astro 7n exam 2

What is the composition of the sun?

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

How is the sun stable?

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

How is gas pressure provided in the suns core?

nuclear fusion reactions

What is the temperature of the suns core?

15 million degrees Kelvin

What is the suns diameter in comparison to earth, what is the mass?

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

What is the luminosity is comparison to a watt light bulbs?

4 × 10^24, 100 Watt light bulbs

Where is 99.9% of all mass in the solar system?

the sun

How long has the sun been shining? how long will it continue to shine?

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

What are sun spots?

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

What is nuclear fusion?

a long lasting energy source for stars

What happens when 4 protons combine?

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

What are the steps of 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

what is created in every step of the proton proton chain?

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

Where does the energy output of the p-p chain come from?

E = m c^2

What is the mass of helium-4 compaired to the mass of 4 protons?

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

What are the layers of the sun from inside to outside?

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

The core of the sun

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

The radiative zone of the sun

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

the convective zone of the sun

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

the photosphere of the sun

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

The chromosphere of the sun

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

The flares of the sun

an eruption coming out of Sun due to magnetic activity

the prominence of the sun

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

the corona of the sun

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

solar winds of the sun

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

What is luminosity?

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

What is brightness?

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

What is the inverse-square law?

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

What happens If two stars have the same luminosity and one is ten times farther away than the other?

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

What is the basic concept of the parallax method to measure distances

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.

what are the opposite ends of the “baseline” is the parallax method?

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

The target star appears to move (relative to more-distant stars)…

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)

Examples of the parallax equation

- 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

Sometimes a star that appears fainter is still ______ then one that appears brighter…

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

What is the Hertsprung-Russell Diagram (the "H-R Diagram")

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)

Horizontal axis of the H-R diagram

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)

stars effective temperature is estimated using its spectral class, what are the major spectral classes

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

mnemonic for spectral classes

“__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”)

What is spectral class deterimined by?

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

Vertical axis of the H-R diagram

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

What is the nearest star?

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

Most nearest stars are…

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

What is the brightest star in the nighttime sky?

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

Do the brightest stars (as we see them from earth) have more veried properties?

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

What are the “brightest stars” biased towards?

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)

what are the properties of a star?

luminosity, mass, size, temperature, and age

Do size and temperature effect luminosity?

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)

Where are main sequence stars located on the H-R diagram?

in a band from lower right across to upper left

What are stars burning on the main sequence?

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)

For stars on the main sequence, higher temperature means…

higher luminosity

What determines where stars on the main sequence live?

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

What do the luminosities of the main sequence stars range from?

10^–3 to 10^6 times that of the Sun

What is the age range for stars on the main sequence?

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

More massive stars…

use their greater fuel supply more rapidly

What types of stars are not on the main sequence?

Red giants (super giants) and white dwarfs

red giants (supergiants)

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

white dwarfs

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

Suns history

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

What is a planetary nebula

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

What is the end state of a former stars core? (for lower mass stars)

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

“brown dwarfs”

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

More massive stars…

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

Burning stages of more massive main sequence stars

-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

What is a type II supernova?

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.

After the star collapses, what happens if the remaining core is about 1.4-3 solar masses

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

After the star collapses, what happens if the core is more then 3 solar masses?

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

what is the end state for the highest-mass stars (about 40 solar masses) left behind after their supernova explosion?

black hole

what happens in a black hole?

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

Do black holes “suck things in”

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

What is the event horizion?

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)

How to measure the black holes mass?

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

What does the intense gravity around a black hole cause?

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

strong tidal forces operate near a black hole…

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

What is at the center of a black hole

a singularity — a point of infinite density

If a star begins with a mass < 8 solar masses

-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

if a star begins with a mass of 8-40 solar masses

-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)

if a star begins with > 40 solar masses

-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.

what is a habitable zone?

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

while it is commonly assumed that life requires liquid water…

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

What is the drake equation?

-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

What are the variables in the drake equation?

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

Low mass stars

-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

medium mass stars

-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

high mass star

-lifetime is a few million years old

-makes elements up through Fe

-end stage is type ll supernova

-leaves behind black hole

Open star clusters

-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

Globular star clusters

-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

how can age of clusters be determined?

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

What happens to many stars that form in clusters?

they later disperse leaving stars more isolated

what leads to symmetry in nebula

bipolar jets

how long do planetary nubulae last

tens of thousands of years

how big are planetary nebulae?

can be a few light years in size

what is at the center of the nebula?

a white dwarf that was the core of a former star

what will be the end state of our sun?

planetray nebulae

What is a nova?

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

steps of a nova

-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

What is a type la supernovae?

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)

supernovae are more _________ than novae, but….

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

what kind of apperance do supernovae have?

filamentary or shredded apperance

where do the different colors in a supernovae come from?

different chemicals, also blue haze from electron glow