Astronomy Exam 3

Density between stars

less dense than the best vacuum we can make on earth—very few atoms.

Interstellar Dust

dust particles are roughly the size of the wavelength of visible light, so it is mostly blocked, but infared radiation (just larger) go right through—results in a dark cloud with “reddish” stars at edges

Preferential scattering

happens at the edges of a dust cloud, or at the horizon at sunset: most blue light is scattered, and red light (at a longer wavelength than dust particles) sometimes makes it through

Interstellar gas

90% hydrogen, 9% helium

Types of nebulae

Emission nebulae= glows reddish due to the radiation from a hot star within the cloud

Dark nebulae= dust cloud

Reflection nebulae=light from surrounding stars bounces off cloud particles to create blue color due to preferential scattering

Dark nebulae

Very cold

Absorb visible light

Emit radio waves

Have strong CO emission lines

Sculpting of dust lanes

radiation blows back less dense dust and leaves just the unique shape of the high-density dust within it

Importance of atomic hydrogen in interstellar gas

when the hydrogen transitions from the electron and proton being in “parallel spin” to “anti-parallel” spin, which reduces it’s energy—causing a lower energy wavelength longer than the typical size of interstellar dust particles.

So, this hydrogen radiation reaches earth unaffected by stellar debris

Molecular Clouds

regions of interstellar gas between 10-20 K, where density is a bit higher due to most gas particles are molecules—only longer wavelength radio waves can escape

Molecular Clouds

H2 (molecular hydrogen)

CO (carbon monoxide)

H2O (water

H2CO (formaldehyde)

The last 3 emit radio waves as they are created by chemical processes within the clouds—called “Tracer molecules”

Ratio of Hydrogen to tracer molecules in molecular clouds

1 billion: 1

Molecular cloud complexes

huge groups of molecular clouds, 10s of parsecs across

Triggers star formation

Some kind of external event causes a molecular cloud to lose it’s hydrostatic equilibrium and gravity dominates over heat, causing it to contract and collapse on itself. 

Seven stages of stellar evolution

1)Interstellar Cloud—begins to collapse and fragment

2) & 3) Contracting Cloud fragment—temp and density increases rapidly

4) Protostar—shrinks as density and temp increase to 1,000,000 K, ignites the proton-proton chain

5) Protostellar Evolution—outward directed pressure grows, heat escapes and contraction slows, luminosity decreases, gas is ionized and temp continues to increase. Strong winds result in bipolar flow and ejects matter perpendicularly

6) Newborn Star—10 million years after stage 4, radius decreases and temp increases. Proton-proton chain begins producing helium, luminosity less than the sun, and radiation is absorbed by dust and reemitted as infrared

7) Main Sequence Star—hydrostatic equilibrium reached, energy emitted is stable

Brown Dwarfs

“Failed stars” whose original fragments were too small, so hydrostatic equilibrium was achieved before temp became high enough to start nuclear fusion—very cool and smaller

Star clusters

a group of stars that all formed from the same parent cloud

Open cluster

type of star cluster that is loose and irregular (often found on Milky Way disc)

Associations

star clusters that are smaller than open clusters, but more spread out and expanding

Globular clusters

spherical star clusters with millions of stars, not found in Milky Way, lack upper-main sequence stars and are the oldest ones

Average lifetime of a star

10 billion year

How stars maintain hydrostatic equilibrium

Due to law of reciprocity:

1) Small decreases in central temp lead to small decreases in pressure—so star contracts and heats

2) Small increase in central temp, increase in pressure, star expands and cools

Hydrogen Shell Burning

nuclear fusion is relegated to the outer surroundings of the core as unburnt helium builds up in the core and less hydrogen is produced there

Red Giant Branch 

Star’s core shrinks and the outer layer expand and cool. It’s luminosity increases due to increased surface area. (stage 9)

Subgiant Branch

Process leading up to stage 9—star’s core cools and shrinks

Helium flash

Stage 10: attempted ignition of the helium in the star when it’s temp has gotten hot enough again after being a Red Giant, but it does not have enough hydrogen fuel. The flash only lasts a few hours. Star reaches equilibrium

Asymptotic Giant Branch

Stage 11: star becomes a Red Giant again! hydrogen burning shell and a helium burning shell

Planetary Nebula

Stage 12: Planetary nebula occur, as it contracts and increases pressure in the core. Outer layers are blown off to form nebula.

