Transcript ref H-R Spectral types

Module :
Swinburne Online Education Exploring Stars and the Milky Way
Activities:
From H-R
Diagrams
Spectral Types
© Swinburne University of Technology
The seven classes
The seven spectral classes are listed below. Traditionally,
astronomers remember the sequence by the mnemonic
“Oh, be a fine girl, kiss me!”.
If you don’t like this, see if you can make up your own!
O
B
A
violet
blue
blue
F
G
K
bluewhite
600075000K
whiteyellow
50006000K
orangered
35005000K
10000>28000K
28000K
750010000K
few visible
absorption
lines, weak
Balmer lines,
ionised
helium lines
weaker
strongest
Balmer lines,
Balmer lines, many lines
other strong including
lines
neutral
metals
neutral
hydrogen
lines, more
prominent
Balmer lines
Balmer lines
weaker still,
neutral metal
dominant
lines most
ionised
prominent
Calcium
lines
M
red
<3500K
strong
neutral metal
lines and
molecular
bands
Some examples
We’ll now browse the
skies for samples of
each classification.
Type O
This is Sirius (“scorching”), the Dog
Star, one of the brightest stars in
the sky. It is the jewel on the collar
of Canis Major (the larger of Orion’s
two hunting dogs).
Sirius A is a type A star, but it has a dwarf companion (Sirius B) which is
a type O but is too small to see here.
Other examples include Meissa, or Orionis lambda in the constellation
Orion (actually, in Orion’s helmet!).
Type
TypeBB
The sky over the Canary
Islands includes the
constellation of Orion the
Hunter.
Rigel is a type B star
clearly visible to the naked
eye below Orion’s “belt”
and his “sword”.
Type A
Sirius is the brightest
star in the constellation
Canis Major (“Big Dog”).
It is also the brightest
star seen from Earth.
One of the two stars that
make up what we call
“Sirius”,Sirius A is a type
A star.
Type F
Procyon I, a type F star,
is the brightest star in
the constellation Canis
Minor (“Little Dog”).
Type G
Our own Sun is a Gtype star. This means
that it is yellow in colour,
and not very hot at all in
stellar terms.
Type K
Arcturus is the bright
star under and to the
left of Comet
Hyakutake in this
photo, which was
taken in 1996 in New
South Wales.
Aracturus is the
brightest star in the
constellation Bootes,
the Bear Driver.
Type M
Betelgeuse is one of the
best-known red giant
stars.
If Betelgeuse were to
swallow the solar
system, it would do so
out as far as Jupiter!
This terrific photo was
taken by the Hubble
Space Telescope.
This activity
In this Activity we have had a look at the Balmer series,
and how its occurrence in the photospheres of stars will
vary with temperature.
The temperature, and hence the colour and spectral line
strength characteristics of stars, is used to classify them
into types O, B, A, F, G, K and M-type stars.
Module :
Swinburne Online Education Exploring Stars and the Milky Way
Activity:
From H-R
© Swinburne University of Technology
Diagrams
Summary:
In this Activity the Herzsprung-Russell (H-R) diagram is
introduced:
• how H-R diagrams came into use
• patterns and trends in H-R diagrams
• and a bit of history about classification of stars
H-R diagrams and spectral classes
We’ll use this
version of an H-R
diagram to show
how spectral
classes appear in
that format.
Supergiants
Giants
Main
sequence
White
dwarfs
Red
dwarfs
O
B
A
F
G
K
M
Looking for patterns
L increasing
Huge, cool stars
appear in the top
right, and small, hot
stars tend to gather in
the bottom left.
But the rest of the
stars lie somewhere
along the main
sequence.
Astronomers
suspected that there
might be some kind of
laws or mathematical
relationships between
L and T and mass.
high
T increasing
temperature
low
It turns out that
there is a very
nice relationship
indeed.
Most of the stars
on the main
sequence lie in
the band shown
on the right.
It means that
these stars all
“work” the same
way, obeying the
same laws.
Luminosity (compared to the Sun)
Mass and luminosity
If you’ve got a line like this,
then you can predict
luminosity from mass,
and vice versa
We do more on how
luminosity and mass
are related in the
next Activities
Mass (compared to the Sun)
What are they doing?
What are those laws?
What could all these
stars be up to that is the
same, and means that
there is a very clear link
between their size and
their energy output?
The answer is: they are
all producing almost all
of their energy by
hydrogen fusion in
the core, and the end
result is helium.
Big stars producing
lots of energy
making helium
Medium-sized stars
producing energy
making helium
Teensy stars
producing a wee
bit of energy
making helium
high
temperature
low
Luminosity (compared to the Sun)
Luminosity classes
Ia = most luminous supergiants
Ib = less luminous supergiants
II = luminous giants
III = normal giants
IV = subgiants
V = main sequence stars
Temperature (decreasing)
The various
families of bright
stars form
luminosity
classes.
(We won’t be
discussing the
white dwarves
here.)
The evolution
of a star is
often drawn
on a similar
diagram,
which can
show how its
temperature
and
luminosity
change with
time (just like
Jim’s hair
colour and
hair length).
Giants
Main Sequence
White Dwarfs
Summary
Cooler and redder
Brighter (usually larger)
H-R diagrams
are often used
to compare the
properties of
stars of different
luminosities and
temperatures.
For instance, a
star to the right
will be cooler
and redder.
A star higher up
will be brighter,
and often
larger.
To keep the picture simple, we’ve left off the
“reverse” arrows (such as “Hotter and bluer”).
In this Activity
In this Activity we had a look at
• how stellar classification came about,
• how H-R diagrams were developed, and
• why the temperature scale goes “the wrong way”.
In the next Activity, we will study a real example by
continuing the story of how a huge, cool molecular
cloud can become a blazing star, and we’ll use H-R
diagrams to help us.
Image Credits
Hubble Space Telescope picture of globular cluster G1 circling galaxy M31:
http://oposite.stsci.edu/pubinfo/pr/96/11/A.jpg
Star trails in the Southern Cross, © David Malin
http://www.aao.gov.au/local/www/dfm/image/s_cross_trails.jpg
Image Credits
Orion - star trail step focus photograph © David Malin
http://antwrp.gsfc.nasa.gov/apod/ap980829.html
Sirius, the Dog Star. Credit: ROSAT, MPE, NASA, Courtesy Skyview
© Oniversity of Leicester
http://antwrp.gsfc.nasa.gov/apod/ap960902.html
Sky over the Canary Islands, showing Rigel. Credit and copyright A. Vannini,
G. Li Causi, A. Ricciardi, A. Garatti
http://antwrp.gsfc.nasa.gov/apod/ap980305.html
Betelgeuse Credit A. Dupree (CfA), R. Gilliland (STScI), NASA
http://antwrp.gsfc.nasa.gov/apod/ap970216.html
Solar Disk in broadband K Ca Data. © J. Harvey, National Solar Ovservatory
(Tucson/Kitt Peak, AZ).
http://www.hao.ucar.edu/public/slides/slide2.html