Astronomy final

Oort cloud

Spherical shell (~2 000–100 000 AU) of icy comet nuclei surrounding the Solar System.

Rotation curve

Graph of orbital speed vs. distance from galaxy centre; “flat” ⇒ unseen mass.

Hubbles law

v = H₀d — galaxies recede at speeds proportional to their distances.

Spiral galaxy

Flattened, rotating disk with spiral arms, gas, dust and a central bulge.

Local group

~60-member cluster containing the Milky Way, Andromeda (M 31) & companions.

dark matter

Invisible gravitating mass detected only by its gravitational effects.

dark energy

Unknown energy component causing the cosmic expansion to accelerate.

Cosmic microwave background (CMB)

2.73 K relic radiation, uniform black-body glow from 380 000 yr after Big Bang.

White-dwarf supernova (Type Ia)

Thermonuclear explosion of a ~1.4 M⊙ C-O dwarf—peak luminosity ~constant.

Gravitational forces

Mutual attraction between masses; binds galaxies, stars, planets.

What does a “flat” Milky Way rotation curve tell astronomers about the Galaxys’ mass distribution

The disk’s luminous stars alone can’t supply the gravity needed to keep outer gas clouds orbiting at ~220 km s⁻¹, so most mass lies in an extended, invisible halo—our key evidence for dark matter.

Explain why Hubble’s Law does not cause the Milky Way or Solar System to expand?

stretches space-time itself, but inside gravitationally bound systems (Solar System, Milky Way) gravity dominates, holding them together against expansion.

Identify the two independent observations that support the existence of dark energy

show distant galaxies are receding faster than expected; (ii) the CMB’s geometry is spatially flat, implying an extra energy component driving acceleration.

Whay are white dwarf (Typer la) supernovae considered reliable standard candles

Type Ia events all detonate near the Chandrasekhar limit, giving them a uniform absolute magnitude; once corrected for decline-rate, they serve as “standard candles” out to billions of light-years.

Define the local group and name the other large spiral that shares it with the Milky way

The Local Group is our mini-cluster; the other giant spiral is Andromeda (M 31).

describe how the blue color of most spiral galaxies relates to their stellar populations

Spiral arms glow blue because they host short-lived, massive O- and B-type stars—evidence of ongoing star formation.

summarize the evidence that dark matter exists in the Milky Way halo

HI 21-cm and stellar velocities stay high far beyond the luminous disk, showing extra mass unaccounted for by visible matter.

where is the Oort cloud located and what does it contain

The Oort Cloud lies beyond the Kuiper Belt, possibly halfway to the nearest star, and stores trillions of dormant comets.c

contrast dark matter with dark energy in one sentence each

Dark matter adds gravity but no pressure; dark energy adds negative pressure, driving repulsion on cosmic scales.

How does the CMB support the idea of a hot Big Bang

The CMB’s perfect 2.7 K black-body spectrum is cooled fossil heat, exactly what a dense, hot origin predicts.

Which evidence directly implies an accelerating universe?

Type la supernovae

A galaxy’s rotation curve is called flat when gas clouds far from the center have orbital speeds that are

nearly the same

The Oort cloud is described as

an icy sphere well beyond Pluto

Spiral galaxies appear blue mainly because they

are dominated by hot massive stars

The local group contains the milky way and is best described as

two large spirals and smaller companions

The unexpectedly high orbital speeds in the outer Milky Way is due to the presence of

dark matter

The primary two observational pillars for dark energy are

accelerated expansion & CMB geometry

Hubbles law fails inside the milky way chiefly because

gravitation binding dominates

A standard candle must have a well-known

absolute luminosity

the cosmic microwave background is residual heat from

the Big Bang

solar nebula

rotating proto-solar gas cloud

Nebular theory

planets form from collapsing nebula

Conservation of angular momentum

L = mvr constant without torque

Planetary disk

flattened gas-dust disk around young star

Angular momentum

rotational “motion quantity” (kg m² s⁻¹)

Mercury

innermost planet, high density, eccentric orbit

Eccentricity

measure of orbit’s deviation from circle • Asteroid belt – rock/metal bodies 2.1–3.3 AU

Kuiper Belt

icy objects 30–50 AU in plane

Newton’s law of gravity

F = Gm₁m₂/r²

Using the nebular-theory explain why the collapsing solar nebula flattened into disk

Cloud collisions cancel vertical motion; gravity plus rotation flatten it into a disk.

Describe how conservation of angular momentum makes the planets orbit in nearly the same plane and direction

As the disk contracts, angular-momentum conservation aligns orbits in one plane, all circling prograde.

Why does Mercury have both the smallest semi major axis and the largest orbital eccentricity in our Solar System?

Mercury formed deep in the Sun’s well and suffered early gravitational tugs from Jupiter, boosting its e ≈ 0.21.

Show how Newton’s version of Kepler’s Third Law lets astronomers calculate the Milky Way’s mass inside the Sun’s orbit

Kepler-Newton form: M=(4π2a3)/(GP2)M=(4π²a³)/(GP²)M=(4π2a3)/(GP2); inserting a = 8 kpc, P = 240 Myr yields ~1 × 10¹¹ M⊙ within the Sun’s orbit.

