# ASTRO Midterm Exam #2: VOCAB (ch. 16-21)

Luminosity
a measure of the light energy emitted into space by stars per second

For stars, the Sun is of average luminosity. For the most part, very dim stars are most common, stars of medium L like the sun are fairly common, and highly luminous stars are very rare.

Luminosity Function
graph that describes how for stars, the Sun is of average luminosity. For the most part, very dim stars are most common, stars of medium L like the sun are fairly common, and highly luminous stars are very rare.

Space Velocity “V”
the speed and direction a star is moving through space

Tangential Velocity – “Vt” –
the component of a star’s velocity that is traveling across the sky plane, OR more importantly perpendicular to our line of sight

the component of a star’s velocity that is traveling parallel to our line of sight, measured from Doppler shifts of the star’s spectral lines

If a star is approaching us, the wavelength decreases (blue shift)

If a star is moving away from us, the wavelength increases (red shift)

Proper Motion
the number of arcseconds that a star appears to move per year on the celestial sphere

Parallax
the apparent displacement of an object caused by the actual change in position of the observer, also the go-to way to measure the distance of the stars. *The closer the object you are viewing, the greater apparent parallax shift*. Most stars in the universe are too far away to accurately measure the angle of apparent shift

Parallax only works for stars that are relatively nearby, and the majority of stars in our galaxy are much too far away to exhibit any apparent shift in position as Earth orbits the Sun.

Stellar parallax
the apparent movement of a nearby star against a background of infinite stars that occurs due to the change in direction from the earth to the star as the Earth orbits the Sun.

Parallax Angle
angle formed by the apparent shift in stellar parallax

Parallax angles are smallest for the most distant stars, so if a parallax angle was measured to be too small then the astronomer would assume the star is farther away than it actually is

Equation for Parallax
D = 1/p

D =distance in parsecs,
p = parallax angle in arcseconds

Hipparcos
High Precision Parallax Collecting Satellite

Satellite created by the European Space Agency that measures parallax angles with an accuracy of up to .001 arcseconds

*If a star’s distance can be known its luminosity can be determined from its apparent brightness*
*If a star’s distance can be known its luminosity can be determined from its apparent brightness*

Brightness (b)
the amount of energy that passes each second through a square meter of a spherical celestial object’s surface area.

Expressed as Watts of power.

Apparent Brightness
same exact thing as brightness.

Concept of Brightness
if you have a star with a brightness of 1000 watts, 1 square meter of a solar panel receives 1000 watts of power from that star

Inverse Square Law
Law of science that describes the inverse relationship between the apparent brightness of a star and the square of its distances.

For example, if the distance between an observer and a star doubles, that is increase by 2 x, the apparent brightness decreases by a factor of 4 (radiation spread over an area that is 4 times larger); If you increase the distance by 3 times your brightness decreases by a factor of 9; 4, 16; 5, 25; you get the idea

Relates a star’s luminosity, distance, and apparent brightness to the corresponding quantities for the Sun
Based on this relationship, we are able to determine the luminosity of a star by comparing the distance of that star and the brightness of that star to the distance of earth to the sun and the apparent brightness of the sun.

A very nearby flashlight will appear much brighter than a very distant spotlight.

Although a large spotlight will emit more light, the light that appears brighter depends on their distances away

Photometry
the process by which scientists measure the apparent brightness of the stars

Apparent Magnitude (brightness)
A measure of a star’s apparent brightness as seen from Earth

Absolute Magnitude (Luminosity)
A true measure of a star’s energy output – The apparent magnitude a star would have if it were located exactly 10 parsecs from earth

The Sun’s absolute magnitude is 14.8

Distance Modulus
The difference between apparent magnitude (m) and absolute magnitude (M).

*= m-M*

The greater the value for distance modulus, the greater distance that star is from earth.

Large discrepancies in absolute and apparent tend to mean the star is comparatively very far from earth

A Star’s color depends on its surface temperature
The intensity of light from a relatively cool star peaks at long wavelengths, making the star look red.

The intensity of light from a relative hot star peaks at short wavelengths, making it look blue.

The shorter the peak wavelength, the higher the temperature, and the higher the frequency.

