ASTR 122 Final: UIUC

Final exam study guide for ASTR 122 cumulative final! Godspeed soldiers.

constellations

In order to more easily locate objects in the sky, we divide the sky into regions named after familiar patterns of stars

celestial sphere

a model of the sky, carrying the celestial objects around the Earth

rotation

The apparent westward motion of the Sun, Moon, and stars across our sky each day

circumpolar

Not all stars in our sky rise and set

Polaris

very close to the North Celestial Pole, and the sky appears to turn around it

Sun's rotation

The Sun moves along the ecliptic eastward - result of the Earth's revolving around the Sun

sidereal day

solar day is shorter by about 4 mins

23.5 degrees

The tilt of the Earth's axis relative to its orbital pole

This results in seasons

How does the tilt cause seasons?

-Changes the amount of direct sunlight over the year

-Changes the length of the day over the year

Why do we see phases of the Moon?

-Half the Moon is lit by the Sun; half is in darkness

-Its appearance to us is determined by the relative positions of Sun, Moon and Earth

lunar eclipses

-Moon passes through the Earth's shadow

-Can only occur at full moon

-Can be total, partial, or penumbral

solar eclipses

-Earth passes through the Moon's shadow

-Can only occur at new moon

-Can be total, annular, or partial

retrograde motion

for a few weeks or months, a planet turns westward relative to the stars

Ptolemaic's model

each planet move on an epicycle (small circle) whose center moves around Earth on a larger circle (deferent)

Copernicus

created a Sun-centered model of the Universe

-Explained retrograde motion without epicycles

Tycho

provided the data needed to improve Copernicus' model

Kepler's Laws of Planetary Motion

1. The orbit of each planet is an ellipse with the Sun at one focus

2. As a planet moves around its orbit it sweeps our equal areas in equal times

3. A planet's orbital period squared is proportional to the semimajor axis cubed: P^2 = a^3 (P in years, a in AU)

Galileo

-Discovered moons orbiting Jupiter; an example of bodies orbiting something other than Earth

-Observed Venus go through a complete set of phases; proved that it revolved around the Sun

Newton's Three Laws of Motion

1. Object moves at constant velocity if no net force is acting.

2. Force = mass × acceleration

3. For every force there is an equal and opposite reaction force

strength of gravity

-Directly proportional to the product of the masses (m1 x m2)

-Inversely proportional to the square of the distance between their centers

center of mass

the mutual mass that objects orbiting each other are actually orbiting

Electromagnetic spectrum

from long to short wavelengths: radio, microwave, infrared, visible light, ultraviolet, X ray and gamma ray

-The energy of a photon increases as the wavelength of the photon decreases

White light

consisted of visible light of different colors

Light

both the properties of waves (electromagnetic waves) and particles (photons)

EM waves

What all objects emit

Wien's law

the higher the temperature, the shorter the wavelength of maximum emission

three basic types of spectra

Continuous spectrum, emission line spectrum, absorption line spectrum

fingerprint

Each element has a unique spectrum line that tells us about its composition

Doppler's effect

The light of a moving source is blue shifted (to shorter wavelength) if it is moving towards us, and is red shifted (to longer wavelength) if it is moving away from us

Refracting telescopes

focus light with lenses

Reflecting telescopes

focus light with mirrors

three powers of a telescope

Light gathering power

Resolving power

Magnifying power - least important

big telescopes

-Light gathering power improves with area of primary mirror

-Resolving power improves with diameter of primary mirror

CCD

more sensitive to eyes and can accumulate light for a long time

adaptive optics

Technology used to correct atmospheric distortion by constantly adjusting a deformable mirror to bring a "star" into focus

terrestrial planets

inner planets closest to the sun; composed primarily of silicate rocks or metals

Jovian planets

gas giants; Jupiter, Saturn, Uranus, and Neptune

Asteroids

small rocky bodies mainly orbit between Mars and Jupiter

Comets

small bodies that contain a substantial amount of ices

Trans-Neptunian objects (TNOs)

small bodies orbiting the Sun beyond Neptune's orbit (30AU)

Kuiper belt

contains small bodies made of ices and rock orbiting the Sun from 30 AU to ~50AU

Dwarf planet

an object that is massive enough to make it round, but not massive enough to "clear the neighborhood around its orbit"

meteoroid

a chunk of rock smaller than 50 m

Meteor (shooting star)

meteoroid burning in the atmosphere

Meteorite

large meteoroid survives its fiery descent and reaches the ground

Sun

huge, glowing ball of gas (actually plasma) at the center of our solar system

-Made mostly of hydrogen and helium

-Generates a tremendous amount of energy via nuclear fusion of hydrogen into helium

hydrostatic equilibrium

balance between gravity pushing in and gas pressure pushing out

thermal equilibrium

(energy balance) between energy generated in the core and energy radiated from the surface

solar activity

Periodic disruptions in the Sun's atmosphere

-Stretching and twisting of magnetic field lines near the Sun's surface causes solar activity

Sunspots

cooler regions of the Sun's surface

Flares

violent explosions in the Sun's atmosphere

Prominences

loops of gas trapped above sunspots

Coronal Mass Ejections

huge bubbles of gas ejected from the Sun

11-year period

solar activity rises and falls

Parallax

tells us distances to the nearest stars

inverse square law for light

If we measure a star's apparent brightness and distance, we can compute its luminosity

Apparent magnitudes

a measure of the apparent brightness of stars

• Smaller magnitude = brighter star!

