Astronomy 103 full semester

Astronomical Unit

1.5 x 10^8

Speed of light

3 x 10^5 km/s

Local group

Cluster of about 30 galaxies

Virgo Cluster

Local group is part of a large collection of smaller clusters and groups of galaxies; has about 2000 galaxies

Cosmic Address

Earth, Solar System, Milky Way Galaxy, Local group, Virgo cluster

Diffraction

bending of light

Light behaves as a wave in that it

Diffracts

Light behaves as a particle in that it

Propagates as a stream of particles

Photons

Particles or parcels of light

Wavelength (λ)

Distance between successive crests; determines colors

Brightness (B)

Amount of light recieved from a source

Inverse-square law

Brightness decreases as the square of the distance (d) from the source increases.

Frequency (ν)

How fast successive crests pass by a given point; Long wavelength light has a low frequency, and short wavelength light has a high frequency.

Planck’s constant

h \= 6.63 × 10^−34 J·s

Order of electromagnetic spectrum (short wavelengths to long)

Gamma, X-rays, Ultraviolet, Visible light, Infrared, Microwaves, Radiowaves

Atmospheric transparency

What Earth’s atmosphere allows us to see; Gamma, X-rays, and Ultraviolet light are blocked by upper atmosphere; Visible light observable from earth with some atmospheric distortion; Infrared spectrum mostly absorbed by atmospheric gasses; Radio waves observable from earth; Long-wavelength Radio Waves blocked

Thermal Radiation

Emitted by an object due to its temperature.

Blackbody

is a hypothetical body that absorbs all energy incident on it (no reflections) and emits energy of all wavelengths; stars behave nearly like perfect blackbodies

Wien’s law

The hotter the body is, the more strongly it will radiate at short wavelengths; Hotter \= Bluer

Temperature (T)

A measure of the motion of atoms in an object; Collisions of particles cause radiation; Objects with high temperatures have atoms that are moving around very rapidly

Stefan-Boltzmann constant

σ \= 5.67×10-8 Watts m-2 K-4

Atoms

What most ordinary matter is made of

What an atom is made of

Protons (+), Neutrons (\=), Electrons (-)

How electrons behave

Sometimes as particles, sometimes as waves

Orbitals

The probability clouds electrons inhabit around the nucleus

Orbitals are quantized

They exist only at particular energies

Ground state

lowest energy orbital

Electronic transitions

a specific amount of energy must be added to move an electron from one orbital to the next

Absorption of a photon

If a photon of exactly the right energy (corresponding to the energy difference between orbitals) strikes an electron, that electron will absorb the photon and move into the next higher orbital

Excited state

when a photon strikes an electron and the electron absorbs it and moves to the next higher orbital

Ionized

If the electron gains enough energy to leave the atom entirely

Emission of a photon

if an atom drops from one orbital to the next lower one, it must first emit a photon with the same amount of energy as the orbital energy difference.

Chemical elements

The number of protons in a nucleus determines the element, and electron orbitals and energy differ for each element; this can be used to tell what elements atoms belong to from light years away

Spectrum

When wavelengths of light get bent by different amounts; allows us to measure for the intensity or amount of radiation as a function of wavelength

Continuous spectrum

Objects that are hot and dense produce continuous spectra (thermal radiation).

Emission line spectrum (hydrogen gas)

Diffuse gasses produce emission or absorption at discrete wavelengths that correspond to particular electronic transitions.

Absorption line spectrum

A diffuse gas in front of a continuous light source will produce absorption on top of the continuous spectrum

The Doppler Effect

The wavelength of spectral line is changed by the relative motion between the source and the observer; this allows us to use spectra to determine an object’s speed towards or away from us

When an object is redshifted, it is

Moving away OR has a low temperature

When an object is blueshifted, it is

Moving closer OR has a high temperature

Refraction Telescopes

Uses lenses to converge light rays upon a focal point; Difficult to make, needs pure glass, can cause blurring

Reflection Telescopes

Uses curved mirrors to focus light; easier to make, can make bigger (but can be compact), doesn’t need pure glass

Radio Telescopes

Reflectors to collect and focus radio waves; Radio waves have long wavelengths so big reflectors are needed

Four ways telescopes & electronic light detectors aid our eyes

1. Light gathering power: the number of photons collected per unit time. Bigger telescopes collect more photon which improves our ability to see faint objects. 2. Resolving power: the ability to reveal fine detail. Bigger telescopes have higher resolution. 3. Magnification: the ability to make images bigger. This is the least important property of modern telescopes. A highly magnified blurry image isn’t of much use! 4. Sensitivity: the ability to collect light at wavelengths that the human eye can’t see.

Telescope aperture

Diameter (D) of the main optical element

Resolution

Ability to see detail

What determines telescope resolution?

