Lecture 13 - Star Patterns Flashcards
what are the axes of the HR diagram?
X: temp (increasing right to left)
Y: luminosity
what are the 3 types of stars on the HR diagram and their properties?
- main sequence (most)
- supergiants, giants –> cooler, brighter, larger
- white dwarfs –> hotter dimmer, smaller
what is the magnitude scale?
LOWER magnitude = BRIGHTER star
why is the magnitude calibrated to the response of the human eye?
our eyes detect brightness better than size –> i.e. if we zoom in on a star, its size doesn’t change much
what is apparent magnitude? and its equation?
what happens to apparent brightness if there is a larger apparent magnitude?
describes how bright a star appears in the sky
equation: (m_1 - m_ref) = -2.5log_10 * (I_1/I_ref)
larger apparent magnitude = dimmer apparent brightness (I)
what is m_ref?
magnitude of the star Vega = 0
if apparent magnitude (m) changes by 5, how does apparent brightness change?
changes by a factor of 100
what is absolute magnitude? and its equation?
what do we need to consider?
describes the intrinsic brightness of a star
m - M = 2.5 log_10 (d/10)^2 = 5(log_10*d - 1)
must consider the distance from earth in parsecs!
if apparent magnitude changes by 5, how does distance change?
distance changes by a factor of 10
if star A has an apparent magnitude of 0 and star B has an apparent magnitude of 5, how much brighter is star A than star B?
if star A and B have the same absolute magnitude, how much further away is star B than star A?
Star A: 100x brighter
Star B: 10x further away (100x dimmer)
what do stellar luminosity classes indicate?
indicate radius and luminosity of a star
what does stellar luminosity classes indicate vs spectral type?
stellar luminosity classes –> radius and luminosity
spectral type –> surface temp and colour
what are the 5 stellar luminosity classes and how does size and luminosity change between them?
I –> supergiant
II –> bright giant
III –> giant
IV –> subgiant
V –> main-sequence star
size and luminosity decrease from I - V
what 3 properties of stars can we infer from HR diagrams that display temp and luminosity?
- colour
- spectral type
- radius
why are red stars usually large?
they emit light with lower energy (red) so to reach a high luminosity, they need to increase their size
what size of star emits blue light vs red light? why?
blue light is high energy so the star is small
red light is low energy so the star is large
what are stars doing when they are in the main sequence? how much of their lifetime is spent in the main sequence?
actively fusing H –> He
stay in main sequence for most of their lifetime in a fixed position on the main sequence
do high mass stars have long or short lifetime? describe why?
high mass stars have shorter lifetime
they have more H so they fuse H –> He very quickly and use it up more quickly, therefore shorter lifetime
what does luminosity indicate about H fusion? what does this indicate about stars with a lot of H?
luminosity indicates how quickly they use up H
therefore, high mass stars (lots of H) are MORE LUMINOUS and HOTTER
If a star’s mass is 10x more massive and 10,000x more luminous than the sun, how much more H does the star have and how much more quickly can it use up the H? what does this indicate about the lifetime of the star?
10x more massive = 10x more H
10,000x more luminous = uses up core H 10,000x more quickly
lifetime is 10/10,000 times as long as the sun (1/1000)
overall describe the 3 qualities of a more luminous star vs a less luminous star
more luminous = hotter = blue = more massive
less luminous = cooler = red = less massive
what is luminosity based on (2)?
what is the range of luminosities for stars?
brightness and distance
10^-4 to 10^6 times the luminosity of the sun
what is temperature based on (2)?
what is the range of temperatures for stars?
colour and spectral type
3000K - 50,000K
what is mass based on (2)?
what is the range of masses for stars?
period and average separation of binary star orbit
0.08 - 100 times the mass of the sun
describe the properties of giants and supergiants
in upper right of HR:
- cooler
- larger
- more luminous
- more massive
- red
when do main sequence stars become giants?
when they run out of core H
what determines the amount of light emitted?
the surface temp determines the amount of light emitted per SA
when do stars become white dwarfs?
when they completely run out of H
describe the properties of white dwarfs
in lower left of HR:
- hotter
- smaller
- less luminous
- less massive
- white
where on the HR diagrams are stars that have changed THE LEAST 10 billion years from now?
bottom right –> cooler and less massive so they run out of core H less quickly and are therefore oldest
where on the HR diagram are stars that are no more than 10 billion years from now?
top left –> hotter and more massive so they run out of core H less quickly and are therefore youngest
what are variable stars? what causes this?
stars that varies significantly in brightness/luminosity with time with pulsations
caused by imbalance btwn power from the core and the power radiated from the surface so the star alternately EXPANDS and CONTRACTS, varying in brightness as it tries to find a balance
when can variable stars occur (2)?
- if there’s excess gravity
- if upper layers of star are too opaque to let energy escape
describe how excess gravity causes variable stars (3 steps)
- excess gravity collapses star and it gets smaller
- radiation pressure increases and it gets larger to reverse the radiation pressure
- then gravity is in excess again and it gets smaller
describe how opaque upper layers cause variable stars (3 steps)
- pressure builds up under the photosphere and the star expands
- outer layers puff up and become transparent so energy can escape
- pressure decreases so the star contracts and has opaque outer layers
where are variable stars located on HR diagram?
in an INSTABILITY STRIP between the main sequence and red giants
what is a star cluster?
group of stars that formed from 1 interstellar cloud
why is it helpful to study star clusters?
- all stars in a cluster are ~ same distance from earth
- all stars in a cluster formed ~ same time
therefore, we can compare the properties of stars with similar ages
what are the 2 types of star clusters?
- open cluster
- globular cluster
what is an open cluster? age? location?
why do they form?
- a few thousand loosely packed stars
- very young
- in the disk of the galaxy
result of star-forming regions in a galaxy with large H2 reservoir that collapsed to make stars
what is a globular cluster? age? location?
describe the stars and their movement
- over a million stars in a dense ball bound by gravity
- older
- in the halo of the galaxy
not actively evolving or making new stars but the stars move around
- if 2 stars pass close to each other, the gravitational pull between them will alter their trajectories/speeds and send them in new directions
- if they eject from the cluster, the cluster gets more compact
describe the age of the purple cluster
YOUNG –> in top left
more luminous and larger with higher temp so they burn H more quickly
describe the age of the teal cluster
trails off the main sequence so it is old enough for stars to die but young enough for H fusion to still be occurring
the main-sequence turnoff point indicates the age of a cluster!
describe the type and age of the yellow cluster
globular cluster
further in bottom right and heaviest ones are now giants so it is OLDER
describe the order of stars dying in terms of their colour
massive blue stars die first, then white, yellow, orange, and red
how can we determine accurate ages of star clusters?
compare models of stellar evolution to the cluster data