Blue is always hotter than red

Red stars are relatively cold, with low surface temps; Blue stars are relatively hot, with high surface temps

UBV Photometry
Photometry technique that utilizes the U, B, and V filters in a telescope that are transparent to UV rays, Blue Rays, and Yellow-Green rays respectively.

Color Ratios
ratio that compares the brightness of stars across the UBV filter, set up as Bv/Bb and Bb/Bu. The higher the surface temp, the lower the ratio for both sets.

The Spectra of stars reveal their chemical composition as well as their surface temperatures
-The process and technique in which scientists analyze the light spectrum of an individual star to determine many of that star’s properties is called spectroscopy

-Absorption lines are created when the light flows outward through the upper layers of the star’s atmosphere

-Atoms absorb wavelengths of the radiation at specific levels depending on the specific atom present (hydrogen, helium, etc) and the pattern of each atom’s spectral lines serve as the comparison for new observations (though they vary a lot)

-Due to the large amount of spectra that comes form the stars, similar looking stellar spectra are placed into spectral classes (OBAFGKM; each letter representing a different class)

– O is most similar to B, B is more similar to F than it is to G

-Within a particular spectral class, the larger numbers correspond to cooler stars. For example, a G2 star is hotter than a G8 star because a G2 star is closer to the hotter F-spectral class in the sequence OBAFGKM

-*For a given luminosity, the greater the surface temperature, the smaller the radius will be.

-*For a given surface temperature, the greater the luminosity the larger the radius must be

Hertzsprung-Russell (H-R) Diagrams
A simple yet important graph that plots the luminosities of stars (or their absolute magnitude) against their spectral types (or surface temperatures).

– increasing luminosity on the vertical axis and increasing temperature on the horizontal axis.

The data points are grouped into a few regions throughout the graph showing that luminosity and surface temperature are correlated

Most stars cluster around the red curve called the main sequence

Makes sense if we remember that the majority of stars have comparatively cooler surface temperatures

As the surface temperature increases, the star glows more intensely and the luminosity of that star increases

As the surface temperature decreases, the star will glow less with a much smaller luminosity

Structure:
As you move to the left of the graph, surface temperature increases

As you move up the graph on the y axis, luminosity increases

Star Varieties (Referring to the Hertzsprung-Russell (H-R) Diagrams):

MAIN SEQUENCE

This variety accounts for the majority of stars (about 90%)

Span from the top left of the HR graph to the bottom right

Gain their energy through the conversion of hydrogen to helium in their cores

The Sun is a main sequence star

Stars like our Sun derive their energy from nuclear reactions in the core, whereas a brown dwarf derives its energy from gravitational contraction.

Star Varieties (Referring to the Hertzsprung-Russell (H-R) Diagrams):

GIANTS

Upper right hand corner of the HR graph; are Luminous and Cool

Since they are cool, they are much less bright (emit less energy). So, in order for these cool stars to be so luminous, they must be incredibly large (Giants)

10 – 100x larger than our Sun.

Between 3000 and 6000K surface Temperature

The cooler giants (3000-4000K) are called Red Giants as they appear red

Super-Giants
Rare stars that are bigger and brighter than red giants with radii of up to 1000x that of our Sun

Together, giants and super-giants make up about 1% of the stars in the sky

White Dwarfs
located in the lower left hand corner of the HR diagram
This means they are extremely hot, but also have very low luminosity, meaning they must be very small aka dwarfs

About the same size as earth

No thermonuclear reactions occur in their cores
Glowing remnants of what used to be giant stars

Brown Dwarfs
lower right hand side of the HR diagram

Cool with low luminosity

Never will become stars, not massive enough for significant fusing of hydrogen in their core

Gets energy from gravitational contraction, not a true star

Details of a star’s spectrum reveal whether it is a giant, a white dwarf, or a main sequence star
The differences between the spectra of stars of different luminosity are due to differences between the stars’ atmospheres which is where absorption lines are produced.

The higher the density and pressure, the more hydrogen collision, the broader the hydrogen spectral lines are

Typically, the smaller the star (comparatively), the higher the density and pressure is, meaning broad and clear absorption lines

Luminosity broken down into 5 classes I – V. The higher the class, the lower the average luminosity for a given surface temperature

If 2 stars have identical surface temperatures, but 1 has a much larger luminosity, you can assume that that star must be of a much more massive size to emit its higher levels of luminosity.