White Dwarf

The core of the blown—off neublae: it can’t become hot enough for fusion of elements heavier than oxygen to take place, so it becomes a white dwarf as it slowly cools down over billions of years. Any luminosity comes only from heat.

Black dwarf

eventually, a white dwarf gets dimmer and cooler, and eventually stops glowing. Like a dark coal in space.

Novas

usually in a double star system—the white dwarf pulls the outer atmosphere of it’s main sequence companion into its orbit and it forms a bright accretion disk with explosive hydrogen burning. Causes a brief, sudden flare up of brightness, and then dimming again. Some novas cycle several times.

Higher mass star’s on H-R Diagram

loop back and forth going upward and right—luminosity stays relatively stable but radii and temp go up and down

Red Supergiant

very high mass stars achieve this—fuse heavier elements like magnesium, silicon, neon, and iron in the inner core

Photodisintegration

as a red supergiant dies and implodes, temps reach 10 billion K and individual photons break iron into smaller and smaller nuclei until only protons and neutrons remain

Main sequence Turnoff

the high-luminosity end of a star’s journey on the main sequence on an H-R diagram

Supernova

occurs when a supergiant’s core collapses and blasts all overlying material into space with a luminosity billions of times brighter than the sun’s in it’s entire lifetime

Type I supernova

Carbon-Detonation Supernova: little hydrogen, and it’s light curve shows a sharp rise of intensity followed by a gradual decline

Type II Supernova

hydrogen rich, and include a plateau in their light curve for a few months after reaching max luminosity—they leave remnants in space, and often result in neutron stars/black holes

Neutron star

can sometimes result from a Type II supernova, if the ball of ultra compressed neutrons in the very center of the core remains after the implosion

Properties of a neutron star

—tiny (size of a city)

—very massive (more than the sun!)

—rotate every fraction of a second

—very strong magnetic field

—can be pulsars

Pulsar

a neutron star that periodically flashes it’s radio radiation towards earth

Lighthouse model

model for pulsars—two hot spots on either side of neutron star diagonal to the rotation axis, as it rotates we get “flashes” of radio waves coming towards our spot in space

Pulsar wind

hot, x-ray emitting gas flowing out of neutron star’s equator at the speed of light

Pulsar/ neutron star relationship

all pulsars are neutron stars, but not all neutron stars are pulsars (due to old age and diminishing magnetic field or rotation rate)

Neutron star binaries

neutron stars (including pulsars) that are in a binary system—can exhibit X-ray activity

X-Ray bursters

occurs on or near neutrons that are part of a binary system—neutron star pulls material from companion’s surface and forms an accretion disk for several seconds as nuclear fusion sputters up and then dies down

Pulsar planets

some millisecond pulsars (very fast ones) who exist in globular clusters show Doppler Shifts and regular intervals that indicate the pull of several planets OUTSIDE our solar system on them

Gamma Ray Bursts

fireballs in space beyond our galaxy that produce expanding superhot jets of gas emitting gamma ray radiation for a brief time—afterglow occurs as fireball expands and cools again. occur about once a day

Theories on Gamma Ray Bursts

1) they are the result of a binary neutron star system in which they merge

2) they are hypernovas in which the core itself collapses, causing a black hole with an erupting accretion disk

Black holes

when neutron stars (whose main sequence mass was more than 25 suns) compress beyond the “set limit” of 3x the sun’s mass, they exert such an intense gravitational pull on their surroundings that any light, radiation or information of any kind from it disappears.

