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April Fools! What's wrong with that orbit?
Stars come in a wide variety! The Sun is an ordinary star--we can see in
these images that stars have all different colors, and thus different
surface temperatures. Stars differ in temperature, size,
composition, age, and rotation.
The brightest stars are 100s of thousands of times brighter than our Sun,
and the most massive are up to around 100 times more massive than our
Sun. The largest are about 1,000 times as large as the Sun, about the
size of the orbit of Jupiter!
On the other end, the least massive stars are less than 10% as
massive as our Sun, cooler than 3,000 K at their surface, about 0.1% as
bright. In fact the least massive stars are the most common!
The H-R diagram is extremely useful to astronomers! Make sure you
are familiar with it, because it will be the basis of much of the 2nd half
of this course!
We need to know two things to make an H-R diagram: the spectral type of a
star (basically its temperature), and how bright the star is (its
luminosity).
Spectral type is labelled with the letters OBAFGKM (which used to be
followed by RNS). Here's more
information on the classification scheme.
The first picture from Astronomy Picture of the Day, and the second from
here.
(In response to a question)--there are new
categories for the very dimmest stars, L and T. They are defined by: L
stars have temperatures from 1300 to 2500 K, show molecular absorption
from CrH and FeH and hardly any from TiO and VO which are commin in M
stars. T stars show lines from methane in their spectrum like Jupiter and
may not really be stars.
Here
is a web site showing an example of a binary star system.
Visual
binaries are where we can actually see the two stars as separate.
Open cluster: Pleiades
More on
the HR diagram and classification of stars
HR Diagram
page, helpful on homework problem 19 on calculating the radius of a
star
Annie
Jump Cannon, the woman who pioneered the classification of stars by
their spectra
Distances to the nearest stars are known through parallax,
as measured most accurately by the Hipparcos
satellite
Great
page on stellar classification
More
on stellar classification, including the new categories of coolest stars,
L and T, with temperatures below 2500 K
OBAFGKM
mnemonics
More
mnemonics
Q: What exactly are those lines that are used to classify based on
spectral type?
A: This is from Donald Clayton's Principles of Stellar Evolution and
Nucleosynthesis (1983):
Class O: Temperatures of 25,000 K and up. Lines of ionized helium are
prominent.... lines of ionized helium will appear only in such an
extremely hot gas. Other atoms in high degrees of ionization are
observed.
Class B: 25,000 to 11,000 K. The lines of hydrogen and neutral helium are
conscpicuous at class B0. Ionized oxygen and ionized carbon become strong
at class B3. Neutral helium lines are strongest at class B5. Hydrogen
lines become progressively stronger in the higher-numbered subdivisions of
this class...
Class A: 11,000 to 7500 K. At class A0 hydrogen and ionized magnesium
lines are strongest, whereas the helium and ionized oxygen lines have
disappeared. Hydrogen lines weaken continuously in the higher
subdivisions of this class, whereas ionized metals (Fe, Ti, Ca, etc.)
strengthen. The hydrogen lines will continue to weaken as we progress
toward cooler stars, because the temperature becomes less and less
sufficient to maintain a significant fraction of hydrogen atoms in the
first excited state. The lines of ionized metals are growing stronger
because the relative number of metals in low degrees of ionization is
increasing...
Class F: 7500 to 6000 K. Class F0 is rich in lines of the ionized
metals... the strongest being... single ionized calcium. Metallic lines,
particularly iron, strengthen and hydrogen lines weaken in the
higher-numbered subdivisions of this class.
Class G: 6000 to 5000 K. In this class the lines of the neutral metals
become strong, whereas the hydrogen lines continue to weaken. Lines of
ionized calcium are very strong. Molecular bands of Cn and CH appear.
The sun belongs to the class G2.
Class K: 5000 to 3500 K. In general, molecular bands and lines of neutral
metals become much stronger, whereas the lines of hydrogen and ionized
metals continue to weaken. At K5 the lines of TiO are weakly visible.
Class M: 3500 to 2200 K. The characteristic feature of the spectrum of
class M stars is the appearance of complex molecular oxide bands, of which
TiO bands are strongest.
You are encouraged to work in groups and hand in a group assignment (up to
3 people).
News: a new
asteroid
A new Earth-approaching asteroid, Hatchett-McCray 2, has just been
discovered! View its orbit here.
Stars -- the Variety of Stars
For a larger view, click here.
Stars
-- The HR Diagram
How we know stellar masses
The most direct way we have of knowing the masses of stars is to look at
the orbits of stars in binary systems.
Clusters of Stars
Hubble images of globular clusters: 1,
2. 
Links
HR
Diagram
Chapter Problems 15 1-10 15 19 