Spectroscopic Parallax
The method for determining the distance of a star using its spectral class, luminosity class, and location of the H-R diagram

Deduce using the formula for the relationship between luminosity, distance, and apparent brightness

No actual parallax involved, just similar because it answers a question of distance

Helpful to make claims about stars that are simply too far to observe true parallax.

H-R diagram plays a big role in determining the luminosity of a star from its spectral type and luminosity class

Can then deduce property of distance from that information

Key Formulas:
L = 4pid^2b
L = 4piR^2T^4

Binary Stars
Stars that orbit each other because of their mutual gravitational attraction

Helps us determine the weight of these stars using Kepler’s 3rd law

Visual Binaries – Binary pairing that you can actually see orbiting each other

Have extremely long orbital periods

Center of Mass
the center of the elliptical orbit of binary stars; a completely imaginary point that lies along a line connecting the 2 stars

The center of mass in a binary star system is always nearer to the more massive star

The two stars are always on opposite ends of the center of mass, so they never collide

If two stars orbiting a common center of mass were moved farther apart, their masses would not change, but the period would increase following Kepler’s 3rd law

22-year solar cycle
The semiregular 22-year interval between successive appearances of sunspots at the same latitude and with the same magnetic polarity

chromosphere
A layer in the atmosphere of the Sun between the photosphere and the corona

CNO cycle
A series of nuclear reactions in which carbon is used as a catalyst to transform hydrogen into helium

conduction
The transfer of heat by directly passing energy from atom to atom

convection
The transfer of energy by moving currents of fluid or gas containing that energy

convective zone
The region in a star where convection is the dominant means of energy transport

corona
The Sun’s outer atmosphere, which has a high temperature and a low density

coronal hole
A region in the Sun’s corona that is deficient in hot gases

coronal mass ejection
An event in which billions of tons of gas from the Sun’s corona is suddenly blasted into space at high speed

differential rotation
The rotation of a nonrigid object in which parts adjacent to each other at a given time do not always stay close together

filament
A portion of the Sun’s chromosphere that arches to high altitudes

granulation
The rice grain-like structure found in the solar photosphere

granule
A convective cell in the solar photosphere

helioseismology
The study of the vibrations of the Sun as a whole

hydrogen fusion
The thermonuclear conversion of hydrogen into helium

hydrostatic equilibrium
A balance between the weight of a layer in a star and the pressure that supports it

limb darkening
The phenomenon whereby the Sun looks darker near its apparent edge, or limb, than near the center of its disk

luminosity (of the Sun)
The rate at which electromagnetic radiation is emitted from a star or other object

magnetic reconnection
An event where two oppositely directed magnetic fields approach and cancel, thus releasing energy

magnetic-dynamo model
A theory that explains the solar cycle as a result of the Sun’s differential rotation acting on the Sun’s magnetic field

magnetogram
An image of the Sun that shows regions of different magnetic polarity

negative hydrogen ion
A hydrogen atom that has acquired a second electron

neutrino
A subatomic particle with no electric charge and very little mass, yet one that is important in many nuclear reactions

neutrino oscillation
The spontaneous transformation of one type of neutrino into another type

photosphere
The region in the solar atmosphere from which most of the visible light escapes into space

plage
A bright region in the solar atmosphere as observed in the monochromatic light of a spectral line

plasma
A hot ionized gas

positron
An electron with a positive rather than negative electric charge; the antiparticle of the electron

prominence
Flamelike protrusions seen near the limb of the Sun and extending into the solar corona

proton-proton chain
A sequence of thermonuclear reactions by which hydrogen nuclei are built up into helium nuclei

The random migration of photons from a star’s center toward its surface

A region within a star where radiative diffusion is the dominant mode of energy transport

solar flare
A sudden, temporary outburst of light from an extended region of the solar surface

solar neutrino
A neutrino emitted from the core of the Sun

solar neutrino problem
The discrepancy between the predicted and observed numbers of solar neutrinos

solar wind
An outward flow of particles (mostly electrons and protons) from the Sun

space weather
Variations in the solar wind and magnetic field, which can affect satellites and astronauts

spicule
A narrow jet of rising gas in the solar chromosphere

sunspot
A temporary cool region in the solar photosphere

sunspot cycle
The semiregular 11-year period with which the number of sunspots fluctuates

sunspot maximum
That time during the sunspot cycle when the number of sunspots is highest

sunspot minimum
That time during the sunspot cycle when the number of sunspots is lowest

supergranule
A large convective feature in the solar atmosphere, usually outlined by spicules

thermal equilibrium
A balance between the input and outflow of heat in a system

thermonuclear fusion
The combining of nuclei under conditions of high temperature in a process that releases substantial energy