Black hole’s escape speed

would be faster than the speed of light—thus, nothing in existence could escape being pulled into the black hole, and no information would exist about it. It is invisible and uncommunicative, with only a single point of gravity betraying it

Schwarzchild radius

the radius a specific object would need to compress to to become a black hole (every object has one)

Event horizon

the imaginary sphere “demarking” the edges of a black hole event—nothing from within that sphere’s surface can be perceived in any way

Michelson-Morely findings

the speed of light is always the same, no matter the relative position of the measurer or the source

What can explain black holes

only Einstein’s theory of general relativity ( integrated from his special relativity, and assuming that spacetime is curved, and gravitational pull is not a real agent).

Black holes

any matter that enters the event horizon of a black hole will be flattened, torn apart, disappear, and be unable to every get out

Gravitational redshift

photons passing nearby a black hole’s event horizon do not slow down, but lose energy—causing their wavelengths to become smaller and smaller, eventually going off the known spectrum past radio. Theoretically, light emitted from on the event horizon itself would be red-shifted to infinitely long wavelengths

Horizontal branch

part of the H-R diagram when large mass stars move from burning hydrogen to burning helium—but not yet carbon

Andromeda galaxy

the nearest galaxy to ours (800 thousand pcs away)—with similar disk, bulge, and halo.

Variable stars

stars whose luminosities change significantly over short periods of time—often the result of binary systems, but sometimes intrinsic to the star itself

Pulsating variable stars

like RR Lyrae and Cepheid— stars that vary cyclically in their luminosity

RR Lyrae stars

pulsating variable stars that are on the low (er) mass end of the horizontal branch—so have started core helium burning—with regular pulses every 0.5-1 day. They have a luminosity of ~ 100x the sun

Cepheid stars

pulsating variables that pulse in a “sawtooth pattern” and have very different periods between pulses (anywhere from 1-100 days)

Period-luminosity relationship

used to infer luminosity of Cepheid stars:

those that vary slowly have the highest luminosity, and those that vary more quickly have lower luminosities.

Discovery of the galactic halo

knowing that many RR Lyrae stars are in globular clusters, were able to map out distances of all known globular clusters and found that they lie on a sphere surrounding the milky way galaxy

Galactic center

not the sun! it is the saggitarius constellation, about 8 K pcs away from the sun

Dust/gas distribution in Milky Way

virtually NONE in galactic halo—in bulge and disk, very common

Population I stars

Stars in the galactic disk—younger, brighter and bluer with more heavy elements

Population II stars

stars in the galactic halo and bulge= older, redder stars with less heavy elements

Differential Rotation of Galaxy

when looking at the area of the galaxy around our sun—stars to the upper right and lower left are blueshifted (moving TOWARDS the sun) and the stars in the lower right and upper left are redshifted (RECEEDING from the sun)

Rotation of Galaxy

in bulge and halo, stars do not share the same differential rotation patten of the disk. Each of them has a random orbit, all filling up a 3D spherical volume

Spiral Arms

pinwheel like sturctures originating close to the galactic bulge (one of which contains our solar system)—each one is 30K pcs across and filled with emission nebulae, O- and B- type stars, and open clusters

Spiral Density Waves

coiled compression waves that move through the arms of the galactic disk, squeezing gas clouds and triggering star formation

Theories on the cause of Spiral Density Waves

1) instabilities in gas near the galactic bulge

2) Gravitational effects of nearby galaxies

3) Elongated shape of the galactic bulge itself

Formula for mass of Milky Way

total mass (s.m)= orbit size (AUs)³// orbital period (yrs)²

Mass of Milky Way

4x 10^11 solar masses, with half of the matter in the Galaxy existing as dark matter beyond the luminous galactic disk

Dark Halo

the extensive region of dark matter surrounding the inner Galactic Halo (with the ancient stars)—accounts for half the Galaxy’s mass

