuclear reaction by which the Sun generates energy

Nuclear fusion in its hot core

H + H + H + H = He + energy

Describe the origins of sunspots; describe the solar cycle and its effects on Earth

Sunspots are regions with strong magnetic fields, which can alter the energy levels in atoms and ions, causing some spectral lines to split into two or more closely spaced lines

Cooler regions on surface (temperature around 3500 C)

Usually seen in pairs

Up to 30,000 km across

Likely due to the Sun's magnetic field

Strong magnetism prevents hot gas from rising up

The sunspot cycle varies both in duration and in the peak number of sunspots from one period to the next; the length of time between maximums averages 11 years, but it has been observed to be as short as 7 years and as long as 15 years

The locations of sunspots on the Sun also vary with the sunspot cycle; as a cycle begins at solar minimum, sunspots from primarily at mid-latitidues (30 to 40 degrees) on the Sun; the sunspots tend to form at lower latitudes as the cycle progresses, appearing very close to the solar equator as the next solar minimum approaches; then the sunspots of the next cycle begin to form near mid-latitidues again

Something peculiar also happens to the Sun's magnetic field at each solar maximum: the Sun's entire magnetic field starts to flip, turning magnetic north into magnetic south and vice versa; the magnetic field thus reverses: in the subsequent solar cycle, the field lines connecting pairs of sunspots point in the opposite direction

The sun's complete magnetic cycle (or solar cycle) thus averages 22 years, since it takes two 11 year sunspot cycles before the magnetic field is back the way it started

Emission

The process by which matter emits energy in the form of light

i.e. a light bulb emits visible light; the energy of the light comes from electrical potential energy supplied to the light bulb

Absorption

the process by which matter absorbs radiative energy

i.e. When you place your hand near an incandescent light bulb, your hand absorbs some of the light, and this absorbed energy warms your hand

Transmission

the process in which light passes through matter without being absorbed

i.e. Some forms of matter, such as glass or air, transmit light, allowing it to pass through

Reflection

Reflection / scattering: the process by which matter changes the direction of light (reflection) or light that is reflected into random directions (scattering)

Light can bounce off matter, leading to what we call reflection when the bouncing is all in the same general direction (i.e. a mirror reflects light along a simple path) or scattering when the bouncing is more random (i.e. a movie screen scatters light in many different directions)

Spectrum

Electromagnetic spectrum = the complete spectrum of light, including radio waves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays

Draw graphs of wavelength vs. light intensity to represent the spectrum of an object

Intensity = a measure of the amount of energy coming from light of specific wavelength in the spectrum of an object

At wavelengths where a lot of light is coming from the object, the intensity is high, while at wavelengths where there is little light, the intensity is low

When the spectra are shown as graphs, absorption lines appear as dips on a background of relatively high-intensity light while emission lines look like spikes on a background with little or no intensity

Understand the relationship between an image of a spectrum and the graph of the same spectrum

Image of the spectrum: what you see

Graph of the spectrum: how much signal there is at each frequency (colour)

Interpret the Sun's spectrum as having both continuous (i.e. "full rainbow") and line features

Continuous spectrum = a spectrum (of light) that spans a broad range of wavelengths without interruption by emission or absorption lines

Emission line spectrum = a bright band (a "line") of single colour, superimposed on a fainter or completely absent rainbow of light, occurring when light viewed though a diffraction element such as a prism shows an excess of photons at or near a specific wavelength

Absorption line spectrum = a dark band (a "line") on an otherwise bright rainbow of light, occurring when light viewed through a diffraction element such as a prism shows a deficit of photons at or near a specific wavelength

Apply the concept of a blackbody spectrum to explain the continuous portion of the Sun's spectrum

Thermal radiation = the spectrum of radiation produced by an opaque object that depends only on the object's temperature; sometimes called blackbody radiation

Any "solid" (opaque) object emits light via "thermal radiation" (i.e. humans, a hot poke in the fire, the Earth, the Sun)

What does light tells us?