Zeeman effect
A splitting or broadening of spectral lines due to a magnetic field

absolute magnitude
The apparent magnitude that a star would have if it were at a distance of 10 parsecs

apparent brightness
The flux of a star’s light arriving at Earth

apparent magnitude
A measure of the brightness of light from a star or other object as measured from Earth

binary star
Two stars orbiting each other

brown dwarf
A starlike object that is not massive enough to sustain hydrogen fusion in its core

center of mass
The point between a star and a planet, or between two stars, around which both objects orbit

color ratio
The ratio of the apparent brightness of a star measured in one spectral region to its brightness measured in a different region

distance modulus
The difference between the apparent and absolute magnitudes of an object

double star
A pair of stars located at nearly the same position in the night sky. Some, but not all, double stars are binary stars

eclipsing binary
A binary star system in which, as seen from Earth, stars periodically pass in front of each other

giant
A star whose diameter is typically 10 to 100 times that of the Sun and whose luminosity is roughly that of 100 Suns

Hertzsprung-Russell diagram (H-R diagram)
A plot of the luminosity (or absolute magnitude) of stars against their surface temperature (or spectral type)

inverse-square law
The statement that the apparent brightness of a light source varies inversely with the square of the distance from the source

light curve
A graph that displays how the brightness of a star or other astronomical object varies over time

luminosity
The rate at which electromagnetic radiation is emitted from a star or other object

luminosity class
A classification of a star of a given spectral type according to its luminosity

luminosity function
The numbers of stars of differing brightness per cubic parsec

magnitude scale
A system for denoting the brightnesses of astronomical objects

main sequence
A grouping of stars on the Hertzsprung-Russell diagram extending diagonally across the graph from hot, luminous stars to cool, dim stars

main-sequence star
A star whose luminosity and surface temperature place it on the main sequence on an H-R diagram; a star that derives its energy from core hydrogen fusion

mass-luminosity relation
A relationship between the masses and luminosities of main-sequence stars

metals
In reference to stars and galaxies, any element other than hydrogen and helium; in reference to planets, a material such as iron, silver, and aluminum that is a good conductor of electricity and of heat

OBAFGKM
The temperature sequence (from hot to cold) of spectral classes

optical double star
Two stars that lie along nearly the same line of sight but are actually at very different distances from us

parallax
The apparent displacement of an object due to the motion of the observer

parsec
A unit of distance; 3.26 light-years

photometry
The measurement of light intensities

proper motion
The angular rate of change in the location of a star on the celestial sphere, usually expressed in arcseconds per year

That portion of an object’s velocity parallel to the line of sight

A plot showing the variation of radial velocity with time for a binary star or variable star

red giant
A large, cool star of high luminosity

space velocity
The speed and direction in which a star moves through space

spectral classes
A classification of stars according to the appearance of their spectra

spectral type
A subdivision of a spectral class

spectroscopic binary
A binary star system whose binary nature is deduced from the periodic Doppler shifting of lines in its spectrum

spectroscopic parallax
The distance to a star derived by comparing its apparent brightness to a luminosity inferred from the star’s spectrum

spectrum binary
A binary star whose binary nature is deduced from the presence of two sets of incongruous spectral lines

stellar parallax
The apparent displacement of a star due to Earth’s motion around the Sun

supergiant
A very large, extremely luminous star of luminosity class I.

tangential velocity
That portion of an object’s velocity perpendicular to the line of sight

UBV photometry
A system for determining the surface temperature of a star by measuring the star’s brightness in the ultraviolet (U), blue (B), and visible (V) spectral regions

visual binary
A binary star in which the two components can be resolved through a telescope

white dwarf
A low-mass star that has exhausted all its thermonuclear fuel and contracted to a size roughly equal to the size of Earth

accretion
The gradual accumulation of matter in one location, typically due to the action of gravity

