AST201 week 5

apparent brightness

how bright stars look in our sky, the amount of power reaching us per unit area, follows inverse square law

luminosity

how bright stars are in an absolute sense, total amount of power that a star emits into space

the amount of energy an object emits per unit time

stellar parallax

small annual shifts in a stars apparent position caused by Earth’s motion around the sun

magnitude system

developed by Greek astronomer Hipparchus, designated the brightest stars by “first magnitude”, “second magnitude” and so on

apparent magnitude

what we now call the magnitude system because it describes how bright stars appear in the sky

absolute magnitude

way of describing stellar luminosity

spectral type

astronomers classify stars according to surface temperature, determined from the spectral lines present in a stars spectrum

binary star systems

systems in which two stars continually orbit each other

three class of binary stars

1. visual binary

2. spectroscopic binary

3. eclipsing binary

visual binary

pair of stars that we can see distinctly as the stars orbit each other, sometimes a star will look like a binary, but the second star is dim

spectroscopic binary

if one star is orbiting another, it periodically moves toward us and away from us in its orbit, spectral lines will show alternating blue and red shifts

eclipsing binary

pair of stars that orbit in the plane of our line of sit, when neither star is eclipsed we see the combined light of the stars

hertzsprung-russell diagram

important tool for astronomers, shows luminosity vs surface temp

main sequence HR

most stars fall somewhere along main sequence, prominent streak running from upper left to lower right

supergiants HR

stars in the upper right because they are very large in addition to being very bright

giants HR

just below supergiants, somewhat smaller in radius and lower in luminosity

white dwarfs HR

stars near the lower left, small in radius and appear white in colour cause of their higher temp

luminosity class HR

stars that fall in between categories

main sequence lifetime

a star is born with limited supply of core hydrogen and therefore can remain as a hydrogen-fusing main sequence star for only a limited time

giants and supergiants are

nearing the ends of their lives, alr exhausted the supply of hydrogen fuel in their central cores

variable star

any star that varies significantly in brightness with time

pulsating variable star

alternately expand and contracts, causing the star to rise and fall in luminosity

luminosity of the sun

one solar luminosity or 1 L⊙ or 4 * 10^26 W

luminosity tells us

how bright something is intrinsically

apparent brightness is

how bright something looks to us

we can measure the distances to nearby stars using

parallax

the bigger the angle

the closer the star is

as the earth orbits the sun

the position of a nearby star appears to shift against the background of more distant stars

of all the light a star emits

earth receives only a small fraction, this small fraction determines the stars apparent brightness as seen from earth

the closer a star is to earth

the higher the fraction of its light we receive

inverse square law of light

the amount of light we receive from a star falls with the square of its distance

inverse square law

the brightness we perceive for a star goes as its luminosity divided by the square of its distance from us

twice as far results in 1/4 the brightness. three times as a far results in 1/9 the brightness

inverse square equation

I = L / (4 pi d^2)

edward pickering

grouped spectra into similar groups and labeled them alphabetically

annie jump cannon

made sense of the vast catalogue of stellar spectra, could get rid of most of the spectral categories keeping only A, B, F, G, K, M, and O

each spectral type is associated

with a different colour of star

Class O

≥ 30,000 K, blue

Class B

10,000 - 30,000 K, blue white

class A

7,500 - 10,000 K, white - blue white

class F

6,000 - 7,500 K, white

class G

5,200 - 6,000 K, yellowish white

class K

3,700 - 5,200, yellow orange

class M

≤ 3,700 K, orange red

chemical composition

the set of lines present in a star’s spectrum tell us this

strength of line

= strength of chemical composition

in the atmosphere of a cool star

electrons are sitting in low energy levels from which they can’t absorb this wavelength so the line is weak

for a warmer star

the electrons are boosted into higher energy levels and can absorb this wavelength, so the line is strong

in neutral gases

the electrons are bound to atoms

plasma

a gas that is so hot, the electrons break free. the nuclei have now been ionized

for a very hot star

most of the atoms are ionized, they have lost their electrons completely, the line is weak again

hot stars break

molecules apart, so they dont produce lines

lots of lines from molecules

in the atmospheres of cooler stars

methods for measuring stellar properties

1. chemical composition

2. temperature

3. distance

4. luminosity