astro 7n exam 2

hydrogen (73 %, by mass) and helium (25 %, by mass)

a balance of gravity (inward force) and gas pressure in its hot interior (outward force)

nuclear fusion reactions

15 million degrees Kelvin

Diameter is 109 x Earth's; mass is 333,000 x Earth's

4 × 10^24, 100 Watt light bulbs

the sun

Sun has been shining for 4.5 billion years, and will continue for about another 5.5 billion years

Sunspots are slightly-cooler regions on the Sun’s surface due to magnetic activity preventing hot material from rising in that region; have 11 year cycles

a long lasting energy source for stars

make helium-4 (2 protons and 2 neutrons) and release energy in gamma rays aka The PROTON-PROTON CHAIN

Step 1 : two protons collide at very high speed, and stick together; one of them changes into a neutron; ends with a Deuterium nucleus ( 2H : 1 proton and 1 neutron, bound together), plus released energy \n Step 2: Deuterium nucleus from Step 1 collides with another proton, and makes a Helium-3 nucleus ( 3He : 2 protons and 1 neutron, bound together), plus some more excess energy released \n Step 3: two Helium-3 nuclei combine to make a Helium-4 nucleus ( 4He : 2 protons and 2 neutrons, bound together), releasing back 2 protons in the process and some more extra energy

Although it requires a lot of energy initially to cause the high-speed collisions between protons and nuclei during each step of the "p-p chain," a little bit of extra energy is generated and released in every step

E = m c^2

less, the mass is converted into energy (gamma-ray photons)

core, radiative zone, convective zone, photosphere, chromosphere, flare, prominence, corona, solar wind

at the center; high density and temperature; where nuclear reactions occur and gamma rays are produced

photons are repeatedly re-absorbed and re-emitted; the energy of an individual photon can take on average 170,000 years to pass through

hot gas rises and cold gas sinks; light traverses in about 1 week

temperature 5,780 K; this is the "surface" of the Sun that we see; photons have been converted to visible wavelengths; can see "granules" due to convection bringing material up and down in cells

red or orange color; temperature about 4,500 K; we see through this, down to the photosphere

an eruption coming out of Sun due to magnetic activity

a hoop-shaped eruption out of Sun due to magnetic activity

low density; temperature about 1 million K; visible during solar eclipses

charged particles coming from Sun’s surface, escaping to deep space; the solar wind permeates the whole Solar System

L, is the absolute power output, at the source (e.g., a star's surface)

B, is apparent output, as observed at some distance ( "d " ) away

the Inverse-Square Law determines how bright a star appears, based upon its luminosity (intrinsic brightness) and its distance; \n B = L / 4 𝝅 d^2 or B ∝ L / d 2

it will appear 1/10^2 times as bright (that is, 100 times fainter)

to view a star from two locations on opposite sides of Sun (Earth, but 6 months apart), and look for minute changes in its apparent position.

These locations in Earth's orbit (every 6 months) (the baseline is 1 au, the distance from the sun to the earth)

by an angle 2 times the parallax angle; so, ... \n D = 1 / p

D is distance is parsecs and p is the parallax angle is arcseconds (1/ 3600 degree)

- a star with measured parallax angle of 0.1 arcsec is 1 / 0.1 = 10 parsecs away \n - a star with a parallax angle of 0.02 arcsec is 1 / 0.02 = 50 parsecs away

closer, because the star that appears brighter is actually more luminous

Diagram of luminosity to temperature (white dwarf under main sequence line, red giants over main sequence line, sun on main sequence line at 1 R)

stars' surface temperature, increasing from right to left (cooler stars are red temps down to around 2300K, hotter stars bluer temps up to around 40000K)

O-B-A-F-G-K-M (hotter to cooler)

“__O__h, __b__e a __f__ine __g__irl/guy, __k__iss __m__e” (or, “__O__nly __b__ored __a__stronomers __f__ind __g__ratification __k__nowing __m__nemonics”)

absorption spectrum, where absorption lines from different chemical elements with different levels of ionization arise at different temperatures

luminosity, expressed in terms of the luminosity of the Sun ( "L⊙" )

Alpha Centauri is the closest star, 4.3 light years (or 1.35 parsecs) from the Sun; actually part of a triple star system, with its brightest member similar to Sun

cool and dim, and fall on the lower right of the H-R diagram; this is because most stars in general have these properties

Sirius (the “Dog Star”), which is twice as massive as Sun; it has a binary companion star, a white dwarf.

yes, red and blue, low and high luminosity, with some on the lower right of the H-R diagram, but others near where the Sun is, some on the upper left, and some on the upper right (red giant region

stars that already have high luminosities, so that they appear bright to us even at large distances; we simply cannot see low-luminosity stars if they are too far away (even if greater in number)

luminosity, mass, size, temperature, and age

yes, L ∝ R^2 x T^4 (a larger size = larger light-emitting surface area = greater luminosity, higher temperature = much greater luminosity (also peaks in bluer colors)

in a band from lower right across to upper left

when on the main sequence (which occupies the majority of a star's lifetime), stars are burning Hydrogen into Helium in their cores (by the p-p chain)

higher luminosity

mass determines where on the main sequence a star lives, and what the main sequence lifetime is for the star; more-massive main sequence stars are on the upper left of H-R diagram; masses range from about 0.1 to 100 times the mass of the Sun; sizes range from 0.1 to 15 times the radius of the Sun