Barnard object
One of a class of dark nebulae discovered by E. E. Barnard

bipolar outflow
Oppositely directed jets of gas expelled from a young star

Bok globule
A small, roundish, dark nebula

circumstellar accretion disk
An accretion disk that surrounds a protostar

cluster (of stars)
A group of stars that formed together and that have remained together because of their mutual gravitational attraction

cocoon nebula
The nebulosity surrounding a protostar

dark nebula
A cloud of interstellar gas and dust that obscures the light of more distant stars

dust grain
A microscopic bit of solid matter found in interplanetary or interstellar space

emission nebula
A glowing gaseous nebula whose spectrum has bright emission lines

evolutionary track
The path on an H-R diagram followed by a star as it evolves

fluorescence
A process in which high-energy ultraviolet photons are absorbed and the absorbed energy is radiated as lower-energy photons of visible light

giant molecular cloud
A large cloud of interstellar gas and dust in which temperatures are low enough and densities high enough for atoms to form into molecules

H II region
A region of ionized hydrogen in interstellar space

Herbig-Haro object
A small, luminous nebula associated with the end point of a jet emanating from a young star

interstellar extinction
The dimming of starlight as it passes through the interstellar medium

interstellar medium
Gas and dust in interstellar space

interstellar reddening
The reddening of starlight passing through the interstellar medium as a result of blue light being scattered more than red

nebula
A cloud of interstellar gas and dust

OB association
A grouping of hot, young, massive stars, predominantly of spectral types O and B

open cluster
A loose association of young stars in the disk of our Galaxy; a galactic cluster

protoplanetary disk
A disk of material encircling a protostar or a newborn star

protostar
A star in its earliest stages of formation

recombination
The process in which an electron combines with a positively charged ion

reflection nebula
A comparatively dense cloud of dust in interstellar space that is illuminated by a star

stationary absorption lines
An absorption line in the spectrum of a binary star that does not show the same Doppler shift as other lines, indicating that it originates in the interstellar medium

stellar association
A loose grouping of young stars

stellar evolution
The changes in size, luminosity, temperature, and so forth that occur as a star ages

supernova remnant
The gases elected by a supernova

supersonic
Faster than the speed of sound

T Tauri star
Young variable stars associated with interstellar matter that show erratic changes in luminosity

alpha particle
The nucleus of a helium atom, consisting of two protons and two neutrons

Cepheid variable
A type of yellow, supergiant, pulsating star

close binary
A binary star system in which the stars are separated by a distance roughly comparable to their diameters

color-magnitude diagram
A plot of the apparent magnitudes (that is, apparent brightnesses) of stars in a cluster versus their color indices (a measure of their surface temperatures)

contact binary
A binary star system in which both members fill their Roche lobes

core helium fusion
The thermonuclear fusion of helium at the center of a star

core hydrogen fusion
The thermonuclear fusion of hydrogen at the center of a star

degeneracy
The phenomenon, due to quantum mechanical effects, whereby the pressure exerted by a gas does not depend on its temperature

degenerate-electron pressure
The pressure exerted by degenerate electrons

detached binary
A binary star system in which neither star fills its Roche lobe

globular cluster
A large spherical cluster of stars, typically found in the outlying regions of a galaxy

helium flash
The nearly explosive beginning of helium fusion in the dense core of a red giant star

helium fusion
The thermonuclear fusion of helium to form carbon and oxygen

horizontal-branch star
A low-mass, post-helium-flash star on the horizontal branch

ideal gas
A gas in which the pressure is directly proportional to both the density and the temperature of the gas; an idealization of a real gas

inner Lagrangian point
The point between the two stars constituting a binary system where their Roche lobes touch; the point across which mass transfer can occur

instability strip
A region of the H-R diagram occupied by pulsating stars

long-period variable
A variable star with a period longer than about 100 days

The total time that a star spends fusing hydrogen in its core, and hence the total time that it will spend as a main-sequence star

mass loss
A process by which a star gently loses matter

mass transfer
The flow of gases from one star in a binary system to the other

metal-poor star
A star that, compared to the Sun, is deficient in elements heavier than helium; also called a Population II star

metal-rich star
A star whose abundance of heavy elements is roughly comparable to that of the Sun; also called a Population I star

open cluster
A loose association of young stars in the disk of our Galaxy; a galactic cluster

overcontact binary
A close binary system in which the two stars share a common atmosphere

