AST201H1 Study Guide - Midterm Guide: Hydrostatic Equilibrium, Stellar Classification, Molecular Cloud
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-A Watt is an unit of energy per unit time. So, in a 100 W rating on a light bulb tells
us how much energy it is using per unit time.
-The Sun’s luminosity tells us how bright the Sun is intrinsically, just like the
wattage of the light bulb.
-They are blackbodies, which means their colours are determined entirely by their
temperatures. Every star emits light of all wavelengths. Their colours are
determined by the colour of light they emit the most of (Colour Surface
-For nearby stars, we use parallax. As Earth orbits the Sun, the position of a nearby
star appears to shift against the background of more distant stars. (Parallax
Distance for nearby stars)
-Of all the light a star emits, Earth receives only a small fraction. This small fraction
determines the star’s apparent brightness or apparent magnitude as seen from
-The amount of light received from a star falls with the square of our distance from it.
This is called the inverse square law of light.
-So, a star’s luminosity and apparent brightness are related as: I = L/4pid2
-Therefore, Distance + Apparent Brightness equals Luminosity. (Luminosity
Distance and Apparent Brightness)
-Pickering and his associates collected stellar spectra, naming each new type
alphabetically: A, B, C, D etc. But, these types of stellar spectra didn’t make any
sense. Annie Jump Cannon, made sense of the vast catalog of stellar spectra. She
realized that you could get rid of most of the spectral categories keeping only O, B,
A, F, G, K, and M. This order formed a pattern.
-Each stellar spectrum has absorption lines corresponding to the chemical elements
in the star’s atmosphere. The strengths of the lines depend on the temperature of
the star. This is a second, more precise way to measure a star’s temperature.
-Example: Hydrogen is weak in cool stars because they are not hot enough to excite
it, strong in warm stars because they are warm enough to excite it, weak again in
hot stars because all the hydrogen is ionized.
-Presence of absorption lines composition, strength of particular absorption lines
temperature (method 2)
-The composition of stars is determined by measuring the spectral emission and
-To measure the masses of stars, we rely on binary stars. Binary stars are pairs of
stars orbiting their common center of gravity. Most stars are actually in binaries –
the Sun is an exception.
-In a visual binary, we can see both members of the system. This only works for
oUsually, we deduce the presence of a binary companion indirectly.
-Spectroscopic binary where we see the Doppler shift in the light from the system as
the two stars orbit one another (blueshift when moving towards us and redshift
when moving away from us).
-Eclipsing binary is where we see the system dim as the stars eclipse one another.
-If a system is BOTH a spectroscopic and an eclipsing binary, we can use Kepler’s
laws to work out the masses of both stars.
-To organize all this information, we plot luminosity vs. surface temperature
(backwards i.e. increasing when heading towards the y-axis). This is known as the
Herztsprung-Russell (H-R) diagram.
-Stars in the main sequence of the H-R diagram are fusing hydrogen in their cores
i.e. they are alive. Confusingly, all stars on the main sequence are called dwarf
stars. This has nothing to do with their size.
-Stars not on the main sequence are either in the process of dying or are dead. They
come in many groups. A Star’s luminosity class tells us which group of stars on the
H-R diagram it belongs to. Supergiants (I), Bright Giants (II), Giants (III), Subgiants
(IV), Main Sequence stars (V). Luminosity class is not equal to luminosity or spectral
-If we know a star’s mass and luminosity, we can figure out how long it will live.
(Lifetime = Mass/Luminosity).
- More massive stars crush their cores more and burn hotter, so they actually use up
their fuel faster.
- Most stars form in clusters. We can use the H-R diagram of a cluster to figure out
how old the cluster is.
-Properties of stars (Summarized):
•Temperature – colour/strength of spectral lines
•Distance – parallax
•Luminosity – distance + apparent brightness
•Spectral Type – Colour/temperature/pattern of spectral lines
•Luminosity Class – Shape of spectral lines
- Most distant stars we can see are blue because although these stars are rare, they
are extremely luminous. But, these stars don’t live long, so they mostly die where
they were born.
-Therefore, to find the sites of star formation, look for blue stars. S
-Taking pictures in multiple parts of the electromagnetic spectrum is essential to
understand star formation.
-Dust absorbs visible light, heats up, and reemits that light in infrared. Where there
is a lot of dust, there are a lot of red nebulae and blue stars.
-For example: Orion Nebula – in visible light we mainly see gas but in infrared light
we see a very young star cluster. In such nebulae, we tend to find lots of dark
patches, which are places where the gas and dust are concentrated enough that they
block out background light. Often these dark, dusty structures are studded with very
young stars. They are the places where stars come from.
-B68 (a molecular cloud) is special because in visible light you don’t see anything, but
in infrared light, we can see right through it. We can see how much light it blocks
and figure out its structure.
-The molecular cloud is in hydrostatic balance (gravity vs. thermal pressure). If the
molecular cloud got a little cooler, the pressure inside would fall and gravity would
win. As it contracted, conservation of angular momentum would cause to spin faster.
It would also flatten. As it contracted, conservation of energy would also cause it to
-From a molecular cloud fragment to… a spinning disk of dust and gas orbiting a
protostar to… a protostar surrounded by a disk and a pair of bipolar jets.
-Protostellar jets are one of the few phenomena in the universe that change fast
enough that we can watch them doing it.
-Steps from how a new star joins the main sequence:
•A protostar assembles from a collapsing cloud fragment. It is concealed beneath
a shroud of dusty gas.
•The protostar shrinks and heats as gravitational potential energy is converted
into thermal energy.
•Surface temperature rises when radiation becomes the dominant mode of energy
flow within the protostar.
•The fusion rate increases until it balances the energy radiated from the star’s
A watt is an unit of energy per unit time. So, in a 100 w rating on a light bulb tells us how much energy it is using per unit time. The sun"s luminosity tells us how bright the sun is intrinsically, just like the wattage of the light bulb. They are blackbodies, which means their colours are determined entirely by their temperatures. Their colours are determined by the colour of light they emit the most of (colour surface. As earth orbits the sun, the position of a nearby star appears to shift against the background of more distant stars. (parallax . Of all the light a star emits, earth receives only a small fraction. This small fraction determines the star"s apparent brightness or apparent magnitude as seen from. The amount of light received from a star falls with the square of our distance from it. This is called the inverse square law of light.