Dark Matter

has mass but emits no electromagnetic radiation, has no detectable elements, and cannot be accounted for by black holes because there simply aren’t enough of those

Subatomic Particle Theory

Dark matter is made up of subatomic particles which were produced in huge amounts during the Big Bang (Creation). They must:

-have mass

-but NOT interact with normal matter or else we would see it

Gravitational Lensing

Explains ½ of galactic dark matter—when looking at an invisible object far away, a dim foreground object will become suddenly brighter when passing in front of it —a way to “see” the effects of dark matter indirectly

Observation of Galactic Center

cannot see visually because interstellar material obstructs our view—must use RADIO and INFARED

Innermost Parsec of Milky Way

densely populated by (in order):

- a cluster of 1 million stars

-huge dust-rich clouds

-400 pc rotating ring of molecular gas

-Saggitarius A region of strong radio waves and powerful magnetic field lines

-Hot X-ray emitting gas from “supernova-like” remnant

-1 pc ring of star-forming molecular gas with streams of matter spiralling inward

-An accretion disk that accelerates particles into “cosmic rays” which is caused by………

-A several MILLION solar mass supermassive Black Hole!

Central Balck Hole

studied by looking at Sgr A*, a compact, relatively low-energy nucleus (still 1 millionX sun) around 10 AU across, powered by the supermassive black hole whose event horizon is only  .08 AU across

Hubble Classification Scheme

classifies galaxies according to appearance:

-Spiral galaxies (Milky Way, Andromeda)

-Barred Spirals

-Ellipticals

-Irregulars

Barred Spiral Galaxies

spiral arms extend from the ends of the huge “bar” of interstellar matter crossing the galaxy: SBa are smallest, SBc are largest

Elliptical Galaxies

no spiral arms or visible disk—just a dense central nucleus and an ellipse shaped ring of stars and interstellar matter—E0= most circular, up to E7, most elongated. They range hugely in size

Irregular Galaxies

a “catch-all” category for all other galaxies

-rich in matter/ young blue stars

-lacks defined structure

-small ones most common

-often found close to “parent galaxy”

-2 subclasses

Irr I Galaxies

irregular galaxies that look like misshapen spirals that contain a lot of gas, dust, supernovas (ongoing star formation) and blue stars

Irr II Galaxies

very rare, with explosive/filamentary appearance—could be result of near collision between two “normal” systems

Hubble Sequence

a diagram of all types of galaxies as outlined by Edwin Hubble:

Standard candles

a bright, easily recognizable object whose luminosity is known (like a planetary nebula or Type I Supernova)—which are used to estimate distances to very distant galaxies

Tully-Fisher Relation

obtaining an estimate of a galaxy’s luminosity from it’s rate of rotation, in order to calculate distance

Extragalactic distance ladder

Local Group

Our galaxy cluster—contains Milky Way and Andromeda as biggest members, plus 50 others

-over 1 Mpc in diameter

Virgo Cluster

galaxy cluster nearest (18 Mpc) to our Local Group

-3 Mpc across

-2,500 galaxies

Hubble Diagrams

plot galaxies’ recessional velocity away from us by their distance from us—shows us that all galaxies are part of a general motion away from us in all direction

Hubble’s Law

the rate at which a galaxy is receding from us is directly proportional to it’s distance from us

Hubble’s constant

recessional velocity= H0 x distance

H0= 70 km/s/Mpc

Active Galaxies

galaxies that look normal, but radiate large amounts of non-stellar radiation in the invisible spectrum

Starburst galaxies

previously normal galaxies that now have intense periods of star formation from an active galactic nuclei—include Seyfert, Radio, and Quasar galaxies

Seyfert galaxies

a type of starburst galaxy that resembles a normal spiral galaxy but has a nucleus up to 10X as bright as our entire galaxy—luminosity can halve or double within the span of a year. Most energy is in the infrared.