What is the explanation for why spiral-arm gas clouds in the Milky Way mass inside the suns orbit

Outer-arm gas exceeds Keplerian speeds due to the Galaxy’s dark-matter halo.

Define the asteroid belt as located in the exam and state its planetary boundaries

Belt of minor planets between Mars & Jupiter (~2–3 AU).

Distinguish between asteroid belt and kuiper belt

Kuiper Belt is icy, lies beyond Neptune; asteroid belt is rocky, inner Solar System.

Explain why planets originally orbiting against the common direction would have been ejected

Retrograde protoplanets would collide with prograde gas, lose energy, and be ejected or absorbed.

“Gravitation forces” are a reason bound systems resists cosmic expansion; give an example within the solar system

Example: Earth and Moon mutually bind against expansion.

Which famous scientists first demonstrated that gravity applies both on Earth and in space, and what was his key publication year?

Sir Isaac Newton, Philosophiae Naturalis Principia Mathematica (1680s).

The collapse of the solar nebula into a disk is chiefly due to ______.

particle collisions in a spinning cloud

The planet closets to the Sun is ______

Mercury

Which planet’s orbit is the most eccentric?

Mercury

The Solar Systems asteroid belt lies between the orbits of ___________.

Mars & Jupiter

The Kuiper Belt is an area _______.

In the plane of the planets containing thousands of comets

The universal law of gravity that applied equally to falling apples and orbiting moons was formulated by ____

Newton

Flat rotation curves in galaxies suggest that mass _____

continues to rise in the halo

Earth is geologically active mainly because it ______.

is larger and retained heat

The Kuiper Belt lies outside the orbit of _______.

Neptune

Mercury’s tidal-locking is not complete; instead the Moons’ synchronous rotation is due to ______.

tidal friction

Axial tilt

23.5 ° spin–orbit angle

Solstice

Sun’s max declination (±23.5°) • Equinox – Sun crosses celestial equator

Angle of incidence

Sun-ray angle to surface normal

Umbra

region of total shadow

Penumbra

partial shadow

Spring tide

extreme high/low tides at syzygy

Tidal friction

torque slowing rotation & locking moons

Solar eclipse

Moon blocks Sun

Lunar eclipse

Earth’s shadow on Moon.

Explain why increasing Earth’s tilt from 23.5 to 40 would intensify seasons

Larger tilt pushes summer pole toward Sun (hotter) and winter pole away (colder)—stronger seasonality.

Describe how axial tilt produces the unequal sunlight received by Northern Canada and the Southern U.S in June

In June the Northern Hemisphere tilts sunward, so Canada gets high-angle, long-daylight sun; the South receives oblique rays.

How does the angle of incidence affect solar-energy intensity at Earth’s surface?

Higher angle ⇒ energy concentrated ⇒ warmer; lower angle spreads energy over larger area.

Why are spring tides highest during New and Full Moon phases?

Sun, Moon and Earth align; lunar-solar gravity adds, producing the highest tides.

Explain why the same lunar hemisphere always faces Earth

Tidal friction has synchronized the Moon’s spin to its orbital period—synchronous rotation.

Describe the alignment that causes a solar eclipse

Sun-Moon-Earth alignment; Moon’s umbra sweeps Earth, causing a solar eclipse.

Predict the tidal change if Earth had no Moon, referring to the forces

Without the Moon, solar tides alone would be ~½ current range—smaller spring tides.

Define Equinox

Day when every place gets ~12 h daylight and 12 h dark (March & September).

State the factor that determines the angle of incidence of sunlight on Earth

Primarily latitude (plus time of year).

Describe the relationship between lunar phase and moonrise time

Each night the Moon rises ~50 min later as it moves eastward 13°/day.

Which eclipse is produced from the order of Sun-Moon-Earth?

Solar eclipse

Tides are highest during which lunar phase?

New & Full

Tides are primarily influenced by _____

gravitation pulls of Moon & Sun

If earth’s axial tilt were zero, seasons would _____

dissapear

Low-angle solar rays deliver______intensity than high angle rays

lower

The astronomical event yielding roughly equal day & night length is the _______.

equinox

A penumbra is the region of ________

partial shadow

tidal friction is the cause of the moons ______

synchronous rotation

Lunar phases affect the time of moonrise; each night it rises about ________.

one hour later

Seasonal variations arise mainly because Earth’s axis is _________

tilted

Hydrogen

Sun’s main fuel

Nuclear fusion

combining light nuclei → heavier, releases energy

Solar cycle

~11-yr sunspot magnetic cycle

Sunspot

cool (~4000 K) magnetized photosphere patch

Coronal Mass Ejection (CME)

billion-ton plasma blast

Coronal hole

open-field gap emitting fast wind

Space weather

solar-driven disturbances near Earth

Magnetosphere

magnetic bubble deflecting solar wind

Solar wind

stream of charged particles

Solar flare

explosive X-ray/UV burst.

What reaction converts hydrogen to helium and powers the Sun?

Proton–proton fusion (⁴1H → ⁴He + energy).

Define the solar cycle in terms of its approximate length and driver

Cycle lasts ~11 yr; driven by the Sun’s winding, reversing magnetic field.