Studied by 6 people
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
visual binary
spectroscopic binary
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
chemical composition
temperature
distance
luminosity
Note 
Flashcard (121)