Temperature: peak of continuum spectrum (blackbody curve)

Composition: which emission/absorption lines are present

Describe light in terms of its wavelength, frequency, energy, and colour; explain how these terms are related

Light has both wave and particle like properties

A wave is described by 3 things:

- Wavelength: distance between two peaks (or troughs)

- Frequency (number of times per second a peak passes you)

- Speed: rate at which a peak (or trough) travels

speed = wavelength x frequency

the longer the wavelength, the lower the frequency, and the shorter the wavelength, the higher the frequency

The wavelength (or frequency) determines the colour and energy of the light

shorter wavelength = higher energy; hotter objects emit more of their light at shorter wavelengths (higher energy)

cooler objects tend to have longer wavelengths and lower frequencies, belonging to the infrared area; hotter objects tend to have shorter wavelengths and higher frequencies (and energy), belonging to the ultraviolet area

Two Laws of Thermal Radiation

1) Each square meter of a hotter object's surface emits more light at all wavelengths.

2) hotter objects emit photons with a higher average energy.

Explain the line features in the solar spectrum as absorption lines

Emission lines or absorption lines occur only at specific wavelengths that correspond to particular energy level transitions in atoms or molecules

Every kind of atom, ion and molecule produces a unique set of spectral lines, so we can determine composition by identifying these lines

How does light interact with matter at the atomic level?

Emission

Absorption

Transmission

reflection/scattering

All matter is made of atoms

Atoms have:

A nucleus made of positively charged protons and neutral neutrons

Negative electrons orbiting outside (actually a quantum probability cloud)

The electrons determine how a material behaves

The number of protons determines which element the atom is composed of

The number of electrons is equal to the number of protons in the nucleus

The number of neutrons can vary, but it is roughly the same as the number of protons

To change an element you must add or remove protons

Doppler effect

the effect that shifts the wavelengths of spectral features in objects that are moving toward or away frm the observer

Blueshift = a Doppler shift where spectral features are shifted to shorter wavelengths, observed when an object is moving toward the observer

Redshift = a Doppler shift where spectral features are shifted to longer wavelengths, observed when an object is moving away from the observer

Describe how spectroscopy can be used to measure the physical characteristics of celestial objects

Light tells us:

What things are made of?

Absorption lines

The temperature of a distant object?

Wavelength of intensity peak

The speed of a distant object?

red/blue shift of absorption lines (Doppler shift)

Give examples that demonstrate the existence of interstellar gas and dust

Interstellar gas = The gas between stars is mostly hydrogen and helium scattered at varying densities between the stars in our galaxy and other galaxies. The proportions of the gases are similar to those in the Sun. Interstellar gas supplies the raw material for star formation.

Interstellar dust = tiny solid flecks of carbon and silicon minerals found in cool interstellar clouds; they resemble particles of smoke and form in the winds of red giant stars

- Not all of the material in a molecular cloud is gaseous; elements such as carbon, silicon, oxygen and iron are often found in tiny solid grains of interstellar dust

There is gas between the stars

It is called the interstellar medium

Mainly hydrogen and helium gas

Most of it is invisible

Spectroscopy is used to measure the abundance of the new elements that stars have added to the interstellar medium

The most straightforward technique is to observe the spectrum of a star whose light has passed through an intervening cloud of interstellar gas; the cloud absorbs some of the star's light, leaving absorption lines in the star's spectrum

The wavelengths of the lines tell us the chemical contents of the cloud, and comparing the amounts of light absorbed by different atoms and molecules allows us to determine the composition of the cloud

The effects of interstellar dust on starlight can be seen by comparing visible-light and infrared photos of a molecular cloud

How did the Sun form?

A molecular cloud collapses

- Something starts a molecular cloud collapsing

- Gravity continues to drag the gas inwards

- Only possible in molecular clouds because they are so cold and so dense

Gravity can overcome outward pressure within a gas cloud

- High density means gravity is strong

- Low temperature means pressure is low

- In most places within our galaxy, gravity is not strong enough to overcome the internal pressure of interstellar gas, which is why star formation does not occur everywhere

Molecular cloud

cool, dense interstellar clouds in which the low temperatures allow hydrogen atoms to pair up into hydrogen molecules (H2)

How and why does a molecular cloud collapse due to the force of gravity?

Stars form in cold, dense regions of space called molecular clouds. When the force of gravity pulling in on the cloud is greater than the strength of internal pressure pushing out, the cloud collapses into a protostar.