Diffraction causes light coming from individual points to be spread out more, which reduces our ability to determine fine detail

Charge-Coupled Device (CCD)

Electronic detectors used to record digital images; are divided into pixels; cannot detect color (color achieved with filters which transmit only over narrow range in wavelengths)

The sun is made of

Mostly Hydrogen and Helium

Plasma

A gas of ions and electrons; the physical state of the sun

Corona

The sun’s outer atmosphere; very low density

Chromosphere

The sun’s lower atmosphere; low density

Photosphere

The surface of the sun; where photons can escape; Gas layer

Convective zone

In the sun where photons are trapped by the cooler, less ionized gas; hot gas rises and cool gas sinks carrying photons to the surface

Radiative zone

In the sun where photons carry energy outward but they are absorbed and reemitted many times - they take over 100,000 years to get out

Core

Very hot and dense center of the sun where fusion occurs

Hydrostatic equilibrium

The balance between gravity and pressure; keeps the sun stable for billions of years, not expanding or collapsing; also allows earth to have an atmosphere

Newton’s Universal Law of Gravity

Every mass exerts a force of attraction on ever other mass. The strength of the force is directly proportional to the product of the masses divided by the square of the distance between them

Pressure

The force per unit area exerted on a surface; In a gas, caused by particles colliding, depends on temperature and on density

Sun’s atmosphere

Density decreases, but temperature increases

Solar wind

Formed by rapidly expanding gas in Corona and sun’s magnetic field

Sunspots

Dark blotches on photosphere which contain magnetic fields preventing hot gas from rising to surface; Appear dark due to being cooler than rest of the sun

Prominences

Large loops of glowing solar plasma, trapped by magnetic fields; Relatively unstable; can release vast quantities of plasma into space very quickly

Solar flares

Huge eruptions of hot gas and radiation in the photosphere in the vicinity of sunspots; Thought to result from highly tangled magnetic fields rapidly re-configuring; can trigger coronal mass ejections

Coronal Mass Ejections

Billions of tons of ionized gas is blasted into space

Aurora Borealis

When material from coronal mass ejections reach earth, it is funneled along earth’s magnetic field towards polar regions; It collides with and excites nitrogen and oxygen atoms in the upper atmosphere; When the electrons return to the ground state, they emit a photon producing a green or pinkish glow

Luminosity (L)

Total energy output per second

Source of sun’s luminosity

Fusion of hydrogen into helium in the sun’s core where temperature and density are higher

Thermonuclear fusion

The process of bonding two or more atomic nuclei into a single heavier one; mass can be converted into energy

Isotopes

Hydrogen atoms go through intermediate stages before being converted into helium

Neutrinos

Can use to measure sun’s luminosity; have tiny mass, can travel near speed of light, and easily pass through matter

Solar cycle

The number of sunspots and associated phenomena increase and decrease roughly every 11 years; We think this has to do with the suns rotation. The sun rotates more quickly along the equator than the poles and this winds up the magnetic field; A more distorted magnetic field leads to more sunspots

Baseline

The distance between two points of vision; used to calculate distance

Parallax

The shift of far away objects relative to nearby stars and the earth’s orbit

As stars’ distance increases…

Parallax decreases

Parsecs

Astronomers’ preferred unit to measure distance; equivalent to 206,265 AU, 3.26 Light Years, 3.086x1013 km

Inverse square law for light

If the distance is doubled, the brightness decreases by a factor of four

Standard candle

Some types of stars have the same luminosity, so they can be used to calculate the distance to the neighborhood these stars live in

Magnitude

Describes how bright a star is

Absolute magnitude (M)

The apparent magnitude a star would have if it was located exactly 10 parsecs from earth

Apparent magnitude (m)

A measure of a star’s brightness as seen from earth

How do we find the surface temperature of stars?

We take their spectrum and measure peak wavelength using absorption lines from the photosphere

Classifications of spectra

O-B-A-F-G-K-M (Over Budget Adult Films Give Knights Merriment)

Kepler’s 1st Law of Planetary Motion

The path of each planet around the sun is an ellipse with the sun at one focus.

Kepler’s 2nd Law of Planetary Motion

As a planet moves in its orbit, a line from the sun to the planet sweeps out equal areas in equal times.

Kepler’s 3rd Law of Planetary Motion

The amount of time a planet takes to orbit the Sun (its period) P is related to its orbit’s size a. By P^2 \= a^3 ; So the larger the orbit, the longer period

Binary stars

A pair of stars orbiting around each other

Visual binary

A type of binary star that we can see from images taken over time

Spectroscopic binary

A type of binary stars that are so close together that their spectra blur together, but we see systematic Doppler shift of spectra lines

Eclipsing binary

A type of binary stars where one star will periodically eclipse the other star in the system, changing the total brightness of the system

Center of mass

In a binary star system, both stars orbit around the common center of mass of the system; The center of mass always lies along the line connecting two stars; If the stars are of equal mass, the center of mass is directly between them; If the stars are of unequal mass, the center of mass is closer to the more massive star.

Angular size

apparent angle subtended by an object in the sky, measured in angle (e.g. degree or arcsecond)

Angular resolution

Larger telescopes (or combinations of separated telescopes) can increase

Angular separation (θ)

The resolution of a telescope is characterized by measuring the smallest angular separation that two points of light will have and still appear well separated

Hertzsprung-Russell (HR) Diagram

A scatter plot of stars showing the relationship of stars’ temperatures (x axis, flipped) and their luminosities (y axis)

Main Sequence location

From top left of HR diagram to bottom right

Main sequence

When stars are fusing hydrogen into helium in their cores; remain stable with hydrostatic equilibrium

Giant location

On right side of HR diagram, above main sequence but below supergiants

(Red/Sub) Giant

Core is hot enough to fuse hydrogen into helium in the shell around the core, core isn’t hot enough to turn helium into carbon, star expands, cooler surface temperature, higher luminosity, no hydrostatic equilibrium

Yellow giant

Once core gets hot enough to burn helium into carbon; elements continue to be fused as core gets hotter