10^–3 to 10^6 times that of the Sun

ages range from a few million years for more massive stars, to much more than 14 billion years — the current age of the Universe — for less-massive stars

use their greater fuel supply more rapidly

Red giants (super giants) and white dwarfs

burning helium or even heavier elements in their cores (not hydrogen anymore); starting to die; size is large, so they are very luminous even though they are relatively cool; top right of H-R diagram

hot, small and dim, so they fall on lower left of H-R diagram; the cooling and fading cores left over from expired low-mass stars

stellar nursery, protostar, main sequence, red giant, planetary nebula, white dwarf

is the ejected envelope (the layers outside the core) of a low- to intermediate-mass star

a white dwarf; held up against gravity not by gas pressure anymore, but by pressure of electrons; about the size of the Earth

have a mass less than 8 % of the Sun, and never heat up enough to have nuclear reactions in their core; also called “failed stars

much shorter lives on the main sequence as stars' mass increases

-hydrogen burning on the main sequence; when core hydrogen is exhausted, then helium burning in the core (while swelling into a red giant

-when core helium supply is exhausted, then carbon burning, and then even-heavier nuclei burning (all while swelling even larger, into a red supergiant)

-leads to the so-called "onion skin" model - concentric shells of fusion zones involving different chemical elements, with the heaviest going on towards the core

-stops burning around iron ("Fe"), because iron is very stable and reactions involving iron do not produce energy — instead they cost more energy than they release, so no further gas pressure support for the star against its own gravitational collapse

violent explosion with the star's core left behind; the explosion itself creates a short-lived highly energetic environment which briefly makes possible the fusion of elements heavier than iron — like gold, silver, etc.

it becomes a neutron star — radius of 5 – 6 km, or roughly "city-sized"

it becomes a black hole — infinitesimally-small radius, "compressed to a point" of infinite density

black hole

the gravity of these collapsed (very dense) objects is so great that even light cannot escape

NO, if you are far enough away from them, they act like normal objects with the same mass

the spherical boundary around a black hole from within whichnothing can escape — not even light (i.e., the "escape velocity" within this distance exceeds the speed of light — "nature's speed limit," nothing can go faster)

the speed of the orbit of a star in a binary system with a black hole

the intense gravity around a black hole warps space, so that a clock appears to slow down as it falls in; that is, light's delivery of the image of the clock to an observer outside is delayed more and more as the clock nears the event horizon

objects are stretched out because the force on the nearer part can be so much greater than the force on farther parts

a singularity — a point of infinite density

-the core becomes a white dwarf — collapse stops because of degeneracy pressure of electrons; core radius same as Earth’s. \n -the white dwarf is surrounded by released outer layers — a planetary nebula

-final core-collapse is preceded by a Type II supernova \n -the core becomes a neutron star — collapse stops because of degeneracy pressure of neutrons; radius 5 – 6 km (city-size)

-final core-collapse is preceded by a Type II supernova \n -the core becomes a black hole — collapse does not stop; all the mass becomes concentrated at a singularity.

the region around a star where liquid water could be present on a planet’s surface — not too hot as to be all boiled off or dried out, and not too cold as to be permanently frozen over

even life on Earth can be found in some "extreme" environments: deep underground; in near or total darkness; high acidity; high radiation; in methane ice; extreme heat and/or pressure

-a method to estimate the number, N, of communicating / technological civilizations in our galaxy at a given time; originally presented by Dr. Frank Drake of the Search for Extraterrestrial Intelligence (SETI) program

-N = R* × fp × ne × fl × fi × fc × L

N = the current number of "intelligent," "communicating" civilizations in the Milky Way Galaxy

R* = the rate of formation of "habitable" \n stars in the galaxy (number per year)

fp = the fraction of these stars (in R*) that \n form planetary systems

ne = the average number of Earth-like \n planets in these systems (from fp)

fl = the fraction of these planets (from ne) \n on which life actually develops

fi = the fraction of planets with life that \n eventually give rise to "intelligent" life

fc = the fraction of intelligent species that \n develop into technological civilizations \n capable of interstellar communication

L = the average lifetime of communicating, \n technological civilizations (in years) \n 10

-lifetime is around a billion or trillion years

-fusion can create low mass elements like He, C, N, O

-end stage, outer lyers are relased and makes planetary nebula

-leaves behind a white dwarf that is earth sized

-lifetime is a few hundred million years

-fusion can create elements between O and Fe

-end stage is a type ll supernova

- leaves behind a neutron star

-neutron star is roughly city sized

-lifetime is a few million years old

-makes elements up through Fe

-end stage is type ll supernova

-leaves behind black hole

-have hundreds, up to thousands, of stars

-the stars formed at about the same time, from the same initial gas & dust cloud

-cluster only stars bound by gravity for a few million years

-tend to have lots of blue stars visible, because of relatively young ages

-hundreds of thousands to millions of stars

-tend to be yellow in color, with a number of red giants

-have many ages around 10 billion years

by seeing what spectral class of star has most recently “turned off” of the main sequence, in the cluster's H-R diagram

they later disperse leaving stars more isolated

bipolar jets

tens of thousands of years

can be a few light years in size

a white dwarf that was the core of a former star

planetray nebulae

is caused by a binary star, wherein one star of the two evolves faster than the othe

-the faster-evolving star, perhaps already a white dwarf, can be close enough to still be partly surrounded by a common envelope of gas from its nearby companion star (which evolves more slowly, but still eventually reaches its own red-giant phase)

-mass transfer of hydrogen from the companion star onto a carbon white dwarf temporarily causes extra burning on the white dwarf's surface

-the white dwarf plus the new material added suddenly brightens; this can happen periodically, repeatedly

occur when very large amounts of material are suddenly added to a white dwarf from a binary companion; the resulting burst destroys the white dwarf (type ll contains hydrogen, type l does NOT because they come from white dwarfs that contain mostly carbon)

luminous, but only happen once (the star is basically destroyed); novae can be a repeating process

filamentary or shredded apperance

different chemicals, also blue haze from electron glow