Gravitational collapse of cloud overcomes thermal pressure

Squeezing gas makes it hot

As the cloud collapses and gets denser, it heats up

- Collisions between molecules create photons which can escape the cloud

- Slows increase in temperature and pressure

A star begins to forms in the centre of the cloud, where temperatures and densities are highest

Inner parts of cloud eventually get too dense for photons to escape

- Temperature rises and collapse slows

A star beings to forms in the centre of the cloud, where temperatures and densities are highest

Outer parts of cloud are still cool and low in density

List the main stages of star formation

Stage 1 - formation of a protostar

Stage 2 - Convective Contraction

Stage 3 - Radiative contraction

Stage 4 - self-sustaining fusion

Contraction of a star-forming cloud:

A contracting cloud begins its transformation into a star when its core starts trapping thermal energy released by gravitational contraction

As pressure begins to push back harder, the contraction slows down and the central part of the cloud becomes a protostar

Meanwhile, matter from the surrounding cloud rains down on the protostar, increasing its mass

Role of rotation in star birth:

Conservation of angular momentum ensures that a young protostar spins rapidly, and much of the material falling inward toward a protostar ends up in a spinning protostellar disk; planets may form in this disk

Friction in the disk can transfer angular momentum away from the inner parts of the disk, allowing gas to accrete more easily onto the protostar

Some protostars drive powerful jets outward along the disk's rotation axis

Along with strong protostellar winds, these jets can disrupt gas in the surrounding molecular cloud

Nuclear fusion in a newborn star:

Nuclear fusion becomes self-sustaining when a protostar's core temperature rises above 10 million K

In order to reach this temperature, the protostar must keep radiating some of its thermal energy, so that it can continue contracting

During the late phases of star formation, the protostar's luminosity declines and its surface temperature increases

protostars

a forming star that has not yet reached the point where sustained fusion can occur in its core

Describe the properties of the young Sun, and how they differ from the Sun's properties today

The Young Sun (4.6 billion years ago):

-spun once a week

-strong wind

-lots of sunspots

-lots of solar flares

-composed of 72% hydrogen, 27% helium

Current Sun:

- spins once a month

-modest wind

-modest sunspots

-some solar flares

-composed of 39% hydrogen, 60% helium

Frost line

the boundary in the solar nebula beyond which ices could condense; only metals and rocks could condense within the frost line

Describe multiple methods for how the age of the solar system can be measured (e.g. radioisotope dating and cratering)

Radiometric dating of the oldest meteorites tells us that accretion began in the solar nebula about 4.56 billion years ago, with the planets forming by about 4.5 billion years ago

- Radiometric dating tells us how long it's been since a rock solidified

Radiometric dating = the process of determining the age of a rock (i.e. the time since it solidified) by comparing the present amount of a radioactive substance to the amount of its decay product

Radiometric dating is based on carefully measuring the proportions of radioactive isotopes and their decay products within rocks

The ratio of the isotopes changes with time in a steady and predictable way that we characterize by an isotope's half-life, the time it takes for half the atoms in a collection to decay

Relate the observed patterns of motion of objects in the solar system to its formation mechanism

Collapse of cloud under gravity causes it to heat up, spin faster, and flatten into a disc

As the solar nebula collapsed under gravity, natural processes caused it to heat up, spin faster, and flatten out as it shrank

The orderly motions we observe today all came from the orderly motion of this spinning disk

Relate the patterns of chemical composition in the solar system to its formation mechanism; correctly use the term frost line

Temperature differences in the solar nebula led to different kinds of condensed materials at different distances from the Sun, sowing the seeds for two kinds of planets.

The inner regions of the solar nebula were relatively hot, so only metal and rock could condense into tiny solid grains; these grains accreted into larger planetesimals that ultimately merged to make the terrestrial planets

Beyond the frost line, cooler temperatures also allowed more abundant hydrogen compounds to condense into ice, building ice-rich planetesimals; some of these grew large enough for their gravity to draw in hydrogen and helium gas, forming the jovian planets

Describe the nebular theory for the formation of the solar system

The scientific theory that describes how our solar system formed from a cloud of interstellar gas and dust

The nebular theory begins with the idea that our solar system was born from the gravitational collapse of an interstellar cloud of gas, called the solar nebula, that collapsed under its own gravity

This cloud gave birth to the Sun at its center and the planets in a spinning disk that formed around the young Sun

Describe our current model for the formation of the Moon

During the formation of the solar system, a marssized planet struck the Earth (giant impact hypothesis)

The moon formed out the debris (through accretion)