Module :

Activities:

From H-R Diagrams

Spectral Types

Swinburne Online Education Exploring Stars and the Milky Way

© Swinburne University of Technology

The seven classesThe 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 F G K M

violet blue blueblue-white

white-yellow

orange-red

red

>28000K10000-28000K

7500-10000K

6000-75000K

5000-6000K

3500-5000K

<3500K

few visible absorption lines, weak Balmer lines, ionised helium lines

neutral hydrogen lines, more prominent Balmer lines

strongest Balmer lines, other strong lines

weaker Balmer lines, many lines including neutral metals

Balmer lines weaker still, dominant ionised Calcium lines

neutral metal lines most prominent

strong neutral metal lines and molecular bands

Some examples

We’ll now browse the skies for samples of each classification.

Type OThis 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 B

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 B

Type ASirius 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 GOur own Sun is a G-type star. This means that it is yellow in colour, and not very hot at all in stellar terms.

Type KArcturus 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 MBetelgeuse 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 :

Activity:

From H-R Diagrams

Swinburne Online Education Exploring Stars and the Milky Way

© Swinburne University of Technology

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

O B A F G K M

low

lum

ino

sit y

hi g

h

White dwarfs

Red dwarfs

Main sequence

Super-giants

Giants

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.

Looking for patterns

high temperature low

low

lum

ino

sit y

hi g

h

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.

T increasingT increasing

L in

crea

sing

L in

crea

sing

Mass increasing

Mass increasing

Mass and luminosityIt turns out that there is a very nice relationship indeed.

Lum

inos

ity (

com

pare

d to

the

Sun

)Lu

min

osity

(co

mpa

red

to t

he S

un)

Mass (compared to the Sun)Mass (compared to the Sun)

If you’ve got a line like this,then you can predict

luminosity from mass,and vice versa

If you’ve got a line like this,then you can predict

luminosity from mass,and vice versa

We do more on howluminosity and mass

are related in thenext Activities

We do more on howluminosity and mass

are related in thenext Activities

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.

What are they doing?

high temperature low

low

lum

ino

sit y

hi g

h

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 producinglots of energymaking helium

Big stars producinglots of energymaking helium

Medium-sized starsproducing energy

making helium

Medium-sized starsproducing energy

making helium

Teensy starsproducing a wee

bit of energymaking helium

Teensy starsproducing a wee

bit of energymaking helium

Luminosity classesThe various families of bright stars form luminosity classes.

(We won’t be discussing the white dwarves here.)

Lum

inos

ity (

com

pare

d to

the

Sun

)Lu

min

osity

(co

mpa

red

to t

he S

un)

Temperature (decreasing)Temperature (decreasing)

Ia = most luminous supergiants Ia = most luminous supergiants

Ib = less luminous supergiants Ib = less luminous supergiants

II = luminous giants II = luminous giants

III = normal giants III = normal giants

IV = subgiants IV = subgiants

V = main sequence stars V = main sequence stars

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

White Dwarfs

Main Sequence

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.

Summary

Cooler and redder

To keep the picture simple, we’ve left off the “reverse” arrows (such as “Hotter and bluer”).

Brig

hter

(us

ually

larg

er)

Bright

er &

coo

ler

(us

ually

larg

er)

Brighter & hotter (bigger/more mass)

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