Absolute magnitudes

compare stars' luminosities

OBAFGKM

From hottest to coolest, the sequence of spectral classes

What is the relationship between luminosity, temperature, and size?

• For stars of the same size, the hotter one has greater luminosity

• For stars of the same temperature, the larger star has greater the luminosity

Hertzsprung-Russell (HR) diagram

plots luminosity of stars versus surface temperature (or color or spectral type)

• 90% of stars are on main sequence

red giants

Cool and luminous stars

white dwarfs

Dim but hot stars,

• Stellar remnant of a low-mass star
• Mostly made of carbon and oxygen

• Solar mass star compressed to the size of the Earth

• High density

• No energy generation, cooling down

• Gravity is supported by electron degeneracy pressure

• The larger the mass, the smaller the size

• Has a mass limit (Chandrasekhar limit): 1.4 solar masses

High-mass star

luminous and blue

Low-mass star

faint and red

average star

70% of nearby stars are red dwarfs, < 0.5 solar masses

-Massive main sequence stars and red giants are rare

protostars

Stars form in dark, cold clouds of gas & dust

-cloud contracts

Pre-main-sequence star

forms when the core are hot enough for the thermal pressure to temporarily balance gravity, but not hot enough to trigger thermonuclear fusion.

brown dwarfs

Starlike objects < 0.08 solar masses

How long do stars "live" on the main sequence?

Stars remain on main sequence as long as they have hydrogen in the cores to sustain fusion

• High-mass stars have shorter lifetimes than low-mass stars

red dwarfs

• Very low fusion rate; fully convective

• Slowly convert all their hydrogen in helium

• No red dwarf has ever left the main sequence

life stages of a Sun-like star

• Main sequence: H fusion in core

• Red giant: H fusion in shell around contracting core

• Helium fusing star: He fusion in core

• Red giant, 2nd time: Double-shell burning

• Ejection of outer layers in a planetary nebula leaves behind a white dwarf

life stages of a high-mass star

• Protostar/pre-main-sequence (contraction, powered by gravity)

• Main sequence: core H fusion

• Post-main-sequence: shell H fusion, followed by a series of nuclear fusions in the core and shells, eventually producing an iron core surrounded by burning shells.

• Iron core collapses, leading to a supernova

neutron star

a very dense stellar corpse consisting of closely packed neutrons in a sphere roughly 20—30 km in diameter.

-The core of a high-mass main sequence star containing between 8 and 25 solar masses becomes a neutron star.

pulsar

A rapidly rotating neutron star with a powerful magnetic field. The spinning field makes the neutron star a source of periodic radio and other electromagnetic pulses. Energy pours out of the polar regions of the neutron star in intense beams that sweep across the sky.

black hole

A massive object whose radius is so small that the escape velocity exceeds the speed of light

Event horizon

point of no return

Singularity

infinitely dense point all a black holes' mass is crushed into

How do we see a black hole?

Gravitational effects on companion stars and/or X rays from material in an accretion disk

central bulge

The galactic center is surrounded by a large distribution of stars

The Shape and Size of the Galaxy

Our Galaxy has a disk about 100,000 ly in diameter and about 2000 ly thick, with a high concentration of interstellar dust and gas in the disk.

galactic halo

The disk of the Galaxy is surrounded by a spherical distribution of globular clusters and old stars

The Sun's Location in the Galaxy

Our Sun lies within the galactic disk, some 8000 pc (26,000 ly) from the center of the Galaxy.

Interstellar dust

obscures our view at visible wavelengths along lines of sight that lie in the plane of the galactic disk. As a result, the Sun's location in the Galaxy was unknown for many years

The Galactic Nucleus

The innermost part of the Galaxy, or galactic nucleus, has been studied through its radio, infrared, and X-ray emissions (which are able to pass through interstellar dust).

Spiral Galaxies

• Disk component: Spiral arms, Contains gas and dust, Stars of all ages

• Bulge (sphere): Few young stars, Light dominated by red giants

Barred Spiral Galaxies

The spiral arms branch off from a straight bar of stars that passes through the central bulge

Elliptical Galaxies

All sphere component, No disk, No spiral arms, Little gas and dust, Yellow-red color, Few young stars, Light dominated by red giants

Irregular Galaxies

Chaotic systems, No disk, No sphere, Blue-white color, Light dominated by O and B-type stars, Tend to be rich in gas and dust

star formation

happens in the spiral arms

dark matter

The remaining 90% of the mass of the galaxy is in some nonvisible form

Cepheid variables

the brightest supergiants, type Ia supernovae; used to calculate distances of galaxies

rich cluster

contains at least a thousand galaxies

poor cluster

may contain only a few dozen up to a thousand galaxies

regular cluster

a nearly spherical shape with a central concentration of galaxies

irregular cluster

the distribution of galaxies is asymmetrical

Local Group

Our Galaxy is a member of a poor, irregular cluster

two galaxies collide

their stars initially pass each other, but their interstellar gas and dust collide violently, either causing gas and dust to be stripped from the galaxies or triggering prolific star formation. The gravitational effects of a galactic collision can cast stars out of their galaxies into intergalactic space.

active galaxy

an extremely luminous galaxy that has one or more unusual features: an unusually bright, starlike nucleus; strong emission lines in its spectrum; rapid variations in luminosity; and jets or beams of radiation that emanate from its core. Active galaxies include quasars, Seyfert galaxies, radio galaxies, double-radio sources, and BL Lacertae objects.

quasar

an object that looks like a star but is actually a very luminous nuclear of a galaxy very far away