AST201 week 5

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

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

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

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

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

way of describing stellar luminosity

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

systems in which two stars continually orbit each other

1. visual binary

2. spectroscopic binary

3. eclipsing 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

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

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

important tool for astronomers, shows luminosity vs surface temp

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

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

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

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

stars that fall in between categories

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

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

any star that varies significantly in brightness with time

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

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

how bright something is intrinsically

how bright something looks to us

parallax

the closer the star is

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

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

the higher the fraction of its light we receive

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

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

I = L / (4 pi d^2)

grouped spectra into similar groups and labeled them alphabetically

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

with a different colour of star

≥ 30,000 K, blue

10,000 - 30,000 K, blue white

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

6,000 - 7,500 K, white

5,200 - 6,000 K, yellowish white

3,700 - 5,200, yellow orange

≤ 3,700 K, orange red

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

= strength of chemical composition

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

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

the electrons are bound to atoms

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

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

molecules apart, so they dont produce lines

in the atmospheres of cooler stars

1. chemical composition

2. temperature

3. distance

4. luminosity