Pauli exclusion principle
A principle of quantum mechanics stating that no two electrons can have the same position and momentum

period-luminosity relation
A relationship between the period and average density of a pulsating star

Population I star
A star whose spectrum exhibits spectral lines of many elements heavier than helium; a metal-rich star

Population II star
A star whose spectrum exhibits comparatively few spectral lines of elements heavier than helium; a metal-poor star

pulsating variable star
A star that pulsates in size and luminosity

red dwarf
A main-sequence star with a mass between about 0.08 solar mass and 0.4 solar mass which has a fully convective interior and which never goes through a red giant stage

red giant
A large, cool star of high luminosity

Roche lobe
A teardrop-shaped volume surrounding a star in a binary inside which gases are gravitationally bound to that star

RR Lyrae variable
A type of pulsating star with a period of less than one day

semidetached binary
A binary star system in which one star fills its Roche lobe

shell hydrogen fusion
The thermonuclear fusion of hydrogen in a shell surrounding a star’s core

triple alpha process
A sequence of two thermonuclear reactions in which three helium nuclei combine to form one carbon nucleus

turnoff point
The point on an H-R diagram where the stars in a cluster are leaving the main sequence

zero-age main sequence
The main sequence of young stars that have just begun to burn hydrogen at their cores

zero-age main-sequence star
A newly formed star that has just arrived on the main sequence

antiparticle
A particle with the same mass as an ordinary particle but with other properties, such as electric charge, reversed

black hole
An object whose gravity is so strong that the escape speed exceeds the speed of light

black hole evaporation
The process by which black holes emit particles

collapsar
A proposed type of supernova in which a black hole forms at the center of a dying star before the outer layers of the star have time to collapse

equivalence principle
In the general theory of relativity, the principle that in a small volume of space, the downward pull of gravity can be accurately and completely duplicated by an upward acceleration of the observer

ergoregion
The region of space immediately outside the event horizon of a rotating black hole where it is impossible to remain at rest

event horizon
The location around a black hole where the escape speed equals the speed of light; the surface of a black hole

gamma-ray burst
Objects found in all parts of the sky that emit a one-time intense burst of high-energy radiation

general theory of relativity
A description of gravity formulated by Albert Einstein. It states that gravity affects the geometry of space and the flow of time

Oscillations of space produced by changes in the distribution of matter

gravitational redshift
The increase in the wavelength of a photon as it climbs upward in a gravitational field

Light and particles made of matter and antimatter that emanate away from a region just outside a black hole’s event horizon. The radiation occurs when one member of a virtual particle pair is captured by the black hole; the other member of the pair becomes a real particle and carries away some of the black hole’s energy (and therefore, some of its mass). This loss of energy leads to black hole evaporation

Heisenberg uncertainty principle
A principle of quantum mechanics that places limits on the precision of simultaneous measurements

intermediate-mass (mid-mass) black hole
A black hole with a mass of hundreds of Suns

length contraction
In the special theory of relativity, the shrinking of an object’s length along its direction of motion

Lorentz transformations
Equations that relate the measurements of different observers who are moving relative to each other at high speeds

no-hair theorem
A statement of the simplicity of black holes

primordial black hole
A type of black hole that may have formed in the very early universe

proper length (proper distance)
A length measured by a ruler at rest with respect to an observer

proper time
A time interval measured with a clock at rest with respect to an observer

The distance from the singularity to the event horizon in a nonrotating black hole

singularity
A place of infinite space-time curvature; the center of a black hole…….. Equations describing the matter within the event horizon indicate that the object’s entire mass is crushed nearly to a single point… That single point is the SINGULARITY (which is at the center of the Black Hole)

spacetime
A four-dimensional combination of time and the three dimensions of space

special theory of relativity
A description of mechanics and electromagnetic theory formulated by Albert Einstein, which explains that measurements of distance, time, and mass are affected by the observer’s motion

stellar-mass black hole
A black hole with a mass comparable to that of a star

supermassive black hole
A black hole with a mass of a million or more Suns

time dilation
The slowing of time due to relativistic motion

virtual pair
A particle and antiparticle that exist for such a brief interval that they cannot be observed

Gravitational Redshifts
caused by time flowing at different rates at different locations

No motion is involved