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JPL Small-Body Database Browser
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Phil Jones, Clifton, Texas
Classification of Stars From Their Colors
(Surface Temperatures)
For now, stars are objects that are currently
generating energy from nuclear fusion in their
cores.
The hottest stars have temperatures ~30,000K.
The coolest stars have temperatures ~3,000K.
Classification of Stars
A.J. Cannon classified ~250,000 stars by
inspecting tiny spectra on photographic
plates.
Spectral lines were not understood in the
context of quantum mechanics until the
1920’s, so they were classified according to
visual impressions, the way you might put the
chimpanzees next to the gorillas in a zoo.
Classification of Stars
For this reason, the spectral classes are
somewhat arcane. From hottest to coolest,
they are:
OB A F G K M
Oh, Be A Fine Girl/Guy
Classification of Stars
For this reason, the spectral classes are
somewhat arcane. From hottest to coolest,
they are:
OB A F G K M
Oh, Be A Fine Girl/Guy/Gorilla
Classification of Stars
For this reason, the spectral classes are
somewhat arcane. From hottest to coolest,
they are:
OB A F G K M
Oh, Be A Fine Girl/Guy/Gorilla Kissed Me
Classification of Stars
These classes refer only to the star’s surface
temperature (color) and say nothing about it’s
diameter or mass. A Red Giant may be larger
in size and cooler than a “bluer” star while
having less mass. We’ll discuss this when we
talk about the lives of stars.
As yet, we have no idea how far away a star is.
Classification of Stars
• Furthermore, the classifications are each
divided into tenths, with labels going from 0
to 9
– e.g. If a star is said to be a G-class star, it could, at
its brightest, be classified as a G9 star, and at its
dimmest, be classified as a G0 star.
• The Sun is classified as a G2 star.
– (There is a further classification based on the
width of spectral lines, but we will not discuss it)
• Note that the stars at the cooler end of the
classifications (K, M classes) don’t emit much
at the blue end of the spectrum (and tend to
show many more absorption lines) while the
starts at the hotter end of the classifications
(O, B, A classes) don’t emit much at the red
end of the spectrum
– Why?
Proper Motion
Proper motion refers to the apparent motion of
a star across the sky dome.
In general this is so slow that it would go
unnoticed except for close telescopic
observations or extremely long time periods.
Proper Motion
Proper motion was discovered by Edmund
Halley. He compared the positions of bright
stars that had been recorded by (Ptolemy).
http://www.hwy.com.au/~sjquirk/images/film/barnard.html
Barnard’s Star has the highest known proper motion of
10.3”/year.
Even at that rate, Barnard’s Star will appear to move
the angular diameter of a Full Moon only after 175
years.
Animation by Steve Quirk – Frog Rock Observatory
Radial Velocity
By looking at the Doppler shift of a spectral line
in a star, you can determine the radial velocity.
That’s the relative velocity on a line between the
observer and the star.
Spectral shift could effectively change how we
would classify a star (if the radial velocity is
great enough), so it is important that we know
the location of particular spectral lines very
well.
Quiz #3
Question 1: A difference of five
magnitudes corresponds to
a factor of _____ in
brightness.
A) Five
B) Ten
C) One Hundred
D) Five Hundred
Question 2: The order of the
four fundamental forces in
physics, from weakest to
strongest is…?
A) Strong, Weak,
Electromagnetic, Gravity
B) Weak, Electromagnetic,
Strong, Gravity
C) Electromagnetic, Weak,
Gravity, Strong
D) Gravity, Electromagnetic,
Weak, Strong.